Single Tests of Thermocline Dwelling Foraminifera Globorotalia inflata as Recorder of Upper Water Column Structure off Mauritania (NW Africa): Methodology and Paleoceanographic Use

Abstract

Thermocline‐dwelling foraminifera calcify over a depth range of several hundred meters; analysis of individual shells therefore allows insight to the hydrography of the upper water column. We analyzed δ18O, δ13C, and Mg/Ca of individual tests of the planktonic foraminifera Globorotalia inflata from a sediment core (GeoB7926‐2) obtained from 20°N in the eastern tropical Atlantic. To facilitate sample throughput, tests were cleaned before Mg/Ca analysis using a pipette robot. The eight samples came from five time periods with contrasting climate states. Median reconstructed temperatures were lowest during the warmth of the Bølling Allerød BA) (11.8°C), while highest temperatures (>14°C) were recorded during the cold periods of the Last Glacial Maximum (LGM), late Heinrich Stadial 1 (HS1), and the Younger Dryas (YD). Southward shift in the subtropical gyre during Northern Hemisphere cold periods and modulation by upwelling could explain the temperature change but not all of the salinity change. δ18Oseawater‐IVC indicated that salinity was higher than the global average during the LGM, with very high salinity excursions in HS1 and a smaller excursion in the YD. The upwelling signature was most strongly imprinted on range in δ13C. The large changes in salinity and δ13C between time slices cannot be explained by upwelling intensity but indicate the presence of a very saline water mass, with low δ13C, in the eastern North Atlantic subsurface during Northern Hemisphere cold periods.

New Perspectives on the 143Nd/144Nd Palaeoceanographic Tracer on Foraminifera: The State‐of‐the‐Art Frontiers of Analytical Methods

“…Analyses of Mn/Ca and Fe/Ca were used as indicators of Fe-Mn crusts/coatings, while Nd/Ca analyses (Table 1) were used to estimate approximate Nd concentrations of particular samples to determine the dominant Nd carrier phases in the analyzed samples (e.g., Kuhnert & Mulitza, 2011;Skinner et al, 2019;Yu et al, 2005). We set critical thresholds of <0.2 mmol/mol for Al/Ca and <1.6 mmol/mol for Sr/Ca (e.g., Barker et al, 2003;Johnstone et al, 2020), (Ayache et al, 2021;Bialik et al, 2019;Colin et al, 2021;Dubois-Dauphin et al, 2017;Duhamel et al, 2020;Freydier et al, 2001;Garcia-Solsona & Jeandel, 2020;Jiménez-Espejo et al, 2015;Osborne et al, 2010;Tachikawa et al, 2004;Wu et al, 2019) whereas the thresholds of <1,500 μmol/mol for Fe/Ca, <150 μmol/mol for Mn/Ca, and <40 μmol/mol for Ba/Ca were selected arbitrarily based on our experience, mainly because there is no general agreement regarding the current application. The elemental analyses were performed using the Element 2™ sector field HR-ICP-MS (Thermo Scientific™) housed at the Institute of Geology of the Czech Academy of Sciences (IG CAS) following the methods outlined in detail in Marchitto (2006).…”