Papers by Douglas Kennett
Accurately predicting future tropical cyclone risk requires understanding the fundamental control... more Accurately predicting future tropical cyclone risk requires understanding the fundamental controls on tropical cyclone dynamics. Here we present an annually-resolved 450-year reconstruction of western Caribbean tropical cyclone activity developed using a new coupled carbon and oxygen isotope ratio technique in an exceptionally well-dated stalagmite from Belize. Western Caribbean tropical cyclone activity peaked at 1650 A.D., coincident with maximum Little Ice Age cooling, and decreased gradually until the end of the record in 1983. Considered with other reconstructions, the new record suggests that the mean track of Cape Verde tropical cyclones shifted gradually north-eastward from the western Caribbean toward the North American east coast over the last 450 years. Since ~1870 A.D., these shifts were largely driven by anthropogenic greenhouse gas and sulphate aerosol emissions. Our results strongly suggest that future emission scenarios will result in more frequent tropical cyclone impacts on the financial and population centres of the northeastern United States. Observational and modelling studies suggest that the recent multidecadal trend of rising sea surface temperatures (SST) in the North Atlantic's Main Development Region (MDR) may have increased Atlantic tropical cyclone (TC) intensity and duration 1–3 , and shifted storm tracks poleward 4,5. Some studies ascribe this oceanic warming to a multi-decadal SST periodicity known as the Atlantic Multidecadal Oscillation (AMO) 6,7 associated with the strength of thermohaline circulation 7,8 or large-scale atmospheric circulation 9,10 , while others implicate rising anthropogenic greenhouse gases (GHGs) 11,12. Deconvolving these effects is critical for predicting how GHG-induced 21 st Century warming may impact future TC activity 13 , however the observational record's brevity complicates assessing the relative influence of natural versus anthropogenic climate forcings on past North Atlantic TC activity. Additionally, multi-model ensemble studies predict that overall TC frequency will decrease through the 21 st Century while the frequency and intensity of the largest storms will increase 14. Although global TC activity and strength predictions are reasonably well constrained, projections for individual basins have considerably more uncertainty 15. Consequently, understanding the drivers of TC strength, frequency, and track for individual basins is critical. Well-dated, high resolution proxy records of total TC activity (including weaker tropical storms) from multiple individual locations are required 16 to identify and characterise long-term trends
Nature Geoscience, Feb 9, 2015
The position of the intertropical convergence zone is an important control on the distribution of... more The position of the intertropical convergence zone is an important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere temperature contrasts1, 2. The release of aerosols by human activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s (refs 1, 3, 4, 5, 6) by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval1, 7, 8, but this proposed shift remains equivocal. Here we reconstruct monthly rainfall over Belize for the past 456 years from variations in the carbon isotope composition of a well-dated, monthly resolved speleothem. We identify an unprecedented drying trend since AD 1850 that indicates a southward displacement of the intertropical convergence zone. This drying coincides with increasing aerosol emissions in the Northern Hemisphere and also marks a breakdown in the relationship between Northern Hemisphere temperatures and the position of the intertropical convergence zone observed earlier in the record. We also identify nine short-lived drying events since AD 1550 each following a large volcanic eruption in the Northern Hemisphere. We conclude that anthropogenic aerosol emissions have led to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes in aerosol emissions should be considered when assessing potential future rainfall shifts in the tropics.
The reasons for the development and collapse of Maya civilization remain controversial and histor... more The reasons for the development and collapse of Maya civilization remain controversial and historical events carved on stone monuments throughout this region provide a remarkable source of data about the rise and fall of these complex polities. Use of these records depends on correlating the Maya and European calendars so that they can be compared with climate and environmental datasets. Correlation constants can vary up to 1000 years and remain controversial. We report a series of high-resolution AMS 14 C dates on a wooden lintel collected from the Classic Period city of Tikal bearing Maya calendar dates. The radiocarbon dates were calibrated using a Bayesian statistical model and indicate that the dates were carved on the lintel between AD 658-696. This strongly supports the Goodman-Martínez-Thompson (GMT) correlation and the hypothesis that climate change played an important role in the development and demise of this complex civilization.
