Hydrothermal vents: A previously unrecognized source of actinium-227 to the deep ocean

Abstract

Actinium-227 (t1/2=21.77y) is produced by decay of protactinium-231 (t1/2=32,760y). Abyssal sediments are enriched in 231Pa and thus provide a source of the more soluble 227Ac to the deep-sea, where it can be used as a tracer of deep ocean diapycnal mixing. In this study, we examine the distribution of dissolved and particulate 227Ac within a neutrally buoyant hydrothermal plume overlying the TAG vent field (26.14°N, 44.83°W, Mid-Atlantic Ridge) collected during the U.S. GEOTRACES North Atlantic Transect (2011). Elevated activities of both particulate and dissolved 227Ac within the plume indicate that hydrothermal vents may be a previously unrecognized source of this isotope to the marine environment. We contend that vent fluids are the main source of the 227Ac enrichment. Based on the activities of dissolved and particulate 227Ac within the plume, a 3He dilution model was used to derive a hydrothermal 227Ac end-member activity of 1.5×104–2.5×104dpmm−3. Assuming that TAG is characteristic of all vents along the Mid-Atlantic Ridge, the hydrothermal source of 227Ac to the deep Atlantic is estimated to be 1.9–5.8% of the flux from Atlantic sediments. Although this hydrothermal flux may not have a large impact on the total oceanic inventory of 227Ac, it must be considered when using this isotope as a tracer of deep ocean mixing in areas where hydrothermal activity is present.