High-resolution Oxygen Research Articles

A high resolution Late-Glacial isotopic record from Hawes Water, Northwest England: Climatic oscillations: calibration and comparison of palaeotemperature proxies

Understanding of the impact of past climate change on the terrestrial environment requires the development of quantitative palaeotemperature indicators that can be readily measured at closely spaced stratigraphic intervals. A high resolution oxygen isotopic record from Hawes Water, Lancashire, UK, reveals a degree of climatic instability that is greater than that recognised in previous isotopic studies of sites from the Atlantic margin of Northwest Europe. Four oscillations in the Late-Glacial interstadial before the onset of the Younger Dryas stadial are marked by sharp negative excursions in the δ 18O record of the carbonate sediments and accompanied by marked changes in chironomid populations and the pollen record. The oscillations are interpreted as cold events: they correlate with events recognised in marine and lacustrine temperature proxy records as well as the isotopic record from the Greenland ice-cores. Results suggest that there has been a significant change in lake water composition since the Late-Glacial but it is argued that the Hawes Water isotopic record, constrained by the results of a study of carbonate precipitation in the modern lake, provides a quantitative record of Late-Glacial temperature change. The 7°C maximum temperature difference inferred from the isotopic data, from the Interstadial optimum to the Younger Dryas stadial, and the magnitude of individual oscillations, compare well with estimates from terrestrial (coleopterid) and marine (diatom sea surface temperature) records: all are significantly greater than the, independently calibrated, 4.2°C change documented from a chironomid record from South Scotland. Such differences need to be resolved if the impacts of climatic events are to be fully understood.

The oxygen isotopic composition of seawater during the Last Glacial Maximum

High-resolution oxygen and hydrogen isotope measurements were made on pore fluids from deep-sea sediments from sites in the North and South Atlantic. The data provide direct measurements of changes in the isotopic composition of bottom waters during the Last Glacial Maximum (LGM). Results from Ocean Drilling Program (ODP) Site 981 in the North Atlantic, currently bathed in North Atlantic Deep Water (NADW) reproduces previous results from the Ceara and Bermuda Rises, constraining the glacial–interglacial change in δ 18O of the deep Atlantic to be 0.7–0.8‰. Results from Site 984, which is located north of Site 981 and at a shallower water depth, yield a similar value (0.8‰), providing insight into the properties of Glacial North Atlantic Intermediate Water (GNAIW). Sites from ODP Leg 177 in the South Atlantic span the modern boundary between northern and southern sources of deep water. Data from the northern site (1088) yield a similar result to sites in the tropical and North Atlantic (0.7‰). At the southern site (1093), located south of the polar front, the change is substantially larger (1.1‰), representing the change in δ 18O of southern source waters since the LGM. These results confirm previous estimates that the global average change in δ 18O of seawater is 1.0±0.1‰. Hydrogen isotopes measured on pore fluids from three sites are consistent with the oxygen isotopes from these locations, giving further support to these results. At all sites studied, the temperature of the deep ocean during the LGM, calculated by combining the pore fluid results with oxygen isotope data from benthic foraminifera, was within 1°C of the freezing point of seawater.

Centennial-scale Holocene climate variability revealed by a high-resolution speleothem delta 18O record from SW Ireland.

Evaluating the significance of Holocene submillennial delta18O variability in the Greenland ice cores is crucial for understanding how natural climate oscillations may modulate future anthropogenic warming. A high-resolution oxygen isotope record from a speleothem in southwestern Ireland provides evidence for centennial-scale delta18O variations that correlate with subtle delta18O changes in the Greenland ice cores, indicating regionally coherent variability in the early Holocene. Evidence for previously undetected early Holocene cooling events is presented, but mid- to late-Holocene ice rafting in the North Atlantic appears to have had little impact on delta18O at this ocean margin site.

Influence of cell-free Hb on local tissue perfusion and oxygenation in acute anemia after isovolemic hemodilution.

