(Table) Specific activities of 137Cs, 90Sr and 239,240Pu in surface waters obtained during cruises BP06-BP09, Indian Ocean

One of the main sources of anthropogenic radionuclides in the ocean is the global fallout resulting from the nuclear tests that had been conducted by the United States, the former Soviet Union, and other countries between 1945 and 1990 mainly in the Northern Hemisphere. The most extensive fallout was observed in the middle latitudes of the Northern Hemisphere in 1963 immediately after the nuclear tests of 1961-1962 conducted by the United States and the Soviet Union. In 2006-2009, under the auspices of an agreement between the Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences and the National Center of Antarctic and Marine Research of the Ministry of Earth Sciences of India, cooperative geological and geochemical investigations were organized in several regions of the Indian Ocean. During these expeditions, the spatial distribution of anthropogenic radionuclides was investigated in the water of the Indian Ocean. The main results of these investigations are reported in this paper.

Radiocarbon datings and temperature reconstructions of sediment core GeoB10038-4 off SW Sumatra

Surface and thermocline conditions of the eastern tropical Indian Ocean were reconstructed through the past glacial-interglacial cycle by using Mg/Ca and alkenone-paleothermometry, stable oxygen isotopes of calcite and seawater, and terrigenous fraction performed on sediment core GeoB 10038-4 off SW Sumatra (~6°S, 103°E, 1819 m water depth). Results show that annual mean surface and thermocline temperatures varied differently and independently, and suggest that surface temperatures have been responding to southern high-latitude climate, whereas the more variable thermocline temperatures were remotely controlled by changes in the thermocline temperatures of the North Indian Ocean. Except for glacial terminations, salinity proxies indicate that changing intensities of the boreal summer monsoon did not considerably affect annual mean conditions off Sumatra during the past 133,000 years. Our results do not show a glacial-interglacial pattern in the thermocline conditions and reject a linear response of the tropical Indian Ocean thermocline to mid- and high-latitude climate change. Alkenone-based surface temperature estimates varied in line with the terrigenous fraction of the sediment and the East Asian winter monsoon proxy records at the precession band suggestive of monsoon (sea level) to be the dominant control on alkenone temperatures in the eastern tropical Indian Ocean on sub-orbital (glacial-interglacial) timescales.

Carbonate accumulation rates of the Ninetyeast Ridge, Indian Ocean

More than 95% of the carbon lost from the "blue-ocean" reservoir to the sedimentary sink appears to be transferred as skeletal CaCO3, produced in the surface waters. This skeletal CaCO3 carries a productivity signal which is much better preserved in the underlying pelagic carbonate sediments than that of the refractory organic carbon accompanying it. Here, we develop a new method to quantify this signal in terms of organic carbon paleoproductivity, using the sedimentary mass accumulation rates of pelagic carbonate. These are converted into carbonate transit-paleofluxes, which are then translated into the corresponding transit-fluxes of organic carbon, via the carbonate to organic carbon ratios reported from deep-moored sediment trap experiments in modern blue-ocean environments. Paleoproductivity can then be estimated quantitatively by using published algorithms describing the relationship between the export production of particulate organic carbon at depth and primary productivity in the euphotic zone. Although our approach seems rather straightforward, it contains several pitfalls, the effects of which are highlighted by an example comprising three Paleocene/Oligocene to Recent pelagic carbonate sequences drilled during ODP Leg 121 in the eastern Indian Ocean. Although some extreme values are likely due to errors, such as poorly constrained datum levels and dissolution peaks, the results for the Quaternary and Neogene correlate well from site to site and are within the productivity range of present-day low to medium latitude open oceans. Our method may provide an opportunity to actually quantify blue-ocean primary productivity in sedimentary carbonate environments, but requires validation by other, more established ones.

