Paleogene Calcite Compensation Depth in the Eastern Subtropical Pacific: Answers and Questions

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Ocean Drilling Program Leg 199 drilled a north-south transect across the Eocene paleoequator in the eastern Pacific, permitting reconstruction the calcite compensation depth (CCD) since earliest Eocene time. The CCD was relatively shallow near the early Eocene Pacific equator, 3200 m, and unlike modern latitudinal CCD gradients deepened to the north (to ∼3600 m; paleolatitude ∼10°N). At 41 Ma the CCD underwent a brief, sharp, transient deepening of 700 m, then remained shallow until the Eocene/Oligocene boundary. At the E/O boundary, the CCD deepened by 1200 m in less than 300 kyr. This rapid deepening served to more than double the area of seafloor subject to CaCO3 deposition. Sea level fall associated with ice volume buildup, and ensuing shelf-basin fractionation, is unlikely to be the sole cause of the increased deep-ocean CaCO3 burial; rather, a sudden, rapid increase in the amount of Ca entering the ocean appears necessary to explain the observations.