Oxygen-carrying solutions are intended to eliminate the blood transfusion trigger. Their ability to maintain microvascular perfusion and to deliver oxygen to tissue when they replace the RBCs as oxygen carriers has not been directly measured. Microvascular response to exchange transfusion with a polymerized bovine cell-free Hb (PBH) solution after acute isovolemic hemodilution with a plasma expander was investigated by using the hamster window model. In vivo functional capillary density (FCD), blood flow, and high-resolution oxygen distribution in microvascular networks were measured by noninvasive methods. Exchange transfusion of PBH solution after a 60-percent isovolemic hemodilution with dextran 70 (MW, 70 kDa) resulted in a Hct of 11 percent and a Hb content of 6.7 g per dL. FCD was 0.37 of baseline. Interstitial pO2 was reduced from 21.0 mm Hg to 0.3 mmHg. Arteriolar and venular blood flows were ratios of 0.75 and 0.76 relative to baseline. In a previous study, tissue pO2 after hemodilution to 5.6 g of Hb per dL with dextran 70 was 23.0 mmHg. Hypervolemic injection of PBH solution increased blood pressure and caused vasoconstriction. Using PBH solution to replace RBC oxygen-carrying capacity during low Hb content conditions (<50%) causes abnormally low tissue oxygenation and FCD, while the same level of hemodilution with dextran maintains normal microvascular conditions.

Transition of Quaternary glacial cyclicity in deep-sea records at Nansha, the South China Sea

High-resolution oxygen isotope records over the last 2249 ka (MIS 1-86) have been obtained from cores of the upper section (105.08 m) at ODP Site 1143 (water depth of 2772 m) drilled in the Nansha area, southern South China Sea. The sampling resolution is at about 2 ka intervals, resulting in one of the best oxygen isotope records over the global ocean. The oxygen isotope curves, displaying details in the Pleistocene glacial cycles, have revealed a nearly 300 ka long stage of transition from a predominant 40 ka to 100 ka periodicity. Therefore, the "Mid-Pleistocene Revolution" should be considered as a process of transition rather than an abrupt change. Within the 100 ka glacial cycles, the changes in tropical sea surface water were found to lead those in high-latitude ice sheet. Our comparisons show that the ice sheet expansion and the glacial stage extension in the Northern Hemisphere with the 100 ka cycles must have been driven not by ice sheet itself, but by processes outside the high latitudes of the Northern Hemisphere.

Competitive and Cooperative Responses to Climatic Instability in Coastal Southern California

AbstractArchaeological data indicates that socially and politically complex hunter-gatherer societies had become well established on the southern California coast by A.D. 1300. Major developmental changes in sociopolitical complexity are generally considered to have taken place rapidly between AD 1150 and 1300. Recently, two hypotheses have been proposed to account for this rapid cultural evolution, both invoking stressful climatic conditions as an important trigger for cultural change. One suggests that the sociopolitical development was stimulated, in part, by multiple marine and terrestrial subsistence stresses, particularly low marine productivity resulting from regional warming. The other suggests that these developments were largely driven by decreases in terrestrial productivity and water availability linked to drought. Resolution of this debate has been hampered by insufficient paleoclimatic and archaeological data. We present a well-dated, relatively high resolution (25-year intervals) oxygen isotopic marine climate record and new archaeological data from the Northern Channel Islands for the last 3,000 years. These data strongly suggest that changes in human behavior associated with increasing cultural complexity: 1) accelerated after A.D. 500 and became dominant by A.D. 1300, 2) occurred during one of the coldest and most unstable marine climatic intervals of the Holocene (A.D. 450-1300), and 3) coincided with cool, dry terrestrial conditions. Incipient cultural complexity emerged during an interval marked by inferred high marine productivity, reduced terrestrial food and water availability, and large, unpredictable variations in terrestrial resource availability. Our records suggest a strong relationship during this time between climatically induced changes in environmental conditions and social, political, and economic responses, including the emergence of more intensified fishing, and increased sedentism, violence, and trade.

Advances in micromilling techniques: a new apparatus for acquiring high-resolution oxygen and carbon stable isotope values and major/minor elemental ratios from accretionary carbonate

A computer-controlled micromilling apparatus that permits discrete sampling of accretionary biogenic carbonate specimens with micron-scale resolution has been developed for the purpose of acquiring high-resolution δ 13C and δ 18O values, and major/minor elemental chemistry. Secular variation in stable isotope ratios and major/minor elemental composition records inter-annual and intra-annual changes in the environmental parameters or animal behavior for extant and extinct species. A polished specimen is attached to a stage beneath a fixed micro-milling head, and viewed on a large-screen monitor via a color digital camera. Growth bands (analogous to tree rings) are generally a result of variable accretion rates in biogenic carbonates. These growth features are first digitized in real-time as a series of three-dimensional coordinates. To better characterize complex growth features, intermediate coordinates are interpolated using a cubic spline fit through the digital points. Intermediate sampling paths, which mimic less visible daily growth banding, are in turn calculated between digitized curves. Sampling path arrays serve to guide three high precision actuators, which position the sample stage relative to the fixed micromilling head. A fourth actuator provides vertical control of the digital color camera (compensating for vertical movement of the z-axis stage actuator) keeping the specimen image focused. This new micromilling device permits high-resolution sampling of complex internal structures via a user-friendly program interface.

