Performance of temperature and productivity proxies based on long-chain alkane-1, mid-chain diols at test: a 5-year sediment trap record from the Mauritanian upwelling

Abstract

Proxies based on long-chain alkane-1, mid-chain diols (diol for short) are obtaining 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), diol saturation index (DSI), and diol chain length index (DCI)), productivity and upwelling intensity proxies (two diol indices DIR and DIW and the combined diol index (CDI)), and the nutrient diol index (NDI) as a 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 (SSTs), subsurface temperatures, productivity, the plankton composition from the trap location, monthly phosphate and nitrate concentrations, wind speed, and wind direction from the nearby Nouadhibou airport. The diol-based SST reconstructions are also compared the long-chain-alkenone-based SST reconstructions. The alkenone SSTs correlate best with satellite SST (r2= 0.60). Amplitude and absolute values agree very well as do the flux-corrected time series averages. For the diol proxies the situation is more complicated. Diol proxies including 1,14 diols lag trade wind speed by 30 d. Since wind is nearly always from the NNE to NNW and induces the upwelling, we relate the variability in these proxies to upwelling-induced processes. Correlation with the abundance of upwelling species and wind speed is best for the NDI and the 1,14 diol-based DCI and DSI. The DIR, DIW, and CDI perform comparatively poorly. A negative correlation between DSI and wind speed may suggest that the DSI reflects wind-speed-forced upwelling-related reductions in temperature rather than irradiation-induced temperatures. The nutrient proxy NDI shows no significant correlation to monthly phosphate and nitrate concentrations in the upper waters and a negative correlation with both wind-induced upwelling (r2=0.28 and lagging 32 d) and the abundance of upwelling species (r2=0.38). It is suggested that this proxy reflects upwelling intensity rather than upper ocean nutrient concentrations. At the trap site, satellite SST lags wind-speed-forced upwelling by about 4 months. The 1,13 and 1,15 diol-based LDI-derived SSTs lag satellite SSTs by 41 d but correlate poorly (r2= 0.17). Absolute as well as flux-corrected LDI SSTs are on average 3 ∘C too high and rather reflect values prevailing during the more oligotrophic summer period. We attribute outliers to low LDI SST to 1,13 diols added during short upwelling-related events. The use of the LDI in regions with higher productivity is therefore not recommended. It appears thus that at the trap site the 1,14 diols primarily reflect conditions relating to upwelling whereas the 1,15C30 and to a lesser extent the 1,13 diols seem to reflect the conditions of the more oligotrophic ocean.