Stable isotope ratios measured on molluscan from the CRP sediment cores in the Ross Sea, Antarctica

Stable isotope analyses of marine bivalve growth increment samples have been used to estimate early Oligocene (29.4 - 31.2) Ma and early Miocene (24.0 Ma) seafloor palaeotemperature from the southwestern continental margin of the Ross Sea. Measured d18O values average +2.5 ? in the early Miocene and range between +1.26 to +3.24 ? in the early Oligocene. The results show that palaeoceanographic conditions in McMurdo Sound during the mid-Cenozoic were significantly different from those of today. The minimum estimated spring through late summer seasonal temperature range was 3°C during the early Miocene and between 1 and 5°C during the early Oligocene. This compares to the equivalent modern day range of <0.5°C within the sound. Absolute seawater temperatures at <100 m depth were of the order of 5 to 7°C during both time slices, compared to modern day values of -1.4 to - 1.9°C in the same area. The results are in broad agreement with early Oligocene Mg/Ca temperature estimates from deep Atlantic foraminifera as well as estimates from local terrestrial palynology and palaeobotany.

Stable isotope analysis of benthic foraminifera and sedimentology on ODP Site 162-984

Sediments recovered at ODP Site 984 on the Reykjanes Ridge provided multicentennial-scale records (SST, planktic and benthic delta18O, IRD and magnetic susceptibility) of Late Pliocene climate change over the onset of Northern Hemisphere glaciation (NHG), 2.95-2.82 Ma. Short-term climate variations prior and after the onset of continent-wide glaciation were compared to test the hypothesis, whether Dansgaard-Oeschger (DO) cycles may have been triggered by continental ice breakouts. By means of spectral analyses for two selected interglacial stages prior to and after NHG (G15 and G1), we found that climate variability resembled that of the Holocene and the mid-Pliocene warm period. In contrast, DO-like periodicities near 1470, 2900, and 4400 yr indeed only occurred in glacial stages after the onset of NHG (G14, G6, and 104), but hardly in stage G20 prior to the onset. These results suggest a causal link between DO cycles and the Late Pliocene onset of major NHG and ice breakouts in the North Atlantic. This data set provides all primary data and spectral analysis related to this scientific work.

Late Neogene carbonate sedimentation and stable isotope record of ODP Site 121-758

The evolution of oceanic and climatic conditions the northeast Indian Ocean during the last 7 m.y. is revealed in the sediments from Site 758. We present detailed and continuous records of d18O and d13C from planktonic foraminifers, weight percent calcium carbonate, weight percent coarse fraction, magnetic susceptibility, and geomagnetic reversals. Sample spacing of the records ranges from 3 to 10 cm and is equivalent to an average time interval of 2000 to 6000 yr. Despite the fact that core recovery ranged between 100% and 105%, recovery gaps as large as 2.7 m occurred at nearly every break between advanced hydraulic piston cores. Approximately 12% of the late Neogene sequence was not recovered in each of the two holes drilled at Site 758. To circumvent the discontinuity introduced by the gaps, a composite depth section was constructed from multiple cores taken from offset holes at Site 758. The resulting composite depth section extends continuously from 0 to 116 mbsf, from the Holocene to the upper Miocene. A detailed chronostratigraphy is based on geomagnetic reversals which extend from the Brunhes Chron to Chron 6, and on d18O stages 1 through 105, which span from 0 to 2.5 Ma. The d18O record is dominated by a ~40-k.y. cycle in the late Pliocene and early Pleistocene, and is followed by a change to a ~100-k.y. cycle in the late Pleistocene. The mid-Pleistocene transition between these two modes of variability occurs between d18O stages 25 and 22 (between 860 and 800 Ka). Thirteen major volcanic ash horizons from the Indonesian arc are observed throughout the sedimentary section and are dated by their relative position within the geomagnetic reversals and the d18O chronostratigraphy. Since 5 Ma, there has been a long-term decline in weight percent CaCO3 and CaCO3 mass accumulation rates, and an associated rise in non-CaCO3 mass accumulation rates. We attribute these changes to a decrease in CaCO3 productivity and an increase in terrigenous sedimentation through enhanced riverine input. Such input may be linked to rapid tectonic uplift of the Himalayas and the Tibetan Plateau via mechanisms such as the intensification of the monsoonal rains, increased fluvial erosion, and regional glaciation. The long-term increase in percent coarse fraction since 5 Ma suggests a gradual increase in CaCO3 preservation. Higher frequency fluctuations in CaCO3 preservation are superimposed on the long-term trend and are related to climate fluctuations. The abrupt drop (-50%) in CaCO3 accumulation at 3.4 Ma signals a dramatic decrease in CaCO3 production that occurred over much of the Indian Ocean.