High Resolution Oxygen Isotope Straigraphy for the Last 150,000 Years in the Southern South China Sea:Core MD972151

This study presents the directional paleomagnetic secular variationpattern of core MD972151 taken from the southwestern South China Seaduring the IMAGES Ⅲ-IPHIS Cruise in 1997.A total of 281 samplestaken from this core were subjected to AF demagnetization to analyze theircharacteristic remanent magnetization.The results indicated a reversed event at depth of between 21.7 and23.8 meters.It was identified as the Blake Event,and has an age interval ofabout 130ka.Another short reversed event was found near the bottom ofthe core.This was tentatively described as an excursion that occurred atabout 150-160ka.These assignments were in very close agreement with theresults of oxygen isotope analysis.Consequently,the core under study wasdetermined to be presenting a record of perhaps the last 160ka probably.And the sedimentation rate can thus be estimated to be of about 17-18cm/ky.In addition to the reversed events,two excursions were also observed,one at the core top to 3.7 meters,and the other at 15.3-15.5m in depth.Thelatter excursion was estimated to be of about 75ka in age,based on comparingthe results of this study to the intensity record made by Valet andMeynadier(1993).However,the former one might well be a recent excursion, under 20ka in age,because comparison with oxygen isotope dataruled out the possibility of being the'Laschamp Event'.Another possiblecause for this abnormal event might be that it was due to deposition processof the sediments.Of course,this question will require further study.Finally,the paleo-longitude variation pattern shows that the VGPsseemed to drift eastward at a record rate of over 4 meters in depth.Thisdeparts from observations of the present-day geomagnetic field that indicate the magnetic pole is drifting westward.The significance of this discrepancy warrants further study too.

High-resolution Carbonate Stratigraphy of IMAGES Core MD972151 from South China Sea

A detailed high-resolution oxygen isotope stratigraphy over the lasttwo glacial-interglacial cycles in the southern South China Sea is presented,based upon analysis of Globigerinoides sacculifer(planktic foraminifera)inCore MD972151.Major characteristic isotopic events were identified andcorrelated to the standard SPECMAP chronology.In addition,radiocarbondating and a few biostratigraphic,geomagnetic and tephrochronologicmarkers were employed to enhance the reliability and resolution of the agemodel.The tephra layer of the Toba Eruption at 71 ka helps to anchor theboundary of Stages 5/4,and provides a yardstick correlating to the coldperiod between interstadials 19 and 20 identified in the GISP2 Greenlandice core.The last appearance of pink”Globigerinoides”ruber and the Blakemagnetic polarity event in the lower part of the core helped us to locate thetransition of Stage 6/5e.Our high-resolution oxygen isotope stratigraphy,however,indicates that the age of the Blake magnetic reversal occurredduring the Stage 6/5e transition instead after the warmth peak of the isotopicevent 5e.The resulting age model shows that the 26.54m sequence from CoreMD972151 provides a continuous record from 153 to 0.94ka,with sedimentationrates varying between 5.6 and 78cm/kyr.The temporal resolutionof the isotopic record is about 60-150 years per sample for the upperpart of the core and 250 years for the lower part,respectively.

Hole states in Eu0.9 − xPrxCa0.1BaSrCu3O7 − δ studied by X-ray absorption spectroscopy

The hole states located on different oxygen sites have been investigated in Eu0.9 – xPrxCa0.1BaSrCu3O7 – δ with increasing Pr3+ substitution by high-resolution oxygen K-edge and copper L-edge X-ray absorption spectra. The results reveal that the hole states are depleted systematically by the substitution and in turn lead to a reduction in Tc. However, the rate at which the depletion occurs is lower in Eu0.9 – xPrxCa0.1BaSrCu3O7 – δ compared to that in Y1 – xPrxBa2Cu3O7 – δ and Dy1 – xPrxBa2Cu3O7 – δ. This is due to the partial substitution of Ba2+ by Sr2+ ions and R (rare earth element) by Ca2+ ions in the present case. The depletion of hole states gives evidence in support of the hole-depletion models based on Pr 4f–O 2pπ hybridization. The possible reasons for the anomalous behavior of Pr3+ ions in RBa2Cu3O7 – δ are also discussed.

