Authors:
Wenzhöfer, Frank, Bienhold, Christina, Boetius, Antje
Abstract:
In this study, we measured a range of biogeochemical parameters down the Laptev Sea continental slope, from around 50 to 3400 m water depth. These included chlorophyll pigments, extracellular enzymatic activity, diffusive oxygen uptake and prokaryotic cell abundance. Our aim was to compare the measurements between two years about two decades apart (1993 and 2012). Benthic oxygen consumption rates were determined from ex situ microsensor measurements of diffusive oxygen uptake in retrieved sediment cores, consistent with Boetius & Damm 1998 (doi (...)
In this study, we measured a range of biogeochemical parameters down the Laptev Sea continental slope, from around 50 to 3400 m water depth. These included chlorophyll pigments, extracellular enzymatic activity, diffusive oxygen uptake and prokaryotic cell abundance. Our aim was to compare the measurements between two years about two decades apart (1993 and 2012). Benthic oxygen consumption rates were determined from ex situ microsensor measurements of diffusive oxygen uptake in retrieved sediment cores, consistent with Boetius & Damm 1998 (doi:10.1016/S0967-0637(97)00052-6). Directly after recovery, cores were stored at in situ temperature and the overlying water gently stirred by rotating small magnets to avoid the development of a stagnate water body above the sediment. High-resolution microprofiles across the sediment water interface were measured using Clark-type O2 microelectrodes equipped with a guard cathode (Revsbach 1989 doi:10.4319/lo.1989.34.2.0474) mounted to motorized micromanipulator (Glud et al. 2009 doi:10.4319/lo.2009.54.1.0001). Sensors were calibrated at in situ temperature against (1) air-saturated bottom water taken from the overlying water from the MUC cores (100% saturation), and (2) dithionate-spiked bottom water (anoxic). Microprofiles across the sediment-water interface were measured with a vertical resolution of 100 μm on a total length of max. 5 cm. The diffusive oxygen uptake (DOU, mmol/m²/d¹) was calculated from the O2 gradient just below the sediment surface and Fick's first law of diffusion (Rasmussen and Jorgensen 1992 doi:10.3354/MEPS081289). DOU = por * Ds * δC/δz where Ds (cm⁻² s⁻¹) = molecular diffusion coefficient in sediment, calculated as Ds = D0 * por/m , where por is the porosity, D0 = diffusion coefficient in water and m = 3, C (μM) = solute concentration, z (cm) = depth in the sediment. Carbon flux (remineralization rates, mg C/m²/d) were calculated from oxygen uptake at the seafloor assuming a molar ratio of C = 0.77 O2 as in Boetius & Damm 1998 (doi:10.1016/S0967-0637(97)00052-6).
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