Sediment Trap Time Series of GDGT Flux in the Northern Gulf of Mexico: Implications for the TEX86 Paleothermometer

Abstract:

The TEX₈₆ proxy, based on the distribution of isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), has been broadly applied in down core marine sediments to reconstruct mean annual sea surface temperature (SST). Despite the fact that GDGTs are produced throughout the entire water column, global core-top calibrations show a robust correlation between mean annual SST and TEX₈₆ of underlying sediments. Published Holocene TEX₈₆ data from the northern Gulf of Mexico (GoM) have been interpreted as summer-weighted, since calibrated TEX₈₆-SSTs are systematically offset 2º C warmer than foraminiferal Mg/Ca-based SST estimates. We measure GDGT flux and TEX₈₆ in a weekly-to-monthly resolution 4-year sediment trap time series (2010-2014) as well as in core-top sediments in the northern GoM to investigate the seasonality of the TEX₈₆ signal in GoM sediments. Overall, the flux of isoprenoid GDGTs is closely tied to mass flux. There is no distinct seasonal cycle in GDGT flux, but peaks occur almost exclusively in winter during our 2010-2014 sampling interval, precluding the interpretation of TEX₈₆ as a summer-weighted SST proxy in the GoM. We use BAYSPAR, a Bayesian spatially-varying regression model, to calibrate sediment trap TEX₈₆ to SST, and find a flux-weighted mean TEX₈₆-SST (24.9 ºC) matches the GoM core-top TEX₈₆-SST, and is consistent with the observed mean annual SST at the site. The sediment trap TEX₈₆-SST varies between 19 ºC and 27 ºC over the 4-year sampling interval, similar to the observed 19 ºC to 31 ºC SST range, however TEX₈₆ maxima and minima are not tied to contemporaneous SST seasonal maxima and minima, suggesting a delay between production and export from the mixed layer. This study supports the conclusion that the TEX₈₆ signal in GoM sediments reflects production in the overlying water column (not terrestrially derived or laterally advected material) and can reasonably be interpreted as a mean annual SST proxy in the GoM.