Ikaite of sediment cores from the Zaire deep-sea fan

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite delta13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate-methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, [SO4]2- and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.

Calcium carbonate and trace element composition of ODP Leg 182 sites

An intensive geochemical investigation was conducted on carbonate sediments recovered during Ocean Drilling Program Leg 182. Four trace elements in 635 sediment samples from Sites 1126-1132 on the Great Australian Bight were examined by atomic absorption spectrometry on the acid-soluble fraction. Downhole profiles of these elements exhibit complicated fluctuations throughout the late Eocene to Pleistocene, principally because of the variations in the acid-soluble fraction. The purpose of this study is to present initial results on the geochemical composition of Cenozoic cool-water carbonates as a basis for a future detailed investigation to determine the paleoenvironment of a carbonate-dominated continental margin during the evolution of the Southern Ocean.

Mineral content, major oxides and trace elements concentrations in clay from different IODP Holes of 331-C0013, 331-C0017, East China sea shelf and different Taiwan rivers

The Okinawa Trough (OT) in the East Asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic-hydrothermal activities. In this study, the clays collected during IODP Expedition 331 to the middle OT (Iheya North Knoll) were analyzed for mineralogical and geochemical compositions. By comparing with the clays from the East China Sea shelf and surrounding rivers, we examine different clay origins. The hydrothermal field in the mid-OT is dominated by Mg-rich chlorite, while the recharge zone has clay mineral assemblages similar to the shelf and rivers, showing high content of illite, subordinate chlorite and kaolinite and scarce smectite. Compared to the terrigenous clays, the hydrothermal clays in the OT have high concentrations of Mg, Mn and Zr but low Fe, Na, K, Ca, Ba, Sr, P, Sc and Ti, while the hydrothermal clays in the mid-ocean ridge are relatively enriched in Fe and V and depleted in Al, Mg, Zr, Sc and Ti. Different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays, diagnostic of variable clay origins. We infer that the OT hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids. The remarkably different compositions of hydrothermal clays between the sediment-rich back arc basin like OT and the sediment-starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid-rock/sediment reactions under various geologic settings.

Geochemistry of ODP Site 202-1233 and river sediments

Bulk sediment chemistry from three Chilean continental margin Ocean Drilling Program sites constrains regional continental erosion over the past 30,000 years. Sediments from thirteen rivers that drain the (mostly igneous) Andes and the (mostly metamorphic) Coast Range, along with existing rock chemistry datasets, define terrestrial provenance for the continental margin sediments. Andean river sediments have high Mg/Al relative to Coast-Range river sediments. Near 36°S, marine sediments have high-Mg/Al (i.e. more Andean) sources during the last glacial period, and lower-Mg/Al (less Andean) sources during the Holocene. Near 41°S a Ti-rich source, likely from coast-range igneous intrusions, is prevalent during Holocene time, whereas high-Mg/Al Andean sources are more prevalent during the last glacial period. We infer that there is a dominant ice-sheet control of sediment sources. At 36°S, Andean-sourced sediment decreased as Andean mountain glaciers retreated after ~17.6 ka, coincident with local oceanic warming and southward retreat of the Patagonian Forest and, by inference, westerly winds. At 41°S Andean sediment dominance peaks and then rapidly declines at ~19 ka, coincident with local oceanic warming and the earliest deglacial sea-level rise. We hypothesize that this decreased flux of Andean material in the south is related to rapid retreat of the marine-based portion of the Patagonian Ice Sheet in response to global sea-level rise, as the resulting flooding of the southern portion of the Central Valley created a sink for Andean sediments in this region. Reversal of the decreasing deglacial Mg/Al trend at 41°S from 14.5 to 13.0 ka is consistent with a brief re-advance of the Patagonian ice sheet coincident with the Antarctic Cold Reversal.