Description: Branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are thought to be transported from soil to marine sediment by rivers, have been used to reconstruct the mean annual air temperature (MAAT) and soil pH of the drainage basin using the methylation index of branched tetraethers (MBT, recently refined as MBT') and cyclization index of branched tetraethers (CBT) from coastal marine sediment records. In this study we are tracing the brGDGTs from source to sink in the Tagus River basin, the longest river system on the Iberian Peninsula, by determining their concentration and distribution in soils, river suspended particulate matter (SPM), riverbank sediments, marine SPM, and marine surface sediments. The concentrations of brGDGTs in river SPM were substantially higher and their distributions were different compared to those of the drainage basin soils. This indicates that brGDGTs are mainly produced in the river itself. In the marine environment, the brGDGT concentrations rapidly decreased with increasing distance from the Tagus estuary. At the same time, the brGDGT distributions in marine sediments also changed, indicating that marine in-situ production also takes place. These results show that there are various problems that complicate the use of the MBT'/CBT for paleoreconstructions using coastal marine sediments in the vicinity of a river. However, if the majority of brGDGTs are produced in the river, it might be possible to reconstruct the environmental (temperature and pH) conditions of the river water using appropriate aquatic calibrations, provided that marine core locations are chosen in such a way that the brGDGTs in their sediments are predominantly derived from riverine in-situ production.

Description: The branched vs. isoprenoid tetraethers (BIT) index, which is based on the relative abundance of non-isoprenoidal, so-called branched glycerol dialkyl glycerol tetraethers (brGDGTs) versus a structurally related isoprenoid GDGT "crenarchaeol", has been used to trace soil organic carbon (OC) from the continent to the ocean. However, it has been found in some locations that the BIT index can be primarily influenced by crenarchaeol concentrations and brGDGT production in fresh water rather than by soil-derived brGDGT concentrations. This may hamper the application of this proxy as an indicator for the input of soil OC. Here we examined the concentration and distribution of brGDGTs as well as variations in the BIT index in marine surface sediments from five transects (Douro, Mondego, Estremadura, Tagus, and Sado) along the southern Portuguese continental margin and in marine suspended particulate matter (SPM) from the Douro and Tagus transects. Higher BIT values and brGDGT concentrations (normalized to OC content) were found close to the river mouths and coast than in deep offshore sites. This clearly indicated the continental input of brGDGTs and revealed that, at least in this setting, the BIT index was primarily influenced by the delivery of brGDGTs from the rivers. However, the BIT index was also, to a minor extent, influenced by crenarchaeol concentrations. This shows that the brGDGT concentration may provide a more straightforward indication of continental OC input than the BIT index. BrGDGT concentrations and distributions in sediments and SPM close to the rivers were similar to those of SPM in the Tagus River. This indicates that degradation processes in the estuaries had no significant effect on the riverine brGDGTs. Therefore, brGDGTs should be a good indicator for the rigid OC fraction transported from the continent to the ocean. Our results also indicate that there are multiple sources of brGDGTs in the marine environment, which complicates the use of the brGDGT distribution as an indicator for terrestrial vs. marine produced brGDGTs.

Description: Soils, fine-grained ice-rafted detritus (IRD), coals, and marine surface sediments in the Arctic realm (Svalbard) were collected in 2007 and 2008 to characterize organic matter (OM) sources in Arctic marine sediments. Bulk geochemical (C:N ratio and stable carbon isotopic composition) parameters suggest a predominant marine contribution to sedimentary OM. The branched and isoprenoid tetraether index (a proxy of soil OM input) indicates that soil OM contribution to the marine sediments is minor. However, the presence of retene (used as an indicator for coal-derived OM), the low carbon preference index and the average chain length of n-alkanes, and the depleted bulk radiocarbon content (D14C value) suggest that ancient OM of both coal-derived and mature IRD-derived OM is being buried in the Kongsfjord-Krossfjord system of Svalbard in the high Arctic. The relatively low retene concentrations in the marine surface sediments other than those in close vicinity of Ny Ålesund, previously a coal-mining town, indicated that input of IRD-derived OM was predominantly responsible for the generally low D14C value. We applied three-end-member models based on D14C and retene and n-alkane concentration data to disentangle relative coal-derived, IRD-derived, and marine OM proportions to sedimentary OM. Sediments were comprised on average 2% ± 4% of coal-derived OM, 37% ± 17% of IRD-derived OM, and 61% ± 18% of marine OM with higher IRD-derived OM deposit in the Kongsvegen glacier front. Our results highlight the important role of ancient OM on carbon dynamics in Arctic environments, in particular for benthic food webs.