August 2009 Geology and GSA Today media ...(mestone sequence from the Precambrian-Ca...)

Date:7/31/2009

mestone sequence from the Precambrian-Cambrian boundary in southwest Namibia. Surprisingly, this 10 million year sequence of limestones contains sulfur isotopes in pyrite that are consistently heavier than the sulfur isotopes in co-deposited sulfate -- which is the inverse of the normally observed trend resulting from bacterial sulfate reduction. This is the first documented occurrence of sulfide that is consistently isotopically heavier than coeval sulfate, and challenges our entire paradigm for interpreting the geologic record of sulfur isotopes. Ries et al. interpret this inverted isotopic offset as evidence for extremely low sulfate concentrations at the Precambrian-Cambrian boundary, most likely driven by very low levels of atmospheric oxygen. Apparently, these low levels of atmospheric oxygen persisted tens of millions of years beyond the origin of animals -- much later than previously reported -- but probably rose prior to the major diversification of animals in Early Cambrian time.

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Pore-fluid Fe isotopes reflect the extent of benthic Fe redox recycling: Evidence from continental shelf and deep-sea sediments
W.B. Homoky et al., National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK. Pages 751-754.

New research by Homoky et al. highlights the potential utility of iron isotopes for addressing important questions in ocean and Earth science. Different biological and chemical processes can leave behind characteristic isotopic "fingerprints." Of specific interest here, iron isotopes in sediment pore fluids may be a unique tracer of bacterial processes that contribute to the input of iron to seawater; an important process contributing to natural carbon sequestration in the oceans. The research shows that iron cycling in the deep sea produces a different iron isotopic fingerprint in the pore fluids compared with previous studies on the coastal shelves. This study confirms the unique isotopic finger
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Contact: Christa Stratton cstratton@geosociety.org 303-357-1093 Geological Society of America Source:Eurekalert

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