Property:Abstract

• Property Type: Text

A summary of a scientific article.

Pages using the property "Abstract"

Showing 25 pages using this property.

View (previous 25 | next 25) (20 | 50 | 100 | 250 | 500)

P
Publication.10.1029/1998PA900015 +	Past sea surface temperatures (SST) in the northern and southern areas of the South China Sea have been reconstructed for the past 220 kyr using the UK37 alkenone index. The SST profiles follow the glacial/interglacial pattern exhibiting differences between Last Glacial Maximum and Holocene that are 1°–3°C larger than those observed at the same latitudes in the Atlantic and Pacific Oceans. In Termination I both planktonic foraminiferal δ18O and SST exhibit well-defined Bølling-Allerød and Younger Dryas events with temperature differences between both periods of 0.8° and 0.4°C in north and south, respectively. SSTs record a constant north-south difference of 1°C in the interglacials and nearly 2.5°C in the glacial stages. These differences define two distinct climatic and water circulation patterns that correspond with glacial/interglacial sea level oscillations which opened and closed water exchange with the tropical Indo-Pacific Ocean through the present Sunda Shelf.  +
Publication.10.1029/2002GL016612 +	Variations in tropical sea surface temperature patterns and the phasing relative to climate change in higher-latitudes provide insight into the mechanisms of climate change on both orbital and shorter time-scales. Here, we present well-dated, high-resolution records of planktonic foraminiferal 18O and Mg/Ca-based SST spanning the last deglaciation from the Sulu Sea, located in the western equatorial Pacific. The results indicate that the last glacial maximum was 2.3 ± 0.5°C cooler than present in the Sulu Sea with a concomitant decrease in sea surface salinity. The similarity between variations in surface salinity in the Sulu Sea, the western and eastern equatorial Pacific, and the Greenland ice-core record suggests that the observed changes in salinity reflect large-scale rearrangement of atmospheric patterns, which were coherent and synchronous throughout the Northern Hemisphere. The results suggest that the glacial equatorial Pacific climate was strongly influencedby both tropical, and extra-tropical forcing, although it is not clear whether interannual (ENSO) variability is a good analogue of glacial-interglacial climate change.  +
Publication.10.1029/2002PA000768 +	Holocene and latest Pleistocene oceanographic conditions and the coastal climate of northern California have varied greatly, based upon high-resolution studies (ca. every 100 years) of diatoms, alkenones, pollen, CaCO3%, and total organic carbon at Ocean Drilling Program (ODP) Site 1019 (41.682°N, 124.930°W, 980 m water depth). Marine climate proxies (alkenone sea surface temperatures [SSTs] and CaCO3%) behaved remarkably like the Greenland Ice Sheet Project (GISP)-2 oxygen isotope record during the Bølling-Allerod, Younger Dryas (YD), and early part of the Holocene. During the YD, alkenone SSTs decreased by >3°C below mean Bølling-Allerod and Holocene SSTs. The early Holocene (ca. 11.6 to 8.2 ka) was a time of generally warm conditions and moderate CaCO3 content (generally >4%). The middle part of the Holocene (ca. 8.2 to 3.2 ka) was marked by alkenone SSTs that were consistently 1-2°C cooler than either the earlier or later parts of the Holocene, by greatly reduced numbers of the gyre-diatom Pseudoeunotia doliolus (<10%), and by a permanent drop in CaCO3% to <3%. Starting at ca. 5.2 ka, coastal redwood and alder began a steady rise, arguing for increasing effective moisture and the development of the north coast temperate rain forest. At ca. 3.2 ka, a permanent ca. 1°C increase in alkenone SST and a threefold increase in P. doliolus signaled a warming of fall and winter SSTs. Intensified (higher amplitude and more frequent) cycles of pine pollen alternating with increased alder and redwood pollen are evidence that rapid changes in effective moisture and seasonal temperature (enhanced El Niño-Southern Oscillation [ENSO] cycles) have characterized the Site 1019 record since about 3.5 ka.  +
