Evolution of Pa-231 and Th-230 in overflow waters of the North Atlantic
Résumé
Many palaeoceanographic studies have sought to use the Pa-231/Th-230 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, Th-230 and Pa-231 in waters immediately following ventilation at the start of Atlantic meridional overturning. To that end, full water-column Pa-231 and Th-230 concentrations were measured along the GEOVIDE section, sampling a range of young North Atlantic deep waters. Th-230 and Pa-231 concentrations in the water column are lower than those observed further south in the Atlantic, ranging between 0.06 and 12.01 mu Bq kg(-1) and between 0.37 and 4.80 mu Bq kg(-1), respectively. Both Th-230 and Pa-231 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 Th-230 and Pa-231 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 Th-230 by a strong correlation between Th-230 and water mass age (though this relationship with age is less clear for Pa-231 and the Pa-231/Th-230 ratio). Scavenged Pa-231 and Th-230 were estimated and compared to their potential concentrations in the water column due to ingrowth. This calculation indicates that more Th-230 is scavenged (similar to 80%) than Pa-231 (similar to 40%), consistent with the relatively higher particle reactivity of Th-230. Enhanced scavenging for both nuclides is demonstrated near the seafloor in young over-flow waters. Calculation of the meridional transport of Th-230 and Pa-231 with this new GEOVIDE dataset enables a complete budget for Th-230 and Pa-231 for the North Atlantic. Results suggest that net transport southward of Th-230 and Pa-231 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 Th-230 and 74% of the production for Pa-231. This result confirms a significantly higher advective loss of Pa-231 to the south relative to Th-230 and supports the use of Pa-231/Th-230 to assess meridional transport at a basin scale.