West Antarctic Ice Sheet retreat driven by Holocene warm water incursions.

Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) incursions onto the West Antarctic continental shelf cause melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet retreat today. Here we present a multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the Holocene epoch (from 11.7 thousand years ago to the present). The chemical compositions of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector from at least 10,400 years ago until 7,500 years ago-when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream-and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models.

Continuous and simultaneous measurement of triple-oxygen and hydrogen isotopes of liquid and vapor during evaporation experiments.

RATIONALE: Oxygen and hydrogen isotopes are important tools for studying the modern and past hydrological cycle. Previous evaporation experiments used episodic measurement of liquid and/or vapor or did not measure all isotopologues of water. Here, we describe an evaporation experimental system that allows all isotopologues of liquid and water vapor to be measured simultaneously and near-continuously at high precision using cavity ring-down laser spectroscopy (CRDS). METHODS: Evaporating liquid is periodically sampled from a closed recirculating loop by a syringe pump that delivers a constant supply of water to the vaporizer, achieving a water vapor concentration of 20,000 ppmV H2 O (±132, 1σ). Vapor is sampled directly from the evaporation chamber. Isotope ratios are measured simultaneously with a Picarro L2140-i CRDS instrument. RESULTS: For liquid measurements, Allan variance analysis indicates an optimum data collection window of 34 min for oxygen isotopes and 27 min for hydrogen isotopes. During these periods, the mean standard error is ±0.0081‰ for δ17 O values, ±0.0081‰ for δ18 O values, and ±0.019‰ for δ2 H values. For the derived parameters 17 O-excess and d-excess, the standard error of the mean is 5.8 per meg and 0.07‰, respectively. For the vapor phase a 12.5 min data window for all isotopologues results in a mean standard error of ±0.012‰ for δ17 O values, ±0.011‰ for δ18 O values, and ±0.023‰ for δ2 H values. For the derived parameters, the standard error of the mean is 9.2 per meg for 17 O-excess and 0.099‰ for d-excess. These measurements result in consistently narrow 95% confidence limits for the slopes of ln(δ17 O + 1) vs ln(δ18 O + 1) and ln(δ2 H + 1) vs ln(δ18 O + 1). CONCLUSIONS: The experimental method permits measurement of fractionation of triple-oxygen and hydrogen isotopes of evaporating water under varying controlled conditions at high precision. Application of this method will be useful for testing theoretical models of evaporation and conducting experiments to simulate evaporation and isotopic equilibration in natural systems.

Evolution of the early Antarctic ice ages.

Understanding the stability of the early Antarctic ice cap in the geological past is of societal interest because present-day atmospheric CO2 concentrations have reached values comparable to those estimated for the Oligocene and the Early Miocene epochs. Here we analyze a new high-resolution deep-sea oxygen isotope (δ18O) record from the South Atlantic Ocean spanning an interval between 30.1 My and 17.1 My ago. The record displays major oscillations in deep-sea temperature and Antarctic ice volume in response to the ∼110-ky eccentricity modulation of precession. Conservative minimum ice volume estimates show that waxing and waning of at least ∼85 to 110% of the volume of the present East Antarctic Ice Sheet is required to explain many of the ∼110-ky cycles. Antarctic ice sheets were typically largest during repeated glacial cycles of the mid-Oligocene (∼28.0 My to ∼26.3 My ago) and across the Oligocene-Miocene Transition (∼23.0 My ago). However, the high-amplitude glacial-interglacial cycles of the mid-Oligocene are highly symmetrical, indicating a more direct response to eccentricity modulation of precession than their Early Miocene counterparts, which are distinctly asymmetrical-indicative of prolonged ice buildup and delayed, but rapid, glacial terminations. We hypothesize that the long-term transition to a warmer climate state with sawtooth-shaped glacial cycles in the Early Miocene was brought about by subsidence and glacial erosion in West Antarctica during the Late Oligocene and/or a change in the variability of atmospheric CO2 levels on astronomical time scales that is not yet captured in existing proxy reconstructions.

Evolution of South Atlantic density and chemical stratification across the last deglaciation.

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.

Drought, agricultural adaptation, and sociopolitical collapse in the Maya Lowlands.

Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (∼800-950 C.E.). Evidence for drought largely derives from the drier, less populated northern Maya Lowlands but does not explain more pronounced and earlier societal disruption in the relatively humid southern Maya Lowlands. Here we apply hydrogen and carbon isotope compositions of plant wax lipids in two lake sediment cores to assess changes in water availability and land use in both the northern and southern Maya lowlands. We show that relatively more intense drying occurred in the southern lowlands than in the northern lowlands during the Terminal Classic period, consistent with earlier and more persistent societal decline in the south. Our results also indicate a period of substantial drying in the southern Maya Lowlands from ∼200 C.E. to 500 C.E., during the Terminal Preclassic and Early Classic periods. Plant wax carbon isotope records indicate a decline in C4 plants in both lake catchments during the Early Classic period, interpreted to reflect a shift from extensive agriculture to intensive, water-conservative maize cultivation that was motivated by a drying climate. Our results imply that agricultural adaptations developed in response to earlier droughts were initially successful, but failed under the more severe droughts of the Terminal Classic period.

Simultaneous analysis of (17) O/(16) O, (18) O/(16) O and (2) H/(1) H of gypsum hydration water by cavity ring-down laser spectroscopy.

RATIONALE: The recent development of cavity ring-down laser spectroscopy (CRDS) instruments capable of measuring (17) O-excess in water has created new opportunities for studying the hydrologic cycle. Here we apply this new method to studying the triple oxygen ((17) O/(16) O, (18) O/(16) O) and hydrogen ((2) H/(1) H) isotope ratios of gypsum hydration water (GHW), which can provide information about the conditions under which the mineral formed and subsequent post-depositional interaction with other fluids. METHODS: We developed a semi-automated procedure for extracting GHW by slowly heating the sample to 400°C in vacuo and cryogenically trapping the evolved water. The isotopic composition (δ(17) O, δ(18) O and δ(2) H values) of the GHW is subsequently measured by CRDS. The extraction apparatus allows the dehydration of five samples and one standard simultaneously, thereby increasing the long-term precision and sample throughput compared with previous methods. The apparatus is also useful for distilling brines prior to isotopic analysis. A direct comparison is made between results of (17) O-excess in GHW obtained by CRDS and fluorination followed by isotope ratio mass spectrometry (IRMS) of O2 . RESULTS: The long-term analytical precision of our method of extraction and isotopic analysis of GHW by CRDS is ±0.07‰ for δ(17) O values, ±0.13‰ for δ(18) O values and ±0.49‰ for δ(2) H values (all ±1SD), and ±1.1‰ and ±8 per meg for the deuterium-excess and (17) O-excess, respectively. Accurate measurement of the (17) O-excess values of GHW, of both synthetic and natural samples, requires the use of a micro-combustion module (MCM). This accessory removes contaminants (VOCs, H2 S, etc.) from the water vapour stream that interfere with the wavelengths used for spectroscopic measurement of water isotopologues. CRDS/MCM and IRMS methods yield similar isotopic results for the analysis of both synthetic and natural gypsum samples within analytical error of the two methods. CONCLUSIONS: We demonstrate that precise and simultaneous isotopic measurements of δ(17) O, δ(18) O and δ(2) H values, and the derived deuterium-excess and (17) O-excess, can be obtained from GHW and brines using a new extraction apparatus and subsequent measurement by CRDS. This method provides new opportunities for the application of water isotope tracers in hydrologic and paleoclimatologic research.

Paleoenvironmental history of the West Baray, Angkor (Cambodia).

Angkor (Cambodia) was the seat of the Khmer Empire from the 9th to 15th century AD. The site is noted for its monumental architecture and complex hydro-engineering systems, comprised of canals, moats, embankments, and large reservoirs, known as barays. We infer a 1,000-y, (14)C-dated paleoenvironmental record from study of an approximately 2-m sediment core taken in the largest Khmer reservoir, the West Baray. The baray was utilized and managed from the time of construction in the early 11th century, through the 13th century. During that time, the West Baray received relatively high rates of detrital input. In the 14th century, linear sedimentation rates diminished by an order of magnitude, yielding a condensed section that correlates temporally with episodes of regional monsoon failure during the late 14th and early 15th century, recorded in tree ring records from Vietnam. Our results demonstrate that changes in the water management system were associated with the decline of the Angkorian kingdom during that period. By the 17th century, the West Baray again functioned as a limnetic system. Ecologic and sedimentologic changes over the last millennium, detected in the baray deposits, are attributed to shifts in regional-scale Khmer water management, evolving land use practices in the catchment, and regional climate change.

Extreme glacial cooling likely led to hominin depopulation of Europe in the Early Pleistocene.

The oldest known hominin remains in Europe [~1.5 to ~1.1 million years ago (Ma)] have been recovered from Iberia, where paleoenvironmental reconstructions have indicated warm and wet interglacials and mild glacials, supporting the view that once established, hominin populations persisted continuously. We report analyses of marine and terrestrial proxies from a deep-sea core on the Portugese margin that show the presence of pronounced millennial-scale climate variability during a glacial period ~1.154 to ~1.123 Ma, culminating in a terminal stadial cooling comparable to the most extreme events of the last 400,000 years. Climate envelope-model simulations reveal a drastic decrease in early hominin habitat suitability around the Mediterranean during the terminal stadial. We suggest that these extreme conditions led to the depopulation of Europe, perhaps lasting for several successive glacial-interglacial cycles.

