Foraminiferal abundances and preservation in ODP Hole 130-805C

Downcore changes in various carbonate dissolution indexes are documented for Hole 805C for the last 1.2 m.y. These indexes include degree of fragmentation of planktonic foraminifers, percent sand, abundance ratio of species of contrasting solution susceptibilities (Globigerinoides sacculifer vs. Pulleniatina, Globorotalia tumida, and Globorotalia menardii), and the difference in d18O between species of contrasting solution susceptibilities (G. sacculifer vs. Pulleniatina). These preservation indexes have been combined into a single composite dissolution index that corresponds closely to the d18O record. The rate of change of the oxygen isotope signal is also important, with glacial-to-interglacial transitions corresponding to maximum preservation events and vice versa. For information on changing productivity (which is important because an increased supply of organic matter may enhance dissolution by lowering pH upon degradation), we present the abundance of coarse-fraction benthic foraminifers per gram and the ratio between two planktonic foraminiferal species, one of which is strongly associated with equatorial upwelling (Globorotalia tumida vs. Pulleniatina). Our results suggest that productivity plays a subordinate role in determining foraminifer preservation. Furthermore, our results confirm previous observations that associate enhanced preservation events with glacial periods and with glacial-to-interglacial transitions. A correlation between preservation and sedimentation rates of these carbonate-rich sediments could not be established. Notable differences are present between the responses of individual dissolution indexes, indicating that processes other than dissolution determine proxy indexes to varying degrees.

Susceptibility, magnetization and S-ratio of ODP Sites 201-1225 and 201-1227

Samples were collected at Sites 1225 and 1227 to investigate the occurrence of fine-grained, biogenic magnetic particles (magnetosomes). Several magnetic methods, including anhysteretic remanent magnetization and isothermal remanent magnetization, were used to characterize the main magnetic carriers in the samples. Extracts were made to isolate the fine-grained fraction, which was then examined under a transmission electron microscope. Grains with the unique characteristics of magnetosomes were found in samples from regions in the core with both high and low concentrations of magnetic minerals. This suggests they have the potential to be a persistent proxy of paleoredox conditions.

Geochemistry and degree of organic carbon preservation in non-bioturbated, shaly sediments

Based on 13 published porewater H2S and sulphate profiles the amount of H2S escaping from non-bioturbated shales varies between some few % to 45% of the amount of bacterially generated H2S. This finding permits calculation of the original organic carbon (TOCor) content of immature nonbioturbated shales using TOC and sulphur content data. In two immature non-bioturbated sequences from Hungary (Toarcian and Oligocene) the first-order correlation between HI and TOC/TOCor was found to be stronger than that between HI and TOC, indicating that sulphate reduction was the leading process both in decrease in TOC content and degradation of kerogen source potential.

Aragonite distribution and preservation of sediment cores from the Brazilian Continental Slope

We present late Quaternary records of aragonite preservation determined for sediment cores recovered on the Brazilian Continental Slope (1790-2585 m water depth) where North Atlantic Deep Water (NADW) dominates at present. We have used various indirect dissolution proxies (carbonate content, aragonite/calcite contents, and sand percentages) as well as gastropodal abundances and fragmentation of Limacina inflata to determine the state of aragonite preservation. In addition, microscopic investigations of the dissolution susceptibility of three Limacina species yielded the Limacina Dissolution Index which correlates well with most of the other proxies. Excellent preservation of aragonite was found in the Holocene section, whereas aragonite dissolution gradually increases downcore. This general pattern is attributed to an overall increase in aragonite corrosiveness of pore waters. Overprinted on this early diagenetic trend are high-frequency fluctuations of aragonite preservation, which may be related to climatically induced variations of intermediate water masses.