Enriched Regions of ²²⁸Ra Along the US GEOTRACES Pacific Meridional Transect (GP15)

The half-life of Ra-228 (5.7 years) aligns well with near-surface and near-bottom ocean mixing timescales. Because Ra-228 is sourced from sediments, regions of enhanced activity represent water that has recently interacted with sediments on the continental margin or seabed. The GP15 meridional transect from Alaska to Tahiti along152 degrees W encountered several regions in the upper ocean where( 228)Ra was enriched. These enrichments follow surface and subsurface ocean current patterns and pair with earlier measurements of Ra-228 and transient radionuclides to reveal the origins of these enriched regions. An enriched region at Alaska margin stations 1-3 was sourced locally but did not extend to the Alaskan trench at station 4. A large shallow region between 47 °rees and 32 degrees N. was sourced from the west by the North Pacific Current; another shallow enriched region between 11 degrees and 5 degrees N was also sourced from the west by the North Equatorial Countercurrent. Subsurface enrichments (100-400 m) between 18 and 47 degrees N were associated with Central Mode Water and North Pacific Intermediate Water. The Ra-228 activities in the upper Pacific were six times lower than activities in the Atlantic. In deep waters the primary enrichment was 27 degrees -47 degrees N. Two stations (32 degrees and 37 degrees N) were especially enriched, having near-bottom inventories several times greater than other stations. With these two exceptions the remaining Pacific stations exhibited averaged inventories lower than those in the Atlantic. There was one region of enriched Ra-223 (half-life = 11 days) above the Puna Ridge near Hawaii.