Publication.10.1029/2002PA000864 +	We present high-resolution paleoceanographic records of surface and deep water conditions within the northern Red Sea covering the last glacial maximum and termination I using alkenone paleothermometry, stable oxygen isotopes, and sediment compositional data. Paleoceanographic records in the restricted desert-surrounded northern Red Sea are strongly affected by the stepwise sea level rise and appear to record and amplify well-known millennial-scale climate events from the North Atlantic realm. During the last glacial maximum (LGM), sea surface temperatures were about 4°C cooler than the late Holocene. Pronounced coolings associated with Heinrich event 1 (~2°C below the LGM level) and the Younger Dryas imply strong atmospheric teleconnections to the North Atlantic. Owing to the restricted exchange with the Indian Ocean, Red Sea salinity is particularly sensitive to changes in global sea level. Paleosalinities exceeded 50 psu during the LGM. A pronounced freshening of the surface waters is associated with the meltwater peaks MWP1a and MWP1b owing to an increased surface-near inflow of "normal" saline water from the Indian Ocean. Vertical delta18O gradients are also increased during these phases, indicating stronger surface water stratification. The combined effect of deglacial changes in sea surface temperature and salinity on water column stratification initiated the formation of two sapropel layers, which were deposited under almost anoxic condition in a stagnant water body.  +
Publication.10.1029/2004PA001061 +	Detailed deglacial and Holocene records of planktonic d18O and Mg/Ca-based sea surface temperature (SST) from the Okinawa Trough suggest that at ~18 to 17 thousand years before present (kyr B.P.), late spring/early summer SSTs were approximately 3°C cooler than today, while surface waters were up to 1 practical salinity unit saltier. These conditions are consistent with a weaker influence of the summer East Asian Monsoon (EAM) than today. The timing of suborbital SST oscillations suggests a close link with abrupt changes in the EAM and North Atlantic climate. A tropical influence, however, may have resulted in subtle decoupling between the North Atlantic and the Okinawa Trough/EAM during the deglaciation. Okinawa Trough surface water trends in the Holocene are consistent with model simulations of an inland shift of intense EAM precipitation during the middle Holocene. Millennial-scale alternations between relatively warm, salty conditions and relatively cold, fresh conditions suggest varying influence of the Kuroshio during the Holocene.  +
Publication.10.1029/2004PA001071 +	High-resolution benthic oxygen isotope and dust flux records from Ocean Drilling Program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the Atlantic from 2.85 Ma back to 5 Ma. Spectral analysis of the delta18O record indicates that the 41-kyr period of Earth's orbital obliquity dominates the Pliocene record. This is shown to be true regardless of fundamental changes in the Earth's climate during the Pliocene. However, the cycles of Sahelian aridity fluctuations indicate a shift in spectral character near 3 Ma. From the early Pliocene to 3 Ma, the periodicities were dominantly precessional (19 and 23 kyr) and remained strong until 1.5 Ma. Subsequent to 3 Ma, the variance at the obliquity period (41 kyr) increased. The timescale tuned to precession suggests that the Pliocene was longer than previously estimated by more than 0.5 m.y. The tuned ages for the magnetic boundaries Gauss/Gilbert and Top Cochiti are about 6-8% older than the ages of the conventional timescale. A major phase of Pliocene northern hemisphere ice growth occurred between 3.15 Ma and 2.5 Ma. This was marked by a gradual increase in glacial Atlantic delta18O values of 1per mil and an increase in amplitude variations by up to 1.5 per mil, much larger than in the Pacific deepwater record (site 846). The first maxima occured in cold stages G6-96 between 2.7 Ma and 2.45 Ma. Prior to 3 Ma, the isotope record is characterized by predominantly low amplitude fluctuations (< 0.7 per mil). When obliquity forcing was at its minimum between 4.15 and 3.6 Ma and during the Kaena interval, delta18O amplitude fluctuations were minimal. From 4.9 to 4.3 Ma, the delta18O values decreased by about 0.5 per mil, reaching a long-term minimum at 4.15 Ma, suggesting higher deepwater temperatures or a deglaciation. Deepwater cooling and/or an increase in ice volume is indicated by a series of short-term delta18O fluctuations between 3.8 and 3.6 Ma.  +
