Brendan Culleton | Penn State University (original) (raw)
Papers by Brendan Culleton
Human Impacts on Seals, Sea Lions, and Sea OttersIntegrating Archaeology and Ecology in the Northeast Pacific, 2011
It's clear that within the sea otter's stabilized foraging range there can be virtually no human ... more It's clear that within the sea otter's stabilized foraging range there can be virtually no human harvest of abalones (Haliotis spp.) (except for a few taken intertidally). d.j. miller, 1974, california department of fish and game marine resources leaflet no. 7
Archaeologists have traditionally relied upon relative ceramic chronologies to understand the occ... more Archaeologists have traditionally relied upon relative ceramic chronologies to understand the occupational histories of large and socially complex polities in the Maya lowlands. High-resolution accelerator mass spectrometry (AMS) radiocarbon dating can provide independent chronological control for more discrete events that reflect cultural change through time. This article reports results of AMS 14C dating of stratified sequences at the residential group Tzutziiy K’in, associated with the major Maya polity of Cahal Pech in the Belize Valley. Cahal Pech is one of the earliest permanently settled sites in the Maya lowlands (1200 cal BC), and was continuously occupied until the Terminal Classic Maya “collapse” (~ cal AD 800). We use Bayesian modeling to build a chronology for the settlement, growth, and terminal occupation of Tzutziiy K’in, and compare our results to chronological data from the monumental site core at Cahal Pech. The analyses indicate that Tzutziiy K’in was first settled by the Late Preclassic period (350–100 cal BC), concurrent with the establishment of several other large house groups and the growth of the Cahal Pech site core. Terminal occupation by high-status residents at this house group occurred between cal AD 850 and 900. This study provides a framework for interpreting patterns of spatial, demographic, and sociopolitical change between households and the Cahal Pech site core.
We synthesise reported stable isotope values for domesticates and wild herbivores from sites span... more We synthesise reported stable isotope values for domesticates and wild herbivores from sites spanning the Neolithic in coastal Croatia, Slovenia, and Italy (6000–3500 calBC). Carbon and nitrogen stable isotope values are analyzed as proxies of diet and environment, with differences between species possibly indicating anthropogenic influence. Results are used to characterise diets and address questions of the origin and development of husbandry strategies, especially transhumance, in early farming communities. Changes in pig carbon and nitrogen isotope values through time suggest alterations in practices, whereas values remain relatively constant for cattle and ovicaprids during most of the Neolithic, despite assumptions of seasonal mobility.
Applied spectroscopy, 2016
Archaeological sediments from mounds within the mangrove zone of far-southern Pacific coastal Chi... more Archaeological sediments from mounds within the mangrove zone of far-southern Pacific coastal Chiapas, Mexico, are characterized in order to test the hypothesis that specialized pyro-technological activities of the region's prehistoric inhabitants (salt and ceramic production) created the accumulations visible today. Fourier transform infrared spectroscopy (FT-IR) is used to characterize sediment mineralogy, while portable X-ray fluorescence (pXRF) is used to determine elemental concentrations. Elemental characterization of natural sediments by both instrumental neutron activation analysis (INAA) and pXRF also contribute to understanding of processes that created the archaeological deposits. Radiocarbon dates combined with typological analysis of ceramics indicate that pyro-industrial activity in the mangrove zone peaked during the Late Formative and Terminal Formative periods, when population and monumental activity on the coastal plain and piedmont were also at their peaks.
Chichén Itzá dominated the political landscape of the northern Yucatán during the Terminal Classi... more Chichén Itzá dominated the political landscape of the northern Yucatán during the Terminal Classic Period (AD 800–1000). Chronological details of the rise and fall of this important polity are obscure because of the limited corpus of dated hieroglyphic records and by a restricted set of radiocarbon dates for the site. Here we compile and review these data and evaluate them within the context of political and climatic change in northern Yucatán at the end of the Classic period. The available data point to the end of elite activity at Chichén Itzá around AD 1000, a century after the collapse of Puuc Maya cities and other interior centers. Evidence supports a population shift in the eleventh century towards some coastal locations during a time associated with the end of monumental construction and art at Chichén Itzá. Our results suggest that regional political disintegration came in two waves. The first was the asynchronous collapse of multiple polities between AD 850 and 925 associated with a regional drying trend and punctuated by a series of multi-decadal droughts in the ninth and tenth centuries. The second wave was the political collapse at Chichén Itzá that coincides with the longest and most severe drought recorded in regional climate records between AD 1000 and 1100. This is a time that some scholars have characterized as a “dark age” across the northern Maya lowlands. Political developments during the Postclassic period (AD 1000–
1517) correspond with a return to higher rainfall. These patterns support a strong relationship between political
disintegration and climatic stress in the Maya lowlands. This research employs Bayesian radiocarbon models in conjunction with calendar dates on carved monuments and climate proxies to evaluate the rise and fall of Maya political centers and serves as an example of the impact of climate change on rainfall-dependent societies in Mesoamerica.
