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

Actinium-227 (t(1/2) = 21.77 y) is produced by decay of protactinium-231 (t(1/2) = 32,760 y). Abyssal sediments are enriched in Pa-231 and thus provide a source of the more soluble Ac-227 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 Ac-227 within a neutrally buoyant hydrothermal plume overlying the TAG vent field (26.14 degrees N, 44.83 degrees W, Mid-Atlantic Ridge) collected during the U.S. GEOTRACES North Atlantic Transect (2011). Elevated activities of both particulate and dissolved (227)AC 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 (227)AC enrichment. Based on the activities of dissolved and particulate Ac-227 within the plume, a He-3 dilution model was used to derive a hydrothermal (227)AC end-member activity of 1.5 x 10(4)-2.5 x 10(4) dpm m(-3). Assuming that TAG is characteristic of all vents along the Mid-Atlantic Ridge, the hydrothermal source of Ac-227 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 Ac-227, it must be considered when using this isotope as a tracer of deep ocean mixing in areas where hydrothermal activity is present. (C) 2015 Elsevier B.V. All rights reserved.