Major and trace elements at DSDP Leg 74 Holes

Deep Sea Drilling Project Leg 74 drilled basement on the Walvis Ridge at Sites 525, 527, and 528. These sites are located on the crest and flanks of the segment of the Ridge about 68 to 70 m.y. old in the central province of the Ridge. Each site has a number of distinct subaqueous flows separated by sediment layers. Although variation in geochemistry among units and sites is related in part to alteration or crystal fractionation, some is caused by small-scale compositional variation in the mantle source of the basalts. Leg 74 basalts are similar to other basalts recovered from the Walvis Ridge and the Rio Grande Rise. They show distinct compositional differences to mid-ocean ridge basalts in general, to those recovered from the South Atlantic at this latitude, and to basalts presently erupting in Tristan da Cunha. The composition of the Walvis Ridge basalts does not suggest simple mixtures of present-day MORB and Tristan da Cunha melts. If the Walvis Ridge represents the trace of the Tristan da Cunha hot spot as the plates separated, then the composition of the mantle source has differed at different times in the past, which suggests mantle heterogeneity.

Absorption characteristics, major elements and chl a content of samples from Pond 19 at Zackenberg, NE Greenland

1. Shallow arctic lakes and ponds have simple and short food webs, but large uncertainties remain about benthic-pelagic links in these systems. We tested whether organic matter of benthic origin supports zooplankton biomass in a pond in NE Greenland, using stable isotope analysis of carbon and nitrogen in the pond itself and in a 13C-enrichment enclosure experiment. In the latter, we manipulated the carbon isotope signature of benthic algae to enhance its isotopic discrimination from other potential food sources for zooplankton. 2. The cladoceran Daphnia middendorffiana responded to the 13C-enrichment of benthic mats with progressively increasing d13C values, suggesting benthic feeding. Stable isotope analysis also pointed towards a negligible contribution of terrestrial carbon to the diet of D. middendorffiana. This agreed with the apparent dominance of autochthonous dissolved organic matter in the pond revealed by analysis of coloured dissolved organic matter. 3. Daily net production by phytoplankton in the pond (18 mg C/m**2/day) could satisfy only up to half of the calculated minimum energy requirements of D. middendorffiana (35 mg C/m**2/day), whereas benthic primary production alone (145 mg C/m**2/day) was more than sufficient. 4. Our findings highlight benthic primary production as a major dietary source for D. middendorffiana in this system and suggest that benthic organic matter may play a key role in sustaining pelagic secondary production in such nutrient-limited high arctic ponds.

Rare earth elements in bottom sediments and Fe-Mn nodules of the Black Sea

Distributions of rare earth element contents in surface layer bottom sediments, in vertical sediment section, and in Fe-Mn nodules of the Black Sea have been studied. An inverse relationship of rare earth element contents and CaCO3 contents has been found in the studied sediments. Fe-Mn nodules of the Black Sea do not concentrate rare earth elements, and their rare earth element composition differs from one of host sediments. It is concluded that rare earth elements are bound with clay minerals of bottom sediments.

Concentrations of tektites and chemical elements in sediments from ODP Holes 121-758B and 124-769A

Recently published studies of Ocean Drilling Project (ODP) cores from near southeast Asia revealed microtektite contents much higher than those in previously studied cores, suggesting that Ir contents might be enhanced in the tektite-bearing horizons. We determined a positive Ir anomaly in ODP core 758B from the Ninetyeast Ridge, eastern Indian Ocean; the peak Ir concentration of 190 pg/ g was ~2X the continuum level. The net Ir fluence is 1.8+/-0.5 ng/cm**2 over the depth interval from 10.87 to 11.32 m; a small additional peak also associated with microtektites contributes another 0.5 ng Ir/cm**2. Concentrations of Ir in core 769A show more scatter, but a small Ir enhancement is associated with the peak microtektite abundance; our best estimate of the poorly constrained fluence is 1.3+/-0.7 ng/cm**2. Data on deep-sea cores show that the microtektite fluence falls exponentially away from southeast Asia, the fluence dropping a factor of 2 in ~400 km. In southeast Asia the trend merges with a roughly estimated mass fluence of ~1.1 g/cm**2 inferred from evidence of a melt sheet in northeast Thailand. Integration of the inferred distribution yields a total mass of Australasian tektites of 3.2x10**16 g, much higher than previous estimates. Assuming a similar fallout distribution for the impactor and a chondritic composition allows us to calculate its mass to be 1.5x10**15 g, about 3 orders of magnitude smaller than the minimum mass of the impactor responsible for the extinctions at the end of the Cretaceous.