Stable isotope ratios and organic carbon accumulation rates of late Pleistocene sediments of ODP Hole 117-724C

Stable isotope records of coexisting benthic foraminifers Uvigerina spp. and Cibicidoides spp. and planktonic G. ruber (white variety) from Site 724 are used to study the late Pleistocene evolution of surface and intermediate water hydrography (593 m water depth) at the Oman Margin. Glacial-interglacial d18O amplitudes recorded by the benthic foraminifers are reduced when compared to the estimated mean ocean changes of d18Oseawater . Epibenthic d13C remains at its modern level or is increased during glacial times. This implies that Red Sea outflow waters which are enriched in d18Oseawater and d13C (Sum CO2) have been replaced during glacial periods by intermediate waters still positive in d13C (Sum CO2) but more negative in d18Oseawater. Glacial-interglacial amplitudes of the planktonic d18O record exceed those of the mean ocean d18Oseawater variation and imply decreased surface water temperatures (SST) during glacial times. Throughout most of the records these cooling events correlate with enhanced rates of carbon accumulation. However, both negative (colder) SST and positive Corg accumulation rate anomalies do not correlate with potential physical upwelling maxima as inferred from the orbital monsoon index. This is in conflict with the established hypothesis that upwelling in the estern Arabia Sea should be strongest during maxima of the southwest monsoon.

210Pb dates, radiocarbon ages and sedimentation and accumulation rates of sediment cores JM99-1198 and MD99-2297

The magnitude of Late Holocene climatic variations are less significant than those that took place during ice ages and deglaciations. However, detailed knowledge about this period is vital in order to understand and model future climate scenarios both as a result of natural climate variation and the effects of global warming. Oceanic heat flux is important for the sensitive climate regime of northern Europe. Our aim is to connect hydrographical changes, reflected by the dinoflagellates cyst (dinocysts) assemblages in the sediments in the Malangen fjord, to local and regional climatic phases. Previous studies have shown that dinocyst assemblages are influenced by temperature, salinity, and the availability of nutrients (e.g. de Vernal et al. 2005, doi:10.1016/j.quascirev.2004.06.014; de Vernal et al. 2001, doi:10.1002/jqs.659; Grosfjeld et al. this volume; Rochon et al. 2008, doi:10.1016/j.marmicro.2008.04.001; Solignac et al. this volume). Dinoflagellates are mostly unicellular organisms that make up one of the main groups of phytoplankton. They are able to regulate their depth within the photic zone and to concentrate along oceanic fronts, which provide nutrient-enriched waters. The dinoflagellate cysts are the hypnozygotes of dinoflagellates naturally produced during the life cycle. Their wall is composed of a highly resistant organic material, which has a high potential to fossilize. Because dinocysts species are linked to particular abiotic and biotic parameters, the dinocyst assemblages provide information about past surface water conditions. Since each fjord has its own hydrographic setting, it is necessary to establish a firm link between the dinocyst composition of the sediment surface samples and the surface water conditions. Indeed the modern dinocyst distribution in subarctic fjords is little known. Thus, in addition to detailing dinocyst results from two shallow cores, several sediment surface samples located along a transect running from the head to the mouth of the fjord, and extending onto the shelf, are also presented.

Age determination and foraminiferal faunas in sediment core MD99-2275

A high-resolution study of palaeoceanographic changes off North Iceland during the time period 8600-5200 cal year BP is based on benthic and planktonic foraminiferal assemblages. The core material (MD99-2275) was obtained from about 440 m water depth on the eastern part of the North Icelandic shelf. Changes in the faunal composition are interpreted to be mainly caused by variations in the strength of the relatively warm, high-salinity Irminger Current and the cold East Icelandic Current, which have been shown to be linked to the climatic changes in the North Atlantic region. Environmental proxies at that site are particularly sensitive to palaeoceanographic changes due to its position close to the marine Polar Front. Benthic assemblages show that relatively cold conditions prevailed at the base of the record. An increase in the influence of Atlantic water masses at the sea floor is seen at around 8400 cal year BP, whereas the surface waters were relatively warm already at 8600 cal year BP. The warming was interrupted by a cold event at around 8100-8000 cal year BP, registered both in the bottom and surface waters and correlated with the so-called 8.2 kyr cooling event. Both the benthic and the planktonic faunal compositions indicate that the Irminger Current had maximum influence in the area between 8000 and about 7300 cal year BP, followed by a gradually decreasing influence through the remaining part of the studied time interval. It is suggested that the contribution of Atlantic water masses from the east and north-east to the Arctic Surface waters off North Iceland increased after around 7000 cal year BP, and that this was further intensified after 6200 cal year BP. At present, the Arctic Surface Water north of Iceland consists of Polar waters, intermittently with direct influence from the East Greenland Current, mixed with Atlantic waters derived from the eastern part of the Nordic Seas. A comparison of the mean values of selected environmental proxies in the interval 8600-5200 cal year BP with the upper part of the same core shows that the water masses north of Iceland were considerably warmer during the Holocene thermal maximum than during the last 2000 cal year. In general, results from core MD99-2275 are in accordance with other marine records from the North Icelandic shelf and the northern North Atlantic region, although a detailed comparison on a centennial time scale is hampered by problems with spatial as well as temporal changes in the marine reservoir ages in the region.

