Late Cretaceous Bottom Waters in South Atlantic Using Benthonic Foraminifera and Stable Isotopes

William B. Gilmour, Robert G. Douglas

Benthonic foraminifera and stable isotopes were used to evaluate the history of bottom waters 70-74 Ma in the south Atlantic. Site 355 was above the CCD at a backtracked depth of 3,400 ± 300 m. The assemblages are interpreted as autochthonous and contain a mixture of upper-to-middle bathyal species (e.g., Gavelinella whitei, Gyroidinoides bandyi, and G. goudkoffi) and abyssal species. In the Argentine basin, Site 358 was below the CCD at a backtracked depth of 3,700 + 300 m. Unlike Site 355, bathyal foraminifera present at Site 358 are interpreted as redeposited below the CCD. The different foraminiferal dissolution depths in the Brazil and Argentine basins indicate different bottom water chemistries and are reflected in oxygen and carbon isotope ratios of foraminife al tests. Average oxygen isotope ratios at Site 355 are heavier than those at Site 358 by 3.3 ^pmil, in part as the result of secondary calcite overgrowths on tests at Site 358. Average carbon isotope ratios at Site 355 are heavier than those at Site 358 by 0.9 ^pmil. Bottom waters in the Brazil basin were different from those in the Argentine basin and may have been warm saline bottom waters (WSBW) like those proposed by Brass et al in 1982. This is supported by broad foraminiferal distributions, light oxygen ratios (warm water mass), heavy carbon ratios (young water mass), and a deeper calcite compensation depth in the Brazil basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.