IMIS - Marine Research Groups

Surface sediments were collected in a transect from the Amazon river delta to open marine sites in the north Atlantic Ocean in order to characterize spatial contrasts in the deposited organic carbon (OC), allowing to uncover the role of the river plume on the sedimentation of OC. Analysis of isoprenoidal and branched glyceroldialkyl glycerol tetraethers (GDGTs), lignin phenols, fatty acids, n-alkanes and bulk parameters were performed to characterize the sedimentary OC. An end-member approach based on biomarkers (lignin phenols and GDGTs), and the stable carbon isotopic composition of bulk organic carbon (δ¹³C_OC), was used to estimatethe fraction of marine (OC_mar) and continental sources (OC _cont) of sedimentary organic carbon. Similar estimates based on lignin phenols were obtained and indicates that the OC_cont was on average 30 ± 37%; based on the GDGTs it was 29 ± 35% while based on the δ¹³C_OC it was 30 ± 32%. The OC content of the surface sediments remained relatively constant from the delta to the marine sites northward (ca. 0.6%) but the fractions of continental and marine OC were variable. In the deltaic region, OC_cont was ca. 84%, while in the open marine sites, OC_mar was ca. 86%. In the stationssouthward the delta, the OC content was about 0.03% with high OC _mar and low OC_cont, which indicates a low influenceof the river plume on this location. The accumulation rates of OC _cont and OC_mar were estimated and showed that both were higher in the deltaic region in comparison to the marine sites. The results suggested that only 10–15% of the OC_cont is deposited offshore and that river plume conditions stimulated primary production and, thus, ultimately the sedimentation of OC_mar in the continental shelf and offshore sediments. Finally, our estimates showed that the Amazon River Delta has an accumulation rate above the global average and the material transported horizontally by the river plume fuels the primary production in adjacent areas, which improves the relevance of the Amazon continental shelf to the global carbon budget .