Transport of branched tetraether lipids from the Tagus River basin to the coastal ocean of the Portuguese margin: consequences for the interpretation of the MBT'/CBT paleothermometer

Abstract

Abstract. 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 trace 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.

Tracing tetraether lipids from source to sink in the Rhône River system (NW Mediterranean)

“…However, the concentrations of CL crenarchaeol in soils were in general higher with increasing soil pH (>7, Figure 8B), resulting in lower BIT values ( Figure 8C). This is in good agreement with previous studies which showed that soil pH was positively related to CL crenarchaeol concentrations and thus negatively associated with BIT values (e.g., Weijers et al, 2006b;Kim et al, 2010;Yang et al, 2011;Zell et al, 2014a). It has been previously suggested that soil pH might more strongly influence the occurrence of the source organisms producing crenarchaeol than those producing brGDGTs (Kim et al, 2010).…”

“…However, the CL DC values in the marine surface sediments were slightly higher than those of the lower Rhône SPM (Figure 9E), showing a similar trend with increasing distance offshore as for the concentrations of CL brGDGTs. Similar studies carried out in other river systems also revealed an increase in the CL DC from soils to the marine environment for a Svalbard fjord (Peterse et al, 2009a), the lower Yangtze River-East China Sea shelf (Zhu et al, 2011), the Pearl River estuary , the Portuguese margin (Zell et al, 2014a), and the Amazon shelf (Zell et al, 2014b). This increase in the DC was assumed to be due to in situ production of brGDGTs in the marine environment, since the pH of the marine environment (pH ≈ 8) is usually higher compared to that of soils and river water.…”

“…An alternative source for the brGDGTs in the lower Rhône SPM would be in situ production. Significantly higher CL brGDGT concentrations in river SPM than in soils have been observed in the Amazon River (Zell et al, 2013a) and in the Tagus River (Zell et al, 2014a). This has been interpreted as evidence for in situ production of brGDGTs in rivers.…”

“…However, in contrast to other river systems, the concentrations of CL brGDGTs in the lower Rhône SPM were comparable to those of the lower Rhône and tributary soils, like in the upper Rhône basin (Figure 9A). Moreover, the IPL percentage of brGDGTs was slightly lower in the lower Rhône SPM (8 ± 6) than in the Amazon River (11 ± 7, Zell et al, 2013a), the Tagus River (12 ± 8, Zell et al, 2014a), and the Yenisei River (10 ± 4, De Jonge et al, 2014). Hence, the proportion of in situ produced brGDGTs to the riverine brGDGTs is possibly smaller in the Rhône River system than in other river systems.…”

“…Further information on the potential in situ production of brGDGTs in rivers can be found in the distribution of brGDGTs with the help of the MBT' and the DC (cf. Zell et al, 2013aZell et al, , 2014a. Therefore, we compared the CL MBT' and the CL DC of the river SPM with those of the soils (Figures 9, 10).…”