Improving temperature estimates derived from Mg/Ca of planktonic foraminifera using X-ray computed tomography–based dissolution index, XDX

• Published in: Paleoceanography
• Main Authors: Johnstone, Heather J. H., Yu, Jimin, Elderfield, Henry, Schulz, Michael
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2011
• Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems; Pacific; Indian; Planktonic foraminifera
• Online Access: http://eprints.esc.cam.ac.uk/2069/; http://eprints.esc.cam.ac.uk/2069/1/Elderfield_Paleoceanography_26_2011.pdf; http://www.agu.org/pubs/crossref/2011/2009PA001902.shtml; https://doi.org/10.1029/2009PA001902

Temperatures derived from Mg/Ca ratios of the calcite tests of planktonic foraminifera are distorted when samples are partially dissolved, and methods are required to quantify this source of inaccuracy. Here we compare a dissolution index (XDX), based on X-ray computed tomography scans, to Mg/Ca of four species of foraminifera ( G. ruber (white), G. sacculifer (without sac), N. dutertrei, and P. obliquiloculata) from core top sediments from the tropical Pacific, Atlantic, and western Indian Ocean. Deepwater calcite saturation values (Δ[CO 3 2−]) at the sites ranged from 55 to −23 μmol/kg. An estimate of ΔMg/Ca (reduction in Mg/Ca due to dissolution) was made for each sample. ΔMg/Ca decreased linearly from deepwater Δ[CO 3 2−] values of between 10 (±4) and 15 (±5) μmol/kg. These values are minimum estimates of the threshold below which Mg/Ca is affected by dissolution, as they are limited by assumptions made in calculating ΔMg/Ca. Sensitivity of Mg/Ca to Δ[CO 3 2−] was greatest for G. ruber, where Mg/Ca decreased by 0.102 (±0.036) mmol/mol per μmol/kg. Sensitivity was similar for G. sacculifer (0.047 ± 0.015 mmol/mol per μmol/kg), N. dutertrei (0.037 ± 0.010 mmol/mol per μmol/kg), and P. obliquiloculata (0.040 ± 0.008 mmol/mol per μmol/kg). Sensitivity was similar at all sites for each species, excepting an apparently greater response for N. dutertrei from the Caribbean compared to other sites. Calibrations between XDX and ΔMg/Ca provide a means to estimate dissolution bias on Mg/Ca. Poor correlation between XDX and δ 18O suggests that, for the small sample size typical for analysis, variability in initial δ 18O overwhelms dissolution effects.