Hole states in (Cd0.5Pb0.5)Sr2(CaxY1 − x)Cu2O7 studied by X-ray absorption spectroscopy

The hole distribution of underdoped and optimum doped states in (Cd0.5Pb0.5)Sr2(CaxY1 – x)Cu2O7 compounds has been investigated by high-resolution oxygen K-edge and copper L-edge X-ray absorption near-edge-structure spectra. Near the oxygen 1s edge, a pre-edge peak with a maximum at ≈528.3 eV was found, which is ascribed to the excitations of oxygen 1s electrons to 2p hole states located in the CuO2 planes. The intensity of this pre-edge peak increased with calcium doping for 0 ⩽ x ⩽ 0.5. This demonstrates that the chemical substitution of Ca2+ for Y3+ in (Cd0.5Pb0.5)Sr2(CaxY1 – x)Cu2O7 gives rise to an increase in hole concentration within the CuO2 planes and a change from a semiconductor to a superconductor with Tc up to 30 K. The results of the copper L-edge absorption are consistent with those observed for oxygen 1s X-ray absorption.

The Mid-Pleistocene climate transition: onset of 100 ka cycle lags ice volume build-up by 280 ka

The Mid-Pleistocene Climate Transition (MPT) is the complex climatic change which brought the Late Pleistocene ice ages. We explore the MPT in the time and frequency domains by new methods of time series analysis. High-resolution oxygen isotope records reveal that the ice volume-related increase in δ 18O mean (amplitude: 0.29 ± 0.05 (1 − σ μ) ‰, transition midpoint: 922 ± 12 ka, duration: 40 ± 9 ka) significantly preceded the abrupt increase in the amplitude of the ∼ 100 ka cycle at 641 ± 9 ka. This finding can be quantitatively simulated using a simple ice-bedrock model in which, due to the additional ice, the calving threshold is exceeded. The simulated calving events prior to ∼ 650 ka are separated by ∼ 77 ka, whereas after ∼ 650 ka they occur pseudo-periodically with a mean period of nearly 100 ka. The cause for the delay of the ∼ 100 ka calving cycle was not a slow bedrock relaxation; rather, the coincidental combination of insolation, existing ice mass, and bedrock depression.

Optical measurement of oxygen and temperature in microscale: strategies and biological applications

Sediments, microbial mats, biofilms and other microbial communities are characterized by steep gradients of physical and chemical parameters. Microsensors are powerful tools to measure these parameters with a sufficient spatial resolution and with a small disturbance of the micro-environment in natural systems. Recently, fiber-optical microsensors have been introduced in the field of aquatic biology as an alternative to existing electrochemical microsensors. Such micro-optodes have already been developed for high-resolution measurement of dissolved oxygen and for temperature measurements. They are easy to fabricate and show an improved long-term and storage stability. An overview is given on the development and characterization of different types of micro-optodes for oxygen and temperature. A luminescence lifetime-based device has been developed which is portable and enables microsensing both in the laboratory and under field conditions. Limitations in practical work with optical microsensors are demonstrated, and strategies to overcome them briefly discussed. A micro-optode array as well as a method for high-resolution oxygen imaging in sediments are presented as two different ways to investigate the two-dimensional oxygen distribution in heterogeneous living systems. Future applications and developments in micro-optode research will be discussed briefly.

High resolution stable isotope analysis of a Tasmanian speleothem

High resolution oxygen and carbon isotope analyses of part of a uniform diameter stalagmite from a cave in northern Tasmania have been used to study the climatic transition from stadial to interstadial conditions during the early Last Glacial (109-95 ka). A significant change in oxygen isotope values suggests that temperature warmed rapidly by approximately 2°-3°C between 100 and 97 ka. A decrease in carbon isotope values berween I 0 I and 98 ka indicates that moisture availability was increasing slightly in advance of rising temperature.

