Abstract: CHANGES in ocean chemistry and circulation have been invoked to explain the lower atmospheric CO2 concentrations of glacial periods observed in ice-core records1. The processes that modulate these concentrations are not well understood, but an increase in the nutrient inventory of the ocean is one mechanism that could lower atmospheric CO2 levels by enhancing oceanic biological productivity and CO2 storage1-3. The oceanic concentrations of one such nutrient, nitrate, may be regulated by changes in the rate at which it is degraded by bacteria (denitrification) in oxygen-deficient subsurface waters. Denitrification constitutes a significant global sink for oceanic nitrate4, and the eastern tropical North Pacific Ocean is particularly important in this respect as it accounts for at least a third of global oceanic fixed-nitrogen removal by water-column denitrification4,5. Here we present 15N/14N records from marine sediment cores, which show that water-column denitrification in the eastern tropical North Pacific Ocean was greatly diminished during glacial periods. We suggest that, because nitrate limits biological productivity in much of the modern ocean, a consequent increase in the oceanic nitrate inventory during glacial periods could have contributed to the observed decrease in atmospheric CO2 concentration.

Abstract: This paper reviews research at two sites in the southern Cape, South Afnca that together span the last 125 ka: Boomplaas and Klasies River main site. Environmental and cultural changes are discussed. The contrasts between the Late Pleistocene and Holocene and between the earlier and later Holocene environments are detailed. Climates have been similar to the present and more favourable for human settlement in the last 5 ka than at any time since the Lasl Interglacial. The Middle-Later Stone Age inteqace predates 21 ka in the Boomplaas sequence. The Howieson's Poort at the base of the Boomplaas sequence is a horizon marker that is also found in the top of the Klasies River main site. Human remains from Klasies River are dated to 90 ka and 120 ka. It is argued that these sites were occupied by morphologically and cognitively modem humans. *Received September 1995

Abstract: Accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes. Cosmogenic beryllium-10 measurements in the Wind River Range indicate that the last glacial maximum (marine oxygen isotope stage 2) was achieved there by 21,700 ± 700 beryllium-10 years and lasted 5900 years. Ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12,100 ± 500 beryllium-10 years.

Abstract: THE eastern equatorial Pacific Ocean (EEP) today sustains up to 30% of global marine productivity1, and the region is one of the largest and most variable marine sources of CO2 to the atmosphere2. This variability is largely controlled by the balance between the physical input of nutrients to the surface ocean and their removal by biological assimilation―the relative nutrient utilization―but the spatial and temporal variability of this balance are poorly understood. Here we use the 15N/14N ratio in sedimentary marine organic matter to show strong spatial gradients in relative nitrate utilization throughout the modern EEP. We interpret down-core decline in this ratio through the Last Glacial Maximum (12-24 kyr ago) as a decrease in relative nitrate utilization; the increase in nitrate supply to surface waters due to upwelling during this period was greater than the apparent increase in nitrogen removal by organic matter export out of surface waters. This interpretation is consistent with cooler sea surface temperatures3 and a higher CO2 flux to the atmosphere4,5 during the Last Glacial Maximum, indicating that the EEP surface waters have remained enriched in nutrients, and have not acted as a net sink for CO2, for at least the past 30,000 years.