Changes in North Atlantic deep-water oxygenation across the Middle Pleistocene Transition.

The oxygen concentrations of oceanic deep-water and atmospheric carbon dioxide (pCO2) are intrinsically linked through organic carbon remineralization and storage as dissolved inorganic carbon in the deep sea. We present a high-resolution reconstruction of relative changes in oxygen concentration in the deep North Atlantic for the past 1.5 million years using the carbon isotope gradient between epifaunal and infaunal benthic foraminifera species as a proxy for paleo-oxygen. We report a significant (>40 micromole per kilogram) reduction in glacial Atlantic deep-water oxygenation at ~960 thousand to 900 thousand years ago that coincided with increased continental ice volume and a major change in ocean thermohaline circulation. Paleo-oxygen results support a scenario of decreasing deep-water oxygen concentrations, increased respired carbon storage, and a reduction in glacial pCO2 across the Middle Pleistocene Transition.

Clay hydroxyl isotopes show an enhanced hydrologic cycle during the Paleocene-Eocene Thermal Maximum.

The Paleocene-Eocene Thermal Maximum (PETM) was an abrupt global warming event associated with a large injection of carbon into the ocean-atmosphere system, as evidenced by a diagnostic carbon isotope excursion (CIE). Evidence also suggests substantial hydrologic perturbations, but details have been hampered by a lack of appropriate proxies. To address this shortcoming, here we isolate and measure the isotopic composition of hydroxyl groups (OH-) in clay minerals from a highly expanded PETM section in the North Sea Basin, together with their bulk oxygen isotope composition. At this location, we show that hydroxyl O- and H-isotopes are less influenced than bulk values by clay compositional changes due to mixing and/or inherited signals and thus better track hydrologic variability. We find that clay OH- hydrogen-isotope values (δ2HOH) decrease slowly prior to the PETM and then abruptly by ∼8‰ at the CIE onset. Coincident with an increase in relative kaolinite content, this indicates increased rainfall and weathering and implies an enhanced hydrologic cycle response to global warming, particularly during the early stages of the PETM. Subsequently, δ2HOH returns to pre-PETM values well before the end of the CIE, suggesting hydrologic changes in the North Sea were short-lived relative to carbon-cycle perturbations.

Drought-Induced Civil Conflict Among the Ancient Maya.

The influence of climate change on civil conflict and societal instability in the premodern world is a subject of much debate, in part because of the limited temporal or disciplinary scope of case studies. We present a transdisciplinary case study that combines archeological, historical, and paleoclimate datasets to explore the dynamic, shifting relationships among climate change, civil conflict, and political collapse at Mayapan, the largest Postclassic Maya capital of the Yucatán Peninsula in the thirteenth and fourteenth centuries CE. Multiple data sources indicate that civil conflict increased significantly and generalized linear modeling correlates strife in the city with drought conditions between 1400 and 1450 cal. CE. We argue that prolonged drought escalated rival factional tensions, but subsequent adaptations reveal regional-scale resiliency, ensuring that Maya political and economic structures endured until European contact in the early sixteenth century CE.

The potential of gypsum speleothems for paleoclimatology: application to the Iberian Roman Human Period.

Carbonate cave deposits (speleothems) have been used widely for paleoclimate reconstructions; however, few studies have examined the utility of other speleothem-forming minerals for this purpose. Here we demonstrate for the first time that stable isotopes (δ17O, δ18O and δD) of structurally-bound gypsum (CaSO4·2H2O) hydration water (GHW) can be used to infer paleoclimate. Specifically, we used a 63 cm-long gypsum stalactite from Sima Blanca Cave to reconstruct the climate history of SE Spain from ~ 800 BCE to ~ 800 CE. The gypsum stalactite indicates wet conditions in the cave and humid climate from ~ 200 BCE to 100 CE, at the time of the Roman Empire apogee in Hispania. From ~ 100 CE to ~ 600 CE, evaporation in the cave increased in response to regional aridification that peaked at ~ 500-600 CE, roughly coinciding with the transition between the Iberian Roman Humid Period and the Migration Period. Our record agrees with most Mediterranean and Iberian paleoclimate archives, demonstrating that stable isotopes of GHW in subaerial gypsum speleothems are a useful tool for paleoclimate reconstructions.

