Planktic foraminiferal I/Ca ratios as a proxy for past seawater oxygen concentrations

Abstract

Marine carbonate I/Ca ratios are an increasingly used proxy for palaeo-oxygen concentrations. Planktic foraminifera I/Ca ratios can give valuable information about past oxygen concentrations in the upper water column and intermediate water ventilation, influencing global geochemical cycles over glacial/interglacial time scales. Three aspects of the I/Ca proxy in planktic foraminifera from the Indian and Pacific Oceans were studied here. The first part addresses questions about the methodology of the analysis. For mass spectrometer analysis, it confirms three different bases as potential stabilisers to prevent iodine loss after acidic sample dissolution. Furthermore, the importance of sufficient cleaning with hydrogen peroxide before analysis is highlighted. In the second part, Indian and Pacific core top samples were used to test the relation of recent water oxygen concentrations with I/Ca of various planktic foraminifera species after selecting the core tops of interglacial age by δ18O stratigraphy and radiocarbon dating. The I/Ca follows thermocline water oxygen concentrations even when the foraminifera live higher up in the mixed layer. Finally, three sediment cores are used from the California Margin and one from the Ontong Java Plateau in the West Pacific to study glacial/interglacial changes in I/Ca of planktic foraminifera. Little change and consistently low values of I/Ca are found along the California margin indicating low oxygen concentrations, which contrasts with previous studies that show significant changes in oxygen concentration at different water depths simultaneous to northern hemisphere climate events like the Bølling/Allerød warm periods the Younger Dryas and the early Holocene. The West Pacific core showed one large swing to higher I/Ca ratios during the last deglacial transition indicating better oxygenation in the upper water column during the Holocene. This is the opposite of what previous reviews have shown for other parts of the Pacific.