Coral reefs - Bomb Radiocarbon
Radiocarbon and the South Pacific Gyre - Climate and Fisheries
Wednesday, August 18, 2021
BOMB-PRODUCED RADIOCARBON ACROSS THE SOUTH PACIFIC GYRE — A NEW RECORD FROM AMERICAN SAMOA WITH UTILITY FOR FISHERIES SCIENCE
Allen H Andrews, Nancy G Prouty, Olivia M Cheriton
Abstract: Coral skeletal structures can provide a robust record of nuclear bomb produced 14C with valuable insight into air-sea exchange processes and water movement with applications to fisheries science. To expand these records in the South Pacific, a coral core from Tutuila Island, American Samoa was dated with density band counting covering a 59-yr period (1953–2012). Seasonal signals in elemental ratios (Sr/Ca and Ba/Ca) and stable carbon (δ13C) values across the coral core corroborated the well-defined annual band structure and highlighted an ocean climate shift from the 1997–1998 El Niño. The American Samoa coral 14C measurements were consistent with other regional records but included some notable differences across the South Pacific Gyre (SPG) at Fiji, Rarotonga, and Easter Island that can be attributed to decadal ocean climate cycles, surface residence times and proximity to the South Equatorial Current. An analysis of the post-peak 14C decline associated with each coral record indicated 14C levels are beginning to merge for the SPG. This observation, coupled with otolith measurements from American Samoa, reinforces the perspective that bomb 14C dating can be performed on fishes and other marine organisms of the region using the post-peak 14C decline to properly inform fisheries management in the South Pacific.
Dated X-ray of the American Samoa coral core that shows the milled extractions as lower density slots. The record begins at the collection date in 2012 and clearly covers regularly spaced bands through to 1953 for a total of 59 yr of growth.
Plot of the 14C record from American Samoa (1953–2012) with other regional coral records from the South Pacific Gyre (Fiji, Rarotonga and Easter Island; Toggweiler et al. 1991; Guilderson et al. 2000; Biddulph et al. 2006). The density band counting performed on the American Samoa coral was corroborated with annual strontium:calcium (Sr/Ca) peaks (8.8–9.8 mmol·mol-1) that can be attributed to annual SST changes (range of ∼27–30oC). The 14C levels associated with a recovery from the 1997–1998 ENSO event function as a regional marker that can be timed to ocean climate patterns. Post-peak 14C levels for each coral record exhibit a monotonic decline after 1985 that appear to converge near 2030 (Sen’s slope: American Samoa = –2.42, Rarotonga = –2.77, and Easter Island = – 3.42 ‰·yr–1). Fish otolith 14C measurements (open grey circles in 2013–2015) follow the declining trend exhibited by the American Samoa coral record (Supplementary Table 1). Prebomb 14C levels recorded in shell and coral fragments among locations were consistent with each regional coral record (black square = American Samoa, grey X = Fiji, blue diamond = Rarotonga, and yellow circle = Easter Island; Petchey et al. 2008).
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