Publication.10.1029/2005PA001208 +	Multiproxy geologic records of d18O and Mg/Ca in fossil foraminifera from sediments under the Eastern Pacific Warm Pool (EPWP) region west of Central America document variations in upper ocean temperature, pycnocline strength, and salinity (i.e., net precipitation) over the past 30 kyr. Although evident in the paleotemperature record, there is no glacial-interglacial difference in paleosalinity, suggesting that tropical hydrologic changes do not respond passively to high-latitude ice sheets and oceans. Millennial variations in paleosalinity with amplitudes as high as 4 practical salinity units occur with a dominant period of 3-5 ky during the glacial/deglacial interval and 1.0-1.5 ky during the Holocene. The amplitude of the EPWP paleosalinity changes greatly exceeds that of published Caribbean and western tropical Pacific paleosalinity records. EPWP paleosalinity changes correspond to millennial-scale climate changes in the surface and deep Atlantic and the high northern latitudes, with generally higher (lower) paleosalinity during cold (warm) events. In addition to Intertropical Convergence Zone (ITCZ) dynamics, which play an important role in tropical hydrologic variability, changes in Atlantic-Pacific moisture transport, which is closely linked to ITCZ dynamics, may also contribute to hydrologic variations in the EPWP. Calculations of interbasin salinity average and interbasin salinity contrast between the EPWP and the Caribbean help differentiate long-term changes in mean ITCZ position and Atlantic-Pacific moisture transport, respectively.  +
Publication.10.1029/2006GC001514 +	Sea surface temperature and oxygen isotopic records from two well-dated Indian Ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the Indian to the Atlantic Ocean, one between 18 and 14.5 ka and the other during the Younger Dryas. Our results imply that during these periods, salt accumulated in the tropical Atlantic, creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming. Furthermore, we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets, and El Niño–Southern Oscillation variability, through changes in the position of the Intertropical Convergence Zone. This interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent.  +
Publication.10.1029/2009PA001740 +	In this study we utilize two organic geochemical proxies, the Uk'37 index and TEX86, to examine past sea surface temperatures (SST) from a site located near the Nile River Delta in the eastern Mediterranean (EM) Sea. The Uk'37 and TEX86 records generally are in agreement and indicate SST ranges of 14°C-26°C and 14°C-28°C, respectively, during the last 27 cal ka. During the Holocene, TEX86-based SST estimates are usually higher than Uk'37-based SST estimates, which is likely due to seasonal differences between the timing of the haptophyte and crenarchaeota blooms in the EM and is related to the onset of the modern flow regime of the Nile River. Both records show that SST varied on centennial to millennial timescales in response to global climate events, i.e., cooling during the Last Glacial Maximum (LGM), Heinrich event 1 (H1), and the Younger Dryas (YD) and warming during the Bølling-Allerød and in the early Holocene during deposition of sapropel S1. The H1 cooling was particularly severe and is marked by a drop in SST of ~4.5°C in comparison to pre-H1 SST, with temperatures >1°C cooler than during the LGM. In contrast to high-latitude and western Mediterranean records, which indicate both an abrupt onset and termination of the YD event, the transition from the YD to the Holocene was much more gradual in the EM.  +
Publication.10.1029/2011GL050202 +	Central Asia is currently a semiarid-arid region, dominated by the Westerlies. It is important to understand mechanisms of climate and precipitation changes here, as water availability in the region is crucial today and in the future. High-resolution, absolutely-dated oxygen isotope (d18O) records of stalagmites from Kesang Cave characterize a dynamic precipitation history over most of the past 500,000 years. This record demonstrates, for the first time, that climate change in the region exhibits a processional rhythm with abrupt inceptions of low d18O speleothem growth at times of high Northern Hemisphere summer insolation followed by gradual d18O increases that track decreases of insolation. These observations and interpretations contrast with the interpretation of nearby, but higher elevation ice core records. The absolutely-dated cave d18O shifts can be used to correlate the regional climate variability by providing chronological marks. Combined with other paleoclimate records, the Kesang observations suggest that possible incursions of Asian summer monsoon rainfall or related moisture into the Kesang site and/or adjacent areas during the high insolation times may play an important role in changing orbital-scale hydrology of the region. Based on our record, arid climate will prevail in this region for the next several millennia, providing that anthropogenic effects do not supersede natural processes.  +