Abstract: SUMMARY Sea-level change during Late Pleistocene and Holocene times is a combination of eustatic, isostatic and tectonic contributions. In the central Mediterranean Sea region of Greece and western Turkey, the isostatic components are important, due to the changing gravitational potential of the Earth, the readjustment of the crust upon the removal of the large ice sheets and the addition of the meltwater into the oceans, including the Mediterranean. Changes in relative sea-level due to these glacio-hydro-isostatic adjustments have reached amplitudes of 1 mm yr−1 during the last few thousand years. A model for the isostatic contribution to sea-level change, including the movement of shorelines due to the combined eustatic-isostatic changes, is developed, based on earth-model and ice-sheet parameters estimated from sea-level observations from other areas. Comparisons of this model with observations of sea-level change permit rates of vertical movement to be estimated for Late Holocene time. Allowance for the isostatic factors makes a significant difference to these rates. The plains of Argolis, Lakonia, Messinia and Navarine, in the southern Peloponnese, for example, appear to be tectonically stable on time-scales of a few thousand years and longer, consistent with the position of the Last Interglacial shoreline close to the present-day sea-level. The observations here, of a rising sea-level, are largely the consequence of the glacio-hydro isostasy and not of a long-term tectonic process. Crete as a whole is subject to tectonic uplift, but at variable rates, and forms part of an arcuate zone of uplift extending from Rhodes and Karpathos in the east to Kithera in the west at rates that locally exceed 4 mm yr−1. The southern shore of the Gulf of Corinthos is also subject to uplift at rates approaching 1.5 mm yr−1. These isostatically corrected estimates for the past few thousand years are in agreement with longer-term estimates based on the position of the Last Interglacial shoreline. Only on the Perachora Peninsula do the tectonic rates over these two time intervals appear to be in disagreement, with the Late Holocene rates being much higher than the long-term rates.

Abstract: We investigated glacial-interglacial changes in the circulation and carbon cycling in the western North Atlantic subtropical gyre using hydrographic data and downcore records of the stable isotopic compositions of individual shells of Bahamian benthic foraminifera. Potential temperature-salinity-depth relations show that modern thermocline (∼200–1000 m) and deep (∼1000–2000 m) waters in the Providence Channels, Bahamas, originate in the Sargasso Sea and are typical of the subtropical gyre. Gradients in the stable isotopic compositions of late Holocene Planulina and Cibicidoides species from the bank margins (∼400 to 1500 m depth) reflect temperature, nutrient, and isotopic gradients of modern subtropical gyre waters. The difference between the δ18O of glacial maximum and late Holocene foraminifera is ∼2.1‰ for the upper 900 m of the water column and ∼1.6‰ for deeper waters, indicating that these waters were ∼4°C and ∼2°C cooler, respectively, during glacial time. The glacial temperature gradient (dT/dz) was similar to today, while the base of the thermocline was ∼100 m shallower. These results differ significantly from our earlier results from multiple shell δ18O analyses, which implied upper thermocline waters were only ∼1°C cooler and dT/dz was greater during the glacial maximum. The difference occurs because bioturbation adversely affects multiple shell analyses of glacial-aged samples from shallow water depths. At all depths above 1500 m, foraminiferal δ13C are greater during the glacial maximum than the late Holocene by at least 0.1 to 0.2‰ (as much as 0.6‰ in the lower thermocline), indicating that nutrient concentrations throughout the thermocline were reduced and there was no oxygen minimum zone during the glacial maximum. This suggests greater, more uniform ventilation of the thermocline. Results of single and multiple shell δ13C analyses of glacial age foraminifera compare favorably because samples most affected by mixing correspond to water depths where the glacial-interglacial change of δ13C was small. Cooler upper ocean waters during the glacial maximum reflect cooler temperatures at the ocean surface where isopycnal surfaces outcrop, including large areas of the subtropical ocean. A shallower thermocline base is consistent with southward migration of the northern edge of the subtropical gyre or increased mode water production. Enhanced thermocline ventilation is consistent with more vigorous winds and all isopycnal surfaces outcropping in the area of Ekman downwelling.

