Sulfur cycling in marine sediments undergoes dramatic changes with changing redox conditions of the overlying waters. The upper sediments of the anoxic Gotland Basin, central Baltic Sea represent a dynamic redox environment with extensive mats of sulfide oxidizing bacteria covering the seafloor beneath the chemocline. In order to investigate sulfur redox cycling at the sediment-water interface, sediment cores were sampled over a transect covering 65 – 174 m water depth in August-September 2013. High resolution (0.25 mm minimum) vertical microprofiles of electroactive redox species including dissolved sulfide and iron were obtained with solid state Au-Hg voltammetric
microelectrodes. This approach enabled a fine-scale comparison of porewater profiles across the basin. The steepest sulfide gradients (i.e. the highest sulfide consumption) occurred within the upper 10 mm in sediments covered by surficial mats (2.10 to 3.08 mmol m-2 day-1). In sediments under permanently anoxic waters (>140m),
voltammetric signals for Fe(II) and aqueous FeS were detected below a subsurface maximum in dissolved sulfide, indicating a Fe flux originating from older, deeper sedimentary layers. Our results point to a unique sulfur cycling in the Gotland basin seafloor where sulfide accumulation is moderated by sulfide oxidation at the sediment surface and by FeS precipitation in deeper sediment layers. These processes may play an important role in minimizing benthic sulfide fluxes to bottom waters around the major basins of the Baltic Sea.
Abstract. Carbon cycling in Peruvian margin sediments (11 and 12° S) was examined at 16 stations, from 74 m water depth on the middle shelf down to 1024 m, using a combination of in situ flux measurements, sedimentary geochemistry and modelling. Bottom water oxygen was below detection limit down to ca. 400 m and increased to 53 μM at the deepest station. Sediment accumulation rates decreased sharply seaward of the middle shelf and subsequently increased at the deep stations. The organic carbon burial efficiency (CBE) was unusually low on the middle shelf (<20%) when compared to an existing global database, for reasons which may be linked to episodic ventilation of the bottom waters by oceanographic anomalies. Deposition of reworked, degraded material originating from sites higher up on the slope is proposed to explain unusually high sedimentation rates and CBE (>60%) at the deep oxygenated sites. In line with other studies, CBE was elevated under oxygen-deficient waters in the mid-water oxygen minimum zone. Organic carbon rain rates calculated from the benthic fluxes alluded to efficient mineralisation of organic matter in the water column compared to other oxygen-deficient environments. The observations at the Peruvian margin suggest that a lack of oxygen does not greatly affect the degradation of organic matter in the water column but promotes the preservation of organic matter in sediments.
Abstract Bacterial sulfate reduction (SR) is often determined by radiotracer techniques using 35 S‐labeled sulfate. In environments featuring simultaneous sulfide oxidation, SR can be underestimated due to re‐oxidation of 35 S‐sulfide. Recycling of 35 S‐tracer is expected to be high in sediment with low concentrations of pore‐water sulfide and high abundance of giant filamentous sulfur‐oxidizing bacteria (GFSOB). Here, we applied a sulfide‐spiking method, originally developed for water samples, to sediments along a shelf‐slope transect (72, 128, 243, 752 m water depth) traversing the Peruvian oxygen minimum zone. Sediment spiked with unlabeled sulfide prior to 35 S‐sulfate injection to prevent radiotracer recycling was compared to unspiked sediment. At stations characterized by low natural sulfide and abundant GFSOB (128 and 243 m), the method revealed 1–3 times higher SR rates in spiked sediment. Spiking had no effect on SR in sediment with high natural sulfide despite presence of GFSOB (72 m). Bioturbated sediment devoid of GFSOB (752 m) showed elevated SR in spiked samples, likely from artificial introduction of sulfidic conditions. Sulfide oxidation rates at the 128 and 243 m station, derived from the difference in SR between spiked and unspiked sediment, approximated rates of dissimilatory nitrate reduction to ammonium by GFSOB. Gross SR contributed considerably to benthic dissolved inorganic carbon fluxes at the three shallowest station, confirming that SR is an important process for benthic carbon respirations within the oxygen minimum zone. We recommend to further explore the spiking method to capture SR in sediment featuring low sulfide concentrations and high sulfur cycling by GFSOB.
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