Performance of long-chain alkane–1,mid–chain–diol based temperature and productivity proxies at test: a five-years sediment trap record from the Mauritanian upwelling

Long-chain alkane–1,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 20 chain-length index (DCI), productivity/upwelling intensity proxies: two diol indices DIR and DIW 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 (SST), subsurface temperatures, productivity, the plankton composition from the trap 25 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 alkenones based SST reconstructions. Gerard Versteegh 14-2-22 18:58 Gelöscht: Gerard Versteegh 14-2-22 18:58 Gelöscht: 30 Gerard Versteegh 14-2-22 18:58 Gelöscht: Gerard Versteegh 14-2-22 18:58 Gelöscht: Gerard Versteegh 15-2-22 12:03 Gelöscht: the Gerard Versteegh 15-2-22 12:03 Gelöscht: (Rampen et al., 2008) Gerard Versteegh 15-2-22 12:04 Gelöscht: (Willmott et al., 2010) 35 Gerard Versteegh 15-2-22 12:05 Gelöscht: SAT Gerard Versteegh 15-2-22 12:04 Gelöscht: PO4 Gerard Versteegh 15-2-22 12:04 Gelöscht: NO3 Gerard Versteegh 15-2-22 12:15 Gelöscht: U37 proxy Gerard Versteegh 15-2-22 12:05 Gelöscht: (SSTUK) 40 2 The alkenone SST correlate best with satellite SST (r = 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 days. Since wind is nearly always from the NNE to NNW and induces the upwelling, we relate the variability in these proxies to upwelling induced 45 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 temperature rather than irradiation induced temperatures. The nutrient proxy NDI shows no significant correlation to monthly phosphate and nitrate concentrations in the 50 upper waters and a negative correlation with both wind-induced upwelling (r=0.28 and lagging 32 days) and the abundance of upwelling species (r=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 SST lag satellite SST by 41 days but correlate poorly (r = 0.17). Absolute as 55 well as flux corrected LDI SST are on average 3°C too high and rather reflect values prevailing during the more oligotrophic summer period. Outliers to low LDI SST we attribute 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 60 whereas the 1,15C30 and to a lesser extent the 1,13 diols seem to reflect the conditions of the more oligotrophic ocean.