Publication.10.1029/2011PA002157 +	The prevailing paradigm of abrupt climate change holds that rapid shifts associated with the most extreme climate swings of the last glacial cycle were forced by changes in the strength and northward extension of Atlantic Meridional Overturning Circulation (AMOC), resulting in an abrupt reorganization of atmospheric circulation patterns with global teleconnections. To determine the timing of tropical Atlantic atmospheric circulation changes over the past 21 ka BP, we reconstruct high resolution sea surface temperature and d18OSW (a proxy for surface salinity) records based on Mg/Ca ratios and oxygen isotope measurements in the planktonic foraminifera Globigerinoides ruber from a sediment core located on the western margin of the Florida Straits. As a proxy for meltwater discharge influence on Florida Straits surface water salinity, we also measured Ba/Ca ratios in G. ruber from the same core. Results show that riverine influence on Florida Straits surface water started by 17.2 ka BP and ended by 13.6 ka BP, 600 years before the start of the Younger Dryas (YD) cold interval. The initiation of the YD is marked by an abrupt increase in Florida Straits d18OSW values, indicating a shift to elevated sea surface salinity occurring in 130 years, most likely resulting from increased regional aridity and/or reduced precipitation. In order to resolve the timing of tropical atmospheric circulation change relative to AMOC variability across this transition, we compare the timing of surface water changes to a recently published record of Florida Current variability in the same core reconstructed from benthic oxygen isotope measurements. We find synchronous changes in atmospheric and ocean circulation on the transition into the YD, consistent with an abrupt reduction in AMOC as the driver of tropical Atlantic atmospheric circulation change at this time.  +
Publication.10.1029/2012PA002284 +	Previous studies showed that sea surface salinity (SSS) in the Florida Straits as well as Florida Current transport covaried with changes in North Atlantic climate over the past two millennia. However, little is known about earlier Holocene hydrographic variability in the Florida Straits. Here, we combine Mg/Ca-paleothermometry and stable oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber (white variety) from Florida Straits sediment core KNR166-2 JPC 51 (24 deg 24.70'N, 83 deg 13.14'W, 198 m deep) to reconstruct a high-resolution (~25 yr/sample) early to mid Holocene record of sea surface temperature and d18OSW (a proxy for SSS) variability. After removing the influence of global d18OSW change due to continental ice volume variability, we find that early Holocene SSS enrichments are associated with increased evaporation/precipitation ratios in the Florida Straits during periods of reduced solar forcing, increased ice rafted debris in the North Atlantic and the development of more permanent El Niño-like conditions in the eastern equatorial Pacific. When considered with previous high-resolution reconstructions of Holocene tropical atmospheric circulation changes, our results provide evidence that variations in solar forcing over the early Holocene had a significant impact on the global tropical hydrologic cycle.  +
Publication.10.1038/NGEO920 +	High sea surface temperatures in the western Pacific warm pool fuel atmospheric convection and influence tropical climate. This region also hosts the Indonesian throughflow, the network of currents through which surface and thermocline waters are transported from the western equatorial Pacific Ocean into the Indian Ocean. Here we show, using records of the d18O and Mg/Ca of planktonic foraminifera from eight sediment cores, that from about 10,000 to 7,000 years ago, sea surface temperatures in the western sector of the western Pacific warm pool were about 0.5°C higher than during pre-industrial times. We also find that about 9,500 years ago, when the South China and Indonesian seas were connected by rising sea level, surface waters in the Makassar Strait became relatively fresher. We suggest that the permanent reduction of surface salinity initiated the enhanced flow at lower, thermocline depths seen in the modern Indonesian throughflow. However, the uniformly warm sea surface temperatures found upstream and downstream of the Indonesian throughflow indicate that the early Holocene warmth in this region was not directly related to reduced heat transport by the throughflow that may have resulted from surface freshening of the Makassar Strait. Instead, we propose that the elevated temperatures were the result of a westward shift or expansion of the boundaries of the western Pacific warm pool.  +
