Background
Just as it is necessary to know how earth's climate varied over the course of the Holocene, in order to know if its behavior subsequent to the Industrial Revolution has been anthropogenically influenced in any way, so is it likewise necessary to know how the health of earth's coral reefs has varied over the same time period, in order to know if recent changes in their state of health might possibly have resulted from anthropogenic activities. And the authors state, in this regard, that on Kodakara Island (29°13'N, 129°19'E) in the northwest Pacific, three trenches resulting from road construction have cut into middle-to-late Holocene reef flats and spurs perpendicular to the coast that allow just such an analysis to be conducted for that part of the planet.

What was done
Hamanaka et al. employed field observations and high-precision in situ coral radiocarbon dating of the excavated trench walls of an uplifted middle-to-late Holocene coral reef on Kodakara Island that provided "evidence of the existence of disturbances with hiatuses in coral reef growth and coral composition differences before and after the disturbances."

What was learned
The six scientists say they found three disconformities in the reef, and that their dating results indicate that these "disturbances with hiatuses in coral reef growth" occurred at approximately 5.9 to 5.8, 4.4 to 4.0 and 3.3 to 3.2 thousand years BP. These disturbances corresponded in time with the cold North Atlantic ice-rafted debris events discovered by Bond et al. (1997, 2001); and they report that ¹⁴C and ¹⁰Be data indicate relatively low solar activity at those particular times, which phenomenon likely led to both the ice-rafted debris events in the North Atlantic and the low sea surface temperatures in the western Tropical Pacific described by Stott et al. (2004). Thus, they conclude that the "disturbances with hiatuses and their links with both cold events in the North Atlantic and low sea surface temperatures in the western tropical Pacific are partly attributable to insolation-forcing changes," citing Liu et al. (2003).

What it means
The Japanese researchers interpret the main cause of the reef growth disturbances they detected as being low sea surface temperatures that were induced by millennial-scale oscillations in solar activity, which means that the coral growth hiatuses they discovered were likely caused by solar-induced global cooling.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Bond, G., Showers, W., Chezebiet, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial scale cycle in North-Atlantic Holocene and glacial climates. Science 278: 1257-1266.

Liu, Z., Brady, E., Lynch-Stieglitz, J. 2003. Global ocean response to orbital forcing in the Holocene. Paleoceanography 18: 10.1029/2002PA000819.

Stott, L., Cannariato, K., Thunell, R., Haug, G.H., Koutavas, A. and Lund, S. 2004. Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch. Nature 431: 56-59.