We used a XAD-purified AMS radiocarbon method to date 62 bison specimens from different contexts ... more We used a XAD-purified AMS radiocarbon method to date 62 bison specimens from different contexts on the very southern extent of the Great Plains of North America to produce a precise chronology of bison population expansions spanning the last 6000 years. Sixty-one of these samples provide stable carbon and nitrogen isotope data indicating relative temperature and moisture conditions during intervals defined by the presence of bison within this time span. This chronology indicates climatic conditions favorable to bison were present in the greater central Texas area, including the uplifted Edwards Plateau and extending to the Coastal Plain during periods from ~5955 to 5815, ~3290 to 3130, 2700 to 2150, and 650 to 530 cal BP. However, isotope results suggest climatic conditions differed for each period. The earliest “Calf Creek” period was characterized by cool but dry conditions, the later “Late Archaic 1 & 2” periods were increasingly warmer and wetter, and the latest “Toyah” period was cooler and drier than the Late Archaic periods, but warmer and wetter than Calf Creek. Both the Calf Creek and Toyah periods had higher variability within these overall trends. Comparison with regional records suggests that these periods represent variation within generally cooledry climates. Human adaptive response to increased bison availability resulted in significant cultural changes across all four periods.
A major cosmic-impact event has been proposed at the onset of the Younger Dryas (YD) cooling epis... more A major cosmic-impact event has been proposed at the onset of the Younger Dryas (YD) cooling episode at ≈12,800 +/- 150 years before present, forming the YD Boundary (YDB) layer, distributed over 150 million km2 on four continents. In 24 dated stratigraphic sections in 10 countries of the Northern Hemisphere, the YDB layer contains a clearly defined abundance peak in nanodiamonds (NDs), a major cosmic impact proxy. Observed ND polytypes include cubic
diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs.We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.
Excavations and regional reconnaissance survey in Mesoamerica’s tropical Maya lowlands of northea... more Excavations and regional reconnaissance survey in Mesoamerica’s tropical Maya lowlands of northeastern Belize document the association of a distinctive orange soil horizon with patinated stone tools dating to the Archaic period (8000-3500 cal BP). Archaic period deposits of this type were found along the Freshwater Creek drainage and on small islands associated with this riverine system. Stone tool
assemblages (n = 778) from these sites indicate a spatial separation of tool use and resharpening at island versus shore sites at Progresso Lagoon and Laguna de On and expand the range of stone tool types reported from the Archaic period in the region. Starch grains (n = 81) were recovered from seven of these stone tools (two unifaces, four bifaces, and a hammer stone) and indicate that preceramic peoples in northern Belize used these implements to harvest or process maize along with several other domesticated plant species. The presence of starch from chili pepper, manioc, and taxa of the bean and squash families is also documented on a number of the same tools. These data are consistent with paleoecological studies in the region suggesting an extended period of horticultural activity in the Maya region prior to the adoption of ceramics and settled village life that occurred after ~3000 cal BP.
The position of the intertropical convergence zone is an
important control on the distribution of... more The position of the intertropical convergence zone is an
important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere
temperature contrasts. The release of aerosols by human
activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval, but this
proposed shift remains equivocal. Here we reconstruct monthly
rainfall over Belize for the past 456 years from variations
in the carbon isotope composition of a well-dated, monthly
resolved speleothem. We identify an unprecedented drying
trend since AD 1850 that indicates a southward displacement of
the intertropical convergence zone. This drying coincides with
increasing aerosol emissions in the Northern Hemisphere and
also marks a breakdown in the relationship between Northern
Hemisphere temperatures and the position of the intertropical
convergence zone observed earlier in the record. We also
identify nine short-lived drying events since AD 1550 each
following a large volcanic eruption in the Northern Hemisphere.
We conclude that anthropogenic aerosol emissions have led
to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes
in aerosol emissions should be considered when assessing
potential future rainfall shifts in the tropics.
Long-distance networks for the transport of exotic goods and the beginnings of specialized craft ... more Long-distance networks for the transport of exotic goods and the beginnings of specialized craft production first appear in Mesoamerica during the Formative Period. The results of
portable X-ray fluorescence (pXRF) analysis of 456 obsidian artefacts imported to the Pacific coast site of La Zanja (Guerrero, Mexico) indicate that long-distance exchange of finished obsidian blades along the coast began during the Early Formative (c.1400–1000 cal BC) and remained constant into the Middle Formative Period (c.800–550 cal BC). Comparisons with
sourcing studies from elsewhere in Mesoamerica indicate the development of a major Pacific coast trade network during the Formative Period that linked coastal Guerrero to the central
Mexican highlands and the Valley of Oaxaca. Weaker connections existed with Gulf coast obsidian trade networks that traversed the Isthmus of Tehuantepec. As the first obsidian sourcing study from coastal Guerrero, these data contribute to a greater understanding of the development of exchange networks in Mesoamerica during the Formative Period.
