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

• Published in: Biogeosciences
• Main Authors: Deng, Feifei, Henderson, Gideon M., Castrillejo, Maxi, Perez, Fiz F., Steinfeldt, Reiner
• Format: Text
• Language: English
• Published: 2018
• Subjects: Irminger Sea; Labrador Sea; North Atlantic
• Online Access: https://doi.org/10.5194/bg-15-7299-2018; https://www.biogeosciences.net/15/7299/2018/

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