988 resultados para STABLE LEAD ISOTOPES
Resumo:
Deep-sea sediment core FR1/97 GC-12 is located 990 mbsl in the northern Tasman Sea, southwest Pacific, where Antarctic Intermediate Water (AAIW) presently impinges the continental slope of the southern Great Barrier Reef. Analysis of carbon (d13C) and oxygen (d18O) isotope ratios on a suite of planktonic and benthic foraminifera reveals rapid changes in surface and intermediate water circulation over the last 30 kyr. During the Last Glacial Maximum, there was a large d13C offset (1.1 per mil) between the surface-dwelling planktonic foraminifera and benthic species living within the AAIW. In contrast, during the last deglaciation (Termination 1), the d13C(planktonic-benthic) offset reduced to 0.4 per mil prior to an intermediate offset (0.7 per mil) during the Holocene. We suggest that variations in the dominance and direction of AAIW circulation in the Tasman Sea, and increased oceanic ventilation, can account for the rapid change in the water column d13C(planktonic-benthic) offset during the glacial-interglacial transition. Our results support the hypothesis that intermediate water plays an important role in propagating climatic changes from the polar regions to the tropics. In this case, climatic variations in the Southern Hemisphere may have led to the rapid ventilation of deep water and AAIW during Termination 1, which contributed to the postglacial rise in atmospheric CO2.
Resumo:
During Leg 177 of the Ocean Drilling Program (ODP), a well-preserved middle Eocene to lower Miocene sediment record was recovered at Site 1090 on the Agulhas Ridge in the Atlantic sector of the Southern Ocean. This new sediment record shows evidence of a hitherto unknown late Eocene opal pulse. Lithological variations, compositional data, mass-accumulation rates of biogenic and lithogenic sediment constituents, grain-size distributions, geochemistry, and clay mineralogy are used to gain insights into mid-Cenozoic environmental changes and to explore the circumstances of the late Eocene opal pulse in terms of reorganizations in ocean circulation. The base of the section is composed of middle Eocene nannofossil oozes mixed with red clays enriched in authigenic clinoptilolite and smectite, deposited at low sedimentation rates (LE 2 cm/ka). It indicates reduced terrigenous sediment input and moderate biological productivity during this preglacial warm climatic stage. The basal strata are overlain by an extended succession (100 m, 4 cm/ka) of biosiliceous oozes and muds, comprising the upper middle Eocene, the entire late Eocene, and the lowermost early Oligocene. The opal pulse occurred between 37.5 and 33.5 Ma and documents the development of upwelling cells along topographic highs, and the utilization of a marine nutrient- and silica reservoir established during the pre-late Eocene through enhanced submarine hydrothermal activity and the introduction of terrigenous solutions from chemical weathering on adjacent continents. This palaeoceanographic overturn probably was initiated through the onset of increased meridional ocean circulation, caused by the diversion of the Indian equatorial current to the south. The opal pulse was accompanied by increased influxes of terrigenous detritus from southern African sources (illite), mediated by enhanced ocean particle advection in response to modified ocean circulation. The opal pulse ended because of frontal shifts to the south around the Eocene/Oligocene boundary, possibly in response to the opening of the Drake Passage and the incipient establishment of the Antarctic Circumpolar Current. Condensed sediments and a hiatus within the early Oligocene part of the section possibly point to an invigoration of the deep-reaching Antarctic Circumpolar Current. The mid-Oligocene to lower Miocene section on long time scale exhibits less pronounced lithological variations than the older section and points to relatively stable palaeoceanographic conditions after the dramatic changes in the late Eocene to early Oligocene.