Abstract: Pollen spectra from three eastern Mediterranean cores have been used to document the paleoclimates of the Levantine Basin borderlands over the last 250 kyr to establish the relationship between this regional climate data set and the global climate as recorded by foraminiferal δ18O and to compare it with proximal land pollen records. Core MD 84 642 with eight sapropels covers the last two climatic cycles up to the early Holocene, MD 84 627 with four sapropels goes back to 125 kyr, and MD 84 629 with one sapropel covers the last 70 kyr. The sedimentation rate decreases from core 629, located at the shallowest depth beneath the Nile River plume, to cores 627 and 642. During the interglacials defined by a low 18O/16O ratio, the abundance of tree pollen is maximum and points to an optimum Mediterranean climate with greatest humidity, including some summer rainfall. During glacial maxima, with highest 18O/16O ratio, the pollen abundance is high for steppe and semidesert plants and low for trees, indicating a definitely more arid, more continental, and probably colder climate. The variations of pollen abundance occur in phase with those of the foraminifer δ18O record. This signifies that the regional climate of the Levantine Basin borderlands had the same temporal pattern as the global ice volume documented by the ice volume curve.

Abstract: Taking advantage of the fact that the Vostok deuterium (δD) record now covers almost two entire climatic cycles, we have applied the orbital tuning approach to derive an age-depth relation for the Vostok ice core, which is consistent with the SPECMAP marine time scale. A second age-depth relation for Vostok was obtained by correlating the ice isotope content with estimates of sea surface temperature from Southern Ocean core MD 88-770. Both methods lead to a close correspondence between Vostok and MD 88-770 time series. However, the coherence between the correlated δD and insolation is much lower than between the orbitally tuned 8D and insolation. This reflects the lower accuracy of the correlation method with respect to direct orbital tuning. We compared the ice and marine records, set in a common temporal framework, in the time and frequency domains. Our results indicate that changes in the Antarctic air temperature quite clearly lead variations in global ice volume in the obliquity and precession frequency bands. Moreover, the average phase we estimated between the filtered δD and insolation signals at precessional frequencies indicates that variations in the southern high latitude surface temperature could be induced by changes in insolation taking place during a large period of the summer in northern low latitudes or winter in southern low latitudes. The relatively large lag found between Vostok δD variations and obliquity-driven changes in insolation suggests that variations in the local radiative balance are not the only mechanism responsible for the variability in surface temperature at those frequencies. Finally, in contrast to the cross-spectral analysis method used in previous studies, the method we use here to estimate the phases can reveal errors in cross-correlations with orbitally tuned chronologies.

Abstract: Data from the Greenland ice sheet and continental records from Europe have indicated climatic fluctuations during the last interglacial (Eemian: Oxygen Isotope Substage 5e). Similar fluctuations have not, however, been documented previously from marine environments. Here, we show the existence of two cold events during substage 5e in two marine, benthic foraminiferal, shelf records from northwest Europe and suggest that these cooling events are a result of fluctuations in the strength of the North Atlantic surface-water circulation.

Abstract: Assessment of changes in surface ocean conditions, in particular, sea-surface temperature (SST), is essential to understand long-term changes in climate especially in regions where continental climate is strongly influenced by oceanographic processes To evaluate changes in SST in the northeast Pacific, we have analyzed long-chain alkenones of prymnesiophyte origin at 38 depths in a piston and associated trigger core collected beneath the contemporary core of the California Current System at 42°N, ∼270 km off the coast of Oregon/California The samples span 30,000 years of deposition at this location Unsaturation patterns (U37K′) in the alkenone series display a statistically significant difference (p ≪ 0001) between interglacial (044 ± 002, n = 11) and glacial (029 ± 004, n = 20) intervals of the cores Detailed examination of other compositional features of the C37, C38, C39 alkenone series and a related C36 alkenoate series measured downcore suggests the published U37K′- temperature calibration (U37K′=0034×T+0039), defined for cultures of a strain of Emiliania huxleyi isolated from the subarctic Pacific, provides best estimates of winter SST at our study site This inference is purely statistical and does not imply, however, that the phytoplankton source of these biomarkers is most productive in winter or at the ocean surface The temperature record for U37K′ implies (1) an ∼4°C shift occurred in winter SST from ∼ 75 ± 11°C at the last glacial maximum to ∼ 117 ± 07°C in the present interglacial period, and (2) this warming trend was confined to the time frame 14–10 Ka within the glacial to interglacial transition period These conclusions are corroborated entirely by results from an independent SST transformation of radiolarian species assemblage data obtained from the same core materials

