Mid-Holocene variability in the marine C reservoir age for northern coastal Papua New Guinea

Changes in oceanic radiocarbon (¹⁴C) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Niño events. Reservoir ages were calculated using paired U–Th and conventional ¹⁴C dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS ¹⁴C and MC-ICPMS U–Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U–Th and conventional ¹⁴C ages. The combined results show average reservoir ages of 185±30 ¹⁴C yr (n=4) for 7220–5850 yr BP compared to 420 ¹⁴C yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air–sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Niño activity around this time would have served to enhance the intrusion of ¹⁴C-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Niño–Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region.