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Birgit Gaye

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The University of Sheffield

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Ege University

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Cochin University of Science and Technology

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Santa Rosa Junior College

Universidad de Oriente, Venezuela

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Papers by Birgit Gaye

The Ballast Effect in the Indian Ocean

In this study, data obtained from a sediment trap experiments off South Java are analyzed and com... more In this study, data obtained from a sediment trap experiments off South Java are analyzed and compared to satellite-derived information on primary production and data collected by deep-moored sediment traps in the Arabian Sea 10 and the Bay of Bengal. The aim was to study the relative importance of primary production and the ballast effect on the organic carbon export and the CO 2 uptake of the biological carbon pumps. Therefore, data obtained from sediment trap experiments carried out in other ocean basins were also integrated into the data analysis and a four-box model was developed. Our data showed that the organic carbon flux in the highly-productive upwelling system in the Arabian Sea was similar to those in the low productive system off South Java. Off South Java as in other river-influenced regions, lithogenic 15 matter supplied from land mainly controls the organic carbon flux via its ballast effect in sinking particles, whereas carbonate produced by marine organisms appears to be the main ballast material in the high productive regions. Since the carbonate flux tends to increase with an increasing export production, it is difficult to quantify the relative importance of productivity and the ballast effect on the organic carbon flux into the deep sea. However, the export of organic matter into the deep sea represents a loss of nutrients for the pelagic ecosystems, which needs to be balanced by mode water nutrient 20 supply into the seasonal thermocline to sustain the productivity of the pelagic system. The amount of preformed nutrients utilized during the formation of the exported organic matter strongly influences the impact of the ballast effect on the CO 2 uptake of the organic carbon pump. Accordingly, this is stronger at higher latitudes where preformed nutrients are formed than at lower latitudes where the euphotic zone is nutrient depleted. Nevertheless, the ballast effect enhances the export of organic matter into the deep sea and favors the sedimentation of organic matter in river-influenced regions. Since globally > 25 80% of organic carbon burial occurs in river-dominated systems, the lithogenic ballast is assumed to play an important role in the Earth's climate system on geological time scales.

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean

Biogeosciences, Jul 12, 2019

The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world'... more The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼ 30 • S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrientdepleted surface waters of the oligotrophic IOSG with nitrate (NO - 3 ) and phosphate (PO 3- 4 ) concentrations of < 3 and < 0.3 µmol kg -1 , respectively (< 300 m; σ < 26.4 kg -1 m -3 ). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ 15 N > 7 ‰; δ 18 O > 4 ‰) at 400-500 m (26.6-26.7 kg -1 m -3 ), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg -1 m -3 ) of the gyre, indicated by an N deficit (N * ∼ -1 to -4 µmol kg -1 ) and by elevated isotopic ratios of

Signals of Holocene climate transition amplified by anthropogenic land-use changes in the westerly–Indian monsoon realm

Climate of The Past, Aug 24, 2021

The Indian summer monsoon (ISM) rainfall is the lifeline for people living on the Indian subconti... more The Indian summer monsoon (ISM) rainfall is the lifeline for people living on the Indian subcontinent today and was possibly the driver of the rise and fall of early agricultural societies in the past. The intensity and position of the ISM have shifted in response to orbitally forced thermal land-ocean contrasts. At the northwestern monsoon margins, interactions between the subtropical westerly jet (STWJ) and the ISM constitute a tipping element in the Earth's climate system because their non-linear interaction may be a firstorder influence on rainfall. We reconstructed marine sea surface temperature (SST), supply of terrestrial material and vegetation changes from a very well-dated sediment core from the northern Arabian Sea to reconstruct the STWJ-ISM interaction. The Holocene record (from 11 000 years) shows a distinct, but gradual, southward displacement of the ISM in the Early to Mid-Holocene, increasingly punctuated by phases of intensified STWJ events that are coeval with interruptions of North Atlantic overturning circulation (Bond events). The effects of the non-linear interactions culminate between 4.6 and 3 ka BP, marking a climatic transition period during which the ISM shifted southwards and the influence of STWJ became prominent. The lithogenic matter input shows an up to 4-fold increase after this time period, probably related to the strengthened influence of agricultural activities of the Indus civilization, with enhanced erosion of soils. This anthropogenic land-use change is amplifying the impact of Bond events and adding to the marine sedimentation rates adjacent to the continent.

Holocene climatic changes in the Westerly-Indian Monsoon realm and its anthropogenic impact

The Indian Summer Monsoon (ISM) with its rainfall is the lifeline for people living on the Indian... more The Indian Summer Monsoon (ISM) with its rainfall is the lifeline for people living on the Indian subcontinent today and possibly was the driver of the rise and fall of early agricultural societies in the past. Intensity and position of the ISM have shifted in response to orbitally forced thermal land-ocean contrasts. At the northwestern monsoon margins, interactions between the subtropical westerly jet (STWJ) and the ISM constitute a tipping element in the Earth's climate system, because their non-linear interaction may be a first-order influence on rainfall. We reconstructed marine sea surface temperature (SST), supply of terrestrial material and vegetation changes from a very well-dated sediment core from the northern Arabian Sea to reconstruct the STWJ-ISM interaction. The Holocene record (from 11,000 years) shows a distinct, but gradual, southward displacement of the ISM in the Early to Mid-Holocene, increasingly punctuated by phases of intensified STWJ events that are coeval with interruptions of North Atlantic overturning circulation (Bond events). Effects of the nonlinear interactions culminate between 4.6-3 ka BP, marking a climatic transition period during which the ISM shifted southwards and the influence of SWTJ became prominent. The lithogenic input shows an up to 4-fold increase after this time period signaling the strengthened influence of agricultural activities of the Indus civilization with enhanced erosion of soils amplifying the impact of Bond events and adding to the marine sedimentation rates adjacent to the continent. Changes in latitude and strength of the ISM were gradual over the course of the Holocene, but were punctuated by distinct climatic events . The ISM was strong during the Early Holocene until approximately 6.5 ka ago, and influenced regions farther north than today . A weakening of the ISM is reflected in many records after the Holocene Optimum Period, commonly attributed to declining summer insolation

The Ballast Effect of Lithogenic Matter and its Influences on the Carbon Fluxes in the Indian Ocean

Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with sa... more Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with satellite observations illustrate the influence of primary production and the ballast effect on organic carbon flux into the deep sea. They suggest that primary production is the main control on the spatial variability of organic carbon fluxes at most of our study sites in the Indian Ocean, except at sites influenced by river discharges. At these sites the spatial variability of organic carbon flux is influenced by lithogenic matter content. To quantify the impact of lithogenic matter on the organic carbon flux, the densities of the main ballast minerals, their flux rates and seawater properties were used to calculate sinking speeds of material intercepted by sediment traps. Sinking speeds in combination with satellite-derived export production rates allowed us to compute organic carbon fluxes. Flux calculations imply that lithogenic matter ballast increases organic carbon fluxes at all sampling sites in the Indian Ocean by enhancing sinking speeds and reducing the time of organic matter respiration in the water column. We calculated that lithogenic matter content in aggregates and pellets enhances organic carbon flux rates on average by 45 % and by up to 62 % at trap locations in the river-influenced regions of the Indian Ocean. Such a strong lithogenic matter ballast effect explains the fact that organic carbon fluxes are higher in the low-productive southern Java Sea compared to the high-productive western Arabian Sea. It also implies that land use changes and the associated enhanced transport of lithogenic matter from land into the ocean may significantly affect the CO 2 uptake of the organic carbon pump in the receiving ocean areas.

