15. OXYGEN AND CARBON ISOTOPE ANALYSES OF TERTIARY AND CRETACEOUS MICROFOSSILS FROM SHATSKY RISE AND OTHER SITES IN THE NORTH PACIFIC OCEAN

• Main Author: Robert G. Douglas
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Hess; Pacific; Planktonic foraminifera
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.610.8683; http://www.deepseadrilling.org/32/volume/dsdp32_15.pdf

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 nan-nofossils, particularly discoasters, are preferentially overgrown and show the largest isotopic adjustment. Nannofossils from mid-Tertiary sediments yield δ θ 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 dif-ferent preservation characteristics, foraminifera are more reliable isotopic paleotemperature indicators than are nannofossils. Microfossils from Shatsky Rise yield oxygen-isotope paleotem-peratures 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 ...