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

We present a high-resolution seawater radiocarbon (Delta C-14) record from a Porites coral collected from Masthead Island in the southern Great Barrier Reef (GBR) covering the years 1945-2017. The Delta C-14 values from 1945-1953 (pre-bomb era) averaged -49 parts per thousand. As a result of bomb-produced C-14 in the atmosphere, Delta C-14 values started to rise rapidly from 1959, levelled off at similar to 131 parts per thousand in the late 1970s and gradually decreased to similar to 40.3 parts per thousand by 2017 due to the decrease in the air-sea C-14 gradient and the overturning of the C-14 ocean reservoir (i.e., surface ocean to subsurface ocean; atmosphere to surface ocean). The Masthead Island record is in agreement with previous C-14 coral records from the southern GBR. A comparison between surface ocean and atmospheric Delta C-14 suggests that, since 2010, the main reservoir of bomb-derived C-14 has shifted from the atmosphere to the surface ocean, potentially resulting in reversed C-14 flux in regions where the CO2 gradient is favorable. The high-resolution Masthead coral Delta C-14 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 C-14 reference series to date other carbonate samples.

Automated summary

The study presents a high-resolution seawater radiocarbon record from the southern Great Barrier Reef covering 1945-2017, showing the impact of nuclear bomb testing on C-14 values and the influence of air-sea exchange rates and ocean reservoirs on C-14 levels. It suggests a potential shift of bomb-derived C-14 from the atmosphere to the surface ocean and provides insights into long-term variability in air-sea exchange dynamics and GBR regional ocean dynamics associated with climate change.