Provenance of branched GDGTs in the Tagus River drainage basin and its outflow in the Atlantic Ocean over the Holocene

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 temperatures (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 in a set of global soil samples were shown to improve the accuracy of reconstructions, 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 that runs in a transect from source to sink along the Tagus River and out to the deep ocean off the Portuguese margin and spans the last 6,000 years in order to determine if it improves paleoclimate reconstructions in this area. 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 relationship between the new degree of cyclization (DC’) of the brGDGTs and MAT not seen before that could be making temperature reconstructions in this and other arid regions difficult.