Radiocarbon sediment ages and sedimentation rates in the Red Sea

Results of radiocarbon dating of 23 cores (81 determinations) collected in the Red Sea rift zone at 8°N are presented. All of the main tectonic structures were dated: the upper and lower tectonic benches, the salt scarp, and the axial zone. Sediments in the upper tectonic bench exhibit normal sedimentation, while all other structures, which have highly dissected relief, show extensive re-deposition or non-accumulation of sediments. Sedimentation rate in Holocene was from two to three times lower than in Late Würm.

Contents of radioactive elements, main chemical composition, and sedimentation rates of bottom sediments from some stations in the Southeast Pacific

Gamma-spectrometric analysis was used for six sediment cores from the area occupied by metalliferous sediments in the Southeast Pacific. In five of these cores vertical distribution curves of 230Th enabled positions of equilibrium points to be determined and sediments to be dated. The ionium curve was normalized for one core. Vertical distribution of 230Th in metalliferous sediments resembles its distribution in normal ocean-floor sediments beyond the area of influence of active ridges. Sedimentation rates lay within the range 0.7-12.3 mm/ky.

Age determination and sedimentation rates of the subpolar northeast Atlantic

In order to monitor the evolution of the British-Irish Ice Sheet (BIIS) and its influence in surface ocean structure during marine isotopic stages (MIS) 2 and 3, we have analyzed the sediments recovered in core MD04-2829CQ (Rosemary Bank, north Rockall Trough, northeast Atlantic) dated between ~41 and ~18 ka B.P. Ice-rafted debris flux and composition, 40Ar/39Ar ages of individual hornblende grains, multispecies planktonic stable isotope records, planktonic foraminifera assemblage data and faunal-based sea surface temperatures (SSTs) demonstrate a close interaction between BIIS dynamics and surface ocean structure and water properties in this region. The core location lies beneath the North Atlantic Current (NAC) and is ideal for monitoring the shifts in the position of its associated oceanic fronts, as recorded by faunal changes. These data reveal a succession of BIIS-sourced iceberg calving events related to low SST, usually synchronous with dramatic changes in the composition of the planktonic foraminifera assemblage and with variations in the stable isotope records of the taxa Neogloboquadrina pachyderma (sinistral coiling) and Globigerina bulloides. The pacing of the calving events, from typically Dansgaard-Oeschger millennial timescales during late MIS 3 to multicentennial cyclicity from ~28 ka B.P., represents the build-up of the BIIS and its growing instability toward Heinrich Event (HE) 2 and the Last Glacial Maximum. Our data confirm the strong coupling between BIIS instabilities and the temperature and salinity of surface waters in the adjacent northeast Atlantic and demonstrate the BIIS's ability to modify the NAC on its flow toward the Nordic Seas. In contrast, subsurface water masses were less affected except during the Greenland stadials that contain HEs, when most intense water column reorganizations occurred simultaneously with the deposition of cream-colored carbonate sourced from the Laurentide Ice Sheet.

Sedimentation rates and geochemistry of the Atlantic and Indic Ocean of Late Miocene - Early Pliocene

Studies of the late Miocene-early Pliocene biogenic bloom typically have focused on high-productivity areas in the Indian and Pacific Oceans in order to achieve high resolution samples. Thus there is a paucity of information concerning whether the Atlantic Ocean, in general or low-productivity regions in all three basins experienced this bloom. This study measured the phosphorus mass accumulation rate (PMAR). in five cores from low-productivity regions of the Atlantic and Indian Oceans. All cores exhibit a peak in productivity 4-5.5 Ma, coincident with the Indo-Pacific bloom. This suggests that nutrients were not shifted away from low-productivity regions nor out of the Atlantic Ocean. Instead, it appears that the bloom was caused by an overall increase in nutrient flux into the world oceans. Four of the cores record the bloom's PMAR peak as bimodal, indicating a pulsed increase in phosphorus to the oceans. This suggests that there may have been multiple causes of the biogenic bloom.

Compilation of sedimentation rates and smear slide analysis of DSDP sites

A history of Mesozoie and Cenozoic palaeoenvironments of the North Atlantie Oeean has been developed based on a detailed analysis of the temporal and spatial distribution of major pelagie sediment facies, of hiatuses. of bulk sediment accumulation rates, and of concentrations and fluxes of the main deep-sea sediment components. The depositional history of the North Atlantic can be subdivided into three major phase: (a) Late Jurassie and Early Cretaceous phase: clastic terrigenous and biogenic pelagic sediment components accumulated rapidly under highly productive surface water masses over the entire occan basin; (b) Late Cretaceous to Early Miocene phase: relatively little terrigenous and pelagic biogenic sediment reached the North Atlantic Ocean floor, intensive hiatus formation occurred at variable rates, and wide stretches of the deep-ocean floor were covered by slowly accumulating terrigenous muds: (c) Middle Miocene to Recent phase: accumulation rates of biogenic and terrigenous deep-sea sediment components increased dramatically up to Quaternary times, rates of hiatus formation and the intensity of deep-water circulation inferred from them seem to have decreased. However, accumulation rate patterns of calcareous pelagic sediment components suggest that large scale reworking and di splacement of deep-sea sediments occurred at a variable rate over wide areas of the North Atlantic during this period.

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.