Cenozoic History and Paleoceanography of the Central Equatorial Pacific Ocean

Publisher Website
Google Scholar
Abstract
This study has three distinct but interrelated objectives: to prepare a geological synthesis of Deep Sea Drilling Project data from the central equatorial Pacific Ocean, to interpret this information in terms of the paleoceanographic history of this region, and to evaluate the usefulness of drill data and develop procedures and strategies for future studies of this kind. The investigation is based on primary data contained in the Initial Reports of the Deep Sea Drilling Project and is supported by information from surface cores. The principal data used are the biostratigraphy, lithology, carbonate content, bulk density, and porosity of the cores. From these properties, sedimentation rates, carbonate and carbonate-free accumulation rates, and paleobathymetric histories of the drill sites were derived with the aid of Berggren’s chronology. Paleopositions of the drill sites and surface cores were determined from rotation parameters of the Pacific and Cocos plates. The present surface and deep circulation, fertility patterns, and sedimentation of the equatorial Pacific constitute a frame of reference for the paleoceanographic evolution. East-west and north-south lithologic profiles show that a zone of maximum deposition approximately parallel to the Equator has existed at least since middle Eocene time. With increasing age, the axis of this zone is found progressively farther north of the Equator. The profiles illustrate a gradual change from calcareous to siliceous deposits with increasing depth at any time, and they indicate an abrupt change from a dominantly siliceous to a dominantly calcareous depositional regime at the Eocene-Oligocene boundary. Large changes with time in the width and sedimentation rate of the calcareous equatorial zone indicate major variations in depositional conditions since Eocene time. Subsidence with age of the oceanic basement, plate rotation, and changes in spreading rate are closely examined in this study. A northward shift of the equatorial zone of maximum deposition with age and trend and ages of linear volcanic island chains (melting spots) define the rotation of the Pacific plate. This gives a pole of rotation at lat 67° N, 59° W, and rotation rates of 0.25°/m.y. before and 0.83°/m.y. after 25 m.y. B.P. This rotation scheme describes the migration with time of the drill sites and, in combination with the subsidence histories of the drill sites, permits the reconstruction of the paleobathymetric evolution of the region. About 50 m.y. ago, the eastern edge of the Pacific plate was located at long. 115° W and migrated rapidly eastward during the next 20 to 30 m.y., becoming stationary at approximately long. 100° to 105° W, notwithstanding a large crestal jump from the Mathematicians-Clipperton ridge system to the present East Pacific Rise 10 to 15 m.y. ago. The ancestral East Pacific Rise was a relatively narrow, symmetric feature with steep upper slopes. About 25 m.y. ago, it developed a much broader and gentler west flank, thereby acquiring its present asymmetry. The early rise crest was much the change with time of the vertical gradient of the carbonate dissolution rate. This gradient appears to have responded mainly to changes in the structure of the deep and bottom waters, whereas the CCD was primarily determined by the global carbonate budget and by changes in the locus of carbonate deposition through time. After changes in carbonate dissolution are taken into account, variations in the carbonate and carbonate-free accumulation rates allow assessment of changes in upwelling and fertility. The paleoceanographic indicators resulting from the study, in context with data from other regions, yield the following oceanographic history for the past 50 m.y. During the initial phase, prior to 38 m.y. B.P., carbonate supply was low, and dissolution was extensive. This resulted in a narrow carbonate zone and low rates of accumulation. Silica was mobilized to form chert, and erosion became widespread. Around 38 m.y. B.P., carbonate input increased abruptly, and solution decreased markedly. As a result, the equatorial carbonate zone widened greatly. Bottom-water characteristics may have changed owing to the first extensive development of sea ice around Antarctica. About 33 m.y. B.P., dissolution rates at depth began to increase again, but this increase was compensated for either by a large increase in the carbonate supply or by a depression of the lysocline. As a result, until about 26 m.y. B.P., a very broad and extensive equatorial carbonate zone with maximal accumulation rates existed. The steepening of the dissolution gradient may be related to a decrease in influx of bottom water resulting from major hydrographic changes around Antarctica. The increase in carbonate supply may have resulted from final closure of the Tethys seaway and the ensuing narrowing and intensification of equatorial upwelling. About 15 m.y. B.P., the rate of carbonate dissolution increased further, and the CCD shoaled. As a result, the equatorial carbonate zone became much narrower, and many hiatuses were formed. At this time, the lysocline may have attained its present position, which suggests that the present configuration of sources and mechanisms of intermediate- and deep-water supply to the Pacific was being developed. This probably resulted from the major development of antarctic glaciation that began during this period and increased the supply of bottom water. Some evidence from the carbonate and carbonate-free accumulation rates also suggests an increase in the fertility of the equatorial region, perhaps resulting from closure of the western Pacific Ocean and the formation of the Cromwell Current at this time. About 3 to 4 m.y. B.P., the onset of arctic glaciation marked the beginning of large and rapid changes in depositional conditions that cannot be followed in detail with the Deep Sea Drilling Project data.

This publication has 0 references indexed in Scilit: