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
Authors: Versteegh Gerard J. M. , Zonneveld Karin A. F. , Hefter Jens , Romero Oscar E. , Fischer Gerhard , Mollenhauer Gesine 
Published: March 2022

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.