Performance of long-chain mid-chan diol based temperature and productivity proxies at test: a five-years sediment trap record from the Mauritanian upwelling

Abstract

Abstract. Long-chain mid-chain diol (shortly diol) based proxies obtain increasing interest to reconstruct past upper ocean temperature and productivity. Here we evaluate performance of the sea surface temperature proxies; long chain diol index (LDI), the diol saturation index (DSI) and the diol chain-length index (DCI), productivity/upwelling intensity proxies: the two diol indices DIR (Rampen et al., 2008) and DIW (Willmott et al., 2010) and the combined diol index (CDI), as well as the nutrient diol index (NDI) as proxy for phosphate and nitrate levels. This evaluation is based on comparison of the diols in sediment trap samples from the upwelling region off NW Africa collected at 1.28 km water depth with daily satellite derived sea surface temperatures (SSTSAT), subsurface temperatures, productivity, the plankton composition from the trap location, monthly PO43− and NO3− concentrations, wind speed and wind direction from the nearby Nouadhibou airport. The diol based SST reconstructions are also compared the long chain alkenones based UK’37 proxy reconstructions (SSTUK). At the trap site, most diol proxies lag wind speed (phase φ = 30 days) and can be related to upwelling. Correlation with the abundance of upwelling species and wind speed is best for the DCI, DSI and NDI whereas the DI and CDI perform comparatively poorly. The nutrient proxy NDI shows no significant correlation to monthly PO43− and NO3− concentrations in the upper waters and a negative correlation with wind-induced upwelling (r2 = 0.28, φ = 32 days) as well as the abundance of upwelling species (r2 = 0.38; Table 4). It is suggested that this proxy reflects upwelling intensity rather than upper ocean nutrient concentrations. At the trap site, SSTSAT lags wind speed forced upwelling by about 4 months (φ = 129 d). The LDI based SST (SSTLDI) correlate poorly (r2 = 0.17) to SSTSAT which we attribute to variability in 1,13 diol abundance unrelated to SST such as productivity. The SSTUK correlates best with SSTSAT (r2 = 0.60). Also amplitude and absolute values agree very well and the flux corrected SSTUK time series average equals the SSTSAT annual average.