Occurrence and chemistry of microtektites of ODP Site 121-758

The Toba lake event, the Australasian microtektite event, and the Cretaceous/Paleogene boundary were analyzed on the basis of foraminifers, carbonate content, trace elements, and spherules (microtektites). The Toba ash event, recovered in Hole 758C, may have had minor influences on the foraminiferal populations. The Australasian tektite event has probably some influence on foraminiferal ecology, because the larger specimens become scarce just above the microtektite layer. Microtektites recovered from Hole 758B closely resemble spherules recovered from several Cretaceous/Paleogene boundary localities in North America. The Cretaceous/Paleogene spherules, however, are usually larger and are completely altered to goyazite in the terrestrial environment and to smectite in a marine environment. The Cretaceous/Paleogene boundary of Hole 752B does not show obvious anomalous trace-element concentrations, and iridium concentrations are below our detection limits. The trace-element pattern is dominated by the alternation of chalk with volcanic ash layers above the Cretaceous/Paleogene boundary.

Chemistry of altered basaltic glass shards and zeolites of volcaniclastic sediments from ODP Hole 157-953C

Trace element concentrations of altered basaltic glass shards (layer silicates) and zeolites in volcaniclastic sediments drilled in the volcanic apron northeast of Gran Canaria during Ocean Drilling Program (ODP) leg 157 document variable element mobilities during low-temperature alteration processes in a marine environment. Clay minerals (saponite, montmorillonite, smectite) replacing volcanic glass particles are enriched in transition metals and rare earth elements (REE). The degree of retention of REE within the alteration products of the basaltic glass is correlated with the field strength of the cations. The high field-strength elements are preferentially retained or enriched in the alteration products by sorption through clay minerals. Most trace elements are enriched in a boundary layer close to the interface mineral-altered glass. This boundary layer has a key function for the physico-chemical conditions of the subsequent alteration process by providing a large reactive surface and by lowering the fluid permeability. The release of most elements is buffered by incorporation into secondary precipitates (sodium-rich zeolites, phillipsite, Fe- and Mn-oxides) as shown by calculated distribution coefficients between altered glasses and authigenic minerals. Chemical fluxes change from an open to a closed system behavior during prograde low-temperature alteration of volcaniclastic sediments with no significant trace metal flux from the sediment to the water column.

Interstitial water chemistry at DSDP Leg 74 Holes

Interstitial water studies from sites drilled during a transect of the Walvis Ridge indicate that concentration increases in calcium and decreases in magnesium toward and into the basement. These trends can be understood principally in terms of reactions taking place in Layer 2 of the oceanic crust. At Site 525, however, some removal of magnesium occurs within the sediment column. Concentration maxima of dissolved strontium clearly indicate that carbonate recrystallization occurs throughout the carbonate sediments, and studies of the Sr/Ca ratio in carbonates indicate that in chalks and limestones recrystallization is essentially complete. Predictions of dissolved strontium maxima generally fail; this can be understood as removal of strontium in basal sediments and/or basalts.

Pore water chemistry of sediment core GeoB15101-7 and water chemistry of CTD casts GeoB15101-1 and GeoB15101-6

Submarine brine lakes feature sharp and persistent concentration gradients between seawater and brine, though these should be smoothed out by free diffusion in open ocean settings. The anoxic Urania basin of the Eastern Mediterranean contains an ultra sulfidic, hypersaline brine of Messinian origin above a thick layer of suspended sediments. With a dual modeling approach we reconstruct its contemporary stratification by geochemical solute transport fundamentals, and show that thermal convection is required to maintain mixing in the brine and mud layer. The origin of the Urania basin stratification was dated to 1650 years before present, which may be linked to a major earthquake in the region. The persistence of the chemoclines may be key to the development of diverse and specialized microbial communities. Ongoing thermal convection in the fluid mud layer may have important, yet unresolved consequences for sedimentological and geochemical processes, also in similar environments.

Water chemistry, and abundance and carbon biomass of under-ice fauna during POLARSTERN cruise ARK-XIX/1 to the Storfjord area in 2003

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7-10/10) prevailed and ice thickness ranged over <0.1-1.6 m covered by <0.1-0.6 m of snow. Air temperatures ranged between -1.8 and -27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0-5 m depth) were not stratified (T=-1.9 to -2.0°C and S=34.2-34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 µg chlorophyll a/l), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2-241.3 and 5.3-16.4 µg/l, respectively, the C/N ratio over 11.2-15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0-1.8 ind/m**2 for Apherusa glacialis, 0-0.7 ind/m**2 for Onisimus spp., and 0-0.8 ind/m**2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181-2,487 ind/m**3 (biomass: 70-2,439 µg C/m**3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34-1,485 ind/m**3), contributing 19-65% to total abundances, followed by copepod nauplii (85-548 ind/m**3) and the cyclopoid copepod Oithona similis (44-262 ind/m**3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.

Chemistry of peridotites from ODP Hole 109-670A

Two types of serpentinized peridotites from Hole 670A of Leg 109 were studied in detail. A small piece of relatively unaltered sample, 109-670A-9R-1, #3 (22-24 cm), is olivine websterite characterized by aluminous chromian spinel with Cr/(Cr + Al) ratio of about 0.2. The other minerals have compositions essentially identical with those in more commonly observed serpentinized harzburgite like 109-670A-9R-01, #12 (94-97 cm). The occurrence of pyroxene-rich peridotite with normal harzburgite suggests that small scale heterogeneity in modal compositions exists in the upper mantle beneath the Mid-Atlantic Ridge. Low Cr/Al ratios of spinel and pyroxenes of those peridotites indicate that they are relatively less refractory among peridotites ever recovered from the oceanic region. Textures and the estimated equilibration temperatures indicate that peridotites recovered from Hole 670A are recrystallized and reequilibrated at subsolidus temperature. The occurrence of serpentinized peridotites from the rift valley of the active mid-oceanic ridge may suggest that they represent direct exposure of upwelling mantle materials rather than serpentine diapirs.