Sediment and pore water geochemistry of sediment cores, Potter Cove, King George Island

Redox-sensitive trace metals (Mn, Fe, U, Mo, Re), nutrients and terminal metabolic products (NO3-, NH4+, PO43-, total alkalinity) were for the first time investigated in pore waters of Antarctic coastal sediments. The results of this study reveal a high spatial variability in redox conditions in surface sediments from Potter Cove, King George Island, western Antarctic Peninsula. Particularly in the shallower areas of the bay the significant correlation between sulphate depletion and total alkalinity, the inorganic product of terminal metabolism, indicates sulphate reduction to be the major pathway of organic matter mineralisation. In contrast, dissimilatory metal oxide reduction seems to be prevailing in the newly ice-free areas and the deeper troughs, where concentrations of dissolved iron of up to 700 µM were found. We suggest that the increased accumulation of fine-grained material with high amounts of reducible metal oxides in combination with the reduced availability of metabolisable organic matter and enhanced physical and biological disturbance by bottom water currents, ice scouring and burrowing organisms favours metal oxide reduction over sulphate reduction in these areas. Based on modelled iron fluxes we calculate the contribution of the Antarctic shelf to the pool of potentially bioavailable iron (Feb) to be 6.9x10**3 to 790x10**3 t/yr. Consequently, these shelf sediments would provide an Feb flux of 0.35-39.5/mg/m**2/yr (median: 3.8 mg/m**2/yr) to the Southern Ocean. This contribution is in the same order of magnitude as the flux provided by icebergs and significantly higher than the input by aeolian dust. For this reason suboxic shelf sediments form a key source of iron for the high nutrient-low chlorophyll (HNLC) areas of the Southern Ocean. This source may become even more important in the future due to rising temperatures at the WAP accompanied by enhanced glacier retreat and the accumulation of melt water derived iron-rich material on the shelf.

Geochemistry of the oceanic crust from ocean drilling samples

This book presents new data on chemical and mineral compositions and on density of altered and fresh igneous rocks from key DSDP and ODP holes drilled on the following main tectonomagmatic structures of the ocean floor: 1. Mid-ocean ridges and abyssal plains and basins (DSDP Legs 37, 61, 63, 64, 65, 69, 70, 83, and 91 and ODP Legs 106, 111, 123, 129, 137, 139, 140, 148, and 169); 2. Seamounts and guyots (DSDP Legs 19, 55, and 62 and ODP Legs 143 and 144); 3. Intraplate rises (DSDP Legs 26, 33, 51, 52, 53, 72, and 74 and ODP Legs 104, 115, 120, 121, and 183); and 4. Marginal seas (DSDP Legs 19, 59, and 60 and ODP Legs 124, 125, 126, 127, 128, and 135). Study results of altered gabbro from the Southwest Indian Ridge (ODP Leg 118) and serpentinized ultramafic rocks from the Galicia margin (ODP Leg 103) are also presented. Samples were collected by the authors from the DSDP/ODP repositories, as well as during some Glomar Challenger and JOIDES Resolution legs. The book also includes descriptions of thin sections, geochemical diagrams, data on secondary mineral assemblages, and recalculated results of chemical analyses with corrections for rock density. Atomic content of each element can be quantified in grams per standard volume (g/1000 cm**3). The suite of results can be used to estimate mass balance, but parts of the data need additional work, which depends on locating fresh analogs of altered rocks studied here. Results of quantitative estimation of element mobility in recovered sections of the upper oceanic crust as a whole are shown for certain cases: Hole 504B (Costa Rica Rift) and Holes 856H, 857C, and 857D (Middle Valley, Juan de Fuca Ridge).

Major element oxides of minerals from a lava pond at ODP Site 206-1256

At Ocean Drilling Program Site 1256 (6°44.2'N, 91°56.1'W), during Leg 206, a thick massive unit was cored in two neighboring penetrations of the uppermost basement, Holes 1256C and 1256D. This thick massive lava flow, commonly referred to as the "Lava Pond," is identified as Unit 18 (>30 m thick) in Hole 1256C and Unit 1 (>74.2 m thick) in Hole 1256D (Wilson et al., 2003, doi:10.2973/odp.proc.ir.206.2003). In the coarse-grained basalt that comprises this lithological unit, low-temperature "background" alteration events are present. This report provides microprobe analyses of both primary and secondary minerals present in this massive lava pond. The analyses of typically magmatic minerals (titanomagnetite, plagioclase, and clinopyroxene) are given for comparison with secondary minerals.

Chemical composition of ferromanganese micronodules and bulk sediments from ODP Hole 199-1216A

Distribution, size, mineral, and chemical compositions of ferromanganese micronodules (FMMNs) and chemical composition of host sediments were examined in a series of red clay samples with ages from Eocene to the present at Ocean Drilling Program Leg 199, Site 1216, south of the Molokai Fracture Zone in the Central Pacific Basin. The number of FMMNs changed drastically throughout the 40-m-long red clay intervals. FMMNs are abundant in the upper 9 m of core, decrease between 9 and 25 meters below seafloor (mbsf) with depth, and are very rare from 30 to 40 mbsf. Chemical composition of FMMNs showed high Mn/Fe ratios and Ni and Cu contents and a distinct positive Ce anomaly because of the existence of buserite. This suggests that FMMNs in the red clay from 25 mbsf to the top of the cored interval were deposited continuously in an oxic diagenetic bottom environment. The red clay below 30 mbsf with higher Mn contents contains few FMMNs but abundant tiny Mn particles within brown silicates coated by Fe (oxy-hydro)oxides. This indicates that the mode of manganese deposition changed between 25 and 30 mbsf.