(Table 1) Adenosine triphosphate, urea, dissolved organic carbon, nitrogen and methane concentrations in bottom water of the northwestern Indian Ocean

Concentrations of adenosine triphosphate (ATP), urea, and dissolved organic carbon in bottom water are shown to be considerable, sometimes several times higher than in the photic and surface layers of the ocean. Urea and ATP concentrations are inversely proportional. Identified biochemical characteristics of bottom water are of great importance in determining the status of the aquatic environment. The highest life activity (maximum ATP content) in bottom water appeared in the vicinity of faults in rift zones of the ocean, where high gas concentrations were also found. Population of chemoautotrophic microorganisms was clearly present under these conditions. Biochemical investigations provide additional criteria for identifying oil and gas prospects. They are also of definite interest in combination with gasometric determinations, which will undoubtedly give us deeper understanding of processes of formation of oil and gas and will help in finding them.

Benthic foraminifera of the central Indian Ocean

Late Oligocene to late Pliocene vertical water-mass stratification along depth traverses in the northern Indian Ocean is depicted in this paper by benthic foraminifer index faunas. During most of this time, benthic faunas indicate well-oxygenated, bottom-water conditions at all depths except under the southern Indian upwelling and in the Pliocene in the southern Arabian Sea. Faunas suggest the initiation of lower oxygen conditions at intermediate depths in the northern Indian Ocean beginning in Oligocene Zone P21a. Lower oxygen conditions intensified during primary productivity pulses, possibly related to increased upwelling vigor, in the latest Oligocene and throughout most of the late middle through late Miocene. During times of elevated primary production, there may be more oxygen flux into sedimentary pore waters and the shallow infaunal habitat may become more oxygenated. One criterion for locating the source of "new" water masses is vertical homogeneity of benthic foraminifer indexes for well-oxygenated water masses from intermediate through abyssal depths. In the northern Mascarene Basin, this type of faunal homogeneity with depth corroborates the proposal that the northern Indian Ocean was an area of sinking well-oxygenated waters through most of the Miocene before Zone N17. Oxygenated, possibly "new" intermediate-water masses in the low- to middle-latitude Mascarene and Central Indian basins first developed in the late Oligocene. These well-oxygenated waters were probably more fertile than the Antarctic Intermediate Waters (AAIW) that cover intermediate depths in these areas today. Production of intermediate waters more similar to modern AAIW is indicated by the sparse benthic population of epifaunal rotaloid species in the northern Mascarene Basin during middle Miocene Zone N9 and from early through late Pliocene time. Deep-water characteristics are more difficult to interpret because of the extensive redeposition at the deeper sites. Redeposited intermediate, rather than shallow, water fossils and erosion from north to south in the Mascarene Basin are incompatible with the sluggish circulation from south to north through the western Indian Ocean basins today. Such erosion could result from the vigorous sinking of an intermediate-depth water mass of northern origin. Before late Oligocene Zone P22, benthic faunas indicate a twofold subdivision of the troposphere, with the boundary between upper and lower well-oxygenated water masses located from 2500-3000 mbsl. No characteristic bottom-water fauna developed before the end of late Oligocene Zone P22. Deep and abyssal benthic indexes suggest the development of water masses similar to those of the present day in the latest Miocene. Faunas containing deep-water benthic indexes, including the uvigerinids, suggestive of a water mass similar to modern Indian Deep Water (IDW), appeared during the late Miocene in the northern Mascarene and Central Indian basins. In the early Pliocene, this deep-water fauna was found only in the Central Indian Basin, whereas a fauna typical of modern Antarctic Bottom Water (AABW) spread through deep waters at 2800 mbsl in the Mascarene Basin. By late Pliocene Zone N21, however, deep-water faunas similar to their modern analogs were developed in both the eastern and western basins. Abyssal faunas, studied only in the Mascarene Basin, show more or less similarity to those under modern AABW. Bottom-water faunas containing Nuttallides umbonifera or Epistominella exiguua were first differentiated at the end of Zone P22, then appeared episodically during the early Miocene. These AABW-type faunas reappeared and migrated updepth into deep waters during the glacial episodes at the end of the Miocene and at the beginning of the Pliocene. By late Pliocene Zone N21, however, a bottom-water fauna similar to that under eastern Indian Bottom Water (IBW) developed in the Mascarene Basin. Modern bottom-water characteristics of the Mascarene Basin must have developed after ZoneN21.