Geochemistry of ODP Sites 123-765 and 123-766

The sediments of the Argo and Gascoyne abyssal plains are generally lean in organic matter, are immature, and contain hydrocarbons trapped during sediment deposition rather than those generated during sediment catagenesis. TOC concentrations in the Argo Abyssal Plain Cenozoic sediments are 0.5 wt%, and organic matter appears to be from mixed marine and reworked, degraded, organic matter sources, with the latter being contributed by turbidity flows from the nearby continental margin. TOC concentrations within the Cenozoic sediments of the Gascoyne Abyssal Plain are mostly undetectable (<0.1 wt%). Biomarker distributions determined by gas chromatography (GC) and gas chromatography-mass spectrometry (GCMS) indicate that organic matter extracted from the Lower Cretaceous sediments from both sites is predominantly marine with varying contributions from terrestrial organic matter. The specific marine biomarker, 24-n-propylcholestane is in relatively high abundance in all samples. In addition, the relatively high abundance of the 4-methylsteranes with the 23,24-dimethyl side chain (in all samples) indicates significant dinoflagellate contributions and marine organic matter. The ratios of n-C27/n-C17 reflect relative contributions of marine vs. terrestrial organic matter. TOC, while generally low at Argo, is relatively high near the Barremian/Aptian boundary (one sample has a TOC of 5.1 wt%) and the Aptian/Albian boundary (up to 1.3 wt% TOC), and two samples from the Barremian and Aptian sections contain relatively high proportions of terrestrial organic carbon. TOC values in the Lower Cretaceous sediments from Gascoyne Abyssal Plain are low (<0.1 wt%) near the Aptian/Barremian boundary. TOC values are higher in older sediments, with maxima in the upper Barremian (1.02 wt%), the Barremian/Hauterivian (0.6 wt%), and Valanginian (1.8 wt%). Sediments from the upper Barremian contain higher amounts of terrestrial organic carbon than older sediments.

(Table 1) Color, Fe-C-S chemistry and carbonate content from ODP Sites 172-1062 and 172-1063

Reflectance spectra collected during ODP Leg 172 were used in concert with solid phase iron chemistry, carbonate content, and organic carbon content measurements to evaluate the agents responsible for setting the color in sediments. Factor analysis has proved a valuable and rapid technique to detect the local and regional primary factors that influence sediment color. On the western North Atlantic drifts, sediment color is the result of primary mineralogy as well as diagenetic changes. Sediment lightness is controlled by the carbonate content while the hue is primarily due to the presence of hematite and Fe2+/Fe3+ changes in clay minerals. Hematite, most likely derived from the Permo-Carboniferous red beds of the Canadian Maritimes, is differentially preserved at various sites due to differences in reductive diagenesis and dilution by other sedimentary components. Various intensities for diagenesis result from changes in organic carbon content, sedimentation rates, and H2S production via anaerobic methane oxidation. Iron monosulfides occur extensively at all high sedimentation sites especially in glacial periods suggesting increased high terrigenous flux and/or increased reactive iron flux in glacials.

Carbonate mineralogical and isotopic analyses of TV-guided grab samples and TV-guided box corer samples from the Shumagin area

Methane seepage leads to Mg-calcite and aragonite precipitation at a depth of 4,850 m on the Aleutian accretionary margin. Stromatolitic and oncoid growth structures imply encrustation of microorganisms (microbial mats) in the host sediment with a unique growth direction downward into the sediment, forming crust-shaped lithologies. Biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol, which contain extremely low d13C values. This indicates the presence of methane-consuming archaea, and d13C values of -42 to -51 per mill PDB indicate that methane is the carbon source for the carbonate crusts. Thus, it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate-reducing bacteria and that carbonate precipitation proceeds downward into the sediment, where ascending cold fluids provide a methane source. Strontium and oxygen isotope analyses as well as 14C ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.

Late Neogene carbonate sedimentation and stable isotope record of ODP Site 121-758

The evolution of oceanic and climatic conditions the northeast Indian Ocean during the last 7 m.y. is revealed in the sediments from Site 758. We present detailed and continuous records of d18O and d13C from planktonic foraminifers, weight percent calcium carbonate, weight percent coarse fraction, magnetic susceptibility, and geomagnetic reversals. Sample spacing of the records ranges from 3 to 10 cm and is equivalent to an average time interval of 2000 to 6000 yr. Despite the fact that core recovery ranged between 100% and 105%, recovery gaps as large as 2.7 m occurred at nearly every break between advanced hydraulic piston cores. Approximately 12% of the late Neogene sequence was not recovered in each of the two holes drilled at Site 758. To circumvent the discontinuity introduced by the gaps, a composite depth section was constructed from multiple cores taken from offset holes at Site 758. The resulting composite depth section extends continuously from 0 to 116 mbsf, from the Holocene to the upper Miocene. A detailed chronostratigraphy is based on geomagnetic reversals which extend from the Brunhes Chron to Chron 6, and on d18O stages 1 through 105, which span from 0 to 2.5 Ma. The d18O record is dominated by a ~40-k.y. cycle in the late Pliocene and early Pleistocene, and is followed by a change to a ~100-k.y. cycle in the late Pleistocene. The mid-Pleistocene transition between these two modes of variability occurs between d18O stages 25 and 22 (between 860 and 800 Ka). Thirteen major volcanic ash horizons from the Indonesian arc are observed throughout the sedimentary section and are dated by their relative position within the geomagnetic reversals and the d18O chronostratigraphy. Since 5 Ma, there has been a long-term decline in weight percent CaCO3 and CaCO3 mass accumulation rates, and an associated rise in non-CaCO3 mass accumulation rates. We attribute these changes to a decrease in CaCO3 productivity and an increase in terrigenous sedimentation through enhanced riverine input. Such input may be linked to rapid tectonic uplift of the Himalayas and the Tibetan Plateau via mechanisms such as the intensification of the monsoonal rains, increased fluvial erosion, and regional glaciation. The long-term increase in percent coarse fraction since 5 Ma suggests a gradual increase in CaCO3 preservation. Higher frequency fluctuations in CaCO3 preservation are superimposed on the long-term trend and are related to climate fluctuations. The abrupt drop (-50%) in CaCO3 accumulation at 3.4 Ma signals a dramatic decrease in CaCO3 production that occurred over much of the Indian Ocean.

(Table 1) Boron, carbon, magnesium and calcium isotope data, post-Marinoan Neoproterozoic carbonate sections, Namibia

A marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger Cryogenian glaciation have been inferred from boron isotope measurements. Calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time. Data from Neoproterozoic sections in Namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate-dominated one. Combined with the effects of primary dolomite formation in the cap dolostones, this caused the ocean to depart from a state of acidification and return to higher pH after climatic amelioration. Differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern Congo craton shelf point to local influences modifying and amplifying the global signal, which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux.