Sulfur isotope variability in biogenic pyrite: Reflections of heterogeneous bacterial colonization?
- Lawrence Livermore National Lab., CA (United States). Inst. for Geophysics and Planetary Physics
- Oak Ridge National Lab., TN (United States). Chemical and Analytical Sciences Div.
- Monterey Bay Aquarium Research Inst., Moss Landing, CA (United States)
- Univ. of California, Santa Cruz, CA (United States). Dept. of Earth Sciences
The top 20 cm of sediments at active cold seeps in Monterey Bay, coastal California, contain framboidal pyrite that occurs as infillings and pseudomorphs of the chambers of the tests of foraminifera and rarely as irregularly shaped grains. Sulfur isotope compositions obtained with the ion microprobe show depletions in {sup 34}S and large variations both within and among these pyrite grains. Backscattered-electron imaging reveals three types of pyrite: isolated framboids in a porous aggregation, agglomerated framboids with cementing interstitial pyrite, and recrystallized pyrite with isolated relicts of framboids. In individual grains, RF-pyrite cores grade into F+I-pyrite toward grain rims, and F+I-pyrite grades into PF-pyrite at the grain edges. These textures are consistent with a paragenetic sequence whereby framboids first agglomerate (PF-pyrite), then cement (F+I-pyrite), and finally recrystallize (RF-pyrite). The {delta}{sup 34}S values of RF-pyrite are generally lower than that of F+I-pyrite; if the paragenetic sequence is correct, then this trend parallels the regular core-rim isotopic zoning observed in some grains. The implied increase in {delta}{sup 14}S with time is consistent with Rayleigh fractionation of sulfur in a closed system. Bacteria are intimately involved in the production of pyrite from the samples, and heterogeneous colonization by bacteria provides a simple explanation for the sulfur isotope heterogeneity among and within grains: The foraminifera provide open space for colonization and local nutrients for bacterial growth, whereas the cell walls of the bacteria may provide a local nucleation site for sulfides.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-48; AC05-96OR22464
- OSTI ID:
- 329216
- Journal Information:
- American Mineralogist, Journal Name: American Mineralogist Journal Issue: 11-12Pt2 Vol. 83; ISSN AMMIAY; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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