Publication.10.1038/nature01779 +	Any assessment of future climate change requires knowledge of the full range of natural variability in the El Niño/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30–150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.  +
Publication.10.1038/nature09751 +	The disintegration of ice shelves, reduced sea-ice and glacier extent, and shifting ecological zones observed around Antarctica (Cook et al., 2005; Stammerjohn et al., 2008) highlight the impact of recent atmospheric (Steig et al., 2009) and oceanic warming (Gille, 2002) on the cryosphere. Observations (Cook et al., 2005; Stammerjohn et al., 2008) and models (Pollard and DeConto, 2009) suggest that oceanic and atmospheric temperature variations at Antarctica's margins affect global cryosphere stability, ocean circulation, sea levels and carbon cycling. In particular, recent climate changes on the Antarctic Peninsula have been dramatic, yet the Holocene climate variability of this region is largely unknown, limiting our ability to evaluate ongoing changes within the context of historical variability and underlying forcing mechanisms. Here we show that surface ocean temperatures at the continental margin of the western Antarctic Peninsula cooled by 3-4 °C over the past 12,000?years, tracking the Holocene decline of local (65° S) spring insolation. Our results, based on TEX86 sea surface temperature (SST) proxy evidence from a marine sediment core, indicate the importance of regional summer duration as a driver of Antarctic seasonal sea-ice fluctuations (Huybers and Denton, 2008, doi:10.1038/ngeo311). On millennial timescales, abrupt SST fluctuations of 2-4 °C coincide with globally recognized climate variability (Mayewski et al., 2004, doi:10.1016/j.yqres.2004.07.001). Similarities between our SSTs, Southern Hemisphere westerly wind reconstructions (Moreno et al., 2010, doi:10.1130/G30962.1) and El Niño/Southern Oscillation variability (Conroy et al., 2008, doi:10.1016/j.quascirev.2008.02.015) indicate that present climate teleconnections between the tropical Pacific Ocean and the western Antarctic Peninsula (Yuan et al., 2004, doi:10.1017/S0954102004002238) strengthened late in the Holocene epoch. We conclude that during the Holocene, Southern Ocean temperatures at the western Antarctic Peninsula margin were tied to changes in the position of the westerlies, which have a critical role in global carbon cycling (Moreno et al., 2010, doi:10.1130/G30962.1; Anderson et al., 2009, doi:10.1126/science.1167441).  +
Publication.10.1038/nature10581 +	Arctic sea ice extent is now more than two million square kilometres less than it was in the late twentieth century, with important consequences for the climate, the ocean and traditional lifestyles in the Arctic1, 2. Although observations show a more or less continuous decline for the past four or five decades3, 4, there are few long-term records with which to assess natural sea ice variability. Until now, the question of whether or not current trends are potentially anomalous5 has therefore remained unanswerable. Here we use a network of high-resolution terrestrial proxies from the circum-Arctic region to reconstruct past extents of summer sea ice, and show that—although extensive uncertainties remain, especially before the sixteenth century—both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years. Enhanced advection of warm Atlantic water to the Arctic6 seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming.  +
Publication.10.1038/ngeo1862 +	Precipitation in southwestern North America has exhibited significant natural variability over the past few thousand years. This variability has been attributed to sea surface temperature regimes in the Pacific and Atlantic oceans, and to the attendant shifts in atmospheric circulation patterns. In particular, decadal variability in the North Pacific has influenced precipitation in this region during the twentieth century, but links to earlier droughts and pluvials are unclear. Here we assess these links using d18O data from a speleothem from southern California that spans AD 854-2007. We show that variations in the oxygen isotopes of the speleothem correlate to sea surface temperatures in the Kuroshio Extension region of the North Pacific, which affect the atmospheric trajectory and isotopic composition of moisture reaching the study site. Interpreting our speleothem data as a record of sea surface temperatures in the Kuroshio Extension, we find a strong 22-year periodicity, suggesting a persistent solar influence on North Pacific decadal variability. A comparison with tree-ring records of precipitation during the past millennium shows that some droughts occurred during periods of warmth in the Kuroshio Extension, similar to the instrumental record. However, other droughts did not and instead must have been influenced by other factors. Finally, we find a significant increase in sea surface temperature variability over the past 150 years, which may reflect an influence of greenhouse gas concentrations on variability in the North Pacific.  +