Abstract: Stable carbon and nitrogen isotopic ratios (δ13C and δ15N) of organic matter were measured in three sediment cores from deep basins of the Bering Sea to investigate past changes in surface nutrient conditions. For surface water reconstructions, hemipelagic layers in the cores were distinguished from turbidite layers (on the basis of their sedimentary structures and 14C ages) and analyzed for isotopic studies. Although δ13C profiles may have been affected by diagenesis, both δ15N and δ13C values showed common positive anomalies during the last deglaciation. We explain these anomalies as reflecting suppressed vertical mixing and low nutrient concentrations in surface waters caused by injection of meltwater from alpine glaciers around the Bering Sea. Appendix tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington , DC 20009. Document P95-003; $2.50. Payment must accompany order.

Abstract: The common mussel Mytilus edulis is an indicator of milder marine conditions in the Arctic, with stronger Atlantic Water influx, during the Holocene and earlier interglacials. Twelve Holocene radiocarbon dates of mytilus from eastern Svalbard fall between ca 8800 and 5000 BP and roughly delimit the marine climatic optimum period there. The beginning of this period in the east coincides with the immigration of boreal extralimital molluscs to western Svalbard, indicating the culmination of Holocene Atlantic influence.

Abstract: The Weinan loess-soil sequence in central China provides lithogenic, geochemical and palaeopedological evidence of past changes in the eastern Asian monsoon climate. The chemical weathering index as defined by Nesbitt and Young (1982) and the SiOJTiOz ratio are found to be sensitive indicators of the strengths of the summer and winter monsoons, respectively. Over the last climatic cycle, strong summer monsoon occurred during six time intervals with an apparent - 20,000 years cycle, the major component of the Earth 's orbital precession. Strengthening of the summer monsoon and weakening of the winter monsoon is approximately in phase. Micromorphological study of the palaeosols reveals a landscape variability from north temperate steppe (Haplic Chernozems) to subtropical forest (Chromic Luvisol) environments for the interstadial and interglacial periods. Changes in both orbitally produced north summer insolation and glacial age boundary condition are necessary to explain the major shifts of the monsoon climate. Within the constraints of the used time scale, the variations of the monsoon climate are basically coeval with the variations of the global ice volume as indicated by the marine 6180 record. Moreover, the amplitudes of monsoon variability and the landscape evolution display _ however striking discrepancies with the global ice volume, which may be better explained by the variations of the summer insolation in the northern I hemisphere with an apparent time lag of several thousands years for the loess sequence.

Abstract: Continuous electrical records covering a climatic cycle are presented for the Greenland Ice Core Project deep ice core from Greenland. Electrical conductivity measurement (ECM) measures the acid content of the ice, and the dielectric profile (DEP) responds to acid, ammonium, and chloride. All features seen can be explained by chemical changes in the ice, and there is no evidence so far for any major change in electrical response with depth or age of the ice. Both records are dominated by the acidity of the ice which varies strongly from acidic in warm periods to alkaline in cold periods, controlled by neutralization by alkaline dust (calcareous and other mineral dust). When Ca is low, the acidity (mainly nitric acid) has a fairly constant background level throughout the cycle, with slightly lower values in ice believed to be from the last interglacial. Ca has to rise only slightly to neutralize the available acidity, so that acidity is a highly nonlinear reflection of climate changes. If neutralization occurred in the aerosol (rather than in the ice), then the number of cloud condensation nuclei over parts of the northern hemisphere could have been reduced, leading to reduced cloud albedo. This nonlinear feedback may have some importance for modeling of climate change. When both acid and ammonium levels are low, the DEP signal can be used to give a rapid indication of chloride trends.