Glacial-Interglacial changes and Holocene variations in Arabian Sea denitrification

What can we learn from amino acids about oceanic organic matter cycling and degradation?

Biogeosciences, Feb 10, 2022

Amino acids (AAs) mainly bound in proteins are major constituents of living biomass and non-livin... more Amino acids (AAs) mainly bound in proteins are major constituents of living biomass and non-living organic material in the oceanic particulate and dissolved organic matter pool. Uptake and cycling by heterotrophic organisms lead to characteristic changes in AA composition so that AAbased biogeochemical indicators are often used to elucidate processes of organic matter cycling and degradation. We analyzed particulate AA in a large sample set collected in various oceanic regions covering sinking and suspended particles in the water column, sediment samples, and dissolved AA from water column and pore water samples. The aim of this study was to test and improve the use of AA-derived biogeochemical indicators as proxies for organic matter sources and degradation and to better understand particle dynamics and interaction between the dissolved and particulate organic matter pools. A principal component analysis (PCA) of all data delineates diverging AA compositions of sinking and suspended particles with increasing water depth. A new sinking particle and sediment degradation indicator (SDI) allows a fine-tuned classification of sinking particles and sediments with respect to the intensity of degradation, which is associated with changes of stable isotopic ratios of nitrogen (δ 15 N). This new indicator is furthermore sensitive to sedimentary redox conditions and can be used to detect past anoxic early diagenesis. A second indicator emerges from the AA spectra of suspended particulate matter (SPM) in the epipelagic and that of the meso-and bathypelagic ocean and is a residence time indicator (RTI). The characteristic changes in AA patterns from shallow to deep SPM are recapitulated in the AA spectra of the dissolved organic matter (DOM) pool, so that deep SPM is more similar to DOM than to any of the other organic matter pools. This implies that there is equilibration between finely dispersed SPM and DOM in the deep sea, which may be driven by microbial activity combined with annealing and fragmentation of gels. As these processes strongly depend on physico-chemical conditions in the deep ocean, changes in quality and degradability of DOM may strongly affect the relatively large pool of suspended and dissolved AA in the ocean that amounts to 15 Pg amino acid carbon (AAC) and 89 ± 29 Pg AAC, respectively.

Nitrate Regeneration and Loss in the Central Yellow Sea Bottom Water Revealed by Nitrogen Isotopes

Frontiers in Marine Science

The Yellow Sea (YS) is an epicontinental sea framed by the densely populated mainland of China an... more The Yellow Sea (YS) is an epicontinental sea framed by the densely populated mainland of China and the Korean peninsula. Human activities over the last decades resulted in heavily increasing discharge of reactive nitrogen into the YS, which created numerous ecological problems. To elucidate the role of central YS in the cycling of reactive nitrogen, specifically the Yellow Sea Cold Water Mass (YSCWM), we determined nutrient concentrations, dual stable isotopes of nitrate (δ15N-NO3- and δ18O-NO3-), and stable isotopes of particulate and sedimentary nitrogen in spring and summer, i.e., in biologically inactive and active periods. The nitrate concentration in spring was higher than that in summer in the northern part of the YSCWM, Nitrate increased in the southern part accompanied by a decrease in δ15N-NO3- and δ18O-NO3-, which are indicative of nitrification that was a significant source of recycled nitrate in the south part of YSCWM. To quantify this regenerated nitrate, we use a mix...

Megacity's fingerprint in Chinese marginal seas

Meereswissenschaftliche Berichte, 2021

Das deutsch-chinesische Verbundvorhaben Megacity's fingerprint in Chinese marginal seas: Inve... more Das deutsch-chinesische Verbundvorhaben Megacity's fingerprint in Chinese marginal seas: Investigation of pollutant fingerprints and dispersal (MEGAPOL) geht nach drei Jahren intensiver Zusammenarbeit (01.08.2017-31.07.2020; verlängert bis 31.12.2020) zu Ende. MEGAPOL hatte zwei regionale Schwerpunkte. Der eine konzentrierte sich auf den Mündungsbereich des Perlflusses im Südchinesischen Meer und der andere auf das Gelbe Meer mit dem Yangtse Fluss sowie auf die Bohai See mit dem Gelben Fluss. Die Verbundpartner auf deutscher Seite sind das Leibniz-Institut für Ostseeforschung Warnemünde, das Institut für Geologie und das Institut für Meereskunde der Universität Hamburg, sowie das Institut für Küstenforschung des Helmholtz Zentrums Geesthacht. Auf chinesischer Seite sind die Verbundpartner der Guangzhou Marine Geological Survey, die School of Oceanography der Shanghai Jiao Tong Universität, das Second Institute of Oceanography SOA, Hangzhou, das National Marine Environmental Moni...

Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

1 Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany (t i... more 1 Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany (t [email protected]) 2 University of Edinburgh, School of Geosciences, James Hutton Road, Edinburgh EH9 3FE, Scotland ([email protected]) 3 Insti tute for Geology, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany (birgit [email protected]) 4 Marine Biology, Department of Marine Biology and Paleoenvironment Lamont Doherty Earth Observatory at Columbia University, 61 Route 9W, Palisades, New York, 10964 (j [email protected]), ([email protected]) 5 Horn Point Laboratory, University of Maryland Center for Environmental Science, P.O. Box 775, Cambridge, MD 21613 ([email protected]) 6 Center for Prototype Climate Modeling (CPCM), NYU Abu Dhabi (zouhair. [email protected]) 7 GEOMAR Helmholtz-Zentrum fuer Ozeanforschung Kiel, Duesternbrooker Weg 20, 24105 Kiel, Germany ([email protected]) 8 Insti tute of Geosciences, Christ ian-Albrechts-Universität Kiel (CAU), Ludewig-Mey...

