Sodium incorporation in foraminiferal calcite: an evaluation of the Na/Ca salinity proxy and evidence for multiple Na-bearing phases
Sodium incorporation in foraminiferal calcite: an evaluation of the Na/Ca salinity proxy and evidence for multiple Na-bearing phases
The ratio of sodium to calcium (Na/Ca) in foraminiferal calcite has been proposed as a proxy for salinity, yet relatively little is known about the incorporation of sodium into the shells of foraminifera. Ongoing debates include the location of Na in the calcite crystal lattice, the possibility that at least some Na might be complexed with organics, and the influence of spines/spine bases. We present new Na/Ca measurements, determined using both solution and laser ablation ICP-MS, of the planktonic foraminifera Globigerinoides ruber (white) from plankton tows and sediment traps spanning a wide salinity range (32.5–40.7 salinity units), laboratory cultures under varying carbonate chemistry, and globally-distributed core-top samples. Our results show that Na/Ca in recently living foraminifera measured by laser ablation ICP-MS is elevated by up to 5 mmol/mol (∼85%) relative to the same samples measured by solution ICP-MS (the same comparison for Mg/Ca shows excellent agreement between the techniques). Na/Ca in recently living foraminifera measured by laser ablation ICP-MS displays a significant relationship with salinity above ∼36 salinity units with a slope of ∼0.7 mmol/mol/salinity unit; however, only a weak relationship is observed between salinity and Na/Ca measured by solution ICP-MS. We propose that Na is incorporated in at least two discrete phases; a primary phase within the CaCO3 mineral, and a (or likely multiple) secondary phase(s). Possibilities for these secondary phases include residual metastable CaCO3, fluid inclusions, high Na/Ca spine bases, and organics. These secondary phases contribute to spatially-resolved analyses (i.e. laser ablation ICP-MS) of recently living foraminifera but are removed by crushing/oxidative cleaning for solution ICP-MS, and during early diagenesis, as evidenced by the agreement between laser analysis of coretop samples and Na/Ca measured by solution. The amount of one of these secondary phases, or the amount of Na within this phase, appears to vary as a function of salinity, and is likely the principal driver of the previously observed steep Na/Ca-salinity relationship in recently living foraminifera analysed by laser ablation. Overall, we find salinity, temperature, carbonate chemistry, and bottom water saturation state (Ωcalcite) all have a significant but relatively weak effect on Na/Ca in the primary calcite phase. As such, Na/Ca in planktonic foraminifera recovered from sediment cores is unlikely to find widespread utility as a salinity proxy.
Calcite, Carbonate chemistry, Na/Ca, Planktonic foraminifera, Salinity
152-164
Gray, William
b6a672d9-2fc9-4ada-b939-377cd98c19a0
Evans, David
878c65c7-eab9-4362-896b-166e165eb94b
Henehan, Michael
29ad287e-77df-4a9a-9446-d9084a180d2a
Weldeab, Syee
c1bb0462-e3ac-4664-b898-c5bea023fb7a
Lea, David W.
1db267be-d3c2-4068-b493-d80fcd8ca18d
Müller, Wolfgang
360a71f7-0b47-4ff3-8c32-1912d70401aa
Rosenthal, Yair
0130f66f-7653-490b-b323-76956e66c9e1
1 May 2023
Gray, William
b6a672d9-2fc9-4ada-b939-377cd98c19a0
Evans, David
878c65c7-eab9-4362-896b-166e165eb94b
Henehan, Michael
29ad287e-77df-4a9a-9446-d9084a180d2a
Weldeab, Syee
c1bb0462-e3ac-4664-b898-c5bea023fb7a
Lea, David W.
1db267be-d3c2-4068-b493-d80fcd8ca18d
Müller, Wolfgang
360a71f7-0b47-4ff3-8c32-1912d70401aa
Rosenthal, Yair
0130f66f-7653-490b-b323-76956e66c9e1
Gray, William, Evans, David, Henehan, Michael, Weldeab, Syee, Lea, David W., Müller, Wolfgang and Rosenthal, Yair
(2023)
Sodium incorporation in foraminiferal calcite: an evaluation of the Na/Ca salinity proxy and evidence for multiple Na-bearing phases.
Geochimica et Cosmochimica Acta, 348, .
(doi:10.1016/j.gca.2023.03.011).
Abstract
The ratio of sodium to calcium (Na/Ca) in foraminiferal calcite has been proposed as a proxy for salinity, yet relatively little is known about the incorporation of sodium into the shells of foraminifera. Ongoing debates include the location of Na in the calcite crystal lattice, the possibility that at least some Na might be complexed with organics, and the influence of spines/spine bases. We present new Na/Ca measurements, determined using both solution and laser ablation ICP-MS, of the planktonic foraminifera Globigerinoides ruber (white) from plankton tows and sediment traps spanning a wide salinity range (32.5–40.7 salinity units), laboratory cultures under varying carbonate chemistry, and globally-distributed core-top samples. Our results show that Na/Ca in recently living foraminifera measured by laser ablation ICP-MS is elevated by up to 5 mmol/mol (∼85%) relative to the same samples measured by solution ICP-MS (the same comparison for Mg/Ca shows excellent agreement between the techniques). Na/Ca in recently living foraminifera measured by laser ablation ICP-MS displays a significant relationship with salinity above ∼36 salinity units with a slope of ∼0.7 mmol/mol/salinity unit; however, only a weak relationship is observed between salinity and Na/Ca measured by solution ICP-MS. We propose that Na is incorporated in at least two discrete phases; a primary phase within the CaCO3 mineral, and a (or likely multiple) secondary phase(s). Possibilities for these secondary phases include residual metastable CaCO3, fluid inclusions, high Na/Ca spine bases, and organics. These secondary phases contribute to spatially-resolved analyses (i.e. laser ablation ICP-MS) of recently living foraminifera but are removed by crushing/oxidative cleaning for solution ICP-MS, and during early diagenesis, as evidenced by the agreement between laser analysis of coretop samples and Na/Ca measured by solution. The amount of one of these secondary phases, or the amount of Na within this phase, appears to vary as a function of salinity, and is likely the principal driver of the previously observed steep Na/Ca-salinity relationship in recently living foraminifera analysed by laser ablation. Overall, we find salinity, temperature, carbonate chemistry, and bottom water saturation state (Ωcalcite) all have a significant but relatively weak effect on Na/Ca in the primary calcite phase. As such, Na/Ca in planktonic foraminifera recovered from sediment cores is unlikely to find widespread utility as a salinity proxy.
Text
na_multiple_phases_v3
- Accepted Manuscript
Restricted to Repository staff only until 4 March 2025.
Request a copy
More information
Accepted/In Press date: 4 March 2023
e-pub ahead of print date: 11 March 2023
Published date: 1 May 2023
Additional Information:
Funding Information:
This research was in part funded by NSF grant NSF-OCE 1260696 awarded to S. Weldeab and D.W. Lea. We thank Birgit Gaye for providing sediment trap samples from the Bay of Bengal, Bobbi Conard for sampling archived sediment trap samples from the Arabian Sea, Minda Monteagudo for help in the preparation of foraminiferal samples for trace element analysis, Ryan Bu for assistance with ICP-MS analysis, and Niki Gruber and Damian Loher for assistance producing the temporally resolved carbonate chemistry estimates. MJH gratefully acknowledges contributions of sample material from Helen Bostock, Brittney Marshall and Michal Kucera, and practical input from Jonathan Erez, Katy Prentice, James Rae, and the Southampton ‘B-Team’. We thank Gernot Nehrke and four anonymous reviewers for their constructive reviews of our manuscript, as well as the Editors for their guidance.
Funding Information:
This research was in part funded by NSF grant NSF-OCE 1260696 awarded to S. Weldeab and D.W. Lea. We thank Birgit Gaye for providing sediment trap samples from the Bay of Bengal, Bobbi Conard for sampling archived sediment trap samples from the Arabian Sea, Minda Monteagudo for help in the preparation of foraminiferal samples for trace element analysis, Ryan Bu for assistance with ICP-MS analysis, and Niki Gruber and Damian Loher for assistance producing the temporally resolved carbonate chemistry estimates. MJH gratefully acknowledges contributions of sample material from Helen Bostock, Brittney Marshall and Michal Kucera, and practical input from Jonathan Erez, Katy Prentice, James Rae, and the Southampton ‘B-Team’. We thank Gernot Nehrke and four anonymous reviewers for their constructive reviews of our manuscript, as well as the Editors for their guidance.
Publisher Copyright:
© 2023 Elsevier Ltd
Keywords:
Calcite, Carbonate chemistry, Na/Ca, Planktonic foraminifera, Salinity
Identifiers
Local EPrints ID: 477528
URI: http://eprints.soton.ac.uk/id/eprint/477528
ISSN: 0016-7037
PURE UUID: 5a4d2356-22d3-425a-9547-dd2d3e9fbf3d
Catalogue record
Date deposited: 07 Jun 2023 17:14
Last modified: 17 Mar 2024 04:18
Export record
Altmetrics
Contributors
Author:
William Gray
Author:
David Evans
Author:
Michael Henehan
Author:
Syee Weldeab
Author:
David W. Lea
Author:
Wolfgang Müller
Author:
Yair Rosenthal
Download statistics
Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.
View more statistics