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Project Progress

Research for the Michael A. Arthur Group

Graduate Students: Anthony Riccardi (co-advised with L. R. Kump), Jamey Fulton (co-advised with K. Freeman) and Chris Junium; Associates and Collaborators: Matt Hurtgen and Galen Halverson (Harvard Univ.), Lee R. Kump, Alexander Pavlov (U. Colorado), Erik Cordes (PSU/Harvard, Biosciences)

Our focus has been the chemistry of and biogeochemical cycling in the Proterozoic Ocean (Namibia, S. Australia, Great Basin U.S.) and in modern euxinic basin analogs (Black Sea, Fayetteville-Green Lake).

At present it is not known what sustains organic-carbon burial for long-lasting black shale depositional events. In many cases black shales are characterized by orgC/Ptot and orgC/Ntot ratios much higher than Redfield stoichiometry. This suggests preferential release of phosphate and nitrogen from particles in the water column or during early diagenesis as a result of anaerobic degradation of organic matter and reduction of metal oxide particles. Recent research suggests that a substantial proportion of what has typically been considered “organic P” is sorbed to metal oxide coatings on cell walls and, therefore, is easily liberated in dysaerobic to anaerobic conditions. Such rapid recycling of P, in particular, could be a strong positive feedback to productivity in surface waters of anoxic basins. However, availability of N is limited because of N consumption during denitrification or annamox reactions in the shallow chemocline of anoxic basins. Thus, without further addition of N, productivity is limited. However, N isotope and limited biomarker data from some black shales seems to indicate that N fixation becomes a dominant mode of organic matter production because of the enhanced P availability and N limitation. However, we are also examining N cycling within euxinic basins (Black Sea and Fayetteville-Green Lake) and the possibility of direct utilization of ammonia as an explanation for ¹⁵N-depleted signatures in organic matter. Fulton and Junium are preparing manuscripts now on nitrogen cycling in the Holocene Black Sea and nitrogen isotopic signatures of Oceanic Anoxic Events, respectively. Junium is pursuing the sedimentology and geochemistry of the Proterozoic Chuar group in this regard. With Kate Freeman we have begun to develop methods for direct analysis of geoporphyrin compounds as a tool to examine possible organism-specific signals as well as to avoid possible diagenetic overprints in ancient sediments.

We have also been examining sulfur cycling in modern and ancient euxinic basins. With Matt Hurtgen and other collaborators (Hurtgen et al., 2004; 2005) we have examined the possibility that the largely euxinic Neoproterozoic ocean was relatively sulfate depleted and that “Snowball Earth” events may have been triggered by sulfate depletion. In addition, we (Kump, Pavlov, and Arthur, 2005) have proposed that CUE (chemocline upward excursion) events may release toxic amounts of sulfide causing extinctions, for example associated with the Permo-Triassic Boundary (PTB). Tony Riccardi is studying PTB sequences in China and Turkey to test this hypothesis.

As an interesting aside we have studied sulfur cycling by tubeworms associated with Gulf of Mexico “cold seeps” and have hypothesized that a sulfide-oxidation – sulfate-recycling loop to the “roots” of tubeworm aggregations coupled with hydrocarbons supplied at seeps fuels continuous bacterial sulfate reduction in sediments and allows rapid and prolonged growth of tubeworm colonies (Cordes et al., 2005).