Author Correction: The potential of gypsum speleothems for paleoclimatology: application to the Iberian Roman Humid Period.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Interglacial instability of North Atlantic Deep Water ventilation.

Disrupting North Atlantic Deep Water (NADW) ventilation is a key concern in climate projections. We use (sub)centennially resolved bottom water δ13C records that span the interglacials of the last 0.5 million years to assess the frequency of and the climatic backgrounds capable of triggering large NADW reductions. Episodes of reduced NADW in the deep Atlantic, similar in magnitude to glacial events, have been relatively common and occasionally long-lasting features of interglacials. NADW reductions were triggered across the range of recent interglacial climate backgrounds, which demonstrates that catastrophic freshwater outburst floods were not a prerequisite for large perturbations. Our results argue that large NADW disruptions are more easily achieved than previously appreciated and that they occurred in past climate conditions similar to those we may soon face.

An astronomically dated record of Earth's climate and its predictability over the last 66 million years.

Much of our understanding of Earth's past climate comes from the measurement of oxygen and carbon isotope variations in deep-sea benthic foraminifera. Yet, long intervals in existing records lack the temporal resolution and age control needed to thoroughly categorize climate states of the Cenozoic era and to study their dynamics. Here, we present a new, highly resolved, astronomically dated, continuous composite of benthic foraminifer isotope records developed in our laboratories. Four climate states-Hothouse, Warmhouse, Coolhouse, Icehouse-are identified on the basis of their distinctive response to astronomical forcing depending on greenhouse gas concentrations and polar ice sheet volume. Statistical analysis of the nonlinear behavior encoded in our record reveals the key role that polar ice volume plays in the predictability of Cenozoic climate dynamics.

The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle.

From 1.25 million to 700,000 years ago, the ice age cycle deepened and lengthened from 41,000- to 100,000-year periodicity, a transition that remains unexplained. Using surface- and bottom-dwelling foraminifera from the Antarctic Zone of the Southern Ocean to reconstruct the deep-to-surface supply of water during the ice ages of the past 1.5 million years, we found that a reduction in deep water supply and a concomitant freshening of the surface ocean coincided with the emergence of the high-amplitude 100,000-year glacial cycle. We propose that this slowing of deep-to-surface circulation (i.e., a longer residence time for Antarctic surface waters) prolonged ice ages by allowing the Antarctic halocline to strengthen, which increased the resistance of the Antarctic upper water column to orbitally paced drivers of carbon dioxide release.

Quantification of drought during the collapse of the classic Maya civilization.

The demise of Lowland Classic Maya civilization during the Terminal Classic Period (~800 to 1000 CE) is a well-cited example of how past climate may have affected ancient societies. Attempts to estimate the magnitude of hydrologic change, however, have met with equivocal success because of the qualitative and indirect nature of available climate proxy data. We reconstructed the past isotopic composition (δ18O, δD, 17O-excess, and d-excess) of water in Lake Chichancanab, Mexico, using a technique that involves isotopic analysis of the structurally bound water in sedimentary gypsum, which was deposited under drought conditions. The triple oxygen and hydrogen isotope data provide a direct measure of past changes in lake hydrology. We modeled the data and conclude that annual precipitation decreased between 41 and 54% (with intervals of up to 70% rainfall reduction during peak drought conditions) and that relative humidity declined by 2 to 7% compared to present-day conditions.

Transient hydrodynamic effects influence organic carbon signatures in marine sediments.

Ocean dynamics served an important role during past dramatic climate changes via impacts on deep-ocean carbon storage. Such changes are recorded in sedimentary proxies of hydrographic change on continental margins, which lie at the ocean-atmosphere-earth interface. However, interpretations of these records are challenging, given complex interplays among processes delivering particulate material to and from ocean margins. Here we report radiocarbon (14C) signatures measured for organic carbon in differing grain-size sediment fractions and foraminifera in a sediment core retrieved from the southwest Iberian margin, spanning the last ~25,000 yr. Variable differences of 0-5000 yr in radiocarbon age are apparent between organic carbon in differing grain-sizes and foraminifera of the same sediment layer. The magnitude of 14C differences co-varies with key paleoceanographic indices (e.g., proximal bottom-current density gradients), which we interpret as evidence of Atlantic-Mediterranean seawater exchange influencing grain-size specific carbon accumulation and translocation. These findings underscore an important link between regional hydrodynamics and interpretations of down-core sedimentary proxies.

Author Correction: Intensified summer monsoon and the urbanization of Indus Civilization in northwest India.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Reducing Uncertainties in Carbonate Clumped Isotope Analysis Through Consistent Carbonate-Based Standardization.

About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve long-standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long-standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories.