5:15 PM - 7:15 PM
[MIS14-P15] Evaluation of U/Ca and U/Mn Analytical Methods for Foraminiferal Shell Coatings Aiming to Reconstruct Bottom Water Oxygen Concentrations off Shimokita Peninsula
Keywords:U/Ca, U/Mn, Bottom water oxygen, Planktonic foraminiferal shell coatings, Benthic foraminiferal shell coatings
In the subarctic region of the North Pacific, sediment analyses have revealed that the waters at several hundred meters depth off the Shimokita Peninsula experienced anoxic conditions during warming events of the deglaciation, the transition from the Last Glacial Period to the Holocene. Bottom water oxygen concentrations (Bottom Water Oxygen: BWO) have been reconstructed using sedimentary structures and benthic foraminiferal assemblages. BWO variations are crucial for understanding past ocean circulation and carbon cycles, and qualitative reconstructions using sedimentary structures and benthic foraminiferal assemblages have been the dominant approach. However, these methods face challenges in quantitatively estimating BWO. Therefore, this study evaluates the applicability of a newly proposed method using U/Ca and U/Mn in foraminiferal shell coatings for quantitative BWO reconstruction.
This study used piston core SMK2-PC (950 m depth) collected during the MR23-05 cruise off the Shimokita Peninsula. The core is located within a depth range strongly influenced by the oxygen minimum zone (OMZ) and is likely to preserve past BWO variations. Four stratigraphic sections, deposited at different time intervals, were selected for analysis. Observations of sedimentary structures, analysis of benthic foraminiferal assemblages, and measurements of U/Ca and U/Mn of foraminiferal shell were conducted. The study assessed the effects of different cleaning methods, size fractions, and species of foraminifera on measurement results to determine the optimal analytical conditions. Additionally, a comparison was made with existing BWO proxies, such as sedimentary structures and benthic foraminiferal assemblages, to verify the consistency of U/Ca and U/Mn as BWO proxies.
In the cleaning comparison experiment, where ultrasonic cleaning, oxidative cleaning, leaching, and reductive cleaning were sequentially added to evaluate the impact of each cleaning step on the measurement values, it was confirmed that reductive cleaning in particular led to the removal of coatings, resulting in the underestimation of U and Mn concentrations. Furthermore, foraminiferal species and size significantly influenced measurement results, highlighting the need to standardize these parameters during analysis. U/Ca and U/Mn values from benthic foraminifera were consistent with sedimentary structures and benthic foraminiferal assemblage analyses, suggesting their potential as reliable BWO proxies. In contrast, values from planktonic foraminifera did not align with other proxies, and layers strongly affected by secondary calcification showed significantly overestimated U/Ca and U/Mn values. Given this clear overestimation in secondary calcification affected layers, U/Ca and U/Mn values from foraminiferal shells influenced by secondary calcification are likely unsuitable for BWO reconstruction.
The results of this study demonstrate that U/Ca and U/Mn in benthic foraminifera are useful proxies for BWO reconstruction and highlight the necessity of considering secondary calcification effects. Future research should compare measurements across different benthic foraminiferal species to improve the accuracy of BWO reconstruction and contribute to understanding long-term deoxygenation events in the waters off the Shimokita Peninsula.
This study used piston core SMK2-PC (950 m depth) collected during the MR23-05 cruise off the Shimokita Peninsula. The core is located within a depth range strongly influenced by the oxygen minimum zone (OMZ) and is likely to preserve past BWO variations. Four stratigraphic sections, deposited at different time intervals, were selected for analysis. Observations of sedimentary structures, analysis of benthic foraminiferal assemblages, and measurements of U/Ca and U/Mn of foraminiferal shell were conducted. The study assessed the effects of different cleaning methods, size fractions, and species of foraminifera on measurement results to determine the optimal analytical conditions. Additionally, a comparison was made with existing BWO proxies, such as sedimentary structures and benthic foraminiferal assemblages, to verify the consistency of U/Ca and U/Mn as BWO proxies.
In the cleaning comparison experiment, where ultrasonic cleaning, oxidative cleaning, leaching, and reductive cleaning were sequentially added to evaluate the impact of each cleaning step on the measurement values, it was confirmed that reductive cleaning in particular led to the removal of coatings, resulting in the underestimation of U and Mn concentrations. Furthermore, foraminiferal species and size significantly influenced measurement results, highlighting the need to standardize these parameters during analysis. U/Ca and U/Mn values from benthic foraminifera were consistent with sedimentary structures and benthic foraminiferal assemblage analyses, suggesting their potential as reliable BWO proxies. In contrast, values from planktonic foraminifera did not align with other proxies, and layers strongly affected by secondary calcification showed significantly overestimated U/Ca and U/Mn values. Given this clear overestimation in secondary calcification affected layers, U/Ca and U/Mn values from foraminiferal shells influenced by secondary calcification are likely unsuitable for BWO reconstruction.
The results of this study demonstrate that U/Ca and U/Mn in benthic foraminifera are useful proxies for BWO reconstruction and highlight the necessity of considering secondary calcification effects. Future research should compare measurements across different benthic foraminiferal species to improve the accuracy of BWO reconstruction and contribute to understanding long-term deoxygenation events in the waters off the Shimokita Peninsula.