Latitudinal gradients in diversity of organisms represent an equilibrium distribution for at least the last 270 x 10(6) years. Faunas endemic to tropical regions evolved significantly faster than extra-tropical faunas. The latitude-dependent difference in rates of evolution also represents an equilibrium condition for at least the last 270 x 10(6) years and has consequences for paleontological correlation of rocks because the attainable resolution depends on rate of evolution and will thus be greater in tropic regions than in extra-tropical ones.
Deep Sea Drilling Project site 289 in the western equatorial Pacific has yielded an extremely detailed record of the carbon and oxygen isotopic changes in the Miocene deep ocean. The isotopic record reflects major changes in paleoclimate and paleoceanography, probably dominated by a major phase of Antarctic ice-cap growth. The transition from a relatively unglaciated world to one similar to today occurred between 16.5 x 10(6) and 13 x 10(6) years before the present, with the greatest change occurring between approximately 14.8 x 10(6) and 14.0 x 10(6) years before the present.
Oxygen and carbon isotopes were determined for nannofossils and foraminifera of Tertiary and Cretaceous age recovered at sites drilled on Shatsky and Hess rises during Leg 32. Additional samples of microfossils from other DSDP sites on Shatsky Rise and on Magellan Rise were also analyzed for comparative purposes.Progressive lithification in pelagic carbonate sediments results in the partial isotopic adjustment of microfossil calcite as dissolved CaCCh is deposited as overgrowth cement. In this process nannofossils, particularly discoasters, are preferentially overgrown and show the largest isotopic adjustment. Nannofossils from midTertiary sediments yield δθ u values that differ from associated planktonic foraminifera by more than 1.0 per mil, although the original isotopic composition of the two groups was similar. Only nannofossils from depths of burial of less than about 200 meters, where little cement overgrowth has occurred, retain their original isotopic composition. Foraminifera, which tend to act as donors in the CaCθ3 transfer, are less affected isotopically by diagenesis. Planktonic foraminifera retain their isotopic composition from sediments as old as Late Cretaceous at Site 305 and benthonic species from Early Cretaceous sediments at Site 306. Because of their different preservation characteristics, foraminifera are more reliable isotopic paleotemperature indicators than are nannofossils.Microfossils from Shatsky Rise yield oxygen-isotope paleotemperatures that suggest a warming trend in the Early Cretaceous, a thermal maximum in the Albian, and a cooling trend in the Late Cretaceous. Bottom-water temperatures were warm, and the difference between surface waters and bottom waters in tropical regions was less than one-half of the present-day value. There is a small drop in temperature across the Cretaceous-Tertiary boundary which begins in the latest Campanian and ends in the Paleocene. The change in the central Pacific corresponds to approximately 2.5°C in surface waters and 3°C in bottom waters.
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