Radiocarbon, Apr 1, 2010
We present the results of 10 AMS radiocarbon dates for Cova de la Pastora (Alcoi, Alicante), a bu... more We present the results of 10 AMS radiocarbon dates for Cova de la Pastora (Alcoi, Alicante), a burial cave attributed to the Late Neolithic/Chalcolithic in eastern Spain. The direct dating of 10 human mandibles from Cova de la Pastora indicates that the cave was used as a burial place from the Late Neolithic/Chalcolithic to the Bronze Age. These dates reveal a continuity of ritual use not previously identified at the site. This case also serves to highlight the utility of revisiting historic excavations and museum collections with modern techniques to shed new light on the prehistoric human record.
Proceedings of the National Academy of Sciences of the United States of America, Jan 24, 2015
Proceedings of the National Academy of Sciences of the United States of America, Jan 27, 2015
The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemis... more The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemisphere deposited the Younger Dryas boundary (YDB) layer, containing peak abundances in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, platinum, and osmium. Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835-12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas. Two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of ...
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.
Journal of Archaeological Science, 2014
ABSTRACT Gatecliff Shelter provides a deeply stratified record of human-environment interaction i... more ABSTRACT Gatecliff Shelter provides a deeply stratified record of human-environment interaction in the Desert West spanning most of the middle and late Holocene. The well-preserved 10 m-deep deposits serve as an important reference sequence for technological and subsistence change in the Great Basin. Archaeological work started at Gatecliff in 1970 and a geochronological framework was established based on 47 uncalibrated radiocarbon (14C) dates obtained from several conventional radiometric laboratories between 1972 and 1982. These radiocarbon dates are not well accommodated within a Bayesian chronological model for these important deposits due to multiple temporal reversals and large analytical errors. This model was discarded in favor of one based on a combination of high-precision AMS 14C dates of short-lived carbonized plant remains supplemented with existing 14C dates culled from the original set using a Bayesian chronological model. Summed probabilities of the modeled posterior distributions of these 14C dates (N = 24) show episodic use of the shelter as a logistic hunting camp between 6050 and 3315 cal BP, a significant hiatus in occupation between 3315 and 2145 cal BP, and more persistent use by family bands after this time. In the future, this high-precision chronology will provide the foundation for reinterpreting broader patterns of technological and subsistence change in the Intermountain West and evaluating these changes relative to high-resolution climate records.
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 Journal of Geology, 2014
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 ע 150 years before present, forming the YD Boundary (YDB) layer, distributed over 150 million km 2 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.
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.
Scientific Reports, 2013
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.
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.
Science, Nov 9, 2012
The role of climate change in the development and demise of Classic Maya civilization (300 to 100... more The role of climate change in the development and demise of Classic Maya civilization (300 to 1000 C.E.) remains controversial because of the absence of well-dated climate and archaeological
sequences. We present a precisely dated subannual climate record for the past 2000 years from Yok Balum Cave, Belize. From comparison of this record with historical events compiled from well-dated stone monuments, we propose that anomalously high rainfall favored unprecedented population expansion and the proliferation of political centers between 440 and 660 C.E. This was followed by a drying trend between 660 and 1000 C.E. that triggered the balkanization of polities, increased warfare, and the asynchronous disintegration of polities, followed by population collapse in the context of an extended drought between 1020 and 1100 C.E.
Human Impacts on Seals, Sea Lions, and Sea OttersIntegrating Archaeology and Ecology in the Northeast Pacific, 2011
It's clear that within the sea otter's stabilized foraging range there can be virtually no human ... more It's clear that within the sea otter's stabilized foraging range there can be virtually no human harvest of abalones (Haliotis spp.) (except for a few taken intertidally). d.j. miller, 1974, california department of fish and game marine resources leaflet no. 7
Archaeologists have traditionally relied upon relative ceramic chronologies to understand the occ... more Archaeologists have traditionally relied upon relative ceramic chronologies to understand the occupational histories of large and socially complex polities in the Maya lowlands. High-resolution accelerator mass spectrometry (AMS) radiocarbon dating can provide independent chronological control for more discrete events that reflect cultural change through time. This article reports results of AMS 14C dating of stratified sequences at the residential group Tzutziiy K’in, associated with the major Maya polity of Cahal Pech in the Belize Valley. Cahal Pech is one of the earliest permanently settled sites in the Maya lowlands (1200 cal BC), and was continuously occupied until the Terminal Classic Maya “collapse” (~ cal AD 800). We use Bayesian modeling to build a chronology for the settlement, growth, and terminal occupation of Tzutziiy K’in, and compare our results to chronological data from the monumental site core at Cahal Pech. The analyses indicate that Tzutziiy K’in was first settled by the Late Preclassic period (350–100 cal BC), concurrent with the establishment of several other large house groups and the growth of the Cahal Pech site core. Terminal occupation by high-status residents at this house group occurred between cal AD 850 and 900. This study provides a framework for interpreting patterns of spatial, demographic, and sociopolitical change between households and the Cahal Pech site core.
