TY - JOUR
T1 - Planktic foraminiferal Na/Ca
T2 - A potential proxy for seawater calcium concentration
AU - Zhou, Xiaoli
AU - Rosenthal, Yair
AU - Haynes, Laura
AU - Si, Weimin
AU - Evans, David
AU - Huang, Kuo Fang
AU - Hönisch, Bärbel
AU - Erez, Jonathan
N1 - Publisher Copyright:
© 2021 Elsevier Ltd
PY - 2021/7/15
Y1 - 2021/7/15
N2 - The reconstruction of seawater calcium concentration ([Ca2+]SW) in the geologic past is crucial to our understanding of biogeochemical processes and elemental cycling as linked to long-term climate change. Published [Ca2+]SW estimates for the Cenozoic differ from each other in both the direction and magnitude of the change, and are associated with large uncertainties. Here we demonstrate the potential of Na/Ca in planktic foraminifera as a new proxy for reconstructing Cenozoic variations in seawater Na/Ca. Because of the long oceanic residence time of Na+ (≫50 Myr), variations in foraminiferal Na/Ca should in principle reflect changes in [Ca2+]SW. Our culture experiments on live planktic species validate the approach, showing that foraminiferal Na/Ca responds to changes in [Ca2+]SW when [Na+]SW is kept constant, consistent with previous experiments on a shallow water benthic foraminifer and on inorganic calcite. The culture study suggests that planktic Na/Ca is affected, to a small extent, by calcite saturation state (Ωcalcite) but not resolvably affected by temperature or salinity. A core tops transect of the planktic foraminifer Trilobatus sacculifer shows similar decreasing trends in Na/Ca and Sr/Ca with water depth that can be associated with dissolution of the tests. The strong covariance with Sr supports the hypothesis that a dominant fraction of the Na resides in lattice positions within the calcite test. Down core planktic foraminiferal records from the Atlantic and Pacific Oceans consistently show 13–28% lower Na/Ca during the mid-Miocene than during the late Pleistocene. After considering the effects of temperature, salinity and diagenesis, we conclude that the observed down core decrease in Na/Ca primarily reflects changes in seawater calcium concentration. Using the calibrations generated from our culture experiments and core top data we reconstructed [Ca2+]SW, suggesting [Ca2+]SW in the mid-Miocene was 46 ± 22% higher than at present, and decreased toward present with a pattern resembling the Neogene climate evolution represented by the benthic δ18O record. The new reconstruction of [Ca2+]SW for the past 16 Myr narrows the range suggested by other [Ca2+]SW proxies, and potentially offers a new way to generate continuous records of seawater calcium concentration at sub-million years resolution.
AB - The reconstruction of seawater calcium concentration ([Ca2+]SW) in the geologic past is crucial to our understanding of biogeochemical processes and elemental cycling as linked to long-term climate change. Published [Ca2+]SW estimates for the Cenozoic differ from each other in both the direction and magnitude of the change, and are associated with large uncertainties. Here we demonstrate the potential of Na/Ca in planktic foraminifera as a new proxy for reconstructing Cenozoic variations in seawater Na/Ca. Because of the long oceanic residence time of Na+ (≫50 Myr), variations in foraminiferal Na/Ca should in principle reflect changes in [Ca2+]SW. Our culture experiments on live planktic species validate the approach, showing that foraminiferal Na/Ca responds to changes in [Ca2+]SW when [Na+]SW is kept constant, consistent with previous experiments on a shallow water benthic foraminifer and on inorganic calcite. The culture study suggests that planktic Na/Ca is affected, to a small extent, by calcite saturation state (Ωcalcite) but not resolvably affected by temperature or salinity. A core tops transect of the planktic foraminifer Trilobatus sacculifer shows similar decreasing trends in Na/Ca and Sr/Ca with water depth that can be associated with dissolution of the tests. The strong covariance with Sr supports the hypothesis that a dominant fraction of the Na resides in lattice positions within the calcite test. Down core planktic foraminiferal records from the Atlantic and Pacific Oceans consistently show 13–28% lower Na/Ca during the mid-Miocene than during the late Pleistocene. After considering the effects of temperature, salinity and diagenesis, we conclude that the observed down core decrease in Na/Ca primarily reflects changes in seawater calcium concentration. Using the calibrations generated from our culture experiments and core top data we reconstructed [Ca2+]SW, suggesting [Ca2+]SW in the mid-Miocene was 46 ± 22% higher than at present, and decreased toward present with a pattern resembling the Neogene climate evolution represented by the benthic δ18O record. The new reconstruction of [Ca2+]SW for the past 16 Myr narrows the range suggested by other [Ca2+]SW proxies, and potentially offers a new way to generate continuous records of seawater calcium concentration at sub-million years resolution.
KW - Neogene
KW - Paleoproxy
KW - Planktic foraminiferal Na/Ca
KW - Seawater Ca concentration
UR - http://www.scopus.com/inward/record.url?scp=85106914396&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2021.04.012
DO - 10.1016/j.gca.2021.04.012
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AN - SCOPUS:85106914396
SN - 0016-7037
VL - 305
SP - 306
EP - 322
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -