Description: Suspended particulate matter (SPM) was collected along the Amazon River in the central Amazon basin and in three tributaries during the risingwater (RW), high water (HW), falling water (FW) and low water (LW) season. Changes in the concentration and the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), i.e., the methylation index of branched tetraethers (MBT) and the cyclization of brGDGTs (CBT), were seen in the Amazon main stem. The highest concentration of core lipid (CL) brGDGTs normalized to particulate organic carbon (POC) was found during the HW season. During the HW season the MBT and CBT in the Amazon main stem was also most similar to that of lowland Amazon (terra firme) soils, indicating that the highest input of soil-derived brGDGTs occurred due to increased water runoff. During the other seasons the MBT and CBT indicated an increased influence of in situ production of brGDGTs even though soils remained the main source of brGDGTs. Our results reveal that the influence of seasonal variation is relatively small, but can be clearly detected. Crenarchaeolwasmostly produced in the river. Its concentration was lower during the HW season compared to that of the other seasons. Hence, our study shows the complexity of processes that influence the GDGT distribution during the transport from land to ocean. It emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT and BIT records for paleoreconstructions in adjacent marine setting.

Description: Branched glycerol dialkyl glycerol tetraethers (brGDGTs) in river fan sediments have been used successfully to reconstruct mean annual air temperature (MAAT) and soil pH of the Congo River drainage basin. However, in a previous study of Amazon deep-sea fan sediments the reconstructed MAATs were ca. 10°C colder than the actual MAAT of the Amazon basin. In this study we investigated this apparent offset, by comparing the concentrations and distributions of brGDGTs in Amazon River suspended particulate matter (SPM) and sediments to those in marine SPM and surface sediments. The riverine brGDGT input was evident from the elevated brGDGT concentrations in marine SPM and surface sediments close to the river mouth. The distributions of brGDGTs in marine SPM and sediments varied widely, but generally showed a higher relative abundance of methylated and cyclic brGDGTs than those in the river. Since this difference in brGDGT distribution was also found in intact polar lipid (IPL)-derived brGDGTs, which were more recently produced, the change in the marine brGDGT distribution was most likely due to marine in-situ production. Consequently, the MAATs calculated based on the Methylation of Branched Tetraethers (MBT) and the Cyclisation of Branched Tetraethers (CBT) were lower and the CBT-derived pH values were higher than those of the Amazon basin. However, SPM and sediments from stations close to the river mouth still showed MBT/CBT values that were similar to those of the river. Therefore, we recommend caution when applying the MBT/CBT proxy, it should only be used in sediment cores that were under high river influence. The influence of riverine derived isoprenoid GDGT (isoGDGT) on the isoGDGT-based TEX86 temperature proxy was also examined in marine SPM and sediments. An input of riverine isoGDGTs from the Amazon River was apparent, but its influence on the marine TEX86 was minor since the TEX86 of SPM in the Amazon River was similar to that in the marine SPM and sediments.

Description: Recently, four bacteriohopanepolyols (BHPs), adenosylhopane, and structurally similar adenosylhopane-type 1, 2-methyl adenosylhopane, and 2-methyl adenosylhopane-type 1, have been suggested to be characteristic of soil microbial communities and therefore can serve as molecular markers for soil organic matter (OM) supply in river, lake, and marine sediments. In this study, we analyzed BHPs in peats and soils collected in the Têt watershed (southern France) and compared them with branched glycerol dialkyl glycerol tetraethers (GDGTs), a more established molecular tracer of soil OM. Adenosylhopane-type I is identified in all of the samples from the study area except one collected near the Têt River mouth with up to three of the related compounds also frequently present, particularly in the surface samples. The concentrations of soil-specific BHPs in peat environments have been shown to increase with lower d15N values, providing evidence that N2-fixing bacteria are probably a major source of soil-specific BHPs in acidic environments. It seems likely that soil pH is a major factor controlling BHP occurrence based on statistical analysis of environmental parameters and BHP concentration data. The comparison of the soil-specific BHP concentrations with those of branched GDGTs shows no clear relationship in the Têt River system, supporting the concept that these two groups of soil-specific compounds are synthesized by different microbial organisms living in different niches in the soil profile (e.g., oxic top versus anoxic deep).