We synthesise reported stable isotope values for domesticates and wild herbivores from sites span... more We synthesise reported stable isotope values for domesticates and wild herbivores from sites spanning the Neolithic in coastal Croatia, Slovenia, and Italy (6000–3500 calBC). Carbon and nitrogen stable isotope values are analyzed as proxies of diet and environment, with differences between species possibly indicating anthropogenic influence. Results are used to characterise diets and address questions of the origin and development of husbandry strategies, especially transhumance, in early farming communities. Changes in pig carbon and nitrogen isotope values through time suggest alterations in practices, whereas values remain relatively constant for cattle and ovicaprids during most of the Neolithic, despite assumptions of seasonal mobility.
Applied spectroscopy, 2016
Archaeological sediments from mounds within the mangrove zone of far-southern Pacific coastal Chi... more Archaeological sediments from mounds within the mangrove zone of far-southern Pacific coastal Chiapas, Mexico, are characterized in order to test the hypothesis that specialized pyro-technological activities of the region's prehistoric inhabitants (salt and ceramic production) created the accumulations visible today. Fourier transform infrared spectroscopy (FT-IR) is used to characterize sediment mineralogy, while portable X-ray fluorescence (pXRF) is used to determine elemental concentrations. Elemental characterization of natural sediments by both instrumental neutron activation analysis (INAA) and pXRF also contribute to understanding of processes that created the archaeological deposits. Radiocarbon dates combined with typological analysis of ceramics indicate that pyro-industrial activity in the mangrove zone peaked during the Late Formative and Terminal Formative periods, when population and monumental activity on the coastal plain and piedmont were also at their peaks.
Chichén Itzá dominated the political landscape of the northern Yucatán during the Terminal Classi... more Chichén Itzá dominated the political landscape of the northern Yucatán during the Terminal Classic Period (AD 800–1000). Chronological details of the rise and fall of this important polity are obscure because of the limited corpus of dated hieroglyphic records and by a restricted set of radiocarbon dates for the site. Here we compile and review these data and evaluate them within the context of political and climatic change in northern Yucatán at the end of the Classic period. The available data point to the end of elite activity at Chichén Itzá around AD 1000, a century after the collapse of Puuc Maya cities and other interior centers. Evidence supports a population shift in the eleventh century towards some coastal locations during a time associated with the end of monumental construction and art at Chichén Itzá. Our results suggest that regional political disintegration came in two waves. The first was the asynchronous collapse of multiple polities between AD 850 and 925 associated with a regional drying trend and punctuated by a series of multi-decadal droughts in the ninth and tenth centuries. The second wave was the political collapse at Chichén Itzá that coincides with the longest and most severe drought recorded in regional climate records between AD 1000 and 1100. This is a time that some scholars have characterized as a “dark age” across the northern Maya lowlands. Political developments during the Postclassic period (AD 1000–
1517) correspond with a return to higher rainfall. These patterns support a strong relationship between political
disintegration and climatic stress in the Maya lowlands. This research employs Bayesian radiocarbon models in conjunction with calendar dates on carved monuments and climate proxies to evaluate the rise and fall of Maya political centers and serves as an example of the impact of climate change on rainfall-dependent societies in Mesoamerica.
Radiocarbon, Apr 1, 2010
We present the results of 10 AMS radiocarbon dates for Cova de la Pastora (Alcoi, Alicante), a bu... more We present the results of 10 AMS radiocarbon dates for Cova de la Pastora (Alcoi, Alicante), a burial cave attributed to the Late Neolithic/Chalcolithic in eastern Spain. The direct dating of 10 human mandibles from Cova de la Pastora indicates that the cave was used as a burial place from the Late Neolithic/Chalcolithic to the Bronze Age. These dates reveal a continuity of ritual use not previously identified at the site. This case also serves to highlight the utility of revisiting historic excavations and museum collections with modern techniques to shed new light on the prehistoric human record.
