Constraints on the application of long chain diol proxies in the Iberian Atlantic margin

Abstract

Long chain diols are lipids that have gained interest over the last years due to their high potential to serve as biomarkers and diol indices have been proposed to reconstruct upwelling conditions and sea surface temperature (SST). However, little is known about the sources of the diols and the mechanisms impacting their distribution. Here we studied the factors controlling diol distributions in the Iberian Atlantic margin, which is characterized by a dynamic continental shelf under the influence of upwelling of nutrient-rich cold deep waters, and fluvial input. We analyzed suspended particulate matter (SPM) of the Tagus river, marine SPM and marine surface sediments along five transects off the Iberian margin, as well as riverbank sediments and soil from the catchment area of the Tagus river. Relatively high fractional abundances of the C32 1,15-diol (normalized with respect to the 1,13- and 1,15-diols) were observed in surface sediments in front of major river mouths and this abundance correlated strongly with the BIT index, a tracer for continental input of organic carbon. Together with an even higher fractional abundance of the C32 1,15-diol in the Tagus river SPM, and the absence of long chain diols in the watershed riverbank sediments and soils, we suggest that this long chain diol is produced in-situ in the river. Further support for this hypothesis comes from the small but distinct stable carbon isotopic difference of 1.3‰ with the marine C28 1,13-diol. The 1,14-diols are relatively abundant in surface sediments directly along the northern part of the coast, close to the upwelling zone, suggesting that diol indices based on 1,14-diols would work well as upwelling tracers in this region. Strikingly, we observed a significant difference in stable carbon isotopic composition between the mono-unsaturated C30:1 1,14- and the saturated C28 1,14-diol (3.8 ± 0.7‰), suggesting different sources, in accordance with their different distributions. In addition, the Long chain Diol Index (LDI), a proxy for sea surface temperature, was applied to the surface sediments. The results correlated well with satellite SSTs offshore, but revealed a significant discrepancy with satellite-derived SSTs in front of the Tagus and Sado rivers. This suggests that river outflow might compromise the applicability of this proxy.