High precision ²³⁰Th and ²³²Th in the Norwegian Sea and Denmark by thermal ionization mass spectrometry

Abstract

Seawater samples (1–2 liters) were collected from the Norwegian Sea and Denmark Strait and analyzed for 230 Th and 232 Th using highly sensitive thermal ionization mass spectrometry (TIMS). Depth profiles of dissolved 230 Th and 232 Th are characterized by surface water minima (<1 fg/kg, <5 pg/kg), subsurface maxima (12 fg/kg, 134 pg/kg), and intermediate concentrations that progressively decrease toward the bottom (∼5 fg/kg, ∼17 pg/kg), respectively. The lack of an increase in 230 Th with depth is suggested to result from the short ventilation age of Norwegian Sea Deep Water combined with enhanced scavenging near the basin margins. The 230 Th maximum is attributed to advection of high 230 Th in the Arctic Intermediate Water, whereas the 232 Th maximum may be related to a particulate source. The low dissolved 230 Th and 232 Th concentrations observed in the NADW formation regions implies a minor advective export of these long‐lived Th tracers to the North Atlantic.