Age determinations and sedimentology on cores GIK17940 and 184-1144

Sedimentological, geochemical and paleomagnetic records were employed to reconstruct the history of East Asian Monsoon variability in the South China Sea (SCS) on orbital- and millennial-to-sub-decadal time scales. A detailed magnetostratigraphy for the southern central SCS was established as well as a stable isotope stratigraphy for ODP Site 1144 for the last 1.2 million years in the northern South China Sea. Furthermore a volcanic tephra layer from the southern central SCS could be identified as the Youngest Toba Ash, which thus re-presents an important age marker and was used to reconstruct paleo wind directions during the eruption 74 ka. Special attention was paid to the high- and ultrahigh-frequency variability in the last glacial-interglacial cycle and the Holocene, and to a precise age control of climate changes in general.

Sedimentology and stable oxygen isotope record of the northern South China Sea

High-resolution sediment records from the South China Sea reveal a winter monsoon dominated glacial regime and a summer monsoon dominated Holocene regime during the last glacial cycle. A fundamental change between regimes occurred during deglaciation through a series of millennial reoccurrences of century-scale changes in the East Asian monsoon (EAM) climate. These abrupt events centered at 17.0, 15.9, 15.5, 14.7, 13.5, 13.9, 13.3, 12.1, 11.5, and 10.7 14C ka correlate well with the millennial-scale events in the Santa Barbara Basin and the Arabian Sea, i.e. a relationship between EAM and El Niño/Southern Oscillation systems. The abrupt increases in summer monsoon imply enhanced heat transport from low-latitude sea area to the midlatitude/high-latitude land area. The phase relationship between events of EAM and ice sheet may reflect a faster EAM response and a slower ice sheet response to the insolation change. A far-reaching conclusion is that the EAM might have triggered the Northern Hemisphere deglaciation.

Age determination and n-Alkane concentration of Arctic Ocean sediment cores

We have reconstructed the surface water environment of the Arctic Ocean over the last ? 50,000 years using measurements of the organic nitrogen and carbon isotope ratios, carbonate and total organic carbon concentrations (TOC), and terrestrial biomarkers (lignin and long-chain n-alkanes) in four multicores. Variations in nitrogen isotope ratios that are concordant with TOC and carbonate concentrations (representing foraminifera and excluding ice-rafted-debris) reflect differences in relative nutrient utilization of phytoplankton in the surface waters. However, d15N variations also appear to be dependent on the stratification of the water column and therefore potentially track the exchange of nutrients between deep and surface waters. Low Last Glacial Maximum (LGM) d15N values and higher Holocene values are opposite to those recorded in the Southern Ocean. The Arctic Ocean with higher nutrient utilization today compared to the LGM therefore acts as a counterpart to the Southern Ocean, although the global impact on carbon dioxide variations compared to the Southern Ocean is probably low.

Age determination and Heinrich Events in sediment cores SU90-08 and KET80-03

The Mediterranean Sea hydrology at the time of the Heinrich formation in the North Atlantic Ocean was analyzed by comparing sea surface temperatures (SSTs) and oxygen isotope composition of seawater (dw) changes during the past 75 kyr in two marine cores. These were compared to the palynological variations derived in the Mediterranean Sea core. During the last glacial the two oceanic SST records show similar and synchronous patterns, with several long-term cooling periods, ending by abrupt SST increases. At the time of the Heinrich events, cold SSTs and low salinity prevailed in the Mediterranean Sea. The freshwater budget was similar to the modern one, permitting the presence of a mixed forest on the Mediterranean borderlands. The post-Heinrich periods are marked by a freshwater budget decrease, limiting oak and fir tree growth in the Mediterranean region. Increase of precipitation or reduction of evaporation is observed before the Heinrich episode, and is associated with a well-developed mixed Mediterranean forest.

