Radiocarbon variability recorded in coral skeletons from the northwest of Luzon Island, Philippines

Abstract

The North Equatorial Current (NEC) bifurcates at the eastern coast of the Philippines and moves northward as the Kuroshio, a North Pacific western boundary current. The NEC bifurcation point and Kuroshio variability are known to be affected by changes in climate such as the El Niño-Southern Oscillation and the pacific decadal oscillation. However, observational data are not sufficient to examine the mechanisms of decadal fluctuation. Here, we report seasonal radiocarbon data recorded from 1968 to 1995 in coral skeletons northwest of Luzon Island. The data suggest that the East Asian winter monsoon is a dominant factor in the seasonal fluctuations in water mass northwest of Luzon Island. Compared with other coral records reported for Guam, Ishigaki, Con Dao, and Hon Tre Island, the data suggest that the area of the Kuroshio loop current through the Luzon Strait decreased from the 1970s to 1980s as a result of the change in Kuroshio transport and the migration of the NEC bifurcation latitude after a regime shift in 1976.

Interannual Coral Δ14C Records of Surface Water Exchange Across the Luzon Strait

“…The SCS records, on the other hand, peaked in 1973 and recorded higher postbomb values than the WPO. Con Dao and Currimao, Philippines, (Hirabayashi, Yokoyama, Suzuki, Miyairi, Aze, Siringan, & Maeda, ) recorded the highest values at 173.8‰ and 165.8‰, respectively, while Hon Tre recorded 155.3‰. Palaui (151.2‰) and Houbihu (147.2 ‰) Δ 14 C values are comparable to Guam.…”

“…We hypothesize that the postbomb peak in 1973 has obscured the NBL‐related surface water entrainment, where the higher and spatially variable Δ 14 C distribution at these sites during this period precedes the mean circulation patterns. Other possible mechanisms include the influence of local oceanographic processes such as occurrence of mesoscale eddies and upwelling (e.g., Ishigaki, Hirabayashi, Yokoyama, Suzuki, Miyairi, Aze, Siringan, & Maeda, ). In contrast with the Palaui‐Guam Δ 14 C difference, Houbihu‐Guam Δ 14 C is not correlated with the NBL migration ( r = 0.22, p = 0.24, n = 31, not shown), likely due to its inward location within the strait where offshore Kuroshio influence is driven by the KI.…”

“…Wang & Li, ), and the ITF (Gordon et al, ). Colored symbols indicate the location of coral Δ 14 C records used in this study: blue circle = Houbihu, Taiwan and red circle = Palaui, Philippines (this study); gray diamond = Guam, USA (Andrews et al, ); light green diamond = Palau (Glynn et al, ); yellow diamond = Ishigaki Island, Japan (Hirabayashi, Yokoyama, Suzuki, Miyairi, & Aze, ); orange triangle = Langkai, Indonesia (Fallon & Guilderson, ); light blue triangle = Lombok Strait, Indonesia (Guilderson et al, ); green square = Currimao, Philippines (Hirabayashi, Yokoyama, Suzuki, Miyairi, Aze, Siringan, & Maeda, ); pink square = Hon Tre, Vietnam (Bolton et al, ); and brown square = Con Dao, Vietnam (Mitsuguchi et al, ). The red line across the strait at 120.8°E running from 18°N to 22°N define the transect where surface zonal currents and sea surface heights from the Simple Ocean Data Assimilation data set (SODA v.2.2.4, http://apdrc.soest.hawaii.edu/dods/public_data/SODA/soda_pop2.2.4) are derived (shown in Figures and c, respectively).…”

Novel reverse radioisotope labelling experiment reveals carbon assimilation of marine calcifiers under ocean acidification conditions

“…Three separate graphitization lines were used to perform steps 1–3. Shell carbonate samples with small masses (0.8–2.2 mg) were analysed in a specialized graphitization line by an AMS procedure capable of handling minute amounts of carbonate (Hirabayashi et al., 2017; Yokoyama, Koizumi, Matsuzaki, Miyairi, & Ohkouchi, 2010), whereas shell carbonate samples with larger masses (>3.0 mg) were processed in a standard graphitization line (Yokoyama, Miyairi, Matsuzaki, & Tsunomori, 2007). The organic samples (2.0–3.0 mg) were loaded into the standard graphitization line for cryogenic purification and reduction.…”

“…In the 1950s and early 1960s, atmospheric nuclear weapons testing produced large amounts of atmospheric 14 C (Grottoli & Eakin, 2007; Hua et al., 2009; Hua, Barbetti, & Rakowski, 2013), which approximately doubled the natural atmospheric 14 C value (Grottoli & Eakin, 2007; Hua et al., 2009, 2013). The transient 14 C enrichment of the biosphere and its subsequent dissipation, the bomb‐ 14 C curve, provides a high‐resolution isotopic chronometer for the last 60 years (Hirabayashi et al., 2017; Hua et al., 2009, 2013) that has been used extensively in environmental (Murphy, Davis, Long, Donahue, & Jull, 1989; Ota et al., 2019; Rethemeyer et al, 2004), forensic (Zoppi et al., 2004) and biogeochemical studies (Furla, Galgani, Durand, & Allemand, 2000; Ishikawa, Hyodo, & Tayasu, 2013; Ishikawa, Uchida, Shibata, & Tayasu, 2010; Ota et al., 2019; Stenström et al., 2010; Wong & Levinton, 2006). Ota et al.…”