Holocene reef-growth dynamics on Kodakara Island (29°N, 129°E) in the Northwest Pacific

The spatial distribution of modern coral reefs in the Northwest Pacific (NWP) is restricted to approximately 30°N. Understanding the high-latitude reef-growth process and its correlation to climate change may provide important insights into future reef growth at even higher latitudes in response to global warming. We conducted field surveys and obtained seven cores from the raised reefs of Kodakara Island (29°N, 129°E) in the NWP to determine the changes in and response of reef growth dynamics to millennial-scale climate change through the Holocene. To reconstruct the timing of the reef growth, 37 coral ages were determined using accelerator mass spectrometry (AMS) radiocarbon dating. The island is characterized by three Holocene reef terraces (Terraces I to III), which were uplifted ca. 2.4 ka, 1.0-0.4 ka, and in the modern era. Three growth hiatuses occurred circa 5.9-5.8 ka, 4.4-4.0 ka, and 3.3-3.2 ka, which correlate with millennial-scale climate changes (Hamanaka et al., 2012). The reef growth began at least 8ka. Relatively rapid vertical growth of 3.6-3.3m kyr^-1 occurred between 8 and 6ka. The reef started growing near land and then gradually extended seaward. The reef growth around the reef slope decelerated after 6ka and most likely coincided with the first and second hiatuses detected landward. In contrast with the reef flat, which resumed its growth after the third hiatus event, the reef mound accretion on the slope ceased definitively. Terrace I was uplifted ca. 2.4 ka. The reef growth reactivated ca. 1.3 ka, and the reef grew at a rate of 9.1m kyr^-1 between 1.3 and 1.0 ka; this rate is the fastest growth recorded in this study. This time interval corresponds to the Medieval Climatic Anomaly (MCA). The cause of the delayed reef growth between 2.4 and 1.4 ka remains unclear, but it may imply the existence of a late Holocene hiatus due to a weaker Kuroshio Current (KC) ca. 1.7ka and to strong El Niño-Southern Oscillation (ENSO) activity between 2 and 1.5ka in the East Pacific. The timing of the hiatuses agrees with the weakening of the KC and the more frequent El Niño events during the positive phase of the Pacific Decadal Oscillation (PDO), whereas the rapid growth coincides with La Niña conditions, such as the MCA, during the negative phase of the PDO. Our results indicate that the climate event at approximately 4. ka caused the largest change in the reef growth, and it may have affected reefs throughout the Pacific region. Specifically, the ENSO and PDO phases may have impacted the reef growth and KC behavior.