Present past and future of the OMZ in the northern Indian Ocean

Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems... more Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems is a major threat to marine ecosystem services because it favors the formation of greenhouse gases such as methane, endangers the growth of economically important species, and increases the loss nitrate. Nitrate is one of the potential primary nutrients, which availability controls the marine productivity. The Arabian Sea and the Bay of Bengal are home to ~59% of the Earth's marine sediments exposed to severe oxygen depletion and approximately 21% of the total volume of oxygen-depleted waters (oxygen minimum zones, OMZs). The balance between physical oxygen supply and the biological oxygen consumption controlled the oxygen concentrations. In the Arabian Sea and most likely also in the Bay of Bengal the supply of oxygen sustained by mixing and advection associated with mesoscale eddies compensated the biological oxygen consumption. These steady states maintain low (hypoxic) oxygen concentrations allowing the competition between anaerobic and aerobic processes. However, due to slightly higher oxygen concentrations, the aerobic nitrite oxidization inhibits the anaerobic nitrite reduction and thus denitrification (the reduction of nitrate to N 2 ) to become significant in the Bay of Bengal. A feedback mechanism caused by the negative influence of decreasing oxygen concentrations on the biological oxygen demand helped to maintain these steady states. Furthermore, it might have also counteracted a reduced physical oxygen supply into the Arabian Sea caused by climate-driven changes in the ocean's circulation during the last 6000 years. However, due to human-induced global changes, the OMZs in Arabian Sea and the Bay of Bengal intensified and expanded, which included also the occurrence of anoxic events on the Indian shelf. This affects benthic ecosystems, and in the Arabian Sea it seems to have initiated a regime shift within the pelagic ecosystem structure. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.

The Second International Indian Ocean Expedition (IIOE-2): Motivating New Exploration in a Poorly Understood Ocean Basin (Volume 2)

Deep Sea Research Part II: Topical Studies in Oceanography, 2019

This Deep-Sea Research II special issue is the second compendium of scientific papers to emerge f... more This Deep-Sea Research II special issue is the second compendium of scientific papers to emerge from the Second International Indian Ocean Expedition (IIOE-2, Fig. 1). By pulling this diverse body of contemporary Indian Ocean research together (some motivated by IIOE-2 and others not) the IIOE-2 has begun the process of achieving its overarching goal, which is to “… advance our understanding of interactions between geologic, oceanic and atmospheric processes that give rise to the complex physical dynamics of the Indian Ocean region, and determine how those dynamics affect climate, extreme events, marine biogeochemical cycles, ecosystems and human populations”

Biogeochemical Regeneration of a Nodule Mining Disturbance Site: Trace Metals, DOC and Amino Acids in Deep-Sea Sediments and Pore Waters

Frontiers in Marine Science, 2018

Increasing interest in deep-sea mineral resources, such as polymetallic nodules, calls for enviro... more Increasing interest in deep-sea mineral resources, such as polymetallic nodules, calls for environmental research about possible impacts of mineral exploitation on the deep-sea ecosystem. So far, little geochemical comparisons of deep-sea sediments before and after mining induced disturbances have been made, and thus long-term environmental effects of deep-sea mining are unknown. Here we present geochemical data from sediment cores from an experimental disturbance area at 4,100 m water depth in the Peru Basin. The site was revisited in 2015, 26 years after a disturbance experiment mimicking nodule mining was carried out and compared to sites outside the experimental zone which served as a pre-disturbance reference. We investigated if signs of the disturbance are still visible in the solid phase and the pore water after 26 years or if pre-disturbance conditions have been re-established. Additionally, a new disturbance was created during the cruise and sampled 5 weeks later to compare short-and longer-term impacts. The particulate fraction and pore water were analyzed for major and trace elements to study element distribution and processes in the surface sediment. Pore water and bottom water samples were also analyzed for oxygen, nitrate, dissolved organic carbon, and dissolved amino acids, to examine organic matter degradation processes. The study area of about 11 km 2 was found to be naturally more heterogeneous than expected, requiring an analysis of spatial variability before the disturbed and undisturbed sites can be compared. The disturbed sites exhibit various disturbance features: some surface sediments were mixed through, others had the top layer removed and some had additional material deposited on top. Pore water constituents have largely regained pre-disturbance gradients after 26 years. The solid phase, however, shows clear differences between disturbed and undisturbed sites in the top 20 cm so that the impact is still visible in the plowed tracks after 26 years. Especially the upper layer, usually rich in manganese-oxide Paul et al. Biogeochemical Regeneration After Mining Disturbance and associated metals, such as Mo, Ni, Co, and Cu, shows substantial differences in metal distribution. Hence, it can be expected that disturbances from polymetallic nodule mining will have manifold and long-lasting impacts on the geochemistry of the underlying sediment.

The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean

Biogeosciences, 2019

Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with sa... more Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with satellite observations illustrate the influence of primary production and the ballast effect on organic carbon flux into the deep sea. They suggest that primary production is the main control on the spatial variability of organic carbon fluxes at most of our study sites in the Indian Ocean, except at sites influenced by river discharges. At these sites the spatial variability of organic carbon flux is influenced by lithogenic matter content. To quantify the impact of lithogenic matter on the organic carbon flux, the densities of the main ballast minerals, their flux rates and seawater properties were used to calculate sinking speeds of material intercepted by sediment traps. Sinking speeds in combination with satellite-derived export production rates allowed us to compute organic carbon fluxes. Flux calculations imply that lithogenic matter ballast increases organic carbon fluxes at all sampling sites in the Indian Ocean by enhancing sinking speeds and reducing the time of organic matter respiration in the water column. We calculated that lithogenic matter content in aggregates and pellets enhances organic carbon flux rates on average by 45 % and by up to 62 % at trap locations in the river-influenced regions of the Indian Ocean. Such a strong lithogenic matter ballast effect explains the fact that organic carbon fluxes are higher in the low-productive southern Java Sea compared to the high-productive western Arabian Sea. It also implies that land use changes and the associated enhanced transport of lithogenic matter from land into the ocean may significantly affect the CO 2 uptake of the organic carbon pump in the receiving ocean areas.

Sources of laminated sediments in the northeastern Arabian Sea off Pakistan and implications for sediment transport mechanisms during the late Holocene

The Holocene, 2018

Laminated sediments of the continental slope off the Makran coast in the northern Arabian Sea are... more Laminated sediments of the continental slope off the Makran coast in the northern Arabian Sea are well-known climate archives and record productivity, as well as supply of material from land. Here, we studied sediment core 275KL off Pakistan in concert with sediment trap, dust and river samples in order to characterize and quantify land-derived material deposited in varves and event layers. We analysed grain sizes, mineral assemblages, bulk components and stable isotopes (δ13C, δ18O) of carbonates. In winter, enhanced river discharge is the main source of lithogenic matter contributing the major amounts to the total annual sedimentation of the northern Arabian Sea. During the late summer season, lithogenic matter accumulation is slightly enhanced, probably carried along with the south-eastward blowing Levar winds from the Balochistan and the Sistan Basins and the summer monsoon discharge maximum of perennial streams. C/N ratios and stable carbon and oxygen isotopes could not be used...

