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- Title
Enriched Regions of <sup>228</sup>Ra Along the U.S. GEOTRACES Pacific Meridional Transect (GP15).
- Authors
Moore, Willard S.; Charette, Matthew A.; Henderson, Paul B.; Hammond, Douglas E.; Kemnitz, Nathaniel; Le Roy, Emilie; Kwon, Eun Young; Hult, Mikael
- Abstract
The half‐life of 228Ra (5.7 years) aligns well with near‐surface and near‐bottom ocean mixing timescales. Because 228Ra 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°W encountered several regions in the upper ocean where 228Ra was enriched. These enrichments follow surface and subsurface ocean current patterns and pair with earlier measurements of 228Ra 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° and 32°N. was sourced from the west by the North Pacific Current; another shallow enriched region between 11° and 5° N was also sourced from the west by the North Equatorial Countercurrent. Subsurface enrichments (100–400 m) between 18 and 47°N were associated with Central Mode Water and North Pacific Intermediate Water. The 228Ra activities in the upper Pacific were six times lower than activities in the Atlantic. In deep waters the primary enrichment was 27°–47°N. Two stations (32° and 37°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 223Ra (half‐life = 11 days) above the Puna Ridge near Hawaii. Plain Language Summary: The international GEOTRACES program aims to identify processes and quantify fluxes that control the distribution of trace elements and isotopes (TEIs) in the ocean. Some TEIs are important micronutrients, which may control biological productivity. Others may become concentrated as they pass up the food web and reach potentially harmful levels in some seafood. But measurements of these TEIs are not adequate to determine controlling processes or quantify their fluxes in the marine environment. Radionuclide tracers provide a means to link TEI measurements with fluxes and processes. Here we employ measurements of 228Radium (half‐life = 5.75 years) to trace water movements and current speeds in the upper Pacific and to determine regions near the seabed where sediment‐water interactions are most intense. The upper ocean measurements confirmed a broad circulation pattern that brought water from the Asian margin to the northeast Pacific at speeds of 2–8 cm/sec, in agreement with other estimates of these current speeds. Near the seabed we discovered a region of enhanced 228Ra activity indicating intense sediment‐water interactions. Such regions may control the release of TEIs from the seabed to the water column. We anticipate others will utilize these data to elucidate aspects of other TEI distributions. Key Points: Several open‐ocean regions of enriched 228Ra were documented in the North and Equatorial Pacific at 152°WThe upper ocean 228Ra‐enriched regions correspond to major surface and subsurface currents transporting water from the Asian marginOne seabed region near 32°N was especially enriched in 228Ra
- Subjects
ALASKA; HAWAII; TAHITI (French Polynesia : Island); OCEANIC mixing; BIOLOGICAL productivity; WATER currents; CONTINENTAL margins; OCEAN currents; FOOD chains; OCEAN bottom; OCEAN mining; OCEAN
- Publication
Journal of Geophysical Research. Oceans, 2024, Vol 129, Issue 3, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2023JC020564