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The transition from carbonate-poor late Eocene sediments to carbonate-rich early Oligocene sediments undoubtedly reflects a major reorganization of the way in which carbonate was fractionated between shelf regions and deep basins. During Eocene time carbonate was being deposited on the shelves marginal to the extensive Tethys Sea, whereas in Oligocene time carbonate sedimentation on the continental shelves decreased, but increased in the deep sea. In the deep sea, this facies change represents a lowering of the carbonate compensation depth (CCD) with the magnitude of the drop varying in a systematic fashion from one ocean basin to another. The CCD drop was most dramatic in the equatorial Pacific, sinking nearly 1,500 m (4,900 ft) in less than 2 m.y. Both the magnitude and abruptness of this drop in the CCD decrease steadily away from the equatorial Pacific. In the South Pacific and South Atlantic, the CCD drop across the Eocene/Oligocene boundary was on the order of 1,000 m (3,280 ft) and 750 m (2,460 ft) respectively. Similarly, in the North Atlantic, not only was the magnitude of this drop reduced to only several hundred meters, but it appears to have occurred over a prolonged period of time.
By early Oligocene time a distinct latitudinal trend in the depth of the CCD existed in the Atlantic Ocean. The maximum depth was centered around 10°N paleolatitude and most probably marked a zone of high productivity. Above 40° north and south lat., there was a definite shoaling of the CCD. This is a feature that also occurs in the present-day Atlantic and may be due to the production of cold bottom waters at high latitudes.
Time series carbonate records from sites located above the CCD show varying responses across the Eocene/Oligocene boundary. DSDP Site 219 (Indian Ocean) and Site 292 (western equatorial Pacific) show no change in carbonate content across this boundary indicating that they were consistently above not only the CCD but the lysocline as well. In contrast, Site 77B (eastern equatorial Pacific), Site 277 (southwest Pacific), Site 363 (southeast Atlantic), and E128 (Gulf of Mexico) all show decreases of 10 to 25% across the boundary in conjunction with a distinct dissolution event. All of these sites may be recording a shoaling of the lysocline that accompanied the drop in the CCD across the Eocene/Oligocene boundary.
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