Studies of climate evolution and Cenozoic cooling published in the past several years strengthen our previous paleontologic and stratigraphic models for significant refrigeration accompanied by lowered sea level during the middle Miocene (±15 to 12 m.y.B.P.).

Currently, there is general acceptance of a eustatic lowering of sea level during late Miocene (Messinian) time (±7 to 5 m.y.B.P.) which caused the isolation and evaporation of the Mediterranean Sea, and this has been attributed to a peak in Cenozoic glaciation. We, however, have proposed that there was, during progressive Cenozoic cooling, a sharp dip in temperature as early as Langhian time (±15 to 14 m.y.B.P.). We also suggested that a short-span eustatic lowering of sea level accompanied this cooling.

Available evidence suggests that the general middle Miocene rise in sea level that resulted in widespread marine transgression was modified by the cooling event that caused a relatively short-lived eustatic sea-level drop.

These concepts are supported by recent oxygen-isotope and continental-migration studies, and by sedimentologic evidence from Deep Sea Drilling Project cores from many parts of the world. Oxygen-isotope studies on tests of planktonic forams have revealed that there was significant buildup of Antarctic ice in the middle Miocene which represented a major glaciation event in the Southern Hemisphere, and that during this time a Circum-Antarctic circulation similar to that of today was established. The isotope data are in agreement with interpretations of paleo-oceanographic and sedimentologic data.

In the Northern Hemisphere, the North Atlantic-Arctic circulation that led to the initial freezing of the Arctic Ocean was well established by Miocene time.

Our models were developed from subsurface paleontologic and stratigraphic data derived from Gulf Coast (south Louisiana) wells. We interpret the sedimentologic data from DSDP cores to be evidence that supports the concept of worldwide middle Miocene cooling.

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