Description: The post-middle Miocene evolution of sedimentary patterns in the eastern equatorial Pacific Ocean has been deduced from a compilation and synthesis of CaCO3, opal, and nannofossil assemblage data from 11 sites drilled during Leg 138. Improvements in stratigraphic correlation and time scale development enabled the construction of lithostratigraphic and chronostratigraphic frameworks of exceptional quality. These frameworks, and the high sedimentation rates (often exceeding 4 cm/k.y.) provided a detailed and synoptic paleoceanographic view of a large and highly productive region. The three highlights that emerge are: (1) a middle late Miocene "carbonate crash" (Lyle et al., this volume); (2) a late Miocene-early Pliocene "biogenic bloom"; and (3) an early Pliocene "opal shift". During the carbonate crash, an interval of dissolution extending from -11.2 to 7.5 Ma, CaCO3 accumulation rates declined to near zero over much of the eastern equatorial Pacific, whereas opal accumulation rates remained substantially unchanged. The crash nadir, near 9.5 Ma, was marked by a brief shoaling of the regional carbonate compensation depth by more than 1400 m. The carbonate crash has been correlated over the entire tropical Pacific Ocean, and has been attributed to tectonically-induced changes in abyssal flow through the Panamanian seaway. The biogenic bloom extended from 6.7 to 4.5 Ma, and was characterized by an overall increase in biogenic accumulation and by a steepening of the latitudinal accumulation gradient toward the equator. The bloom has been observed over a large portion of the global ocean and has been linked to increased productivity. The final highlight, is a distinct and permanent shift in the locus of maximum opal mass accumulation rate at 4.4 Ma. This shift was temporally, and perhaps causally, linked to the final closure of the Panamanian seaway. Before 4.4 Ma, opal accumulation was greatest in the eastern equatorial Pacific Basin (near 0°N, 107°W). Since then, the highest opal fluxes in the equatorial Pacific have occurred in the Galapagos region (near 3°S, 92°W).

Description: In the eastern and central Pacific Ocean the most profound change in Neogene calcium carbonate deposition occurred at the late/middle Miocene boundary (about 10 Ma), when carbonate mass accumulation rates (MARs) abruptly dropped. East of the East Pacific Rise (EPR), carbonate deposition essentially ceased. The carbonate compensation depth (CCD) in the Guatemala Basin, for example, rose by 800 m in less than 0.5 Ma. Even the rise crests suffered carbonate losses - Site 846, at the time less than 300 meters deeper than the EPR axis, experienced intervals between 10 and 9 Ma where no carbonate at all was buried. By about 8 Ma carbonate deposition resumed and was concentrated along an equatorial band, suggestive of high surface water carbonate production. East of the EPR, however, CCDs remained shallow since 10 Ma. This event which we have termed the late Miocene carbonate crash marks a fundamental paleoceanographic change that occurred in the eastern Pacific Ocean. Here, we document the changing pattern of carbonate deposition from 13 Ma to 5 Ma by using maps of carbonate MAR reconstructed from ODP Leg 138 and DSDP data. Comparisons to modern oceanographic conditions demonstrate that the late Miocene carbonate crash could not have been caused by an abrupt increase in productivity at 10 Ma or by loss of Corg from continental shelves. Instead it was probably caused by a relatively small reduction in deep-water exchange between the Atlantic and Pacific Oceans through the Panama Gateway prior to the emergence of the isthmus. A small restriction of deep-water exchange through this gateway is sufficient to radically change carbonate MARs in the eastern Pacific.