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Open Access Publications from the University of California

Biogeochemical Cycling in Costa Rica Margin Sediments as Recorded in Stable Sulfur Isotopes

  • Author(s): Gott, Caroline Susanne
  • Advisor(s): Lyons, Timothy
  • et al.

Complex sedimentary and tectonic behavior of the Costa Rica margin has a poorly understood impact on biogeochemical cycles. Although it is known that continental margins feature high rates of sulfur reduction and fluid flow within sediments, the biogeochemical limits of such environments is not well known. In this research, we focus on sulfur geochemistry from sediments collected during the IODP Expedition 344 at Holes U1381C, U1413B and U1414A. This research’s goal is to investigate this dynamic system’s biogeochemical processes, particularly the sulfur cycle, and how these processes are reflected in the geologic record.

In the sediments investigated, non steady-state conditions were suggested by several factors. Hole U1381C features indications of microbial reduction of sulfate, with depleted sulfate concentrations and heavy sulfate isotope fractionation (δ34S >+60 ‰). However, the product of this reaction, H2S, is notably absent from the pore waters. Hole U1413B’s deposits are characterized by a shallow (~15mbsf) sulfate-methane transition zone (SMT) where released hydrogen sulfide reacts with reactive iron to form iron sulfides. At Hole U1414A pore water sulfate is present at several hundreds of meters sediment depth, while the concentration of hydrogen sulfide is low (<4 µM). Sequential extractions of iron oxides reveal the presence of reactive iron phases in low concentrations (< 1.1 wt.%), indicating ongoing alteration of iron oxides. The presence of reactive iron minerals in the deep sediments at Hole U1414A has implications for the biosphere – as microbes could still utilize those minerals.

The cause of such extreme pore water sulfate fractionation may be biotic, although previous research has not noted such high values as those found in Hole U1414A ((δ34S >+140 ‰). Using chemical profiles such as those of total organic and inorganic carbon and iron compound, we attempt to determine the source of fractionation. In addition, we use Rayleigh modeling to find the fractionation factor of the most isotopically heavy sediments to compare to known values for various causes of sulfate fractionation.

The unique biogeochemistry of these sediments can provide insight to the processes found in continental margins globally and provide clues to characterize such regions in more ancient sediments.

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