Early Eocene palaeoceanography and palaeoclimatology of the eastern North Atlantic: stable isotope results for DSDP Hole 550

Abstract High-resolution oxygen and carbon isotope records for benthonic and planktonic foraminifera have been established through the early Eocene at DSDP Site 550, Goban Spur in the North Atlantic. Benthonic stable isotopic records from Site 550 (estimated palaeodepth c. 4000 m) represent the first documentation of actual deep water properties during the early Eocene. At Site 550 in the earliest Eocene (end of NP10 Zone), a 0.5‰ vertical δ 18 O (i.e. thermal) gradient between subsurface and bottom water was eliminated, probably reflecting upward displacement of subsurface water by intermediate and deep water. Thereafter vertical isothermal conditions prevailed for the rest of the early Eocene. This is consistent with earlier findings suggesting unusually low vertical and latitudinal temperature gradients in the early Eocene ocean. A comparison of the isotopic records from Site 550 and nearby DSDP Site 401 (palaeodepth 1800 m), reflecting intermediate water properties, confirms the isothermal nature of the water column in the eastern North Atlantic. This is in contrast to earlier findings for the Antarctic region (ODP Sites 689 and 690) in the early Eocene, where supposedly warm bottom waters originating in low latitudes were overlain by cooler intermediate waters originating in high latitudes. Both the oxygen and the carbon isotopic records for Sites 550 and 401 indicate that the eastern North Atlantic region was influenced by a single source of intermediate-to-deep water through most of the early Eocene. The benthonic δ 18 O and δ 13 C values for Site 550 are generally low compared with coeval sections worldwide, indicating that during most of the early Eocene the North Atlantic was dominated by warm and nutrient-enriched deep water formed in a low latitude region. The earliest stages of the long-term cooling trend, which characterizes the second part of the Paleogene, can be traced by a 2°C decrease in bottom water temperatures across the NP13–NP14 transition at both Sites 550 and 401.

High-resolution oxygen absorption spectrum obtained with an external-cavity continuously tunable diode laser

We have measured the electronic absorption spectrum of the oxygen (O(2)) A band b(1)Sigma(+)(g)(nu' =0)? X(3)Sigma(-)(g)(nu'' = 0) near 760 nm in room air by using a newly available external-cavity diode laser. The external-cavity diode laser permitted a continuous, single-mode scan of the P branch of ambient oxygen by 2f modulation spectroscopy. The external-cavity diode laser opens up many applications limited in the past by the noncontinuous tunability of standard monolithic Fabry-Perot cavity laser designs. Specifically, absorption transitions that coincide with he gain bandwidth of any available diode laser (635 to 1550 mn) may now be reached without the problems and imitations imposed by mode hops inherent in standard Fabry-Perot cavity designs.

The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. [1] from ODP site 677 in the equatorial Pacific differs significantly from previous ones [e.g., 2–5], yielding estimated ages for the last Earth magnetic reversals that are 5–7% older than the K Ar values [6–8] but are in good agreement with recent Ar Ar dating [9–11]. These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in δ 18O are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal [12]. Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the specmap composite curves [4,13], and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. [1]. The precession peaks found in the δ 18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our δ 18O record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 ± 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. [1] and recent radiochronological Ar Ar datings on lavas [9–11]. We developed a new low-latitude, Upper Pleistocene δ 18O reference record by stacking and tuning the δ 18O records from core MD900963 and site 677 to orbital forcing functions.

The history of coastal upwelling off Peru (11°S, ODP Leg 112, Site 680B) over the past 650 000 years

Abstract The coarse sediment fraction (&gt;63 μm) and benthic foraminiferal composition of Quaternary sediment cores from ODP LEG 112 Hole 680B (252.5 m water depth) were sampled from the upwelling area off Peru. High-resolution oxygen isotope stratigraphy based on analyses of the benthic foraminifera Bolivina seminuda allows a detailed reconstruction of the upwelling history since oxygen isotope Stage 16. Variations in the relative abundances of the benthic foraminiferal species Nonionella auris, Bolivina costata , and Bolivina seminuda document changes in fertility, which were found to be enhanced during interglacial Stages 1, 3, 5, 7, 9 and 11. Generally, bioturbated sediments are dominated by the genera Cassidulina, Epistominella, Gyroidina, Trifarina and Valvulineria . These benthic foraminifera are members of the ‘Undercurrent Fauna’, which is commonly associated with sediments that have a high terrigenous detrital component and a low organic carbon content (&lt;3 wt%). These sedimentary deposits are indicative of high near-bottom current velocities of the Peruvian Undercurrent and well oxygenated bottom waters (0.5 to &gt;6 ml/l O 2 ). The entire core interval investigated in Hole 680B appears to have been affected by the Peruvian Undercurrent. Generally, undercurrent-dominated sections were found to correspond with glacial oxygen isotope stages.

