Effect of preservation state of planktonic foraminifera tests on the decrease in Mg/Ca due to reductive cleaning and on sample loss during cleaning

• Published in: Chemical Geology
• Main Authors: Johnstone, Heather J. H., Lee, William, Schulz, Michael
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Foraminifera; Mg/Ca; Reductive cleaning; Calcite preservation; Cd-cleaning; Mg-cleaning; /dk/atira/pure/core/subjects/maths; Mathematics; Planktonic foraminifera
• Online Access: https://doi.org/10.1016/j.chemgeo.2015.10.045; https://researchportal.port.ac.uk/portal/en/publications/effect-of-preservation-state-of-planktonic-foraminifera-tests-on-the-decrease-in-mgca-due-to-reductive-cleaning-and-on-sample-loss-during-cleaning(5dfb0f4e-9d48-4f34-bff5-9c647f660e63).html; https://researchportal.port.ac.uk/ws/files/5134011/2015_Johnstone_RedOxClYd_HK_WL.pdf

Four species of planktic foraminifera from core-tops spanning a depth transect on the Ontong Java Plateau were prepared for Mg/Ca analysis both with (Cd-cleaning) and without (Mg-cleaning) a reductive cleaning step. Reductive cleaning caused etching of foraminiferal calcite, focused on Mg-rich inner calcite, even on tests which had already been partially dissolved at the seafloor. Despite corrosion, there was no difference in Mg/Ca of Pulleniatina obliquiloculata between cleaning methods. Reductive cleaning decreased Mg/Ca by an average (all depths) of ~ 4% for Globigerinoides ruber white and ~ 10% for Neogloboquadrina dutertrei . Mg/Ca of Globigerinoides sacculifer (above the calcite saturation horizon only) was 5% lower after reductive cleaning. The decrease in Mg/Ca due to reductive cleaning appeared insensitive to preservation state for G. ruber , N. dutertrei and P. obliquiloculata . Mg/Ca of Cd-cleaned G. sacculifer appeared less sensitive to dissolution than that of Mg-cleaned. Mg-cleaning is adequate, but SEM and contaminants (Al/Ca, Fe/Ca and Mn/Ca) show that Cd-cleaning is more effective for porous species. A second aspect of the study addressed sample loss during cleaning. Lower yield after Cd-cleaning for G. ruber , G. sacculifer and N. dutertrei confirmed this to be the more aggressive method. Strongest correlations between yield and Δ[CO 3 2− ] in core-top samples were for Cd-cleaned G. ruber (r = 0.88, p = 0.020) and Cd-cleaned P. obliquiloculata (r = 0.68, p = 0.030). In a down-core record (WIND28K) correlation, r, between yield values > 30% and dissolution index, XDX, was − 0.61 (p = 0.002). Where cleaning yield < 30% most Mg-cleaned Mg/Ca values were biased by dissolution.