Radionuclides and sedimentation rates of sediment cores from the eastern South Atlantic

High-resolution records of the natural radionuclide 230Th were measured in sediments from the eastern Atlantic sector of the Antarctic circumpolar current to obtain a detailed reconstruction of the sedimentation history of this key area for global climate change during the late Quaternary. High-resolution dating rests on the assumption that the 230Thex flux to the sediments is constant. Short periods of drastically increased sediment accumulation rates (up to a factor of 8) were determined in the sediments of the Antarctic zone during the climate optima at the beginning of the Holocene and the isotope stage 5e. By comparing expected and measured accumulation rate of 230Thex, lateral sediment redistribution was quantified and vertical particle rain rates originating from the surface water above were calculated. We show that lateral contributions locally were up to 6.5 times higher than the vertical particle rain rates. At other locations only 15% of the expected vertical particle rain rate were deposited.

Description of manganese deposits collected by R/V Nothwind during the 1963 expedition in the Laptev sea

The 400-km-wide, low gradient Laptev Sea continental shelf consists of flat terrace-like features at regular depth intervals from 10 to 40 m below present sea level. The five large submarine valleys traversing the shelf do not continuously grade seaward, but contain elongated, closed basins. These terraces and closed basins plus deltaic sediments associated with the submarine valleys quite possibly mark sea level Stillstands, and enable reconstruction of the paleogeography of the Laptev Sea shore line at five periods during post-Wisconsin (Holocene) time. Radiocarbon dates on the silty-clay to clayey-silt sediments from cores of the northeastern Laptev Sea indicate average sedimentation intensity of 2 to 15 mg/cm2/yr. The presence of manganese nodules and crusts in surface samples from less than 55 m depths and a general decrease in total foraminiferal abundances with depth in the cores suggest that the present deposition rate is less than when sea level was lower. The main components of the shelf deposits are near- shore sediments which were spread over the shelf as Holocene sea level fluctuated and marine currents distributed modern fine sediment. Rare silty-sand layers and the coarser nuclei of the manganese crusts and nodules indicate ice rafting. However, this mechanism is probably only locally important as a significant transporting agent.

Sedimentation rates and geochemical analysis of surface sediments from the South Atlantic

Extensive investigations of sedimentary barium were performed in the southern South Atlantic in order to assess the reliability of the barium signal in Antarctic sediments as a proxy for paleoproductivity. Maximum accumulation rates of excess barium were calculated for the Antarctic zone south of the polar front where silica accumulates at high rates. The correspondence between barium and opal supports the applicability of barium as a proxy for productivity. Within the Antarctic zone north of today's average sea ice maximum, interglacial vertical rain rates of excess barium are high, with a maximum occurring during the last deglaciation and early Holocene and during oxygen isotope chronozone 5.5. During these periods, the maximum silica accumulation was supposedly located south of the polar front. Glacial paleoproductivity, instead, was low within the Antarctic zone. North of the polar front, significantly higher barium accumulation occurs during glacial times. The vertical rain rates, however, are as high as in the glacial Antarctic zone. Therefore there was no evidence for an increased productivity in the glacial Southern Ocean.

Radionuclide analyses, and sedimentation and accumulation rates of sediment core PS2319-1

There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.

Location and ages of terrestrial peatlands located on continental shelves and in coastal sediments

This synthesis dataset contains records of freshwater peat and lake sediments from continental shelves and coastal areas. Information included is site location (when available), thickness and description of terrestrial sediments as well as underlying and overlying sediments, dates (when available), and references.

Age and sedimentation rates of bottom sediments from the Baltic Sea

An automated system is described for collecting and processing information from radiometric devices used in determination of isotopic age and sedimentation rate by radiocarbon and lead-210 methods. The system is used in a day-and-night continuous measuring mode, with six radiocarbon liquid counters and two low-background beta-counters. Examples are given of use of the system to determine average sedimentation rates in the Baltic Sea.

Lithological and chemical composition, stages of sediment accumulation, and sedimentation rates in Core PSh24-2537, West Gotland Basin, Baltic Sea

We studied the grain-size, mineral and chemical compositions, physical properties, radiocarbon age, spore-pollen spectra, and diatom composition in sediments from Core PSh24-2537 sampled in the West Gotland Basin. Four lithological-stratigraphic units were distinguished: varved clays of the Baltic Ice Lake, black and black-gray (sulfide) clays of the Yoldian Sea, gray clays of Ancylus Lake, and greenish-gray sapropel-like littorine and post-littorine silts of the marine stage of Holocene. These units differ from each other both in their matter composition and in plant remains. In the littorine silts organic carbon concentra¬tion reached from 1.5 to 10.35%. Conditions of sediment accumulation and the stages of evolution of the West Gotland Basin over the post-glacial time are characterized.

(Table 1) Sedimentation rates in the Reykjanes rift near 58° for time intervals 0-11 ka, 11-18 ka, and 18-21 ka

Sedimentation rates in the Reykjanes rift near 58°N have been measured on the base of radiocarbon datings of bottom sediments collected during Cruise 4 of R/V Akademik Mstislav Keldysh.

Averaged sedimentation rates in the Northwest Pacific and Bering Sea during the first and the second oxygen isotope stages

Isotope chronostratigraphy of Upper Quaternary sediments from the Northwest Pacific and the Bering Sea was established by oxygen isotope records in planktonic and benthic foraminifera. The main regularities of temporal variations of calcium carbonate, organic carbon and opal contents, as well as of magnetic susceptibility in sediments of the study region with regard to climatic variations and productivity were established by means of isotopic-geochemical and lithophysical analyses of bottom sediments. Correlation of volcanogenic interbeds in the sediments was carried out, and their stratigraphy and age were preliminarily ascertained. Correlation was accomplished of A.P. Jouse diatom horizons determined by an analysis of the main ecological variations in diatom assemblages in Upper Quaternary sediments of the Northwest Pacific, Bering and Okhotsk Seas, and their comparison with similar variations in sediment cores with standard oxygen isotope stages. Also variations in lithology and contents of biogenic components in sediments of the region and in the cores were taken into account. A ratio of abundance of "neritic" species to the sum of "neritic" and oceanic species abundance (coefficient Id) can be a criterion of ecological changes of diatom assemblages in the studied region. It is determined by climate variability and mostly by sea ice influence. Schemes of average sedimentation rates in the Northwest Pacific and Bering Sea for periods of the first and the second oxygen isotope stages (12.5-1 and 24-12.5 ka, respectively) were plotted on the basis of obtained results and correlation of diatom horizons and lithological units in early studied cores with the oxygen isotope stages. Closure of the Bering Strait and aeration of the north-eastern shelf of the Bering Sea during the second stage induced increase of sedimentation rates in the Bering Sea, as compared with the first stage, and suspended material transport from the Bering Sea through the Kamchatka Strait into the Northwest Pacific and its accumulation in the southeast direction.

Particle flux characterisation and sedimentation patterns of protistan plankton during the iron fertilisation experiment LOHAFEX in the Southern Ocean

The taxonomic composition and types of particles comprising the downward particle flux were examined during the mesoscale artificial iron fertilisation experiment LOHAFEX. The experiment was conducted in low-silicate waters of the Atlantic Sector of the Southern Ocean during austral summer (January-March 2009), and induced a bloom dominated by small flagellates. Downward particle flux was low throughout the experiment, and not enhanced by addition of iron; neutrally buoyant sediment traps contained mostly faecal pellets and faecal material apparently reprocessed by mesozooplankton. TEP fluxes were low, <5 mg GX eq/m**2/day, and a few phytodetrital aggregates were found in the sediment traps. Only a few per cent of the POC flux was found in the traps consisting of intact protist plankton, although remains of taxa with hard body parts (diatoms, tintinnids, thecate dinoflagellates and foraminifera) were numerous, far more so than intact specimens of these taxa. Nevertheless, many small flagellates and coccoid cells, belonging to the pico- and nanoplankton, were found in the traps, and these small, soft-bodied cells probably contributed the majority of downward POC flux via mesozooplankton grazing and faecal pellet export. TEP likely played an important role by aggregating these small cells, and making them more readily available to mesozooplankton grazers.

Late Quaternary deep sea sedimentation off the Niger delta

The oxygen isotopes ratios of benthic foraminifera and detailed radiocarbon ages of the organic matter of an over 15 m long sediment core from the outer Niger delta allow us to date the oxygen isotope stage boundaries 1/2 to 11500 (+/- 650) years BP, 2/3 to approximately 23000 (+/- 2000) years BP. The composition of the predominantly terrigenous clays and accessory pelagic fossils reflects the evolution of the climate over the southwestern Sahel zone and the response of the Eastern Tropical Atlantic to these climatic fluctuations during the Late Quaternary. The dilution of the pelagic fossil concentrations by the terrigenous material and the oxygen isotopes ratios of planktonic foraminifera indicate large fluctuations in the freshwater discharge from the Niger, with high precipitations over the drainage area of this river from 4500 (+/- 300) to 11500 (+/- 650) years BP and from 11800 (+(- 600) to 13000 (+/- 600) years BP while the time intervals in between were as dry as today. Relative increase of kaolinite during wet phases and the association of smectite, chlorite and attapulgite during dry ones characterize the response of the weathering in the Niger drainage basins to the climatic fluctuations. The occurrence of 10-14 A mixed-layers prior to 26000 years BP is correlated with moderate alteration of the crystalline substratum outcrops from the middle-lower part of the Niger Basin. High quartz concentrations are particularly typical for the transition between oxygen isotope stages 1 and 2 at the inception of heavy precipitations in the southern Sahel zone. Sedimentation rates were quite constant, 30-35 cm/1000 years; they became unusually large at the beginning of the Holocene from 10900 (+/- 650) to 11500 (+/- 650) years BP where they reached more than 600 cm/1000 years. Bottom waters around 1100 m depth in the Gulf of Guinea responded to changes in paleo-oceanography of the entire Atlantic Ocean as well as to local influences. Abnormal carbon isotopes ratios and the drastic changes from a highly diversified fauna (during stages 2 and 3. and during the last part of stage 1 after approx. 7000 years BP) to a poorly diversified fauna in the intervenin time span point to the development of a local benthic environment which cannot easily be compared with the corresponding continental and slope environments of the entire Atlantic Ocean.