Stable isotope ratios of foraminifera from sediment cores off northeastern Brazil

The western South Atlantic boundary currents represent a sensitive system within the global thermohaline circulation (THC). We investigated the impact of deglacial THC changes on the western tropical Atlantic studied in six high resolution sediment cores from the upper continental slope of Brazil. The stratigraphy of the cores is mainly based on 14C AMS dating of monospecific foraminiferal samples. Changes in the upper layer tropical ocean during the deglaciation are inferred from stable oxygen isotope measurements on planktic and benthic foraminifera. Variations in the delta18O residuals are assumed to be mainly temperature related. During the Oldest and Younger Dryas cooling periods, two major deglacial THC disturbances are reported from North Atlantic sediment cores. Concomitant to the repeated THC slowdown, we observe an upper layer warming in the tropical ocean. A reduced northward heat export from the tropical areas during these periods (weak North Brazil Current) is additionally reflected by low meridional gradients in the stable oxygen records. This generally agrees with results from coupled ocean atmosphere models.

Sea surface temperatures during Marine Isotope Stage (MIS) 11

The Marine Isotope Stage (MIS) 11 (424-374 ka) was characterized by a protracted deglaciation and an unusually long climatic optimum. It remains unclear to what degree the climate development during this interglacial reflects the unusually weak orbital forcing or greenhouse gas trends. Previously, arguments about the duration and timing of the MIS11 climatic optimum and about the pace of the deglacial warming were based on a small number of key records, which appear to show regional differences. In order to obtain a global signal of climate evolution during MIS11, we compiled a database of 78 sea surface temperature (SST) records from 57 sites spanning MIS11, aligned these individually on the basis of benthic (N = 28) or planktonic (N = 31) stable oxygen isotope curves to a common time frame and subjected 48 of them to an empirical orthogonal function (EOF) analysis. The analysis revealed a high commonality among all records, with the principal SST trend explaining almost 49% of the variability. This trend indicates that on the global scale, the surface ocean underwent rapid deglacial warming during Termination V, in pace with carbon dioxide rise, followed by a broad SST optimum centered at ~410 kyr. The second EOF, which explained ~18% of the variability, revealed the existence of a different SST trend, characterized by a delayed onset of the temperature optimum during MIS11 at ~398 kyr, followed by a prolonged warm period lasting beyond 380 kyr. This trend is most consistently manifested in the mid-latitude North Atlantic and Mediterranean Sea and is here attributed to the strength of the Atlantic meridional overturning circulation. A sensitivity analysis indicates that these results are robust to record selection and to age-model uncertainties of up to 3-6 kyr, but more sensitive to SST seasonal attribution and SST uncertainties >1 °C. In order to validate the CCSM3 (Community Climate System Model, version 3) predictive potential, the annual and seasonal SST anomalies recorded in a total of 74 proxy records were compared with runs for three time slices representing orbital configuration extremes during the peak interglacial of MIS11. The modeled SST anomalies are characterized by a significantly lower variance compared to the reconstructions. Nevertheless, significant correlations between proxy and model data are found in comparisons on the seasonal basis, indicating that the model captures part of the long-term variability induced by astronomical forcing, which appears to have left a detectable signature in SST trends.

Isotope ratios of osmiun, carbon and oxygen of Miocene sediments and benthic foraminifera

Seawater 187Os/188Os ratios for the Middle Miocene were reconstructed by measuring the 187Os/188Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187Os/188Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187Os/188Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187Os/188Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87Sr/86Sr ratios at 16 Ma. A large increase of 1? in benthic foraminiferal delta18O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate delta13C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187Os/188Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187Os/188Os ratios and also nearly constant 87Sr/86Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187Os/188Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187Os/188Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187Os/188Os variations may correlate inversely with short-term benthic foraminiferal delta18O and bulk carbonate delta13C variations that reflect glacioeustatic events.