Recent evolution of the Ryukyu Arc Coral Reef Front

In the present day, the Kuroshio Current (the western boundary current of the North Pacific Subtropical Gyre) flows through the East China Sea, alon the northwestern flank of the Ryukyu Island Arc. The Kuroshio Current is associated with a large heat transport, elevating surface temperatures in the East China Sea. As a result, southern Japan is home to amongst the northernmost warm water coral reefs on Earth. However, during the Last Glacial Maximum (LGM) around 20,000 years ago, the Earth was cooler than the present and sea-level fell due to the formation of expansive ice sheets across Eurasia and North America. It has previously been suggested that this fall in sea-level may have obstructed the Kuroshio Current from entering the East China Sea, which would have had severe implications for the habitability of the East China Sea for coral reefs during the LGM.

Through an ensemble of high-resolution ocean simulations with LGM boundary conditions (from the PMIP3 project), we found that the present-day path of the Kuroshio is robust with respect to absolute and relative sea-level changes associated with glacial-interglacial climate change and tectonics. In other words, we do not believe that a diverted Kuroshio at the LGM is physically consistent. We also found that simulations with the best model-proxy agreement predicted the least contraction of the coral reef habitable range (insofar as sea-surface temperature is concerned), suggesting that much of the East China Sea may have remained habitable for coral reefs during the LGM. This lends support to recent seismic evidence for glacial-aged coral reefs in the northern Ryukyu Islands. We also found that the Kuroshio Current axis shifted towards the Ryukyu Arc in our simulations, which may have promoted poleward larval dispersal and improved reef resilience at the coral reef front during this time of environmental stress.

This study was published in Paleoceanography and Paleoclimatology.

Estimates of the LGM coral reef front (black line) compared to the pre-industrial coral reef front (white line) in the four members of our ensemble, compared to confirmed (black star) and suspected (white stars) LGM reefs. The bottom pannel shows the model-proxy agreement for SST in each ensemble member.