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical South Indian Ocean

Biogeosciences Discussions, 2019

Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocea... more Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~30°S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of <5.9 µM NO3 -and <0.5 µM PO4 3-(<310 m; σ<26.4 kg/m³). Increased nutrient concentrations towards the equator represent the northern end of the gyre, characterized by typical strong horizontal gradients of the outcropping nutriclines. Measurements of stable isotopes of nitrate (δ 15 N and δ 18 O) indicate isotopic maxima of δ 15 N (>7 ‰) and δ 18 O (>4 ‰) centred at 400-500 m, representing the preformed nitrate exported from the Southern Ocean with mode water and induced by partial N-assimilation there. Additionally, a residue of nitrate affected by denitrification in the Arabian Sea is imported into the sub-thermocline of the gyre, indicated by a strong N deficit (N*<-1 µM) within the northern study area, accompanied by elevated isotopic ratios of nitrate (δ 15 N>7 ‰; δ 18 O>3 ‰). The subtropical South Indian Ocean is thus supplied by nitrate from lateral influx of water masses that have similar isotopic character, but antagonistic origin (preformed versus regenerated). A significant contribution of N2-fixation within the Indian Ocean subtropical gyre (17° S-25° S) is promoted by low nitrate to phosphate ratios in the surface layer, where approximately one-third of the nitrate in the upper ocean is derived by newly fixed N.

Contrasting pattern of hydrological changes during the past two millennia from central and northern India: Regional climate difference or anthropogenic impact?

Global and Planetary Change, 2018

High resolution reconstructions of the India Summer Monsoon (ISM) are essential to identify regio... more High resolution reconstructions of the India Summer Monsoon (ISM) are essential to identify regionally different patterns of climate change and refine predictive models. We find opposing trends of hydrological proxies between northern (Sahiya cave stalagmite) and central India (Lonar Lake) between 100 and 1300 CE with the strongest anti-correlation between 810 and 1300 CE. The apparently contradictory data raise the question if these are related to widely different regional precipitation patterns or reflect human influence in/around the Lonar Lake. By comparing multiproxy data with historical records, we demonstrate that only the organic proxies in the Lonar Lake show evidence of anthropogenic impact. However, evaporite data (mineralogy and δ 18 O) are indicative of precipitation/evaporation (P/E) into the Lonar Lake. Back-trajectories of air-mass circulation over northern and central India show that the relative contribution of the Bay of Bengal (BoB) branch of the ISM is crucial for determining the δ 18 O of carbonate proxies only in north India, whereas central India is affected significantly by the Arabian Sea (AS) branch of the ISM. We conclude that the δ 18 O of evaporative carbonates in the Lonar Lake reflects P/E and, in the interval under consideration, is not influenced by source water changes. The opposing trend between central and northern India can be explained by (i) persistent multidecadal droughts over central India between 810 and 1300 CE that provided an effective mechanism for strengthening sub-tropical westerly winds resulting in enhancement of wintertime (non-monsoonal) rainfall over northern parts of the Indian subcontinent, and/or (ii) increased moisture influx to northern India from the depleted BoB source waters.

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Biogeosciences, 2014

The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mi... more The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mid-water oxygen minimum zone (OMZ) in which bio-available nitrate (NO - 3 ) is reduced to dinitrogen gas (N 2 ). As the nitrogen cycle can respond fast to climate-induced changes in productivity and circulation, the Arabian Sea sediments are an important palaeoclimatic archive. In order to understand seasonal and interannual variations in the nitrogen cycle, nutrient data were obtained from the literature published prior to 1993, evaluated, and compared with data measured during five expeditions carried out in the framework of the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea in 1995 and during a research cruise of RV Meteor in 2007. The data comparison showed that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63 % larger in 1995 than a similarly determined estimate based on pre-JGOFS data. This area, referred to as the core of the denitrifying zone, showed strong seasonal and interannual variations driven by the monsoon. During the SW monsoon, the SNM retreated eastward due to the inflow of oxygenenriched Indian Ocean Central Water (ICW). During the NE monsoon, the SNM expanded westward because of the reversal of the current regime. On an interannual timescale, a weaker SW monsoon decreased the inflow of ICW from the equatorial Indian Ocean and increased the accumulation of denitrification tracers by extending the residence time of water in the SNM. This is supported by palaeoclimatic studies showing an enhanced preservation of accumulative denitrification tracers in marine sediments in conjunction with a weakening of the SW monsoon during the late Holocene. → 291.17CO 2 + 150.23N 2 + 1.75N 2 O + 2.75H 3 PO 4 + 487.1H 2 O. Approximately 30 % of the global oceanic water-column denitrification occurs in the oxygen minimum zone (OMZ)

Glacial-Interglacial changes and Holocene variations in Arabian Sea denitrification

Biogeosciences Discussions, 2017

At present the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between abou... more

A contemporary sediment and organic carbon budget for the Kara Sea shelf (Siberia)

Marine Geology, 2005

It has recently been realized that the Arctic undergoes drastic changes, probably resulting from ... more It has recently been realized that the Arctic undergoes drastic changes, probably resulting from global change induced processes. This acts on the cycling of matter and on biogenic elements in the Arctic Ocean having feedback mechanisms with the global climate, for example by interacting with atmospheric trace gas concentration. A contemporary budget for biogenic elements as well as suspended matter for the Arctic Ocean as a baseline for comparison with effects of further global change is, thus, needed. Available budgets are based on the late Holocene sedimentary record and are therefore quiet different from the present which has already been affected by the intense anthropogenic activity of the last centuries. We calculated a contemporary suspended matter and organic carbon budget for the Kara Sea utilizing the numerous available data from the recent literature as well as our own data from Russian-German SIRRO (Siberian River Run-off) expeditions. For calculation of the budgets we used a multi-box model to simplify the Kara Sea shelf and estuary system: input was assumed to comprise riverine and eolian input as well as coastal erosion, output was assumed to consist of sedimentation and export to the Arctic Ocean. Exchange with the adjacent seas was considered in our budget, and primary production as well as recycling of organic material was taken into account. According to our calculations, about 18.5 Â 10 6 t yr À 1 of sediments and 0.37 Â 10 6 t yr À 1 of organic carbon are buried in the estuaries, whereas 20.9 Â 10 6 t yr À 1 sediment and 0.31 Â10 6 t yr À 1 organic carbon are buried on the shelf. Most sources and sinks of our organic carbon budget of the Kara Sea are in the same order of magnitude, making it a region very sensitive to further changes.

