Ages, sedimentation rates and geochemical composition of ODP Site 162-983 sediments

A geochemical study of sediments from Ocean Drilling Program Site 983 was conducted to examine low-frequency variations in carbonate content as expressed by blue-band reflectance (450-500 nm) over the last 1.2 Ma. Sedimentary percent organic carbon, percent carbonate, and excess barium (Ba[ex]) were used as the primary tools to evaluate the factors responsible for these long-term changes. We observe positive correlation between the mass-accumulation rate of various biogenic components and the mass-accumulation rate of Ba(ex), especially in sediments younger than ~600 ka. Deeper in the section (~600-1200 ka), the correlation between Ba(ex) and the other biogenic tracers is weak. The lack of correlation between Ba(ex) and biogenic carbonate likely results either from a higher supply of terrigenous material at that time (which confounds Ba[ex] estimation), or remobilization of Ba resulting from low pore-water sulfate ion concentrations, or both. Nonbiogenic sediments at Site 983, represented by Th, K2O, and the molar Ti/Al ratio, exhibit cyclic variations that represent mixing between continental and oceanic (i.e., basaltic) terrigenous sources. The timing of these cycles matches that of the major glacial-interglacial cycles, which suggests that they result from the supply of continental material as ice-rafted debris during glacial periods and fine-grained basaltic material by bottom currents during interglacial periods. Given these observations, the most likely causes for the low-frequency carbonate variations observed in the Site 983 sediments are shifts in surface productivity and, to a lesser extent, dilution by the input of terrigenous material.

Grain size analysis and sedimentation rates of ODP Hole 175-1085A off the Namibian coast

Sediments from the ODP Site 1085A were studied to investigate the impacts of global cooling in the Middle and Late Miocene on the climate in Southwestern Africa. The size composition of the sediment was analysed emphasising the silt fraction. A comparison with the modern grain size distribution and suitable transport processes made it possible to assign specific transport processes to the grain size composition. Three processes are considered for transport of terrigeneous silt: while there was no evidence found for (1) transport by ocean currents, the analyses showed signals of (2) wind transport indicating dry conditions associated with a cool climate and (3) fluvial transport that points to humid and warm conditions. Three climatic phases were defined. The first phase from 13.8 to 11.8 Myr reveals a stable humid climate in Southwest Africa independent of the Antarctic glaciations. During the second phase from 11.8 to 10.4 Myr the regional climate cooled considerably but was not drier. Additionally, the climate during this phase reacted to the Antarctic glaciations. This cooling-trend continued during phase 3 from 10.4 to 9.0 Myr with a significant increase in dust input, pointing to overall drier conditions. However, fluvial transport still remained as the main source.

Ages and sedimentation rates of bottom sediments from the south arid zone of the Pacific Ocean

Radiocarbon datings are used to calculate mean sedimentation rates of metalliferous sediments in the southern arid zone of the Pacific Ocean adjoining the axis of the East Pacific rise (20°S). Owing to low sedimentation rates and intense mixing, only averaged figures could be obtained for ages less than 16 ky. Sedimentation rate varies from 0.3 to about 1 cm/1000 years in the surface layer and is several times higher in the time interval from 20 ky to 45 ky ago. Formulas for calculating mean sedimentation rates with allowance for benthic mixing are presented.

Sedimentation rates and foraminiferal assemblages from ODP Site 162-184

Sedimentological and faunal records from the transitional period marking the onset of widespread northern hemisphere glaciation have been investigated at Ocean Drilling Program Site 984. The late Pliocene interglacial sediments of the northeast Atlantic are carbonate rich and show evidence of vigorous bottom water circulation at intermediate water depths. Contrasting this, the late Pliocene glacial sediments are characterised by carbonate dissolution and slower bottom current velocities. Weak or "leaky" Norwegian Sea overflows, undersaturated with respect to carbonate, influenced this region during the late Pliocene glacials. The early Pleistocene pattern of intermediate water circulation appears to have changed radically in the northeast Atlantic. At this time, interglacial carbonate values and inferred bottom current velocities are low. This suggests slow-flowing, undersaturated Norwegian Sea water bathing the site. The overflow increased during the early Pleistocene interglacials as the exchange between the Atlantic and Norwegian-Greenland Seas improved. The most significant feature of the early Pleistocene glacials is the increase in inferred bottom current velocity. These changes reflect a switch in deep North Atlantic convection to shallower depths during glacial periods, possibly in a manner similar to the increasing contribution of glacial intermediate water to the North Atlantic during the late Pleistocene glacials. Our results suggest that the late Pleistocene climate variability of the North Atlantic is a pervasive feature of the late Pliocene-early Pleistocene record.

(Table 1) Thickness of the Holocene layer and sedimentation rates in the Atlantic Ocean

On the basis of planktonic foraminifera study, thickness of Holocene sediments has been ascertained in 60 sediment cores from various regions of the Atlantic Ocean. Ratios of species reflect warming of the upper water layer at the Pleistocene-Holocene boundary over the entire ocean. The Holocene boundary can be determined not only from microfaunal data, but also from lithologic ones including textural and structural features. Increase in CaCO3 contents in Holocene sediments as compared to Pleistocene is from 5-7% to 60-70% in different parts of the ocean.