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Organic matter (OM) remineralization may be considered a key function of the benthic compartment of marine ecosystems and in this study we investigated if the input of labile organic carbon alters mineralization of indigenous sediment OM (OM priming). Using 13C-enriched diatoms as labile tracer carbon, we examined shallow-water sediments (surface and subsurface layers) containing organic carbon of different reactivity under oxic versus anoxic conditions. The background OM decomposition rates of the sediment used ranged from 0.08 to 0.44 μmol C mlws−1 day−1. Algal OM additions induced enhanced levels of background remineralization (priming) up to 31% and these measured excess fluxes were similar to mineralization of the added highly degradable tracer algal carbon. This suggests that OM priming may be important in marine sediments.

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Nitrogenase activity, indicative of N 2 fixation, was measured in the surface waters along a north-south transect in the eastern Atlantic Ocean, from Texel (The Netherlands, 53uN) to Cape Town (South Africa, 35uS) using a sensitive on-line, near real-time acetylene reduction assay. From the beginning of January to the end of February 2000 nitrogenase activity was detected in varying rates, but only between 14uN and 13uS latitudes. Dark incubations yielded an average activity of 2.2 (6 2.4) mmol m 22 d 21 N, but light increased the activity to 3.7 (6 2.9) mmol m 22 d 21 N. However, nitrogenase activity in the light was sensitive to O 2 doubling to 7.6 (6 12.7) mmol m 22 d 21 N when the incubation was anaerobic. In the area where N 2 fixation occurred, phosphate concentrations were fourfold lower than in the area where N 2 fixation was absent, while silicate levels were higher. The water temperature in the area with N 2 fixation was 28uC, while in the adjacent area the temperature was 3uC lower, which might have prevented the proliferation of diazotrophic cyanobacteria. Action spectra revealed that chlorophyll a, phycocyanin, and phycoerythrin are the light-harvesting pigments supporting nitrogenase activity. In one area in the northern latitudes, potential nitrogenase activity was highest during daytime, which is characteristic for Trichodesmium. In areas with a high potential nitrogenase activity, surface waters were dominated by a phycoerythrin-containing cyanobacterium. Since nitrogenase activities were highest at night, these cells may have been unicellular cyanobacteria like Crocosphaera.

The Long chain Diol Index (LDI) is a relatively new organic geochemical proxy for sea surface temperature (SST), based on the abundance of the C30 1,15-diol relative to the summed abundance of the C28 1,13-, C30 1,13-and C30 1,15-diols. Here we substantially extend and reevaluate the initial core top calibration by combining the original dataset with 172 data points derived from previously published studies and 262 newly generated data points. In total, we considered 595 globally distributed surface sediments with an enhanced geographical coverage compared to the original calibration. The relationship with SST is similar to that of the original calibration but with considerably increased scatter. The effects of freshwater input (e.g., river runoff) and long-chain diol contribution from Proboscia diatoms on the LDI were evaluated. Exclusion of core-tops deposited at a salinity < 32 ppt, as well as core-tops with high Probosciaderived C28 1,12-diol abundance, resulted in a substantial improvement of the relationship between LDI and annual mean SST. This implies that the LDI cannot be directly applied in regions with a strong freshwater influence or high C28 1,12-diol abundance, limiting the applicability of the LDI. The final LDI calibration (LDI=0.0.0325×SST+0.1082; R 2 = 0.88; n = 514) is not statistically different from the original calibration of Rampen et al. (2012)