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

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.