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

[摘要] Proxies based on long-chain alkane-1, mid-chain diols (diol for short)are obtaining increasing interest to reconstruct past upper oceantemperature and productivity. Here we evaluate performance of the seasurface temperature proxies (long-chain diol index (LDI), diolsaturation index (DSI), and diol chain length index (DCI)),productivity and upwelling intensity proxies (two diol indices DI R andDI W and the combined diol index (CDI)), and the nutrient diolindex (NDI) as a proxy for phosphate and nitrate levels. This evaluation isbased on comparison of the diols in sediment trap samples from the upwellingregion 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, monthlyphosphate and nitrate concentrations, wind speed, and wind direction from thenearby Nouadhibou airport. The diol-based SST reconstructions are alsocompared the long-chain-alkenone-based SST reconstructions. The alkenone SSTs correlate best with satellite SST ( r 2 =  0.60). Amplitude and absolute values agree very well as do the flux-corrected time series averages. For the diol proxies the situation is morecomplicated. Diol proxies including 1,14 diols lag trade wind speed by 30 d. Sincewind is nearly always from the NNE to NNW and induces the upwelling, werelate the variability in these proxies to upwelling-induced processes.Correlation with the abundance of upwelling species and wind speed is bestfor the NDI and the 1,14 diol-based DCI and DSI. The DI R , DI W , andCDI perform comparatively poorly. A negative correlation between DSI andwind 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 phosphateand nitrate concentrations in the upper waters and a negative correlationwith both wind-induced upwelling ( r 2 =0.28 and lagging 32 d) andthe abundance of upwelling species ( r 2 =0.38 ). It is suggested thatthis proxy reflects upwelling intensity rather than upper ocean nutrientconcentrations. 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 by41 d but correlate poorly ( r 2 = 0.17). Absolute as well as flux-corrected LDI SSTs are on average 3  ∘ C too high and rather reflectvalues prevailing during the more oligotrophic summer period. We attribute outliers tolow LDI SST to 1,13 diols added during short upwelling-related events. The use of the LDI in regions with higher productivity istherefore not recommended. It appears thus that at the trap site the 1,14 diols primarily reflectconditions relating to upwelling whereas the 1,15 C 30 and to a lesserextent the 1,13 diols seem to reflect the conditions of the moreoligotrophic ocean.

[学科分类] 大气科学