Publication.10.1038/ngeo2510 +	The oceans mediate the response of global climate to natural and anthropogenic forcings. Yet for the Common Era (past 2000 years) global ocean sea surface temperature (SST) variations, and the underlying driving mechanisms, are poorly constrained. Here we present a global synthesis of Common Era SSTs derived solely from marine archives. The synthesis uses 57 individual SST reconstructions that meet strict quality control criteria. We find an SST cooling trend during 0-1800 CE that is robust against explicit tests for potential biases in the reconstructions. The surface cooling trend for 801-1800 CE is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with SST simulated by an ensemble of climate model simulations using best estimates of past forcings. Single and cumulative forcing climate model simulations suggest that the ocean SST cooling trend is not a response to orbital forcing, which is globally weak over this interval, but rather to increased frequency of explosive volcanism in recent centuries. This finding provides support for the hypothesis that ocean dynamics transform episodic volcanic forcing into a centennial and global-scale cooling trend.  +
Publication.10.1038/ngeo2603 +	The Indian monsoon, the largest monsoon system on Earth, responds to remote climatic forcings, including temperature changes in the North Atlantic. The monsoon was weak during two cool periods that punctuated the last deglaciation - Heinrich Stadial 1 and the Younger Dryas. It has been suggested that sea surface cooling in the Indian Ocean was the critical link between these North Atlantic stadials and monsoon failure; however, based on existing proxy records it is unclear whether surface temperatures in the Indian Ocean and Arabian Sea dropped during these intervals. Here we compile new and existing temperature proxy data from the Arabian Sea, and find that surface temperatures cooled whereas subsurface temperatures warmed during both Heinrich Stadial 1 and the Younger Dryas. Our analysis of model simulations shows that surface cooling weakens the monsoon winds and leads to destratification of the water column and substantial subsurface warming. We thus conclude that sea surface temperatures in the Indian Ocean are indeed the link between North Atlantic climate and the Indian monsoon.  +
Publication.10.1038/ngeo277 +	Palaeoclimate records and numerical model simulations indicate that changes in tropical and subtropical sea surface temperatures and in the annual average position of the intertropical convergence zone are linked to high-latitude climate changes on millennial to glacial-interglacial timescales. It has recently been suggested that cooling in the high latitudes associated with abrupt climate-change events is evident primarily during the northern hemisphere winter, implying increased seasonality at these times. However, it is unclear whether such a seasonal bias also exists for the low latitudes. Here we analyse the Mg/Ca ratios of surface-dwelling foraminifera to reconstruct sea surface temperatures in the northeastern Gulf of Mexico for the past 300,000 years. We suggest that sea surface temperatures are controlled by the migration of the northern boundary of the Atlantic Warm Pool, and hence the position of the intertropical convergence zone during boreal summer, and are relatively insensitive to winter conditions. Our results suggest that summer Atlantic Warm Pool expansion is primarily affected by glacial-interglacial variability and low-latitude summer insolation. Because a clear signature of rapid climate-change events, such as the Younger Dryas cold event, is lacking in our record, we conclude that high-latitude events seem to influence only the winter Caribbean climate conditions, consistent with the hypothesis of extreme northern-hemisphere seasonality during abrupt cooling events.  +
