Oceanic crust at Sites 767 and 770 in the northern Celebes Sea is overlain by Eocene and Oligocene pelagic sediments. Brown clay accumulated below the calcite compensation depth (CCD) at the deeper Site 767 throughout this time interval. At the shallower Site 770, nannofossil clay accumulated just above the CCD during middle to late Eocene and early Oligocene time, interrupted by a brief episode of noncalcareous clay deposition in earliest Oligocene time. Depth backtracking of these sites and the alternation of calcareous and noncalcareous sediments at Site 770 indicate that an abrupt lowering of the CCD by as much as 500 m occurred in the Celebes Sea region in earliest Oligocene time. This event was synchronous with an equally abrupt but larger-magnitude drop in the CCD in the Pacific and Atlantic Oceans, which has been attributed to changes in ocean circulation, increasing biological productivity, and the accelerated influx of cold Antarctic Bottom Water into the deep ocean basins. The presence of this paleoceanographic signal in the pelagic sediments at Site 770 indicates that there were open deep-water connections between the Celebes Sea and the Pacific or Indian Oceans during the Eocene and Oligocene. This conclusion supports the notion that the Celebes Sea originated in an open-ocean setting and became a trapped marginal basin as a result of Neogene tectonic events.

Supplement to: Smith, Randall B; von Breymann, Marta T; Huang, Zehui (1991): Site backtracking and the Eocene-Oligocene calcite compensation depth in the Celebes Sea. In: Silver, EA; Rangin, C; von Breymann, MT; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 124, 447-457