Proceedings of the National Academy of Sciences of the United States of America, Jan 24, 2015
Proceedings of the National Academy of Sciences of the United States of America, Jan 27, 2015
The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemis... more The Younger Dryas impact hypothesis posits that a cosmic impact across much of the Northern Hemisphere deposited the Younger Dryas boundary (YDB) layer, containing peak abundances in a variable assemblage of proxies, including magnetic and glassy impact-related spherules, high-temperature minerals and melt glass, nanodiamonds, carbon spherules, aciniform carbon, platinum, and osmium. Bayesian chronological modeling was applied to 354 dates from 23 stratigraphic sections in 12 countries on four continents to establish a modeled YDB age range for this event of 12,835-12,735 Cal B.P. at 95% probability. This range overlaps that of a peak in extraterrestrial platinum in the Greenland Ice Sheet and of the earliest age of the Younger Dryas climate episode in six proxy records, suggesting a causal connection between the YDB impact event and the Younger Dryas. Two statistical tests indicate that both modeled and unmodeled ages in the 30 records are consistent with synchronous deposition of ...
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.
Journal of Archaeological Science, 2014
ABSTRACT Gatecliff Shelter provides a deeply stratified record of human-environment interaction i... more ABSTRACT Gatecliff Shelter provides a deeply stratified record of human-environment interaction in the Desert West spanning most of the middle and late Holocene. The well-preserved 10 m-deep deposits serve as an important reference sequence for technological and subsistence change in the Great Basin. Archaeological work started at Gatecliff in 1970 and a geochronological framework was established based on 47 uncalibrated radiocarbon (14C) dates obtained from several conventional radiometric laboratories between 1972 and 1982. These radiocarbon dates are not well accommodated within a Bayesian chronological model for these important deposits due to multiple temporal reversals and large analytical errors. This model was discarded in favor of one based on a combination of high-precision AMS 14C dates of short-lived carbonized plant remains supplemented with existing 14C dates culled from the original set using a Bayesian chronological model. Summed probabilities of the modeled posterior distributions of these 14C dates (N = 24) show episodic use of the shelter as a logistic hunting camp between 6050 and 3315 cal BP, a significant hiatus in occupation between 3315 and 2145 cal BP, and more persistent use by family bands after this time. In the future, this high-precision chronology will provide the foundation for reinterpreting broader patterns of technological and subsistence change in the Intermountain West and evaluating these changes relative to high-resolution climate records.
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 Journal of Geology, 2014
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 ע 150 years before present, forming the YD Boundary (YDB) layer, distributed over 150 million km 2 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.
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.
Scientific Reports, 2013
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.
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.
Science, Nov 9, 2012
The role of climate change in the development and demise of Classic Maya civilization (300 to 100... more The role of climate change in the development and demise of Classic Maya civilization (300 to 1000 C.E.) remains controversial because of the absence of well-dated climate and archaeological
sequences. We present a precisely dated subannual climate record for the past 2000 years from Yok Balum Cave, Belize. From comparison of this record with historical events compiled from well-dated stone monuments, we propose that anomalously high rainfall favored unprecedented population expansion and the proliferation of political centers between 440 and 660 C.E. This was followed by a drying trend between 660 and 1000 C.E. that triggered the balkanization of polities, increased warfare, and the asynchronous disintegration of polities, followed by population collapse in the context of an extended drought between 1020 and 1100 C.E.
The development of food production in Mesoamerica was a complex and protracted process. We argue ... more The development of food production in Mesoamerica was a complex and protracted process. We argue
that while maize had been cultivated for many millennia, this cereal grain assumed a markedly more
important role in the political economy of the Soconusco (and elsewhere in Mexico, Guatemala and
Belize) only after 1000 cal BC. Macrobotanical data from the long-occupied village of Cuauhtémoc
document low-level maize production from 1900 to 1400 cal BC with a significant increase during the
final centuries of the site’s occupation after 1000 cal BC. Botanical evidence of increased maize consumption
at this time occurred with evidence for changing groundstone use, intensified exploitation of dog and
deer as well as iconography linking maize with rulership. This was also when monumental architecture
was first built to mark a regional hierarchy of political centers. Changes evident in the Soconusco at
1000 cal BC parallel transformations in both highland and lowland regions of Mesoamerica when
ceramic-using villagers expanded into new environments, farther away from the permanent water
sources favored by Late Archaic and Early Formative peoples. We interpret the changes evident at
1000 cal BC in terms of both proximate historical factors as well as ultimate adaptive causes to produce
a fuller understanding of changing Mesoamerican food production practices.