Multidecadal oceanographic changes in the western Pacific detected through high‐resolution bomb‐derived radiocarbon measurements on corals

Abstract

High‐resolution measurements of radiocarbon (14C) in corals can be used to reconstruct past variability in ocean conditions. Here we report seasonal Δ14C changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Δ14C from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Δ14C spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956, and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Δ14C difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Δ14C difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward‐propagating mesoscale eddies in the Subtropical Countercurrent region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Δ14C difference between Ishigaki and Guam after 1976.

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

“…Compared with the WPO and SCS annual records (Figure ), Houbihu prebomb Δ 14 C values are lower than most other records in the region (i.e., −64.6 ± 2.5‰, n = 8, 1 σ , Figure b). In contrast, Palaui prebomb Δ 14 C (−58.4 ± 1.6‰, n = 12, 1 σ ) is regionally comparable to Hon Tre, Vietnam (−57.1 ± 3.7‰, n = 10, 1 σ , Bolton et al, ), Ishigaki, Japan (−57.9 ± 7.6‰, n = 18, 1 σ , Hirabayashi, Yokoyama, Suzuki, Miyairi, & Aze, ), Guam (−54.0 ± 5.4‰, n = 18, 1 σ , Andrews et al, ), and Palau (−54.9 ± 2.4‰, n = 9, 1 σ , Glynn et al, ; Figure b). Con Dao, Vietnam (−47.9 ± 3.3‰, n = 6, 1 σ , Mitsuguchi et al, ) on the SCS shelf has the highest Δ 14 C values during this period (Figure b).…”

“…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).…”

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

“…3a). The average Δ 14 C value in the 1970s was 146.7 ‰, which lies between the average Δ 14 C values in Guam (Andrews et al 2016) and Ishigaki (Hirabayashi et al 2017a) and is lower than that in Con Dao (Mitsuguchi et al 2007) (Fig. 2).…”

“…We also compare these data with the coral data reported from Ishigaki (Hirabayashi et al 2017a), Guam (Andrews et al 2016), Con Dao (Mitsuguchi et al 2007), and Hon Tre Island (Bolton et al 2016) to understand the relationship between North Pacific western boundary currents and SCS circulation around the Luzon Strait.…”

“…This is attributed to the fact that Hon Tre Island coral site is affected by shallow coastal upwelling at ~12°N during the summer, whereas this upwelling-originated water mass does not reach Con Dao. The Δ 14 C value of Ishigaki was affected by the mixing of water masses from the Pacific Ocean and Luzon Strait because of the variation in the Kuroshio path (Hirabayashi et al 2017a). Because Kuroshio intrusion in the Pacific water mass takes different paths through the Luzon Strait because of eddies on various timescales (e.g., Li et al 1998;Hu et al 2000;Wu and Chiang 2007;Qu et al 2009), and because surface circulation in the SCS exhibits seasonal variation, we discuss the changes in water masses separately for different seasons (summer and winter) and decades (1970s and 1980s) (Fig.…”