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.