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.