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.