The Ballast Effect in the Indian Ocean

In this study, data obtained from a sediment trap experiments off South Java are analyzed and com... more In this study, data obtained from a sediment trap experiments off South Java are analyzed and compared to satellite-derived information on primary production and data collected by deep-moored sediment traps in the Arabian Sea 10 and the Bay of Bengal. The aim was to study the relative importance of primary production and the ballast effect on the organic carbon export and the CO 2 uptake of the biological carbon pumps. Therefore, data obtained from sediment trap experiments carried out in other ocean basins were also integrated into the data analysis and a four-box model was developed. Our data showed that the organic carbon flux in the highly-productive upwelling system in the Arabian Sea was similar to those in the low productive system off South Java. Off South Java as in other river-influenced regions, lithogenic 15 matter supplied from land mainly controls the organic carbon flux via its ballast effect in sinking particles, whereas carbonate produced by marine organisms appears to be the main ballast material in the high productive regions. Since the carbonate flux tends to increase with an increasing export production, it is difficult to quantify the relative importance of productivity and the ballast effect on the organic carbon flux into the deep sea. However, the export of organic matter into the deep sea represents a loss of nutrients for the pelagic ecosystems, which needs to be balanced by mode water nutrient 20 supply into the seasonal thermocline to sustain the productivity of the pelagic system. The amount of preformed nutrients utilized during the formation of the exported organic matter strongly influences the impact of the ballast effect on the CO 2 uptake of the organic carbon pump. Accordingly, this is stronger at higher latitudes where preformed nutrients are formed than at lower latitudes where the euphotic zone is nutrient depleted. Nevertheless, the ballast effect enhances the export of organic matter into the deep sea and favors the sedimentation of organic matter in river-influenced regions. Since globally > 25 80% of organic carbon burial occurs in river-dominated systems, the lithogenic ballast is assumed to play an important role in the Earth's climate system on geological time scales.

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical southern Indian Ocean

Biogeosciences, Jul 12, 2019

The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world'... more The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼ 30 • S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrientdepleted surface waters of the oligotrophic IOSG with nitrate (NO - 3 ) and phosphate (PO 3- 4 ) concentrations of < 3 and < 0.3 µmol kg -1 , respectively (< 300 m; σ < 26.4 kg -1 m -3 ). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ 15 N > 7 ‰; δ 18 O > 4 ‰) at 400-500 m (26.6-26.7 kg -1 m -3 ), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg -1 m -3 ) of the gyre, indicated by an N deficit (N * ∼ -1 to -4 µmol kg -1 ) and by elevated isotopic ratios of

Signals of Holocene climate transition amplified by anthropogenic land-use changes in the westerly–Indian monsoon realm

Climate of The Past, Aug 24, 2021

The Indian summer monsoon (ISM) rainfall is the lifeline for people living on the Indian subconti... more The Indian summer monsoon (ISM) rainfall is the lifeline for people living on the Indian subcontinent today and was possibly the driver of the rise and fall of early agricultural societies in the past. The intensity and position of the ISM have shifted in response to orbitally forced thermal land-ocean contrasts. At the northwestern monsoon margins, interactions between the subtropical westerly jet (STWJ) and the ISM constitute a tipping element in the Earth's climate system because their non-linear interaction may be a firstorder influence on rainfall. We reconstructed marine sea surface temperature (SST), supply of terrestrial material and vegetation changes from a very well-dated sediment core from the northern Arabian Sea to reconstruct the STWJ-ISM interaction. The Holocene record (from 11 000 years) shows a distinct, but gradual, southward displacement of the ISM in the Early to Mid-Holocene, increasingly punctuated by phases of intensified STWJ events that are coeval with interruptions of North Atlantic overturning circulation (Bond events). The effects of the non-linear interactions culminate between 4.6 and 3 ka BP, marking a climatic transition period during which the ISM shifted southwards and the influence of STWJ became prominent. The lithogenic matter input shows an up to 4-fold increase after this time period, probably related to the strengthened influence of agricultural activities of the Indus civilization, with enhanced erosion of soils. This anthropogenic land-use change is amplifying the impact of Bond events and adding to the marine sedimentation rates adjacent to the continent.

Holocene climatic changes in the Westerly-Indian Monsoon realm and its anthropogenic impact

The Indian Summer Monsoon (ISM) with its rainfall is the lifeline for people living on the Indian... more The Indian Summer Monsoon (ISM) with its rainfall is the lifeline for people living on the Indian subcontinent today and possibly was the driver of the rise and fall of early agricultural societies in the past. Intensity and position of the ISM have shifted in response to orbitally forced thermal land-ocean contrasts. At the northwestern monsoon margins, interactions between the subtropical westerly jet (STWJ) and the ISM constitute a tipping element in the Earth's climate system, because their non-linear interaction may be a first-order influence on rainfall. We reconstructed marine sea surface temperature (SST), supply of terrestrial material and vegetation changes from a very well-dated sediment core from the northern Arabian Sea to reconstruct the STWJ-ISM interaction. The Holocene record (from 11,000 years) shows a distinct, but gradual, southward displacement of the ISM in the Early to Mid-Holocene, increasingly punctuated by phases of intensified STWJ events that are coeval with interruptions of North Atlantic overturning circulation (Bond events). Effects of the nonlinear interactions culminate between 4.6-3 ka BP, marking a climatic transition period during which the ISM shifted southwards and the influence of SWTJ became prominent. The lithogenic input shows an up to 4-fold increase after this time period signaling the strengthened influence of agricultural activities of the Indus civilization with enhanced erosion of soils amplifying the impact of Bond events and adding to the marine sedimentation rates adjacent to the continent. Changes in latitude and strength of the ISM were gradual over the course of the Holocene, but were punctuated by distinct climatic events . The ISM was strong during the Early Holocene until approximately 6.5 ka ago, and influenced regions farther north than today . A weakening of the ISM is reflected in many records after the Holocene Optimum Period, commonly attributed to declining summer insolation

The Ballast Effect of Lithogenic Matter and its Influences on the Carbon Fluxes in the Indian Ocean

Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with sa... more Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with satellite observations illustrate the influence of primary production and the ballast effect on organic carbon flux into the deep sea. They suggest that primary production is the main control on the spatial variability of organic carbon fluxes at most of our study sites in the Indian Ocean, except at sites influenced by river discharges. At these sites the spatial variability of organic carbon flux is influenced by lithogenic matter content. To quantify the impact of lithogenic matter on the organic carbon flux, the densities of the main ballast minerals, their flux rates and seawater properties were used to calculate sinking speeds of material intercepted by sediment traps. Sinking speeds in combination with satellite-derived export production rates allowed us to compute organic carbon fluxes. Flux calculations imply that lithogenic matter ballast increases organic carbon fluxes at all sampling sites in the Indian Ocean by enhancing sinking speeds and reducing the time of organic matter respiration in the water column. We calculated that lithogenic matter content in aggregates and pellets enhances organic carbon flux rates on average by 45 % and by up to 62 % at trap locations in the river-influenced regions of the Indian Ocean. Such a strong lithogenic matter ballast effect explains the fact that organic carbon fluxes are higher in the low-productive southern Java Sea compared to the high-productive western Arabian Sea. It also implies that land use changes and the associated enhanced transport of lithogenic matter from land into the ocean may significantly affect the CO 2 uptake of the organic carbon pump in the receiving ocean areas.

Glacial-Interglacial changes and Holocene variations in Arabian Sea denitrification

What can we learn from amino acids about oceanic organic matter cycling and degradation?

Biogeosciences, Feb 10, 2022

Amino acids (AAs) mainly bound in proteins are major constituents of living biomass and non-livin... more Amino acids (AAs) mainly bound in proteins are major constituents of living biomass and non-living organic material in the oceanic particulate and dissolved organic matter pool. Uptake and cycling by heterotrophic organisms lead to characteristic changes in AA composition so that AAbased biogeochemical indicators are often used to elucidate processes of organic matter cycling and degradation. We analyzed particulate AA in a large sample set collected in various oceanic regions covering sinking and suspended particles in the water column, sediment samples, and dissolved AA from water column and pore water samples. The aim of this study was to test and improve the use of AA-derived biogeochemical indicators as proxies for organic matter sources and degradation and to better understand particle dynamics and interaction between the dissolved and particulate organic matter pools. A principal component analysis (PCA) of all data delineates diverging AA compositions of sinking and suspended particles with increasing water depth. A new sinking particle and sediment degradation indicator (SDI) allows a fine-tuned classification of sinking particles and sediments with respect to the intensity of degradation, which is associated with changes of stable isotopic ratios of nitrogen (δ 15 N). This new indicator is furthermore sensitive to sedimentary redox conditions and can be used to detect past anoxic early diagenesis. A second indicator emerges from the AA spectra of suspended particulate matter (SPM) in the epipelagic and that of the meso-and bathypelagic ocean and is a residence time indicator (RTI). The characteristic changes in AA patterns from shallow to deep SPM are recapitulated in the AA spectra of the dissolved organic matter (DOM) pool, so that deep SPM is more similar to DOM than to any of the other organic matter pools. This implies that there is equilibration between finely dispersed SPM and DOM in the deep sea, which may be driven by microbial activity combined with annealing and fragmentation of gels. As these processes strongly depend on physico-chemical conditions in the deep ocean, changes in quality and degradability of DOM may strongly affect the relatively large pool of suspended and dissolved AA in the ocean that amounts to 15 Pg amino acid carbon (AAC) and 89 ± 29 Pg AAC, respectively.

Nitrate Regeneration and Loss in the Central Yellow Sea Bottom Water Revealed by Nitrogen Isotopes

Frontiers in Marine Science

The Yellow Sea (YS) is an epicontinental sea framed by the densely populated mainland of China an... more The Yellow Sea (YS) is an epicontinental sea framed by the densely populated mainland of China and the Korean peninsula. Human activities over the last decades resulted in heavily increasing discharge of reactive nitrogen into the YS, which created numerous ecological problems. To elucidate the role of central YS in the cycling of reactive nitrogen, specifically the Yellow Sea Cold Water Mass (YSCWM), we determined nutrient concentrations, dual stable isotopes of nitrate (δ15N-NO3- and δ18O-NO3-), and stable isotopes of particulate and sedimentary nitrogen in spring and summer, i.e., in biologically inactive and active periods. The nitrate concentration in spring was higher than that in summer in the northern part of the YSCWM, Nitrate increased in the southern part accompanied by a decrease in δ15N-NO3- and δ18O-NO3-, which are indicative of nitrification that was a significant source of recycled nitrate in the south part of YSCWM. To quantify this regenerated nitrate, we use a mix...

Megacity's fingerprint in Chinese marginal seas

Meereswissenschaftliche Berichte, 2021

Das deutsch-chinesische Verbundvorhaben Megacity's fingerprint in Chinese marginal seas: Inve... more Das deutsch-chinesische Verbundvorhaben Megacity's fingerprint in Chinese marginal seas: Investigation of pollutant fingerprints and dispersal (MEGAPOL) geht nach drei Jahren intensiver Zusammenarbeit (01.08.2017-31.07.2020; verlängert bis 31.12.2020) zu Ende. MEGAPOL hatte zwei regionale Schwerpunkte. Der eine konzentrierte sich auf den Mündungsbereich des Perlflusses im Südchinesischen Meer und der andere auf das Gelbe Meer mit dem Yangtse Fluss sowie auf die Bohai See mit dem Gelben Fluss. Die Verbundpartner auf deutscher Seite sind das Leibniz-Institut für Ostseeforschung Warnemünde, das Institut für Geologie und das Institut für Meereskunde der Universität Hamburg, sowie das Institut für Küstenforschung des Helmholtz Zentrums Geesthacht. Auf chinesischer Seite sind die Verbundpartner der Guangzhou Marine Geological Survey, die School of Oceanography der Shanghai Jiao Tong Universität, das Second Institute of Oceanography SOA, Hangzhou, das National Marine Environmental Moni...

Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

1 Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany (t i... more 1 Leibniz Centre for Tropical Marine Research (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany (t [email protected]) 2 University of Edinburgh, School of Geosciences, James Hutton Road, Edinburgh EH9 3FE, Scotland ([email protected]) 3 Insti tute for Geology, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany (birgit [email protected]) 4 Marine Biology, Department of Marine Biology and Paleoenvironment Lamont Doherty Earth Observatory at Columbia University, 61 Route 9W, Palisades, New York, 10964 (j [email protected]), ([email protected]) 5 Horn Point Laboratory, University of Maryland Center for Environmental Science, P.O. Box 775, Cambridge, MD 21613 ([email protected]) 6 Center for Prototype Climate Modeling (CPCM), NYU Abu Dhabi (zouhair. [email protected]) 7 GEOMAR Helmholtz-Zentrum fuer Ozeanforschung Kiel, Duesternbrooker Weg 20, 24105 Kiel, Germany ([email protected]) 8 Insti tute of Geosciences, Christ ian-Albrechts-Universität Kiel (CAU), Ludewig-Mey...

Present past and future of the OMZ in the northern Indian Ocean

Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems... more Decreasing concentrations of dissolved oxygen and the resulting expansion of anaerobic ecosystems is a major threat to marine ecosystem services because it favors the formation of greenhouse gases such as methane, endangers the growth of economically important species, and increases the loss nitrate. Nitrate is one of the potential primary nutrients, which availability controls the marine productivity. The Arabian Sea and the Bay of Bengal are home to ~59% of the Earth's marine sediments exposed to severe oxygen depletion and approximately 21% of the total volume of oxygen-depleted waters (oxygen minimum zones, OMZs). The balance between physical oxygen supply and the biological oxygen consumption controlled the oxygen concentrations. In the Arabian Sea and most likely also in the Bay of Bengal the supply of oxygen sustained by mixing and advection associated with mesoscale eddies compensated the biological oxygen consumption. These steady states maintain low (hypoxic) oxygen concentrations allowing the competition between anaerobic and aerobic processes. However, due to slightly higher oxygen concentrations, the aerobic nitrite oxidization inhibits the anaerobic nitrite reduction and thus denitrification (the reduction of nitrate to N 2 ) to become significant in the Bay of Bengal. A feedback mechanism caused by the negative influence of decreasing oxygen concentrations on the biological oxygen demand helped to maintain these steady states. Furthermore, it might have also counteracted a reduced physical oxygen supply into the Arabian Sea caused by climate-driven changes in the ocean's circulation during the last 6000 years. However, due to human-induced global changes, the OMZs in Arabian Sea and the Bay of Bengal intensified and expanded, which included also the occurrence of anoxic events on the Indian shelf. This affects benthic ecosystems, and in the Arabian Sea it seems to have initiated a regime shift within the pelagic ecosystem structure. Consequences for biogeochemical cycles are unknown, which, in addition to the poor representation of mesoscale features reduces the reliability of predictions of the future OMZ development in the northern Indian Ocean.

The Second International Indian Ocean Expedition (IIOE-2): Motivating New Exploration in a Poorly Understood Ocean Basin (Volume 2)

Deep Sea Research Part II: Topical Studies in Oceanography, 2019

This Deep-Sea Research II special issue is the second compendium of scientific papers to emerge f... more This Deep-Sea Research II special issue is the second compendium of scientific papers to emerge from the Second International Indian Ocean Expedition (IIOE-2, Fig. 1). By pulling this diverse body of contemporary Indian Ocean research together (some motivated by IIOE-2 and others not) the IIOE-2 has begun the process of achieving its overarching goal, which is to “… advance our understanding of interactions between geologic, oceanic and atmospheric processes that give rise to the complex physical dynamics of the Indian Ocean region, and determine how those dynamics affect climate, extreme events, marine biogeochemical cycles, ecosystems and human populations”

Biogeochemical Regeneration of a Nodule Mining Disturbance Site: Trace Metals, DOC and Amino Acids in Deep-Sea Sediments and Pore Waters

Frontiers in Marine Science, 2018

Increasing interest in deep-sea mineral resources, such as polymetallic nodules, calls for enviro... more Increasing interest in deep-sea mineral resources, such as polymetallic nodules, calls for environmental research about possible impacts of mineral exploitation on the deep-sea ecosystem. So far, little geochemical comparisons of deep-sea sediments before and after mining induced disturbances have been made, and thus long-term environmental effects of deep-sea mining are unknown. Here we present geochemical data from sediment cores from an experimental disturbance area at 4,100 m water depth in the Peru Basin. The site was revisited in 2015, 26 years after a disturbance experiment mimicking nodule mining was carried out and compared to sites outside the experimental zone which served as a pre-disturbance reference. We investigated if signs of the disturbance are still visible in the solid phase and the pore water after 26 years or if pre-disturbance conditions have been re-established. Additionally, a new disturbance was created during the cruise and sampled 5 weeks later to compare short-and longer-term impacts. The particulate fraction and pore water were analyzed for major and trace elements to study element distribution and processes in the surface sediment. Pore water and bottom water samples were also analyzed for oxygen, nitrate, dissolved organic carbon, and dissolved amino acids, to examine organic matter degradation processes. The study area of about 11 km 2 was found to be naturally more heterogeneous than expected, requiring an analysis of spatial variability before the disturbed and undisturbed sites can be compared. The disturbed sites exhibit various disturbance features: some surface sediments were mixed through, others had the top layer removed and some had additional material deposited on top. Pore water constituents have largely regained pre-disturbance gradients after 26 years. The solid phase, however, shows clear differences between disturbed and undisturbed sites in the top 20 cm so that the impact is still visible in the plowed tracks after 26 years. Especially the upper layer, usually rich in manganese-oxide Paul et al. Biogeochemical Regeneration After Mining Disturbance and associated metals, such as Mo, Ni, Co, and Cu, shows substantial differences in metal distribution. Hence, it can be expected that disturbances from polymetallic nodule mining will have manifold and long-lasting impacts on the geochemistry of the underlying sediment.

The ballast effect of lithogenic matter and its influences on the carbon fluxes in the Indian Ocean

Biogeosciences, 2019

Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with sa... more Data obtained from long-term sediment trap experiments in the Indian Ocean in conjunction with satellite observations illustrate the influence of primary production and the ballast effect on organic carbon flux into the deep sea. They suggest that primary production is the main control on the spatial variability of organic carbon fluxes at most of our study sites in the Indian Ocean, except at sites influenced by river discharges. At these sites the spatial variability of organic carbon flux is influenced by lithogenic matter content. To quantify the impact of lithogenic matter on the organic carbon flux, the densities of the main ballast minerals, their flux rates and seawater properties were used to calculate sinking speeds of material intercepted by sediment traps. Sinking speeds in combination with satellite-derived export production rates allowed us to compute organic carbon fluxes. Flux calculations imply that lithogenic matter ballast increases organic carbon fluxes at all sampling sites in the Indian Ocean by enhancing sinking speeds and reducing the time of organic matter respiration in the water column. We calculated that lithogenic matter content in aggregates and pellets enhances organic carbon flux rates on average by 45 % and by up to 62 % at trap locations in the river-influenced regions of the Indian Ocean. Such a strong lithogenic matter ballast effect explains the fact that organic carbon fluxes are higher in the low-productive southern Java Sea compared to the high-productive western Arabian Sea. It also implies that land use changes and the associated enhanced transport of lithogenic matter from land into the ocean may significantly affect the CO 2 uptake of the organic carbon pump in the receiving ocean areas.

Sources of laminated sediments in the northeastern Arabian Sea off Pakistan and implications for sediment transport mechanisms during the late Holocene

The Holocene, 2018

Laminated sediments of the continental slope off the Makran coast in the northern Arabian Sea are... more Laminated sediments of the continental slope off the Makran coast in the northern Arabian Sea are well-known climate archives and record productivity, as well as supply of material from land. Here, we studied sediment core 275KL off Pakistan in concert with sediment trap, dust and river samples in order to characterize and quantify land-derived material deposited in varves and event layers. We analysed grain sizes, mineral assemblages, bulk components and stable isotopes (δ13C, δ18O) of carbonates. In winter, enhanced river discharge is the main source of lithogenic matter contributing the major amounts to the total annual sedimentation of the northern Arabian Sea. During the late summer season, lithogenic matter accumulation is slightly enhanced, probably carried along with the south-eastward blowing Levar winds from the Balochistan and the Sistan Basins and the summer monsoon discharge maximum of perennial streams. C/N ratios and stable carbon and oxygen isotopes could not be used...

