Prebomb to Postbomb ¹⁴C History From the West Side of Palawan Island: Insights Into Oceanographic Changes in the South China Sea

Abstract

Compared to the natural production of 14C, thermonuclear tests have produced a globally abnormal 14C signal. To examine and reconstruct ocean circulation in the South China Sea (SCS), we generated a prebomb to postbomb 14C time series from a Porites lobata coral from the west side of Palawan Island, Philippines. Results show a diluted early bomb 14C intrusion in late 1955. We use this record to extend our understanding of the surface ocean circulation in and around the SCS by tracing 14C labeled water from nuclear test sites. Our postbomb Δ14C peaked at 154‰ in 1975, which lags ~10 years from the atmospheric peak, consistent with other marine records. This coral record also displayed clear seasonal Δ14C cycles indicating different surface waters passed the Palawan Island coral site due to seasonal variations of the East Asian Monsoon.

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

“…This provides an opportunity to examine oceanographic and climate variability using radiocarbon as a seawater tracer over multiple time scales. Many studies have used the 14 C in coral skeletons to track ocean circulation (Druffel 1987;Druffel andGriffin 1995, 1999;Guilderson et al , 2000Guilderson et al , 2009Fallon and Guilderson 2008;Andrews et al 2016a;Hirabayashi et al 2017a;Hirabayashi 2017b;Wu and Fallon 2020). Seawater 14 C has also been shown to be sensitive to large-scale climate phenomena such as the El Niño-Southern Oscillation (ENSO) due to the movement of 14 C labelled water (Druffel andGriffin 1993, 1995;.…”

Radiocarbon in the Atmosphere and Seawater in the South China Sea: Flux, Inventory and Air‐Sea CO2 Exchange Rate Tracing

“…The Δ 14 C values of surface seawater DIC are 23.6 ± 2.5‰ higher than the atmospheric Δ 14 C values in the SCS (Figure 2). Due to the atmospheric nuclear bomb test since the 1950s, the atmospheric Δ 14 C values in the Northern Hemisphere (NH) recorded by tree rings (Hua et al., 2013) are higher than corresponding surface seawater Δ 14 C values recorded by coral in the SCS (Bolton et al., 2016; Mitsuguchi et al., 2007; Ramos et al., 2019; Wu & Fallon, 2020) before 2003, resulting in a positive air‐sea Δ 14 C gradient. The gradient gradually switched from positive to negative, due to fossil fuel emissions (Graven, Gruber, et al., 2012).…”

“…A previous study showed that the uncertainties of bomb 14 C inventories using pre‐bomb 14 C depth profiles at stations from the same latitudinal belt were less than ±10% (Bhushan et al., 2000). Thus, the pre‐bomb 14 C profile in the SCS was reconstructed approximately from the pre‐bomb Δ 14 C data in the studies in the western Pacific Ocean (Bien et al., 1965; Östlund & Stuiver, 1980), and the SCS (Bolton et al., 2016; Mitsuguchi et al., 2007; Ramos et al., 2019; Wu & Fallon, 2020).…”

“…Based on the coral Δ 14 C measurements in the SCS (Bolton et al., 2016; Mitsuguchi et al., 2007; Ramos et al., 2019; Wu & Fallon, 2020), pre‐bomb surface seawater Δ 14 C (Δ 14 C°) is approximately −60‰ for this region. Because this study has only one Δ 14 C depth profiles at station 74, the Δ 14 C depth profiles from six other stations from the SCS during almost the same period (Ding et al., 2020) are also included to study bomb 14 C inventories in this region.…”

“…The addition of bomb 14 C in the environment can be used as a tracer to study carbon movement in the global carbon cycle (Nydal, 1968; Nydal & Gislefoss, 1996), and to study the ocean circulation pattern (Fallon & Guilderson, 2008; Wu & Fallon, 2020). The first extensive measurements of Δ 14 C in ocean dissolved inorganic carbon (DIC) were carried out as part of the Geochemical Ocean Sections (GEOSECS) program (Stuiver & Östlund, 1980; Östlund & Stuiver, 1980), and bomb 14 C has been used to trace the air‐sea CO 2 exchange rate (Bhushan et al., 2000; Broecker et al., 1985; Dutta & Bhushan, 2012; Stuiver, 1980).…”