Holocene evolution of sea-surface temperature and salinity in the Gulf of Mexico

Planktic foraminiferal geochemistry was used to reconstruct Holocene SST and δ18Osw variations in the northwestern GoM. Marine sediment core 2010-GB2-GC1 was raised from the Garrison Basin (26.67°N, 93.92°W) at a water depth of 1776 meters (m), aboard the R/V Cape Hatteras in April 2010. An age model was built using a Monte Carlo approach on the Bayesian posterior ages from 15 radiocarbon measurements on upper-ocean planktic foraminifera. Researchers measured the δ18O (number (n) = 265) and magnesium to-calcium (Mg/Ca) ratio (n = 275) composition in the calcareous shells of the white variety of planktic foraminifera Globigerinoides ruber (hereafter G. ruber). For this study, researchers did not discriminate between different morphotypes of G. ruber. Approximately 70-100 individual shells were picked from 0.5 centimeter (cm) sediment intervals (corresponding to an average time resolution of ∼45 years (yrs) per sample), after which they were gently crushed and homogenized. A large fraction of this mixture was kept aside for trace metal analysis whereas the remnant portion was used for stable isotope analysis, which was performed on an isotope ratio mass spectrometer housed at the University of Texas at Austin. All δ18O values are reported in permil relative to the Vienna Pee Dee Belemnite standard notation (‰, VPDB). Based on an external standard, NBS-19 (n = 89), the 1σ analytical precision was ±0.03‰ for δ13C and ±0.06‰ for δ18O, consistent with the long-term precision of the instrumental setup (0.06‰ for δ13C and 0.08‰ for δ18O). Note that reconstructed and observed δ18Osw values are reported in permil relative to the Vienna Standard Mean Ocean Water notation (‰, VSMOW). Elemental ratios were measured at the USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) following established foraminiferal cleaning techniques (Barker and others, 2003; note that the reductive cleaning step was not used). Mg/Ca analysis was performed using inductively coupled plasma optical emission spectrometry utilizing protocols similar to those reported in Thirumalai and others (2018). Along with an internal gravimetric standard which was used to monitor drift and accuracy, repeat measurements on external, matrix-matched standards yielded an analytical precision of ±0.15% (1σ = 0.02 mmol mol−1). Researchers inferred sampling uncertainty from replicate measurements of different sets of G. ruber tests from the same sample depth (n = 30), which was ±3% (0.10 mmol/mol) and ±0.08‰ respectively for Mg/Ca and δ18O analysis. Both analytical and sampling uncertainty were taken into account while computing SST and δ18Osw.