Publication.10.1073/pnas.1222804110 +	The dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles, ocean circulation through its influence on atmospheric circulation, and sea-surface temperatures. However, few records from the summer North American Monsoon system are available to test for a synchronous response with other global monsoons to shared forcings. In particular, the monsoon response to widespread atmospheric reorganizations associated with disruptions of the Atlantic Meridional Overturning Circulation (AMOC) during the deglacial period remains unconstrained. Here, we present a high-resolution and radiometrically dated monsoon rainfall reconstruction over the past 22,000 y from speleothems of tropical southwestern Mexico. The data document an active Last Glacial Maximum (18-24 cal ka B.P.) monsoon with similar d18O values to the modern, and that the monsoon collapsed during periods of weakened AMOC during Heinrich stadial 1 (ca. 17 ka) and the Younger Dryas (12.9-11.5 ka). The Holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation. We conclude that the Mesoamerican monsoon responded in concert with other global monsoon regions, and that monsoon strength was driven by variations in the strength and latitudinal position of the Intertropical Convergence Zone, which was forced by AMOC variations in the North Atlantic Ocean. The surprising observation of an active Last Glacial Maximum monsoon is attributed to an active but shallow AMOC and proximity to the Intertropical Convergence Zone. The emergence of agriculture in southwestern Mexico was likely only possible after monsoon strengthening in the Early Holocene at ca. 11 ka.  +
Publication.10.1126/science.1140461 +	A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial-scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. EEA Mg/Ca-based sea surface temperatures (SSTs) were decoupled from northern high-latitude millennial-scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low-latitude solar insolation. The onset of enhanced monsoon precipitation lags behind the changes in EEA SSTs by up to 7000 years during glacial interglacial transitions. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.  +
Publication.10.1126/science.1143791 +	We investigate the relationship between tropical Pacific and Southern Ocean variability during the Holocene using the stable oxygen isotope and magnesium/calcium records of co-occurring planktonic and benthic foraminifera from a marine sediment core collected in the western equatorial Pacific. The planktonic record exhibits millennial-scale sea surface temperature (SST) oscillations over the Holocene of ~0.5°C while the benthic δ18Oc document ~0.10‰ millennial-scale changes of Upper Circumpolar Deep Water (UCDW), a water mass which outcrops in the Southern Ocean. Solar forcing as an explanation for millennial-scale SST variability requires (1) a large climate sensitivity and (2) a long 400-year delayed response, suggesting that if solar forcing is the cause of the variability, it would need to be considerably amplified by processes within the climate system at least at the core location. We also explore the possibility that SST variability arose from volcanic for! cing using a simple red noise model. Our best estimates of volcanic forcing falls short of reproducing the amplitude of observed SST variations although it produces power at low-frequency similar to that observed in the MD81 record. Although we cannot totally discount the volcanic and solar forcing hypotheses, we are left to consider that the most plausible source for Holocene millennial-scale variability lies within the climate system itself. In particular, UCDW variability coincided with deep North Atlantic changes, indicating a role for the deep ocean in Holocene millennial-scale variability.  +
Publication.10.1130/G32471.1 +	Droughts are a recurring feature of Mexican climate, but few high-resolution data are available to test for climate-change forcing of Mesoamerican civilizations. We present a quantitative 2400 yr rainfall reconstruction for the Basin of Mexico, from a precisely dated and highly resolved speleothem, that documents highly variable rainfall over the past 2400 yr. Dry conditions peaked during a 150-yr-long late Classic (ca. 600–900 CE) (Common Era) mega drought that culminated at 770 CE which followed centuries of climatic drying that spanned the fall of the city of Teotihuacán ca. 550 CE. The wettest conditions in the 1450s CE were associated with flooding in the Basin of Mexico. Our data suggest that rainfall variability was likely forced by the El Niño-Southern Oscillation, and impacts on spring-fed irrigation agriculture may have been a stressor on Mesoamerican civilizations.  +
Publication.10.1175/JCLI-D-11-00510.1 +	Any assessment of future climate change requires knowledge of the full range of natural variability in the El Niño/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30–150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.  +