Mid‐Miocene isotope stratigraphy in the deep sea: High‐resolution correlations, paleoclimatic cycles, and sediment preservation

Mid‐Miocene pelagic sedimentary sections can be correlated using intermediate and high resolution oxygen and carbon isotopic records of benthic foraminifera. Precision of a few tens of thousands of years is readily achievable at sites with high sedimentation rates, for example, Deep Sea Drilling Project sites 289 and 574. The mid‐Miocene carbon isotope records are characterized by an interval of high δ13C values between 17 and 13.5 Ma (the Monterey Excursion of Vincent and Berger 1985) upon which are superimposed a series of periodic or quasi‐periodic fluctuations in δ13C values. These fluctuations have a period of approximately 440 kyr, suggestive of the 413 kyr cycle predicted by Milankovitch theory. Vincent and Berger proposed that the Monterey Excursion was the result of increased organic carbon burial in continental margins sediments. The increased δ13C values (called 13C maxima) superimposed on the generally high mid‐Miocene signal coincide with increases in δ18O values suggesting that periods of cooling and/or ice buildup were associated with exceptionally rapid burial of organic carbon and lowered atmospheric CO2 levels. It is likely that during the Monterey Excursion the ocean/atmosphere system became progressively more sensitive to small changes in insolation, ultimately leading to major cooling of deep water and expansion of continental ice. We have assigned an absolute chronology, based on biostratigraphic and magneto‐biostratigraphic datum levels, to the isotope stratigraphy and have used that chronology to correlate unconformities, seismic reflectors, carbonate minima, and dissolution intervals. Intervals of sediment containing 13C maxima are usually better preserved than the overlying and underlying sediments, indicating that the δ13C values of TCO2 in deep water and the corrosiveness of seawater are inversely correlated. This again suggests that the 13C maxima were associated with rapid burial of organic carbon and reduced levels of atmospheric CO2. The absolute chronology we have assigned to the isotopic record indicates that the major mid‐Miocene deepwater cooling/ice volume expansion took 2 m.y. and was not abrupt as had been reported previously. The cooling appears abrupt at many sites because the interval is characterized by a number of dissolution intervals. The cooling was not monotonic, and the 2 m.y. interval included an episode of especially rapid cooling as well as a brief return to warmer conditions before the final phase of the cooling period. The increase in δ18O values of benthic foraminifera between 14.9 and 12.9 Ma was greatest at deeper water sites and at sites closest to Antarctica. The data suggest that the δ18O value of seawater increased by no more than about 1.1 per mil during this interval and that the remainder of the change in benthic δ18O values resulted from cooling in Antarctic regions of deepwater formation. Equatorial planktonic foraminifera from sites 237 and 289 exhibit a series of 0.4 per mil steplike increases in δ13C values. Only one of these increases in planktonic δ13C is correlated with any of the features in the mid‐Miocene benthic carbon isotope record.

Global nature of the Younger Dryas cooling event inferred from oxygen isotope data from Sulu Sea cores

THE Younger Dryas, an approximately 1,000-year-long return to near-glacial conditions, interrupted the glacial/Holocene climate transition during which most of the Northern Hemisphere ice sheets melted. Evidence for the Younger Dryas event has been found mainly in sediments from the North Atlantic Ocean and northwest Europe, and this has led to the idea that the event was caused by an injection of meltwater into the North Atlantic Ocean1–3. This model, however, has been recently questioned in the light of coral-reef data on the rate of sea level changes during this transition4. Here we present high-resolution oxygen isotope records from benthic and planktonic foraminifera from two radiocarbon-dated cores in the Sulu Sea, showing that the Younger Dry as occurred synchronously in the surface and deep waters of the Sulu Sea and the northern Atlantic Ocean. By combining our results with other palaeoclimate data, we suggest that the Younger Dry as event was a global phenomenon, and we believe it to have been caused by low atmospheric CO2concentrations.