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.

Highlights

  • 1.1 Regional oceanography The Mauritania upwelling system is part of the Canary Current (CC) Eastern Boundary Upwelling Ecosystem (CC–EBUE). 65 The coastal upwelling off Mauretania is driven by the NNW to NNE trade winds and occurs where the southward CC flowing along the coast meets the northward Cap Verde Current (CVC) and Poleward Undercurrent (PUC)

  • 115 Relative amounts of alkyl diol isomers were estimated from peak areas of specific ions resulting from α–cleavage of trimethylsilyl-ethers at various mid-chain positions As an example, for C28 diols peaks at m/z 299 (1,14-diol) and m/z 313 (1,13-diol), and for C30 diols peaks at m/z 313 (1,15 diol) and m/z 327 (1,14-diols) were integrated (Rampen et al, 2008; Versteegh et al, 1997)

  • If the diol peaks consisted of a coeluting isomeric mixture, isomer proportions were quantified by their relative ratios as obtained by analysis of isomer-specific m/z ions from gas chromatograph (GC)/time of flight mass spectrometer (TOF–MS)

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Summary

Introduction

Mauritania upwelling system is part of the Canary Current (CC) Eastern Boundary Upwelling Ecosystem (CC–EBUE). 65 The coastal upwelling off Mauretania is driven by the NNW to NNE trade winds and occurs where the southward CC flowing along the coast meets the northward Cap Verde Current (CVC) and Poleward Undercurrent (PUC). 65 The coastal upwelling off Mauretania is driven by the NNW to NNE trade winds and occurs where the southward CC flowing along the coast meets the northward Cap Verde Current (CVC) and Poleward Undercurrent (PUC) These currents are deflected offshore, resulting in the SW directed Cape Verde Frontal Zone (CVFZ) (Fig. 1). After the solvent vent time, the split valve was closed, the CIS heated at 12 °C s-1 to 340 °C and held for 2 min for sample transfer to the GC-column. For the diol quantification, selected reference samples containing alkyl diols in high relative amounts and without other interfering compounds (as indicated by GC/TOF–MS analyses) were analyzed by 120 GC–FID on an Agilent 6890 GC equipped with a 60m x 0.32mm i.d. DB1–MS (Agilent J&W) column (film thickness: 0.25μm).

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