Chemical and isotopic compositions of Archean magmatic and metasedimentary rocks and minerals from the Podolia domain, Ukrainian Shield

Zircons from the oldest magmatic and metasedimentary rocks in the Podolia domain of the Ukrainian shield were studied and dated by the U-Pb method on a NORDSIM secondary-ion mass spectrometer. Age of zircon cores in enderbite gneisses sampled in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of terrigenous zircons in quartzites from the Odessa quarry as well as in garnet gneisses from the Zaval'e graphite quarry have age within 3650-3750 Ma. Zircon rims record two metamorphic events around 2750-2850 Ma and 1900-2000 Ma. Extremely low U content in zircons of the second age group indicates conditions of the granulite facies metamorphism in Paleoproterozoic within the Podolia domain. Measured data on orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest existence of the ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of formation and geochemical features of the primordial crust.

Chemical and isotopic compositions of altered MORB from ODP Holes 187-1154A, 187-1155B, and 187-1160B

Boron and Pb isotopic compositions together with B-U-Th-Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4-0.6 ppm B. Similarly the d11B values range from 5.5? to 15.9? in the altered basalts and require interaction with a d11B enriched fluid similar to seawater ~39.5? and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and 87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, d11B shows the largest isotopic shift to +16? in the least altered basalts, followed by a continual decrease to +5-6? in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy d11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of 238U/204Pb and 206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U-Pb isotope system confirms that some alteration events occurred early in the 21.5 Ma history of these rocks, even though a significant second pulse of alteration happened at ~12 Ma after formation of the crust. The U-Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust.

Sulphate concentrations and sulphur isotopic composition in pore fluids from ODP Leg 168 sites

A numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid SO4**2- and d34S data from four sites drilled during Ocean Drilling Program (ODP) Leg 168 in the Cascadia Basin at 48°N, where basement temperatures reach up to 62°C. There is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow, which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures. Pore fluid d34S data show the highest values (135 per mil) yet found in the marine environment. The bacterial sulfur isotopic fractionation factor, a, is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and Rayleigh fractionation plots yield erroneous values for a by as much as 15 per mil in diffusive and advective pore fluid regimes. Model results are consistent with a = 1.077+/-0.007 with no temperature effect over the range 1.8 to 62°C and no effect of sulfate reduction rate over the range 2 to 10 pmol/ccm/day. The reason for this large isotopic fractionation is unknown, but one difference with previous studies is the very low sulfate reduction rates recorded, about two orders of magnitude lower than literature values that are in the range of µmol/ccm/day to tens of nmol/ccm/day. In general, the greatest 34S depletions are associated with the lowest sulfate reduction rates and vice versa, and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates.

Carbon isotopic record of CO2 from the Holocene of the Dome C ice core

Reconstructions of atmospheric CO2 concentrations based on Antarctic ice cores reveal significant changes during the Holocene epoch, but the processes responsible for these changes in CO2 concentrations have not been unambiguously identified. Distinct characteristics in the carbon isotope signatures of the major carbon reservoirs (ocean, biosphere, sediments and atmosphere) constrain variations in the CO2 fluxes between those reservoirs. Here we present a highly resolved atmospheric d13C record for the past 11,000 years from measurements on atmospheric CO2 trapped in an Antarctic ice core. From mass-balance inverse model calculations performed with a simplified carbon cycle model, we show that the decrease in atmospheric CO2 of about 5 parts per million by volume (p.p.m.v.) and the increase in d13C of about 0.25% during the early Holocene is most probably the result of a combination of carbon uptake of about 290 gigatonnes of carbon by the land biosphere and carbon release from the ocean in response to carbonate compensation of the terrestrial uptake during the termination of the last ice age. The 20 p.p.m.v. increase of atmospheric CO2 and the small decrease in d13C of about 0.05% during the later Holocene can mostly be explained by contributions from carbonate compensation of earlier land-biosphere uptake and coral reef formation, with only a minor contribution from a small decrease of the land-biosphere carbon inventory.