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.

Geochemistry of Mid-Atlantic Ridge sediments

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the epsilon-Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater epsilon-Nd values. Nd isotope data from these cores span the period of 4-14 ka (14C ages) and yield epsilon-Nd values for North East Atlantic Deep Water (NEADW) that are higher (-9.3 to -11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (-12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water epsilon-Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.

Vertical microalgae fluxes at the mooring station LOMO-2 over the Lomonosov Ridge in the period from September 15, 1995 till August 16, 1996

The first studies of microalgae fluxes over the Lomonosov Ridge in the northern Laptev Sea were carried out with a sediment trap at the year-long mooring station LOMO-2, installed at 150 m depth from September 15, 1995 to August 16, 1996. These studies demonstrated essential seasonal variations of vertical microalgae flux. It was shown that in summer diverse flora (composed mainly of cryophylic diatoms) growed intensively beneath the permanent ice cover. Strongly pronounced seasonal variations of microalgae growth correlate closely with solar radiation. Exactly during the maximum insolation period, from the middle of July until the end of September, the microalgae flux was hundreds of times higher than that in the rest of the year. Summer values of the microalgae flux over the Lomonosov Ridge in the northern Laptev Sea were similar to those in the Weddell Sea (Antarctic) and exceeded summer flux values in the Norwegian and Greenland Seas and in the St. Anna Trough (northwestern Kara Sea).