Articles | Volume 13, issue 20
Research article
17 Oct 2016
Research article |  | 17 Oct 2016

Examining the provenance of branched GDGTs in the Tagus River drainage basin and its outflow into the Atlantic Ocean over the Holocene to determine their usefulness for paleoclimate applications

Lisa Warden, Jung-Hyun Kim, Claudia Zell, Geert-Jan Vis, Henko de Stigter, Jérôme Bonnin, and Jaap S. Sinninghe Damsté

Abstract. The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are transported from the soils where they are predominantly produced to marine sediments via rivers, have been applied in reconstructing mean annual air temperature (MAT) and pH of soils. However, paleoclimate reconstructions using sedimentary brGDGTs have proven difficult in arid regions, including the Iberian Peninsula. Recently, six novel 6-methyl brGDGTs have been described using new analytical methods (in addition to the nine 5-methyl brGDGTs previously used for climate reconstructions), and so new pH and MAT calibrations have been developed that were shown to improve the accuracy of reconstructions in a set of global soil samples, especially in arid regions. Because of this we decided to apply the new method to separate the 5- and 6-methyl isomers along with the novel calibrations to a sample set from the Iberian Peninsula to determine whether it improves paleoclimate reconstructions in this area. This set includes samples that run in a transect from source to sink along the Tagus River and out to the deep ocean off the Portuguese margin spanning the last 6000 years. We found that although pH reconstructions in the soils were improved using the new calibration, MAT reconstructions were not much better even with the separation of the 5- and 6-methyl brGDGTs. This confirmed the conclusion of previous studies that the amount of aquatically produced brGDGTs is overwhelming the soil-derived ones in marine sediments and complicating MAT reconstructions in the region. Additionally, the new separation revealed a strong and until now unseen relationship between the new degree of cyclization (DC') of the brGDGTs and MAT that could be making temperature reconstructions in this and other arid regions difficult.

Short summary
Enhanced analytical techniques were applied to characterize fossilized microbial cell membrane lipids from samples in the Tagus River basin spanning the last 6000 years. Using the novel methods and calibration, the pH estimates were improved upon, and this study reveals new factors that should be considered when using this proxy as well as affirms the importance of examining the provenance of these lipids before applying them for paleoclimate reconstructions.
Final-revised paper