Geochemistry and foraminiferal stable isotoe record of sediment core SCS90-36

Changes in the Southeast Asia monsoon winds and surface circulation patterns since the last glaciation are inferred using multiple paleoceanographic indicators including planktic foraminifer faunal abundances, fauna and alkenones sea-surface temperature (SST) estimates, oxygen and carbon isotopes of planktic and benthic foraminifers, and sedimentary fluxes of carbonates and organic carbon obtained from deep-sea core SCS90-36 from the South China Sea (SCS) (17°59.70'N, 111°29.64'E at water depth 2050 m). All these paleoceanographic evidences indicate marked changes in the SCS ocean system over the last glacial toward the Holocene. Planktic foraminiferal faunal SST estimates show stable warm-season SST of 28.6°C, close to the modern value, throughout the glacial-interglacial cycle. In contrast, cold-season SST increases gradually from 23.6°C in the last glacial to a mean value of 26.4°C in the Holocene with a fluctuation of about 3°C during 13-16 ka. SST estimates by UK'37 method reveal less variability and are in average 1-3°C lower than the fauna-derived winter-season SST. These patterns reveal that the seasonality of the SST is not only higher by about 3-4°C in the glacial, but also a function of the winter season SST. Sedimentation rates decrease from the last glacial-deglacial stage to the Holocene due to a reduction in supply of terrigenous components, which led to an increase of carbonate contents. Total organic carbon (TOC) contents of primarily marine sources decrease from the last glacial-deglacial to the Holocene. The last deglaciation is also characterized by high surface productivity as indicated by increased ketones abundances and high mass accumulation rates (MAR) of the TOC and carbonates. The gradient of planktic foraminifer ocygen and carbon isotopes of between surface dwellers and deep dwellers increases significantly toward Termination I and Holocene, and is indiscernibly small in the carbon isotope gradient of between 14 and 24 ka, revealing a deep-mixing condition in surface layers prior to 10 ka. The glacial-interglacial fluctuation of the carbon isotope value of a benthic foraminifer is 0.61%. which is significantly larger than a global mean value. The large carbon isotope fluctuation indicates an amplification of marginal-sea effects which is most likely resulted from an increase in surface productivity in the northern SCS during the last glacial-deglacial stage. The multiple proxies consistently indicate that the last glacial-deglacial stage winter monsoon in the Southeast Asia was probably strengthened in the northern SCS, leading to a development of deep-mixing surface layer conditions and a more efficient nutrient cycling which supports more marine organic carbon production.

Accumulation and composition of terrigenous material on Ceara rise over the past 55 Ma

Ceara Rise, located east the Amazon River mouth, is covered with a thick blanket of pelagic carbonate and hemipelagic terrigenous sediment. The terrigenous component has been extracted from 57 bulk sediment samples at Ocean Drilling Program (ODP) Sites 925 and 929 on Ceara Rise to obtain a Cenozoic record of riverine discharge from northern South America. From the early Eocene to early Miocene (55-20 Ma), terrigenous accumulation was dominated by moderate amounts of generally large-grained, gray to green sediment especially depleted in elements that are enriched in post-Archaean shale (e.g. Cs, Th, Yb). However, pulsed inputs of relatively small-grained, gray to green terrigenous sediment less depleted in the above elements occurred in the late Eocene and Oligocene. The accumulation of terrigenous sediment decreased significantly until 16.5 Ma. In the middle Miocene (16.5-13 Ma), terrigenous accumulation was dominated by small amounts of small-grained, tan sediment notably depleted in Na and heavy rare earth elements. The accumulation rate of terrigenous sediment increased markedly from the latest Miocene (10 Ma) to the present day, a change characterized by deposition of gray-green sediment enriched in elements that are enriched in post-Archaean shale. Observed changes in terrigenous sediment at Ceara Rise record tectonism and erosion in northern South America. The Brazil and Guyana shields supplied sediment to the eastern South American margin until the middle Miocene (20-16.5 Ma) when a period of thrusting, shortening and uplift changed the source region, probably first to highly weathered and proximal Phanerozoic sediments. By the late Miocene (9 Ma), there was a transcontinental connection between the Andes and eastern South America. Weathering products derived from the Andes have increasingly dominated terrigenous deposition at Ceara Rise since the Late Miocene and especially since the late Pliocene.

Alkenone accumulation and SST reconstruction for the Benguela upwelling zone

A feature of Pliocene climate is the occurrence of "permanent El Niño-like" or "El Padre" conditions in the Pacific Ocean. From the analysis of sediment cores in the modern northern Benguela upwelling, we show that the mean oceanographic state off Southwest Africa during the warm Pliocene epoch was also analogous to that of a persistent Benguela "El Niño". At present these events occur when massive southward flows of warm and nutrient-poor waters extend along the coasts of Angola and Namibia, with dramatic effects on regional marine ecosystems and rainfall. We propose that the persistent warmth across the Pliocene in the Benguela upwelling ended synchronously with the narrowing of the Indonesian seaway, and the early intensification of the Northern Hemisphere Glaciations around 3.0-3.5 Ma. The emergence of obliquity-related cycles in the Benguela sea surface temperatures (SST) after 3 Ma highlights the development of strengthened links to high latitude orbital forcing. The subsequent evolution of the Benguela upwelling system was characterized by the progressive intensification of the meridional SST gradients, and the emergence of the 100 ky cycle, until the modern mean conditions were set at the end of the Mid Pleistocene transition, around 0.6 Ma. These findings support the notion that the interplay of changes in the depth of the global thermocline, atmospheric circulation and tectonics preconditioned the climate system for the end of the warm Pliocene epoch and the subsequent intensification of the ice ages.