Geochemistry of surface and groundwaters from lower Tom Basin wetland, western Siberia

New data are reported on the major- and trace-component compositions of acidic and weakly acidic low-concentration wetland waters and other water types. Special attention was given to dissolved organic compounds: fulvic and humic acids, bitumens, and hydrocarbons. The first comprehensive data are presented for organic trace components in the wetland waters of western Siberia: alkanes, pentacyclic terpenoids, steranes, alkylbenzenes, naphthalenes, phenanthrenes, tetraarenes, etc.

Interstitial-water chemistry and stable isotope record of Prydz Bay sediments

Leg 119 of the Ocean Drilling Program (ODP) provided the first opportunity to study the interstitial-water chemistry of the eastern Antarctic continental margin. Five sites were cored in a northwest-southeast transect of Prydz Bay that extended from the top of the continental slope to within 30 km of the coastline. Geological studies of the cores reveal a continental margin that has evolved through terrestrial, glacial, and glacial-marine environments. Chemical and stable isotopic analyses of the interstitial-waters were performed to determine the types of depositional environments and the diagenetic and hydrologic processes that are operating in this unusual marine environment. Highly compacted glacial sediments provide an effective barrier to the vertical diffusion of interstitial-water solutes. Meteoric water from the Antarctic continent appears to be flowing into Prydz Bay sediments through the sequence of terrestrial sediments that lie underneath the glacial sediments. The large amounts of erosion associated with glacial advances appear to have had the effect of limiting the amount of marine organic matter that is incorporated into the sediments on the continental shelf. Although all of the sites cored in Prydz Bay exhibit depletions in dissolved sulfate with increasing depth, the greatest bacterial activity is associated with a thin layer of diatom ooze that coats the seafloor of the inner bay. Results of alkalinity modeling, thermodynamic calculations, and strontium analyses indicate that (1) ocean bottom waters seaward of Site 740 are undersaturated with respect to both calcite and aragonite, (2) interstitial waters at each site become saturated or supersaturated with respect to calcite and aragonite with increasing depth, (3) precipitation of calcium carbonate reduces the alkalinity of the pore waters with increasing depth, and (4) recrystallization of aragonite to calcite accounts for 24% of the pore-water strontium. Weathering of unstable terrestrial debris and cation exchange between clay minerals and pore fluids are the most probable chemical processes affecting interstitial water cation gradients.

Chemical and Sr isotopic compositions of 3 step leachates and residues of bulk samples, zeolites, and smectites from Lower Miocene sediments at ODP Site 166-1007

Massive clinoptilolite authigenesis was observed at about 1105 meters below sea floor (mbsf) in lower Miocene wellcompacted carbonate periplatform sediments from the Great Bahama Bank [Ocean Drilling Program, ODP Leg 166, Site 1007]. The diagenetic assemblage comprises abundant zeolite crystallized within foraminifer tests and sedimentary matrix, as well as Mg smectites. In carbonate-rich deposits, the formation of the zeolite requires a supply of silica. Thus, the objective of the study is to determine the origin of the silica supply, its diagenetic evolution, and consequently the related implications on interpretation of the sedimentary record, in terms of local or global paleoceanographic change. For lack of evidence for any volcaniclastic input or traces of Si-enriched deep fluids circulation, an in situ biogenic source of silica is validated by isotopic data and chemical modeling for the formation of such secondary minerals in shallow-water carbonate sequences. Geochemical and strontium isotopic data clearly establish the marine signature of the diagenetic zeolite, as well as its contemporaneous formation with the carbonate deposition (Sr model ages of 19.6-23.2 Ma). The test of saturation for the pore fluids specifies the equilibrium state of the present mineralogical assemblage. Seawater-rock modeling specifies that clinoptilolite precipitates from the dissolution of biogenic silica, which reacts with clay minerals. The amount of silica (opal-A) involved in the reaction has to be significant enough, at least 10 wt.%, to account for the observed content of clinoptilolite occurring at the most zeolite-rich level. Modeling also shows that the observed amount of clinoptilolite (~19%) reflects an in situ and short-term reaction due to the high reactivity of primary biogenic silica (opal-A) until its complete depletion. The episodic occurrence of these well-lithified zeolite-rich levels is consistent with the occurrence of seismic reflectors, particularly the P2 seismic sequence boundary located at 1115 mbsf depth and dated as 23.2 Ma. The age range of most zeolitic sedimentary levels (biostratigraphic ages of 21.5-22 Ma) correlates well with that of the early Miocene glaciation Mi-1 and Mi-1a global events. Thus, the clinoptilolite occurrence in the shallow carbonate platform environment far from volcanogenic supply, or in other sensitive marine areas, is potentially a significant new proxy for paleoproductivity and oceanic global events, such as the Miocene events, which are usually recognized in deep-sea pelagic sediments and high latitude deposits.

Geochemistry of interstitial waters of Lau Backarc Basin sediment samples

Interstitial water samples from Sites 834 through 839, drilled during Ocean Drilling Program Leg 135 in the backarc Lau basin (Southwestern Pacific), have been analyzed for major elements, manganese, copper, strontium, barium, vanadium, and 87Sr/86Sr isotopic composition values. The concentration-depth profiles of the major chemical components show almost straight concentration gradients at all sites, and seem to reflect slight alteration of volcanic material. However, in the lower part of the sedimentary cover, where volcanogenic material is abundant and where diagenetic minerals occur, systematic decreases in calcium, strontium, manganese, copper, and vanadium concentrations are observed. A downwelling flow of bottom seawater, which affected the diagenetic chemical signature of the interstitial water, is probably responsible for the recorded chemical features. This hypothesis is supported by strontium isotope data obtained from interstitial water samples at Site 835. It is also in accordance with data from heat flow and physical properties.