Analyses of terminal long count dates from stone monuments in the Maya lowlands have played a cen... more Analyses of terminal long count dates from stone monuments in the Maya lowlands have played a central role in characterizing the rise and “collapse” of polities during the Late and Terminal Classic periods (A.D. 730–910). Previous studies propose a directional abandonment of large political centers from west-to-east. We retest the west-to-east hypothesis, using Geographic Information Systems (GIS) and spatial statistics to analyze an updated dataset of 89 terminal dates from the Maya Hieroglyphic Database (MHD). Our results do not support a directional collapse, but instead suggest a contraction of Terminal Classic polities around seven core areas in the Maya lowlands. Three regions demonstrate distinct subregional abandonments of monument carving over a period of 24 to 127 years, consistent with independent archaeological data for each region. Advances in GIS, spatial statistics, and related methods applied to an increasingly detailed and comprehensive epigraphic and archaeological database provide a foundation for examining long-term sociopolitical dynamics in the Maya lowlands.
The Journal of Island and Coastal Archaeology, 2011
ABSTRACT In this article we report ceramic stylistic and compositional (Instrumental Neutron Acti... more ABSTRACT In this article we report ceramic stylistic and compositional (Instrumental Neutron Activation-INAA) data from two Early Formative Period (3,4002,800 cal yrs. BP) coastal sites from the Acapetahua region of southern Mexico, datasets potentially ...
Radiocarbon 56(3):1019-1038
The eastern Adriatic is a key area for understanding the mechanisms and effects of the spread of ... more The eastern Adriatic is a key area for understanding the mechanisms and effects of the spread of agriculture. This article presents an accelerator mass spectrometry (AMS) radiocarbon chronology for the introduction and subsequent development of farming villages on the eastern shore of the Adriatic (~6000-4700 cal BC) and evaluates this in comparison with the established pottery chronology based on stylistic data from Pokrovnik (Drniš) on the Dalmatian coast of Croatia. Models for the spread of agriculture rely heavily on changing pottery styles to define cultural groups and trace geographic relationships. Based on AMS 14 C dates presented here, Impressed Wares first appear in central Dalmatia by 6000 cal BC and persist until 5300 cal BC, well into what is generally termed the Middle Neolithic. Similarly, a typical Middle Neolithic ware, figulina, appeared earlier than anticipated. These findings stand in contrast to cave and rockshelter assemblages in the eastern Adriatic, but mirror assemblages from farming villages on the Italian Adriatic coast. This study argues that the similarities in ceramic assemblage composition and change through time may have less to do with direct contacts between areas, but more with the nature of ceramic production and consumption at village sites in general. These data shed light on the limitations of regional ceramic chronologies in the eastern Adriatic and highlight the necessity for systematic expansion of 14 C chronologies to address the social, economic, and ecological relevance of early farming in the Adriatic for the spread of agriculture in Europe and the Mediterranean.
Environmental Archaeology, 2014
We examine bone samples of known domesticates (sheep, goat, cattle and pig) from five open-air vi... more We examine bone samples of known domesticates (sheep, goat, cattle and pig) from five open-air village sites spanning most of the Neolithic period in Dalmatia, Croatia (cal 6000–4700 BC) to characterise diets of domestic animals and address questions of the origin and development of animal husbandry strategies in early farming communities. Carbon and nitrogen stable isotope values are analysed as proxies of diet and local environment that may indicate differences in herding and management practices between domesticated species. Results are compared to those reported for faunal remains found at other Neolithic sites from coastal Croatia and the wider Adriatic region. We find that isotopic values remain stable for cattle and ovicaprids during most of the Neolithic, suggesting that husbandry of these species remained fundamentally the same throughout the period in much of the Adriatic. However, temporal differences identified among pigs indicate changes in associated management practices through time, and may be a result of different foddering practices.

Using targeted survey, excavation, and radiocarbon dating, we assess the extent to which human se... more Using targeted survey, excavation, and radiocarbon dating, we assess the extent to which human settlement patterns on California's northern Channel Islands fit predictions arising from the ideal free distribution (IFD): (1) people first established and expanded permanent settlements in the regions ranked high for environmental resource suitability; (2) as population grew, they settled in progressively lower ranked habitats; and (3) changes in the archaeological record associated with high population levels such as increases in faunal diversity and evenness in high-ranked habitats are coincident with the expansion to other areas. On Santa Rosa Island, the early permanent settlements were located in both high-and middle-ranked locations, with the most extensive settlement at the highest ranked locations and only isolated sites elsewhere. Settlement at a low-ranked habitat was confined to the late Holocene (after 3600 cal BP). Drought influenced the relative rank of different locations, which is an example of climate adding a temporal dimension to the model that episodically stimulated population movement and habitat abandonment. Because the IFD includes a wide range of cultural and environmental variables, it has the potential to be a central model for guiding archaeological analysis and targeted field research.
Science, May 16, 2014
Because of differences in craniofacial morphology and dentition between the earliest American ske... more Because of differences in craniofacial morphology and dentition between the earliest American skeletons and modern Native Americans, separate origins have been postulated for them, despite genetic evidence to the contrary. We describe a near-complete human skeleton with an intact cranium and preserved DNA found with extinct fauna in a submerged cave on Mexico’s Yucatan Peninsula. This skeleton dates to between 13,000 and 12,000 calendar years ago and has Paleoamerican craniofacial characteristics and a Beringian-derived mitochondrial DNA (mtDNA) haplogroup (D1). Thus, the differences between Paleoamericans and Native Americans probably resulted from in situ evolution rather than separate ancestry.
ABSTRACT: The measure of the “Mayacene,” a microcosm of the Early Anthropocene that occurred from... more ABSTRACT: The measure of the “Mayacene,” a microcosm of the Early Anthropocene that occurred from c. 3000 to 1000 BP, comes from multiple Late Quaternary paleoenvironmental records. We synthesized the evidence for Maya impacts on climate, vegetation, hydrology and the lithosphere, from studies of soils, lakes, floodplains, wetlands and other ecosystems. Maya civilization had likely altered local to regional ecosystems and hydrology by the Preclassic Period (3000-1700 BP), but these impacts waned by 1000 BP. They altered ecosystems with vast urban and rural infrastructure that included thousands of reservoirs, wetland fields and canals, terraces, field ridges, and temples. Although there is abundant evidence that indicates the Maya altered their forests, even at the large urban complex of Tikal as much as 40% of the forest remained intact through the Classic period. Existing forests are still influenced by ancient Maya forest gardening, particularly by the large expanses of ancient stone structures, terraces, and wetland fields that form their substrates. A few studies suggest deforestation and other land uses probably also warmed and dried regional climate by the Classic Period (1700-1100 BP). A much larger body of research documents the Maya impacts on hydrology, in the form of dams, reservoirs, canals, eroded soils and urban design for runoff. Another metric of the “Mayacene” are paleosols, which contain chemical evidence for human occupation, revealed by high phosphorus concentrations and carbon isotope ratios of C4 species like maize in the C3–dominated tropical forest ecosystem. Paleosol sequences exhibit “Maya Clays,” a facies that reflects a glut of rapidly eroded sediments that overlie pre-Maya paleosols. This stratigraphy is conspicuous in many dated soil profiles and marks the large-scale Maya transformation of the landscape in the Preclassic and Classic periods. Some of these also have increased phosphorous and carbon isotope evidence of C4 species. We synthesize and provide new evidence of Maya-period soil strata that show elevated carbon isotope ratios (δ13C), indicating the presence of C4 species in typical agricultural sites. This is often the case in ancient Maya wetland systems, which also have abundant evidence for the presence of several other economic plant species. The “Mayacene” of c. 3000 to 1000 BP was thus a patchwork of cities, villages, roads, urban heat islands, intensive and extensive farmsteads, forests and orchards. Today, forests and wetlands cover much of the Maya area but like so many places, these are now under the onslaught of the deforestation, draining, and plowing of the present Anthropocene.
