Evolution of 231Pa and 230Th in overflow waters of the North Atlantic

Deng, Feifei; Henderson, Gideon M.; Castrillejo, Maxi; Perez, Fiz F.; Steinfeldt, Reiner

Many palaeoceanographic studies have sought to use the inline-formula231Pa∕230Th ratio as a proxy for deep ocean circulation rates in the North Atlantic. As of yet, however, no study has fully assessed the concentration of, or controls on, inline-formula230Th and inline-formula231Pa in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column inline-formula231Pa and inline-formula230Th concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. inline-formula230Th and inline-formula231Pa concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 inline-formulaµBq kginline-formula−1 and between 0.37 and 4.80 inline-formulaµBq kginline-formula−1, respectively. Both inline-formula230Th and inline-formula231Pa profiles generally increase with water depth from surface to deep water, followed by decrease near the seafloor, with this feature most pronounced in the Labrador Sea (LA Sea) and Irminger Sea (IR Sea). Assessing this dataset using extended optimum multi-parameter (eOMP) analysis and CFC-based water mass age indicates that the low values of inline-formula230Th and inline-formula231Pa in water near the seafloor of the LA Sea and IR Sea are related to the young waters present in those regions. The importance of water age is confirmed for inline-formula230Th by a strong correlation between inline-formula230Th and water mass age (though this relationship with age is less clear for inline-formula231Pa and the inline-formula231Pa∕230Th ratio). Scavenged inline-formula231Pa and inline-formula230Th were estimated and compared to their potential concentrations in the water column due to ingrowth. This calculation indicates that more inline-formula230Th is scavenged (inline-formula∼80 %) than inline-formula231Pa (inline-formula∼40 %), consistent with the relatively higher particle reactivity of inline-formula230Th. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young overflow waters. Calculation of the meridional transport of inline-formula230Th and inline-formula231Pa with this new GEOVIDE dataset enables a complete budget for inline-formula230Th and inline-formula231Pa for the North Atlantic. Results suggest that net transport southward of inline-formula230Th and inline-formula231Pa across GEOVIDE is smaller than transport further south in the Atlantic, and indicate that the flux to sediment in the North Atlantic is equivalent to 96 % of the production of inline-formula230Th and 74 % of the production for inline-formula231Pa. This result confirms a significantly higher advective loss of inline-formula231Pa to the south relative to inline-formula230Th and supports the use of inline-formula231Pa∕230Th to assess meridional transport at a basin scale.

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Deng, Feifei / Henderson, Gideon M. / Castrillejo, Maxi / et al: Evolution of 231Pa and 230Th in overflow waters of the North Atlantic. 2018. Copernicus Publications.

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