231Pa/230Th ratios in the Atlantic and Indian Oceans and their implications for the application of the ratios as an ocean circulation proxy

Unsupported ²³¹Pa/²³⁰Th ratios have been used widely as a paleoproxy for ocean circulation and as a paleoproductivity proxy; however, some of the inherent assumptions for these proxies have not been thoroughly tested, which would impact how the ratio is interpreted in different regions. Both applications of the ratio influence interpretations of past climate changes, so it is important to determine the extent to which each process impacts the sedimentary ²³¹Pa/²³⁰Th values. Here, we compare ²³¹Pa/²³⁰Th ratios between the Atlantic, Indian, and Pacific Oceans, and within the Atlantic Ocean, as a test of whether or not this ratio serves as a reliable proxy for water mass ventilation age, which is closely related to ocean circulation.

In this study, we present new ²³¹Pa and ²³⁰Th measurements from Indian and Atlantic Ocean sediments and from Indian Ocean seawater samples, alongside previously published Atlantic Ocean water column and sedimentary ²³¹Pa/²³⁰Th ratios. The observed water column dissolved ²³¹Pa/²³⁰Th profiles do not show the expected increase in ratio values with water mass age, which disagrees with the conceptual model for the use of ²³¹Pa/²³⁰Th as a paleocirculation proxy. Dissolved ²³¹Pa/²³⁰Th varies between 0.35 and 0.75 within the depth range of NADW, and there is no apparent correlation with water mass age. The observed ²³¹Pa/²³⁰Th values and profiles are strikingly similar between the western North Atlantic, Western Indian, and North Pacific Oceans, even though these basins have significantly different water mass histories and deep water mass ages.

The water column ²³¹Pa/²³⁰Th values then determine the ratio value of the underlying sediments. So, we then compare ²³¹Pa/²³⁰Th values from Holocene sediments between the ocean basins to determine if the observations agree with our understanding of modern ocean circulation. Atlantic and Indian Ocean sediments are indistinguishable from each other with respect to their ²³¹Pa/²³⁰Th ratios. Taken together, these results indicate that factors other than ventilation age must significantly impact ²³¹Pa/²³⁰Th ratios, and the evidence suggests that fractionation and scavenging intensity may strongly impact particulate and sedimentary ²³¹Pa/²³⁰Th.