(Tables 2) The bPSi:POC ratio of the total diatom community, and abundance of full, empty, and broken diatom cells compared to heavily silicified diatoms during the EIFEX experiment

During the European Iron Fertilisation Experiment (EIFEX), performed in the Southern Ocean, we investigated the reactions of different phytoplankton size classes to iron fertilization, applying measurements of size fractionated pigments, particulate organic matter, microscopy, and flow cytometry. Chlorophyll a (Chl a) concentrations at 20-m depth increased more than fivefold following fertilization through day 26, while concentrations of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) roughly doubled through day 29. Concentrations of Chl a and particulate organic matter decreased toward the end of the experiment, indicating the demise of the iron-induced phytoplankton bloom. Despite a decrease in total diatom biomass at the end of the experiment, biogenic particulate silicate (bPSi) concentrations increased steadily due to a relative increase of heavily silicified diatoms. Although diatoms >10 µm were the main beneficiaries of iron fertilization, the growth of small diatoms (2-8 mm) was also enhanced, leading to a shift from a haptophyte- to a diatom-dominated community in this size fraction. The total biomass had lower than Redfield C : N, N : P, and C : P ratios but did not show significant trends after iron fertilization. This concealed various alterations in the elemental composition of the different size fractions. The microplankton (>20 µm) showed decreasing C : N and increasing N : P and C : P ratios, possibly caused by increased N uptake and the consumption of cellular P pools. The nanoplankton (2-20 µm) showed almost constant C : N and decreasing N : P and C : P ratios. Our results suggest that the latter is caused by a shift in composition of taxonomic groups.

Proto-kerogens in Holocene sediments of ODP Hole 165-1002C

Proto-kerogens were isolated, by extraction and HF/HC1 treatment, from core samples of Holocene sediments of the Cariaco Trench, with interpolated ages of 900, 2850 and 6000 years, and examined via a combination of microscopic, spectroscopic and pyrolytic methods. It appears that these proto-kerogens were chiefly formed from phytoplanktonic components via the degradation-recondensation pathway. The natural sulfurisation pathway only afforded a minor contribution, in spite of the conditions prevailing in the water column and sediments that correspond to those generally considered as especially favourable for the formation of sulfurised organic matter. Proto-kerogen formation via sulfurisation, i.e. the endpoint of the continuum leading to insoluble high molecular weight structures cross-linked by sulfur and resistant to acid hydrolysis, is therefore a rather slow process under these conditions. However, the contribution of sulfurised moieties to the total proto-kerogen substantially increased with depth due to continuous sulfurisation in the time/depth interval, whereas formation through degradation-recondensation is almost complete for the 900 years old sample onwards. Proto-kerogen formation via carbohydrate sulfurisation is faster than lipid sulfurisation and only sulfurised carbohydrates were detected in the shallowest sample. In contrast, sulfurised lipids occur in the other two proto-kerogens. Moreover, their contribution relative to sulfurised carbohydrates increases with depth, probably due to the higher resistance of lipids to mineralisation compared to carbohydrates.