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical South Indian Ocean

Biogeosciences Discussions, 2019

Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocea... more Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~30°S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of <5.9 µM NO3 -and <0.5 µM PO4 3-(<310 m; σ<26.4 kg/m³). Increased nutrient concentrations towards the equator represent the northern end of the gyre, characterized by typical strong horizontal gradients of the outcropping nutriclines. Measurements of stable isotopes of nitrate (δ 15 N and δ 18 O) indicate isotopic maxima of δ 15 N (>7 ‰) and δ 18 O (>4 ‰) centred at 400-500 m, representing the preformed nitrate exported from the Southern Ocean with mode water and induced by partial N-assimilation there. Additionally, a residue of nitrate affected by denitrification in the Arabian Sea is imported into the sub-thermocline of the gyre, indicated by a strong N deficit (N*<-1 µM) within the northern study area, accompanied by elevated isotopic ratios of nitrate (δ 15 N>7 ‰; δ 18 O>3 ‰). The subtropical South Indian Ocean is thus supplied by nitrate from lateral influx of water masses that have similar isotopic character, but antagonistic origin (preformed versus regenerated). A significant contribution of N2-fixation within the Indian Ocean subtropical gyre (17° S-25° S) is promoted by low nitrate to phosphate ratios in the surface layer, where approximately one-third of the nitrate in the upper ocean is derived by newly fixed N.

Contrasting pattern of hydrological changes during the past two millennia from central and northern India: Regional climate difference or anthropogenic impact?

Global and Planetary Change, 2018

High resolution reconstructions of the India Summer Monsoon (ISM) are essential to identify regio... more High resolution reconstructions of the India Summer Monsoon (ISM) are essential to identify regionally different patterns of climate change and refine predictive models. We find opposing trends of hydrological proxies between northern (Sahiya cave stalagmite) and central India (Lonar Lake) between 100 and 1300 CE with the strongest anti-correlation between 810 and 1300 CE. The apparently contradictory data raise the question if these are related to widely different regional precipitation patterns or reflect human influence in/around the Lonar Lake. By comparing multiproxy data with historical records, we demonstrate that only the organic proxies in the Lonar Lake show evidence of anthropogenic impact. However, evaporite data (mineralogy and δ 18 O) are indicative of precipitation/evaporation (P/E) into the Lonar Lake. Back-trajectories of air-mass circulation over northern and central India show that the relative contribution of the Bay of Bengal (BoB) branch of the ISM is crucial for determining the δ 18 O of carbonate proxies only in north India, whereas central India is affected significantly by the Arabian Sea (AS) branch of the ISM. We conclude that the δ 18 O of evaporative carbonates in the Lonar Lake reflects P/E and, in the interval under consideration, is not influenced by source water changes. The opposing trend between central and northern India can be explained by (i) persistent multidecadal droughts over central India between 810 and 1300 CE that provided an effective mechanism for strengthening sub-tropical westerly winds resulting in enhancement of wintertime (non-monsoonal) rainfall over northern parts of the Indian subcontinent, and/or (ii) increased moisture influx to northern India from the depleted BoB source waters.

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Biogeosciences, 2014

The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mi... more The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mid-water oxygen minimum zone (OMZ) in which bio-available nitrate (NO - 3 ) is reduced to dinitrogen gas (N 2 ). As the nitrogen cycle can respond fast to climate-induced changes in productivity and circulation, the Arabian Sea sediments are an important palaeoclimatic archive. In order to understand seasonal and interannual variations in the nitrogen cycle, nutrient data were obtained from the literature published prior to 1993, evaluated, and compared with data measured during five expeditions carried out in the framework of the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea in 1995 and during a research cruise of RV Meteor in 2007. The data comparison showed that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63 % larger in 1995 than a similarly determined estimate based on pre-JGOFS data. This area, referred to as the core of the denitrifying zone, showed strong seasonal and interannual variations driven by the monsoon. During the SW monsoon, the SNM retreated eastward due to the inflow of oxygenenriched Indian Ocean Central Water (ICW). During the NE monsoon, the SNM expanded westward because of the reversal of the current regime. On an interannual timescale, a weaker SW monsoon decreased the inflow of ICW from the equatorial Indian Ocean and increased the accumulation of denitrification tracers by extending the residence time of water in the SNM. This is supported by palaeoclimatic studies showing an enhanced preservation of accumulative denitrification tracers in marine sediments in conjunction with a weakening of the SW monsoon during the late Holocene. → 291.17CO 2 + 150.23N 2 + 1.75N 2 O + 2.75H 3 PO 4 + 487.1H 2 O. Approximately 30 % of the global oceanic water-column denitrification occurs in the oxygen minimum zone (OMZ)

Glacial-Interglacial changes and Holocene variations in Arabian Sea denitrification

Biogeosciences Discussions, 2017

At present the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between abou... more

A contemporary sediment and organic carbon budget for the Kara Sea shelf (Siberia)

Marine Geology, 2005

It has recently been realized that the Arctic undergoes drastic changes, probably resulting from ... more It has recently been realized that the Arctic undergoes drastic changes, probably resulting from global change induced processes. This acts on the cycling of matter and on biogenic elements in the Arctic Ocean having feedback mechanisms with the global climate, for example by interacting with atmospheric trace gas concentration. A contemporary budget for biogenic elements as well as suspended matter for the Arctic Ocean as a baseline for comparison with effects of further global change is, thus, needed. Available budgets are based on the late Holocene sedimentary record and are therefore quiet different from the present which has already been affected by the intense anthropogenic activity of the last centuries. We calculated a contemporary suspended matter and organic carbon budget for the Kara Sea utilizing the numerous available data from the recent literature as well as our own data from Russian-German SIRRO (Siberian River Run-off) expeditions. For calculation of the budgets we used a multi-box model to simplify the Kara Sea shelf and estuary system: input was assumed to comprise riverine and eolian input as well as coastal erosion, output was assumed to consist of sedimentation and export to the Arctic Ocean. Exchange with the adjacent seas was considered in our budget, and primary production as well as recycling of organic material was taken into account. According to our calculations, about 18.5 Â 10 6 t yr À 1 of sediments and 0.37 Â 10 6 t yr À 1 of organic carbon are buried in the estuaries, whereas 20.9 Â 10 6 t yr À 1 sediment and 0.31 Â10 6 t yr À 1 organic carbon are buried on the shelf. Most sources and sinks of our organic carbon budget of the Kara Sea are in the same order of magnitude, making it a region very sensitive to further changes.