Rates of C, N, P and Si recycling and denitrification at the US Mid-Atlantic continental slope depocenter

Abstract

Benthic fluxes of O 2, titration alkalinity (TA), total inorganic carbon (TIC), Ca 2+, NO 3 −, NH 4 +, PO 4 3−, and Si(OH) 4 were measured by in situ benthic flux chamber incubations at 13 locations on the North Carolina continental slope. The majority of measurements were made at water depths of approximately 700–850 m, in the previously identified upper slope depocenter. This region is characterized by extremely high organic matter deposition rates and near saturation bottom water oxygen concentrations. Measured benthic fluxes of TA are reasonably correlated with O 2 benthic fluxes. Because bottom waters are supersaturated with respect to calcite and aragonite at these shallow water depths, these results demonstrate the importance of metabolically driven dissolution in this region. Subtraction of the calcium carbonate dissolution contributions from the TIC benthic fluxes suggests rates of organic matter remineralization ranging from 0.97 to 3.9 mol C m −2 yr −1 at the depocenter sites, a factor of 3–10 greater than estimated for the adjacent continental rise and upper slope areas. Because biological primary production in the overlying waters does not follow this pattern, these extremely high values are most likely supported by lateral inputs of highly reactive organic matter. Mass balance calculations indicate that despite the oxygenated bottom water conditions, 68% of the organic nitrogen released during organic matter remineralization processes is ultimately denitrified. The release of PO 4 3− from the depocenter sediments is equivalent to or larger than that predicted from the remineralization of Redfield organic matter. This implies either that PO 4 3− is preferentially released in this setting and that the accumulating sediments must be depleted in PO 4 3− relative to organic carbon or that another, non-organic, phase is contributing PO 4 3− to the system. The molar ratio of the Si benthic flux and organic carbon remineralization rate ranges from 0.30 to 0.86. This is significantly greater than the ratio reported for most pelagic diatoms. Possible reasons for this high ratio include the deposition of benthic diatoms that may have a larger Si : C ratio than pelagic diatoms, the near-bottom lateral input of partially reworked organic matter that may have an elevated Si : C ratio relative to fresh diatoms, preferential loss of carbon in sinking particulates or the release of Si from non-opaline materials.