Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p51a1393r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P51A-1393
Mathematics
Logic
0424 Biosignatures And Proxies, 0454 Isotopic Composition And Chemistry (1041, 4870), 1041 Stable Isotope Geochemistry (0454, 4870), 4845 Nutrients And Nutrient Cycling (0470, 1050), 4870 Stable Isotopes (0454, 1041)
Scientific paper
The oxygen isotope composition of phosphate is a useful indicator of biological P cycling in low to moderate temperature environments, such as those that characterize most of the habitable zone of Earth. In microbially active systems, phosphate oxygen isotope compositions are driven towards a temperature- dependent, thermodynamic equilibrium offset from water. Enzymatic reactions involving organophosphorus compounds, pyrophosphate, and polyphosphates promote the exchange of oxygen atoms between water and phosphate. These enzyme driven reactions are key to the attainment of isotopic equilibrium under conditions in which the rate of inorganic oxygen exchange is slow. We have examined the phosphate oxygen isotope systematics of the Lost City hydrothermal vent system, which is located on a gabbroic and peridotitic massif, 15km off-axis of the Mid Atlantic Ridge. The Lost City hydrothermal system's fluid chemistry and heat budget are controlled by serpentinization reactions. Fluids vent at temperatures up to around 80°C and with a pH around 9-10. Vent mineralogy is dominated by calcite, aragonite, and brucite, with mineral layers intercalated by biofilms. Phosphorus content ranges from 400 - 1000 ppm (by mass as P2O5) in the vent samples we have analyzed. The oxygen isotope composition of phosphate extracted from the vent solids is a few per mil lighter than that of phosphate dissolved in ambient sea water. This oxygen isotope composition reflects exchange of phosphate oxygen with water oxygen at elevated temperature. We show that under a wide range of conditions, abiological reaction rates are too slow to produce these isotopic compositions. This suggests that cycling of the phosphate by the vent system's microbial community has imprinted the phosphate with a stable isotope signature of biological activity. The oxygen isotope composition of lattice-bound phosphate preserves well in the geologic record, commending phosphate oxygen isotope measurements as a tool for the detection of life in ancient terrestrial and in extraterrestrial rocks.
Colman A. S.
Fogel Marilyn L.
Kelley Deborah S.
Robinson Karen L.
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