The measure of the “Mayacene,” a microcosm of the Early Anthropocene that occurred from c. 3000 t... more The measure of the “Mayacene,” a microcosm of the Early Anthropocene that occurred from c. 3000 to 1000 BP, comes from multiple Late Quaternary paleoenvironmental records. We synthesized the evidence for Maya impacts on climate, vegetation, hydrology and the lithosphere, from studies of soils, lakes, floodplains, wetlands and other ecosystems. Maya civilization had likely altered local to regional ecosystems and hydrology by the Preclassic Period (3000-1700 BP), but these impacts waned by 1000 BP. They altered ecosystems with vast urban and rural infrastructure that included thousands of reservoirs, wetland fields and canals, terraces, field ridges, and temples. Although there is abundant evidence that indicates the Maya altered their forests, even at the large urban complex of Tikal as much as 40% of the forest remained intact through the Classic period. Existing forests are still influenced by ancient Maya forest gardening, particularly by the large expanses of ancient stone structures, terraces, and wetland fields that form their substrates. A few studies suggest deforestation and other land uses probably also warmed and dried regional climate by the Classic Period (1700-1100 BP). A much larger body of research documents the Maya impacts on hydrology, in the form of dams, reservoirs, canals, eroded soils and urban design for runoff. Another metric of the “Mayacene” are paleosols, which contain chemical evidence for human occupation, revealed by high phosphorus concentrations and carbon isotope ratios of C4 species like maize in the C3–dominated tropical forest ecosystem. Paleosol sequences exhibit “Maya Clays,” a facies that reflects a glut of rapidly eroded sediments that overlie pre-Maya paleosols. This stratigraphy is conspicuous in many dated soil profiles and marks the large-scale Maya transformation of the landscape in the Preclassic and Classic periods. Some of these also have increased phosphorous and carbon isotope evidence of C4 species. We synthesize and provide new evidence of Maya-period soil strata that show elevated carbon isotope ratios (δ13C), indicating the presence of C4 species in typical agricultural sites. This is often the case in ancient Maya wetland systems, which also have abundant evidence for the presence of several other economic plant species. The “Mayacene” of c. 3000 to 1000 BP was thus a patchwork of cities, villages, roads, urban heat islands, intensive and extensive farmsteads, forests and orchards. Today, forests and wetlands cover much of the Maya area but like so many places, these are now under the onslaught of the deforestation, draining, and plowing of the present Anthropocene.
https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/18100, Oct 2014
Archaeologists working in the Belize Valley have argued for the persistence of Maya populations f... more Archaeologists working in the Belize Valley have argued for the persistence of Maya populations from the Classic (AD 300–900) through Postclassic (AD 900–1500) periods since Gordon Willey’s groundbreaking settlement survey and excavation work in the 1950s. This is contrary to the trajectory recorded in some parts of the Maya region where there is clear evidence for political disruption and population decline at the end of the Classic period. The argument for continuous Classic to Postclassic occupation in the Belize Valley remains ambiguous due to researchers’ reliance on relative ceramic chronologies. This article reports the results of direct accelerator mass spectrometry (AMS) radiocarbon dating of human skeletons (n = 12) from the important center of Baking Pot, Belize, which is thought to provide some of the best ceramic evidence for continuity in the valley. The AMS dates show a long span of mortuary activity between the Middle Preclassic and Late Classic periods (405 cal BC to cal AD 770), with a hiatus in activity during the Early Postclassic (cal AD 900–1200) and subsequent activity in the Late Postclassic (cal AD 1280–1420). These results are not consistent with the idea that Baking Pot was occupied continuously from the Classic through Postclassic periods. This work highlights the need for additional AMS 14C work at Baking Pot and elsewhere to establish absolute chronologies for evaluating the political and demographic collapse of Classic Maya regional centers.
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Papers by Douglas Kennett
diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs.We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.
assemblages (n = 778) from these sites indicate a spatial separation of tool use and resharpening at island versus shore sites at Progresso Lagoon and Laguna de On and expand the range of stone tool types reported from the Archaic period in the region. Starch grains (n = 81) were recovered from seven of these stone tools (two unifaces, four bifaces, and a hammer stone) and indicate that preceramic peoples in northern Belize used these implements to harvest or process maize along with several other domesticated plant species. The presence of starch from chili pepper, manioc, and taxa of the bean and squash families is also documented on a number of the same tools. These data are consistent with paleoecological studies in the region suggesting an extended period of horticultural activity in the Maya region prior to the adoption of ceramics and settled village life that occurred after ~3000 cal BP.
important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere
temperature contrasts. The release of aerosols by human
activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval, but this
proposed shift remains equivocal. Here we reconstruct monthly
rainfall over Belize for the past 456 years from variations
in the carbon isotope composition of a well-dated, monthly
resolved speleothem. We identify an unprecedented drying
trend since AD 1850 that indicates a southward displacement of
the intertropical convergence zone. This drying coincides with
increasing aerosol emissions in the Northern Hemisphere and
also marks a breakdown in the relationship between Northern
Hemisphere temperatures and the position of the intertropical
convergence zone observed earlier in the record. We also
identify nine short-lived drying events since AD 1550 each
following a large volcanic eruption in the Northern Hemisphere.
