Survey of porewater geochemistry within deep marine hydrocarbon seep sediments of the Scotian Slope, Canada

Abstract

<p>The ocean floor surface sediments of the Scotian Slope, Nova Scotia are host to a complex network of microbially mediated reactions that knit together the carbon, sulfur, and nitrogen biogeochemical cycles. Limited diffusion between the upper water column and ocean floor muds pore space, coupled with competitive microbial ecological niche partitioning, leads to theformation of biogeochemically controlled redox gradients. The energetics of such gradients are further governed by microbial heterotrophy with the deposition of detrital organic matter that is primarily sourced from terrestrial runoff and upper water column productivity. However, these microbial biogeochemical zones change if surface sediments are impregnated by hydrocarbon seepage that migrates up from deeper within the basin. Porewater profiles of F^-, NO₂^-, NO₃^-, CO₃^2- and SO₄^2- were used to reconstruct biogeochemical stratification depth profiles that can providecomparative evidence for anion behaviour in active cold seep sites. These profiles define microbialmetabolic processes within the sediment subsurface. To test this hypothesis, 50 samples stratigraphically collected across 9 sediment cores were separated, centrifugated and analyzed using ion chromatography. A comparative study between two methods of data analysis was applied to the samples. The internal calibration method of standard addition proved to be a better method than external calibration method of calibration curve to measure porewater anion concentrations of natural samples with complex matrices and a varying range of concentrations. For this reason, porewater anion concentrations were compared using the standard addition method. Fluoride concentration seems uncorrelated to sediment depth and unique sample heterogeneity might be the leading factor to its variation. However, NO₂^-, NO₃^-, and SO₄^2-display decreasing concentrations with increasing depths. Carbonate displays a steady variation in concentration possibly attributing to precipitated aggregates of carbonate rocks in the ocean floor surface sediments not being subject to a massive change. Sulfate concentration decreases dramatically in both ambient and hydrocarbon impacted marine benthic sediments although, in hydrocarbon impacted sites, it appears to occur at a much shallower depth suggesting that the redox gradient is much more pronounced and as much sulfate reduction has not yet transpired with the ambient sediments at the same depth. Nitrate and NO₂^- trends also show similar pronounced reduction patterns occurring at shallower depths for hydrocarbon impacted sediments suggesting widespread increased microbial and bacterial activity in these regions.</p>