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The Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea. A large fraction of this terrestrial carbon is either buried or mineralized in the sediments close to the river mouth. This mineralization follows aerobic and anaerobic pathways, with a range of impacts on calcium carbonate precipitation and dissolution in the sediment near the sediment-water interface. This study focuses on the production of dissolved inorganic carbon (DIC) and total alkalinity (TA) by early diagenesis, consequential pH variations and the effect on calcium carbonate precipitation or dissolution. The sediment porewater chemistry was investigated along a transect from the Rhône River outlet to the continental shelf. TA and concentrations of DIC, SO42- and Ca2+ were analyzed on bottom waters and extracted sediment porewaters, whereas pH and oxygen concentrations were measured in situ using microelectrodes. The average oxygen penetration depth into the sediment was 1.7±0.4mm close to the river mouth and 8.2±2.6 mm in the continental shelf sediments, indicating intense respiration rates. Diffusive oxygen fluxes through the sediment-water interface ranged between 3 and 13 mmol O2 m-2 d-1. In the first 30 cm of the sediment, TA and DIC porewater concentrations increased with depth up to 48 mmol L-1 near the river outlet and up to 7 mmol L-1 on the shelf as a result of aerobic and anaerobic mineralization processes. Due to aerobic processes, at all stations pH decreased by 0.6 pH units in the oxic layer of the sediment accompanied by a decrease of the saturation state regarding calcium carbonate. In the anoxic layer of the sediments, sulfate reduction was the dominant mineralization process and was associated with an increase of porewater saturation state regarding calcium carbonate. Ultimately anoxic mineralization of organic matter caused calcium carbonate precipitation demonstrated by a large decrease in Ca2+ concentration with depth in the sediment. Carbonate precipitation decreased in the offshore direction, together with the carbon turnover and sulfate consumption in the sediments. The large production of porewater alkalinity characterizes these sediments as an alkalinity source to the water column, which may increase the CO2 buffering capacity of these coastal waters. Estuarine sediments should therefore receive more attention in future estimations of global carbon fluxes. [ABSTRACT FROM AUTHOR]

The Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea, and a large fraction thereof is buried or mineralized in the sediments close to the river mouth. The mineralization follows aerobic and anaerobic pathways with varying impacts on the carbonate chemistry in the sediment pore waters. This study focused on the production of dissolved inorganic carbon (DIC) and total alkalinity (TA) by early diagenesis at the sediment water-interface, consequential pH variations and the effect on calcium carbonate precipitation or dissolution. The sediment pore water chemistry was investigated during the DICASE cruise along a transect from the Rhône River outlet to the continental shelf. The concentrations of DIC, TA, SO42- and Ca2+ were analyzed on bottom waters and extracted pore waters, whereas pH and oxygen concentrations were measured in situ using microelectrodes. The average oxygen penetration depth into the sediment was 1.7 ± 0.4 mm in the proximal domain and 8.2 ± 2.6 mm in the distal domain, indicating intense aerobic respiration rates. Diffusive oxygen fluxes through the sediment water interface range between 3 and 13 mmol O2 m-2 d-1. The DIC and TA concentrations increased with depth in the sediment pore waters up to 48 mmol L-1 near the river outlet and up to 7 mmol L-1 on the shelf as a result of aerobic and anaerobic mineralization processes. Due to oxic processes, the pH decreased by 0.6 pH units in the oxic layer of the sediment accompanied by a decrease of the saturation state regarding calcium carbonate. In the anoxic part of the sediments, sulfate reduction was seen to be the dominant mineralization process and was associated to an increase of pore water saturation state regarding calcium carbonate. Ultimately anoxic mineralization of organic matter caused calcium carbonate precipitation as shown by large decrease in Ca2+ concentration with depth in the sediment. The saturation state and carbonate precipitation decreased in offshore direction, together with the carbon turnover and sulfate consumption in the sediments. [ABSTRACT FROM AUTHOR]

Phospholipid-derived fatty acids (PLFA) and respiratory quinones (RQ) are microbial compounds that have been utilized as biomarkers to quantify bacterial biomass and to characterize microbial community structure in sediments, waters, and soils. While PLFAs have been widely used as quantitative bacterial biomarkers in marine sediments, applications of quinone analysis in marine sediments are very limited. In this study, we investigated the relation between both groups of bacterial biomarkers in a broad range of marine sediments from the intertidal zone to the deep sea. We found a good log-log correlation between concentrations of bacterial PLFA and RQ over several orders of magnitude. This relationship is probably due to metabolic variation in quinone concentrations in bacterial cells in different environments, whereas PLFA concentrations are relatively stable under different conditions. We also found a good agreement in the community structure classifications based on the bacterial PLFAs and RQs. These results strengthen the application of both compounds as quantitative bacterial biomarkers. Moreover, the bacterial PLFA- and RQ profiles revealed a comparable dissimilarity pattern of the sampled sediments, but with a higher level of dissimilarity for the RQs. This means that the quinone method has a higher resolution for resolving differences in bacterial community composition. Combining PLFA and quinone analysis as a complementary method is a good strategy to yield higher resolving power in bacterial community structure. [ABSTRACT FROM AUTHOR]