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Evaluating shallow-water carbonates as archives for seawater zinc isotope compositions: implications for palaeoceanographic studies.

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Title: Evaluating shallow-water carbonates as archives for seawater zinc isotope compositions: implications for palaeoceanographic studies.
Authors: Zhang, Chengyu1 (AUTHOR), Cao, Mengchun1 (AUTHOR) cmc@nju.edu.cn, Li, Na1 (AUTHOR)
Source: Geochimica et Cosmochimica Acta. Feb2026, Vol. 414, p1-14. 14p.
Subjects: Paleoceanography, Marine productivity, Marine phytoplankton, Sediments, Zinc compounds, Sedimentation & deposition, Seawater composition, Marine sediments
Geographic Terms: South China Sea
Abstract: Zinc (Zn) is an essential micronutrient for marine phytoplankton and exhibits a strong linear correlation with the major nutrient silicon in modern oceans. In recent years, Zn isotopes (δ66Zn) in marine carbonates have been increasingly used as proxies for reconstructing past marine productivity. However, improved understanding of the modern marine Zn cycle suggests that this potential link may be obscured by multiple biotic and abiotic factors. Additionally, the diverse mineralogical types and broad δ66Zn ranges reported for carbonate archives have raised ongoing debates about the reliability of marine carbonates in recording seawater zinc isotopic composition. Here, we present new δ66Zn data from 39 bulk carbonate sediment samples collected from two shallow-water push cores (Jiuzhang Atoll) in the South China Sea. The samples yield relatively consistent δ66Zn values across depth profiles, averaging +0.28 ± 0.11 ‰ (2SD), suggesting that shallow-water carbonates may serve as promising archives for palaeoceanographic reconstructions. However, comprehensive comparison with other recent carbonates—considering both Zn isotopic compositions and concentrations—suggests that the original seawater δ66Zn signatures recorded in ancient carbonate rocks were likely modified by depositional and diagenetic processes. These processes include mixing between inorganic primary calcite and biogenic carbonates, and the redox conditions of pore waters during deposition. Furthermore, the low Zn concentrations in modern primary carbonates—only about 3 % of those in ancient carbonate rocks—highlight their high susceptibility to diagenetic alteration. In addition, the wide spatial variability of modern surface seawater δ66Zn complicates the establishment of a consistent isotopic offset for calibrating ancient carbonates. Together, our findings call for caution when employing carbonate-hosted Zn isotopes as proxies for past ocean productivity, particularly given the unresolved uncertainties regarding the relationship between seawater δ66Zn and marine primary production. [ABSTRACT FROM AUTHOR]
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Abstract:Zinc (Zn) is an essential micronutrient for marine phytoplankton and exhibits a strong linear correlation with the major nutrient silicon in modern oceans. In recent years, Zn isotopes (δ66Zn) in marine carbonates have been increasingly used as proxies for reconstructing past marine productivity. However, improved understanding of the modern marine Zn cycle suggests that this potential link may be obscured by multiple biotic and abiotic factors. Additionally, the diverse mineralogical types and broad δ66Zn ranges reported for carbonate archives have raised ongoing debates about the reliability of marine carbonates in recording seawater zinc isotopic composition. Here, we present new δ66Zn data from 39 bulk carbonate sediment samples collected from two shallow-water push cores (Jiuzhang Atoll) in the South China Sea. The samples yield relatively consistent δ66Zn values across depth profiles, averaging +0.28 ± 0.11 ‰ (2SD), suggesting that shallow-water carbonates may serve as promising archives for palaeoceanographic reconstructions. However, comprehensive comparison with other recent carbonates—considering both Zn isotopic compositions and concentrations—suggests that the original seawater δ66Zn signatures recorded in ancient carbonate rocks were likely modified by depositional and diagenetic processes. These processes include mixing between inorganic primary calcite and biogenic carbonates, and the redox conditions of pore waters during deposition. Furthermore, the low Zn concentrations in modern primary carbonates—only about 3 % of those in ancient carbonate rocks—highlight their high susceptibility to diagenetic alteration. In addition, the wide spatial variability of modern surface seawater δ66Zn complicates the establishment of a consistent isotopic offset for calibrating ancient carbonates. Together, our findings call for caution when employing carbonate-hosted Zn isotopes as proxies for past ocean productivity, particularly given the unresolved uncertainties regarding the relationship between seawater δ66Zn and marine primary production. [ABSTRACT FROM AUTHOR]
ISSN:00167037
DOI:10.1016/j.gca.2025.12.012