SURFACE OCEAN RADIOCARBON FROM A PORITES CORAL RECORD IN THE GREAT BARRIER REEF: 1945–2017

Abstract

We present a high-resolution seawater radiocarbon (Δ14C) record from a Porites coral collected from Masthead Island in the southern Great Barrier Reef (GBR) covering the years 1945–2017. The Δ14C values from 1945–1953 (pre-bomb era) averaged –49‰. As a result of bomb-produced 14C in the atmosphere, Δ14C values started to rise rapidly from 1959, levelled off at ∼131‰ in the late 1970s and gradually decreased to ∼40.3‰ by 2017 due to the decrease in the air-sea 14C gradient and the overturning of the 14C ocean reservoir (i.e., surface ocean to subsurface ocean; atmosphere to surface ocean). The Masthead Island record is in agreement with previous 14C coral records from the southern GBR. A comparison between surface ocean and atmospheric Δ14C suggests that, since 2010, the main reservoir of bomb-derived 14C has shifted from the atmosphere to the surface ocean, potentially resulting in reversed 14C flux in regions where the CO2 gradient is favorable. The high-resolution Masthead coral Δ14C sheds light on long-term variability in air-sea exchange and GBR regional ocean dynamics associated with climate change and in conjunction with the previous records provides a robust seawater 14C reference series to date other carbonate samples.

Atmospheric Radiocarbon for the Period 1950–2019

“…After reaching its peak levels, atmospheric Δ 14 C in the NH and SH has decreased since 1963-1964 and 1964-1965, respectively. Decreases in atmospheric Δ 14 C from the mid-1960s to mid-1980s are mainly due to rapid exchange between the atmosphere and the biosphere and oceans (Oeschger et al 1975;Druffel and Suess 1983;Levin and Hesshaimer 2000), while combustion of fossil fuels free of 14 C is the main causal factor for the Δ 14 C decline since the late 1980s and early 1990s (Levin et al 2010;Graven et al 2012a). Since the early and late 2000s, the atmospheric Δ 14 C values have been lower than those of the surface waters in the North and South Pacific Gyres, respectively, indicating the oceans might become a net 14 C source (instead of a net 14 C sink) of the atmosphere (Andrews et al 2016(Andrews et al , 2021Wu et al 2021). The last data points in our compiled monthly data at 2019.375 have respective F 14 C values of 1.0084 and 1.0195 for the NH and SH (see Supplementary Tables 2a-e), which are very close to the pre-bomb F 14 C value of slightly lower than 1.…”

WHEAT SEED (TRITICUM AESTIVUM L.) RADIOCARBON CONCENTRATION OVER THE LAST 75 YEARS

“…Furthermore, since the early 2000s, the North and South Pacific surface ocean have become a source of 14 C, as indicated by 14 C measurements of annually precipitated carbonate of sub-surface living corals of Porites sp. (Andrews et al 2016;Wu et al 2021), influencing the overlying northern and southern hemispheric airmasses. To what extent the North Atlantic also became a source of enriched 14 C would need actualized studies such as those from Scourse et al (2012).…”

“…This level may even have been reached already (Graven 2015; see also the discussion section). The ocean, on the other hand, may have become a source of 14 C for atmospheric CO 2 as suggested by Andrews et al (2016) and Wu et al (2021).…”

Bomb-Produced Radiocarbon Across the South Pacific Gyre—a New Record From American Samoa With Utility for Fisheries Science

“…Many of these Pacific coral 14 C records provide evidence of post-peak 14 C declines that point to a convergence among the various 14 C records that may be useful in other age validation studies (Andrews et al 2016a(Andrews et al , 2016cRamos et al 2019;Wu et al 2021). Specifically, bomb 14 C dating of fishes typically requires that the specimen lived through the late 1950s to mid-1960s to use the diagnostic 14 C rise period as a chronological reference, but some fishes may not have a long lifespan (on the order of 1-3 decades) or archived otoliths available that would provide birth years in the bomb 14 C rise period; however, the post-peak 14 C decline period provides a novel approach to determining the age of shorter-lived fishes from recent collections, such as the ulua or giant trevally (Caranx ignobilis) of the Hawaiian Islands that was validated with bomb 14 C dating to live 25 yr using this approach (Andrews 2020).…”

“…Coral records of bomb-produced radiocarbon ( 14 C) from atmospheric testing of thermonuclear devices are now sparsely available across the tropical and subtropical Pacific Ocean, ranging from the most northerly in the world at Kure Atoll (Hawaiian Archipelago; Dana 1971;Andrews et al 2016c) to Easter Island (Rapa Nui, Chile;Biddulf et al 2006) in the south, with various records across the Indo-Pacific region from the Great Barrier Reef to Japan and the Indian Ocean (e.g., Grumet et al 2004;Mitsuguchi et al 2016;Ramos et al 2019, Wu et al 2021. In general, bomb-produced 14 C in the 1950s entered the mixed layer of tropical and subtropical oceans through air-sea diffusion in the late 1950s (Grottoli and Eakin 2007).…”