HR: 17:30h
AN: B54C-07    [Abstracts]
TI: Geochemical Variations in Paleoseeps Over Short Time Scales: Stable Isotope Results From the Panoche-Tumey Hills Paleoseep (PTHP), California
AU: * Sample, J C
EM: James.Sample@nau.edu
AF: Dept. Geology, N. Ariz. Univ., Flagstaff, AZ 86011, United States
AU: Csar, A
EM: ajc98@nau.edu
AF: Dept. Geology, N. Ariz. Univ., Flagstaff, AZ 86011, United States
AU: Schwartz, H
EM: hildes@es.ucsc.edu
AF: Earth & Planetary Sci. Dept., Univ. Calif., Santa Cruz, CA 95064, United States
AB: Fluctuations in fluids compositions over varying time scales are widely recognized from modern cold seep deposits. We present results from a detailed study of carbonate mineral proxies for fluid flow from the PTHP. This cold seep deposit extends for 20 km along strike and was fed by a system of sandstone injectites during its mid- Paleocene lifespan. We sampled carbonate-bearing portions of the seep horizons both laterally and vertically over an area of approximately 5 km². We analyzed the elemental and stable isotopic chemistry to examine temporal variations. Carbonate occurrences include dolomite to low-Mg calcite mineralogies in matrix cements, cemented pipe structures, laminated carbonates (fossilized bacterial mats?), and carbonate veins. In all cases the direction of carbonate mineral growth can be determined. Previous work on bulk samples has yielded a range of δ¹³C of –-54 to +3 ‰ and δ¹⁸O of --7 to +7 ‰ (all values VPDB). Our more detailed sampling shows a smaller range, but significant variation of isotopic results over mm's to cm's (δ¹³C values vary in individual features from as much as –-28.19 to +3.58 ‰; δ¹⁸O values from –-8.68 to +4.71 ‰). Sequential sampling by micromilling of transects across carbonate infill show the greatest range in carbon isotopes. Individual transects show ranges in carbon isotopes minimum and maximum of 4.8 and 29.7 ‰, respectively. Oxygen isotopes vary less – from 4.1 to 10.1 ‰ for different transects. The influence of meteoric diagenesis on the oxygen is difficult to constrain well. Coupled with carbonate and sulfate/sulfide mineralogy, the variations in isotopic compositions can be related to fine-scale variations in microchemical environments, perhaps due to repeated changes in the chemistry of fluids fluxing through the features examined. Given the age range of the seep horizon, and the size of individual carbonate features, we estimate that total cementation times of individual features, and hence fluid flow variation time scales, is at the most on the order of a few thousands of years.
DE: 0460 Marine systems (4800)
DE: 0463 Microbe/mineral interactions
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3004 Gas and hydrate systems
DE: 8104 Continental margins: convergent
SC: Biogeosciences [B]
MN: 2007 Fall Meeting