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

被引:120
作者
Kohn, MJ [1 ]
Riciputi, LR
Stakes, D
Orange, DL
机构
[1] Univ Calif Lawrence Livermore Natl Lab, Inst Geophys & Planetary Phys, Livermore, CA 94550 USA
[2] Oak Ridge Natl Lab, Div Chem & Analyt Sci, Oak Ridge, TN 37831 USA
[3] Monterey Bay Aquarium Res Inst, Moss Landing, CA 95039 USA
[4] Univ Calif Santa Cruz, Dept Earth Sci, Santa Cruz, CA 95062 USA
关键词
D O I
10.2138/am-1998-11-1234
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
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 S-34 (delta(34)S = -41 to -5 parts per thousand, CDT), and large variations both within and among these pyrite grains. Intergranular 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 core to rim or from one foraminiferal chamber to another, but irregular in others. The regular zoning is consistent with an increase in S-34 through time. Backscattered-electron imaging reveals three types of pyrite: isolated framboids in a porous aggregation ("PF-pyrite"), agglomerated framboids with cementing interstitial pyrite ("F+I-pyrite"), and recrystallized pyrite with isolated relicts of framboids ("RF-pyrite"). 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(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(34)S With time is consistent with Rayleigh fractionation of sulfur in a closed system. Bacteria are intimately involved in the production of pyrite from our 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. If so, then initial colonization is reflected in lower delta(34)S values, whereas later bacterial emigration to other foraminifera chambers is indicated by higher delta(34)S values.
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收藏
页码:1454 / 1468
页数:15
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