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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. INTRODUCTION Drilling at Ocean Drilling Program Sites 767 and 770 in the northern Celebes Sea confirmed that oceanic crust of late middle Eocene age underlies each site (Shipboard Scientific Party, 1990b and 1990c). Although the sites now lie within a marginal basin largely surrounded by Neogene arc terranes (Rangin and Silver, 1990), the history of sedimentation in the basin is complex. Pelagic, terrigenous turbidite, and volcanogenie sedimentation have each dominated during different stages in the basin's history. Eocene and Oligocene strata at both sites are dominantly pelagic, with little turbidite or volcanogenic sediment. The pelagic character of the first 20 m.y. of the sedimentary record at these sites is persuasive evidence that the crust of the Celebes Sea originated in an open-ocean environment rather than in its present back-arc basin setting (Shipboard Scientific Party, 1990d). We have examined the record of Eocene to early Oligocene pelagic carbonate sedimentation at Sites 767 and 770 for further clues to the early paleoceanographic setting of the Celebes Sea. We conclude that variations in carbonate content at Site 770 record the global drop in the calcite compensation depth (CCD) near the Eocene-Oligocene boundary, suggesting that deep-water connections existed between the Celebes Sea sites and the Pacific or Indian Oceans. CARBONATE CONTENT OF EOCENE-OLIGOCENE PELAGIC SEDIMENTS Site 767 was drilled on the deep basin floor (present water depth 4905 m). Brown pelagic clay stone with less than 1% 1 Silver, E. A., Rangin, C , von Breymann, M. T., et al., 1991. Proc. ODP, Sci. Results, 124: College Station, TX (Ocean Drilling Program). 2 Department of Geology, California State University, Fresno, CA 93740. 3 Present affiliation: Department of Geology, Sonoma State University, Rohnert Park, CA 94928. 4 Ocean Drilling Program, 1000 Discovery Drive, College Station, TX 77845-9547, U.S.A. 5 Department of Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada. carbonate makes up the entire Eocene-Oligocene interval at this site, indicating that deposition occurred well below the CCD. Site 770 was drilled on a raised basement fault block about 400 m shallower (present water depth 4516 m) than Site 767. The relief on the basement surface between the two sites is even greater, about 750 m, reflecting a more condensed sedimentary sequence at Site 770. The middle Eocene through lower Oligocene section at Site 770 consists predominantly of brown nannofossil clay stone, but several meters of noncalcareous claystone separates the dated upper Eocene and lower Oligocene calcareous intervals (Fig. 1). To define the variations in carbonate content through this interval more precisely, we determined the carbonate content of 29 samples from Cores 124-770B-11R through -16R. The data were merged with the shipboard carbonate analyses to produce a database of 44 analyses within the depth range from 360 to 421 meters below seafloor (mbsf), averaging 1 to 2 samples per meter of recovered core. Both shipboard and shore-based analyses were carried out using procedures outlined by the Shipboard Scientific Party (1990a). In routine shipboard work, such analyses have been highly reproducible, with standard deviations for replicate analyses usually less than 1%. The resulting values are listed in Table 1 and plotted on Figure 1. The carbonate content of the middle to upper Eocene nannofossil claystone in Cores 124-770B-15R and -16R ranges from 5% to 25%, with a mean of 19.2%. The lower Oligocene nannofossil claystone has somewhat higher carbonate contents, ranging from 15% to 45% with a mean of 31.2%. Deposition in both of these intervals took place above, but close to, the CCD, as evidenced by the relatively low carbonate content, moderate to poor preservation of calcareous nannoplankton, and the presence of only dissolution-resistant planktonic foraminifers (Shipboard Scientific Party, 1990c; Nederbragt, this volume). About 1.7 m of noncalcareous (<1% CaCO3) brown claystone underlies the oldest dated Oligocene calcareous claystone at Site 770 (assigned to nannofossil zone NP21), but the base of this noncalcareous interval was not recovered; a 4.5-m coring gap separates it from the top of the upper Eocene

Site Backtracking and the Eocene-Oligocene Calcite Compensation Depth in the Celebes Sea