Origin and fate of branched tetraether lipids in river drainage systems. Implications for the use of the MBT/CBP proxy as a continental palaeothermometer in marine sedimentary records

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids ofbacteria. The number of methyl groups of the alkyl chains in brGDGTs correlates with the mean annual air temperature (MAAT) and soil pH, while the number of cyclopentane moieties correlates with soil pH. Based on this observation the methylation index of branched tetraethers (MBT) and the cyclisation index of branched tetraethers (CBT) were introduced. Since brGDGTs are found in large quantities in soil, it was assumed that they are washed into rivers by erosion and transported to the ocean. Hence, the MBT/CBT proxy can be used in river influenced marine settings to reconstruct past MAAT and soil pH of the river basin. In this study the applicability of the MBT/CBT proxy in diverse river systems was assessed, by detailed studies of two river systems that are different in terms of river size and climate setting (Amazon River and Tagus River). BrGDGTs in drainage basin, soils and river suspended particulate matter (SPM) were compared to those in marine SPM and surface sediments. Core lipid (CL) and intact polar lipid (IPL)-derived brGDGTs were analyzed, in order to distinguish recently produced (IPL-derived) GDGTs from older (CL) GDGTs. The majority of brGDGTs in the Amazon River is derived from lowland basin soils. However, the distribution of CL and IPL-derived brGDGTs was slightly different to that of the soils, which showed that in-situ production of brGDGTs also played a role. The influence of seasonal variation on the brGDGT distribution is relatively small, but significant differences were detected. During the high water season the input of brGDGTs from soil increased, which was seen in increased brGDGT concentrations and a brGDGT distribution that was most similar to that of soil. During the other seasons the influence of in-situ production of brGDGTs increased. In the Amazon shelf and fan, the delivery of brGDGTs from the Amazon River was evident. However, further away from the river, the distributions of CL brGDGTs in marine SPM and sediments varied widely, since this difference was also found in IPL-derived brGDGTs it is most likely due to marine in-situ production. In the Tagus basin soils the brGDGT distributions were very variable and did not reflect the local MAAT and soil pH. The concentrations of brGDGTs in river SPM were substantially higher and the distributions were different compared to those of the drainage basin soils. This indicates that brGDGTs are mainly produced in the river itself. Therefore, the MAAT and soil pH cannot be reconstructed with the commonly used soil calibrations. Along the Portuguese margin the brGDGT concentrations rapidly decreased with increasing distance from the shore. The brGDGT distributions in marine SPM and surface sediments also changed, indicating that marine in-situ production takes place. In conclusion, this study emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT records for paleoreconstructions in adjacent marine setting. In general, it is recommendable to apply the MBT/CBT proxy only to sediment which was under strong river influence.