We conclude that anthropogenic aerosol emissions have led
to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes
in aerosol emissions should be considered when assessing
potential future rainfall shifts in the tropics.
portable X-ray fluorescence (pXRF) analysis of 456 obsidian artefacts imported to the Pacific coast site of La Zanja (Guerrero, Mexico) indicate that long-distance exchange of finished obsidian blades along the coast began during the Early Formative (c.1400–1000 cal BC) and remained constant into the Middle Formative Period (c.800–550 cal BC). Comparisons with
sourcing studies from elsewhere in Mesoamerica indicate the development of a major Pacific coast trade network during the Formative Period that linked coastal Guerrero to the central
Mexican highlands and the Valley of Oaxaca. Weaker connections existed with Gulf coast obsidian trade networks that traversed the Isthmus of Tehuantepec. As the first obsidian sourcing study from coastal Guerrero, these data contribute to a greater understanding of the development of exchange networks in Mesoamerica during the Formative Period.

diamonds, lonsdaleite-like crystals, and diamond-like carbon nanoparticles, called n-diamond and i-carbon. The ND abundances in bulk YDB sediments ranged up to ≈500 ppb (mean: 200 ppb) and that in carbon spherules up to ≈3700 ppb (mean: ≈750 ppb); 138 of 205 sediment samples (67%) contained no detectable NDs. Isotopic evidence indicates that YDB NDs were produced from terrestrial carbon, as with other impact diamonds, and were not derived from the impactor itself. The YDB layer is also marked by abundance peaks in other impact-related proxies, including cosmic-impact spherules, carbon spherules (some containing NDs), iridium, osmium, platinum, charcoal, aciniform carbon (soot), and high-temperature melt-glass. This contribution reviews the debate about the presence, abundance, and origin of the concentration peak in YDB NDs.We describe an updated protocol for the extraction and concentration of NDs from sediment, carbon spherules, and ice, and we describe the basis for identification and classification of YDB ND polytypes, using nine analytical approaches. The large body of evidence now obtained about YDB NDs is strongly consistent with an origin by cosmic impact at ≈12,800 cal BP and is inconsistent with formation of YDB NDs by natural terrestrial processes, including wildfires, anthropogenesis, and/or influx of cosmic dust.
assemblages (n = 778) from these sites indicate a spatial separation of tool use and resharpening at island versus shore sites at Progresso Lagoon and Laguna de On and expand the range of stone tool types reported from the Archaic period in the region. Starch grains (n = 81) were recovered from seven of these stone tools (two unifaces, four bifaces, and a hammer stone) and indicate that preceramic peoples in northern Belize used these implements to harvest or process maize along with several other domesticated plant species. The presence of starch from chili pepper, manioc, and taxa of the bean and squash families is also documented on a number of the same tools. These data are consistent with paleoecological studies in the region suggesting an extended period of horticultural activity in the Maya region prior to the adoption of ceramics and settled village life that occurred after ~3000 cal BP.
important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere
temperature contrasts. The release of aerosols by human
activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval, but this
proposed shift remains equivocal. Here we reconstruct monthly
rainfall over Belize for the past 456 years from variations
in the carbon isotope composition of a well-dated, monthly
resolved speleothem. We identify an unprecedented drying
trend since AD 1850 that indicates a southward displacement of
the intertropical convergence zone. This drying coincides with
increasing aerosol emissions in the Northern Hemisphere and
also marks a breakdown in the relationship between Northern
Hemisphere temperatures and the position of the intertropical
convergence zone observed earlier in the record. We also
identify nine short-lived drying events since AD 1550 each
following a large volcanic eruption in the Northern Hemisphere.
We conclude that anthropogenic aerosol emissions have led
to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes
in aerosol emissions should be considered when assessing
potential future rainfall shifts in the tropics.
portable X-ray fluorescence (pXRF) analysis of 456 obsidian artefacts imported to the Pacific coast site of La Zanja (Guerrero, Mexico) indicate that long-distance exchange of finished obsidian blades along the coast began during the Early Formative (c.1400–1000 cal BC) and remained constant into the Middle Formative Period (c.800–550 cal BC). Comparisons with
sourcing studies from elsewhere in Mesoamerica indicate the development of a major Pacific coast trade network during the Formative Period that linked coastal Guerrero to the central
Mexican highlands and the Valley of Oaxaca. Weaker connections existed with Gulf coast obsidian trade networks that traversed the Isthmus of Tehuantepec. As the first obsidian sourcing study from coastal Guerrero, these data contribute to a greater understanding of the development of exchange networks in Mesoamerica during the Formative Period.