Sulfur isotope variability in biogenic pyrite: Reflections of heterogeneous bacterial colonization?

The top 20 cm of sediments at active cold seeps in Monterey Bay, coastal Ca lifornia, contain framboidal pyrite that occurs as infillings and pseudomor phs 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 S-34 (delta(34)S = -41 to -5 parts per thousand, CDT), and large variations both within and among these pyrite grains. Intergranul ar differences in delta(34)S values in the same sediment are as large as 35 parts per thousand, and intragranular zoning reaches 15 parts per thousand . Zoning is regular in some grains, with systematic isotope changes from co re to rim or from one foraminiferal chamber to another, but irregular in ot hers. The regular zoning is consistent with an increase in S-34 through tim e. Backscattered-electron imaging reveals three types of pyrite: isolated f ramboids in a porous aggregation ("PF-pyrite"), agglomerated framboids with cementing interstitial pyrite ("F+I-pyrite"), and recrystallized pyrite wi th isolated relicts of framboids ("RF-pyrite"). In individual grains, RF-py rite cores grade into F+I-pyrite toward grain rims, and F+I-pyrite grades i nto PF-pyrite at the grain edges. These textures are consistent with a para genetic sequence whereby framboids first agglomerate (PF-pyrite), then ceme nt (F+I-pyrite), and finally recrystallize (RF-pyrite). The delta(34)S valu es of RF-pyrite are generally lower than that of F+I-pyrite; if the paragen etic sequence is correct, then this trend parallels the regular core-rim is otopic zoning observed in some grains. The implied increase in delta(34)S W ith time is consistent with Rayleigh fractionation of sulfur in a closed sy stem. Bacteria are intimately involved in the production of pyrite from our samples, and heterogeneous colonization by bacteria provides a simple expl anation for the sulfur isotope heterogeneity among and within grains: The f oraminifera provide open space for colonization and local nutrients for bac terial growth, whereas the cell walls of the bacteria may provide a local n ucleation site for sulfides. If so, then initial colonization is reflected in lower delta(34)S values, whereas later bacterial emigration to other for aminifera chambers is indicated by higher delta(34)S values.