Environmental Control of Holocene Changes to the World's Most Northern Hermatypic Coral Outcrop

Tateyama, near Tokyo (35° N lat.), is the site of the world's most northern occurrence of living hermatypic corals and is also the site (in the Numa beds) of a substantial outcrop of Holocene fossil corals with a radiocarbon date of 5000-6000 yr B.P. This extraordinary co-occurrence provides the opportunity for a detailed reconstruction of environmental change during the Holocene, especially change in sea-surface temperature. The present study, combined with a series of previous studies, reveals 72 coral species in the Numa beds, of which 53 have been identified with reasonable certainty; and 34 species of extant corals at Tateyama, of which 25 have been located and identified. These data are compared with recently completed studies of the distribution of extant corals of Japan, and sea-surface temperatures of the principal regions of extant corals. Nearly one-half of all species from the Numa beds have remained extant at Tateyama until recent times, 85% are extant as far north as Kushimoto on the Kii Peninsula (33.5° N lat.), and all except two have been recorded extant somewhere in mainland Japan. There has been a major change in species dominance at Tateyama. The identified species from the Numa beds and those of the Izu Peninsula and Tateyama show a high degree of dissimilarity compared with other coral communities of mainland Japan. The closest extant fauna to the corals from the Numa beds appears to be that of Kushimoto. Based on six ways of measuring the temperature regimes of coral communities of modern mainland Japan, over the past 40 yr, this geographic comparison corresponds to a mean sea-surface temperature increase of 1.7°C. Although there are several assumptions in arriving at this number, the increase is clearly less than 2.1 °C, which is the temperature difference corresponding to the substantially richer coral communities of Tanegashima at the southern tip of mainland Japan. This study shows that an increase in sea-surface temperature of < 2°C, such as is widely predicted in response to the "greenhouse effect," should result in a greatly increased diversity of corals in high-latitude locations. It also shows that this temperature increase is sufficient to create a "high latitude subtropical" community in a region that appears almost devoid of corals in a fossil sequence