The influence of salinity on Mg/Ca in planktic foraminifers – Evidence from cultures, core-top sediments and complementary δ18O

Hönisch, Bärbel and Allen, Katherine A. and Lea, David W. and Spero, Howard J. and Eggins, Stephen M. and Arbuszewski, Jennifer and deMenocal, Peter and Rosenthal, Yair and Russell, Ann D. and Elderfield, Henry (2013) The influence of salinity on Mg/Ca in planktic foraminifers – Evidence from cultures, core-top sediments and complementary δ18O. Geochimica Et Cosmochimica Acta, 121. pp. 196-213. ISSN 0016-7037 DOI 10.1016/j.gca.2013.07.028

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Abstract

The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers. This study compiles new and published laboratory culture experiment data from the planktic foraminifers Orbulina universa, Globigerinoides sacculifer and Globigerinoides ruber, in which salinity was varied but temperature, pH and light were held constant. Combining new data with results from previous culture studies yields a Mg/Ca-sensitivity to salinity of 4.4 ± 2.3%, 4.7 ± 1.2%, and 3.3 ± 1.7% per salinity unit (95% confidence), respectively, for the three foraminifer species studied here. Comparison of these sensitivities with core-top data suggests that the much larger sensitivity (27 ± 4% per salinity unit) derived from Atlantic core-top sediments in previous studies is not a direct effect of salinity. Rather, we suggest that the dissolution correction often applied to Mg/Ca data can lead to significant overestimation of temperatures. We are able to reconcile culture calibrations with core-top observations by combining evidence for seasonal occurrence and latitude-specific habitat depth preferences with corresponding variations in physico-chemical environmental parameters. Although both Mg/Ca and δ18O yield temperature estimates that fall within the bounds of hydrographic observations, discrepancies between the two proxies highlight unresolved challenges with the use of paired Mg/Ca and δ18O analyses to reconstruct paleo-salinity patterns across ocean basins. The first step towards resolving these challenges requires a better spatially and seasonally resolved δ18Osw archive than is currently available. Nonetheless, site-specific reconstructions of salinity change through time may be valid.

Item Type: Article
Uncontrolled Keywords: 2013AREP; IA66;
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: Geochimica Et Cosmochimica Acta
Volume: 121
Page Range: pp. 196-213
Identification Number: 10.1016/j.gca.2013.07.028
Depositing User: Sarah Humbert
Date Deposited: 09 Nov 2013 16:20
Last Modified: 15 Nov 2013 14:19
URI: http://eprints.esc.cam.ac.uk/id/eprint/2900

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