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

Actinium-227 (t_1/2=21.77y) is produced by decay of protactinium-231 (t_1/2=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.