Hisato Yasuda2


Changes in deep-water environments of the western Atlantic over the past six million years were investigated by studying benthic foraminiferal abundance changes in 335 samples from Sites 926 (3598 m), 928 (4010 m), and 929 (4358 m) on the Ceara Rise. The location of these sites on a transect at lower abyssal depths provides a unique opportunity to evaluate changes in water mass properties as well as in the supply of organic matter from the surface waters, especially because other Leg 154 studies provided a precise time scale for correlations between the sites.

Relative abundances and fluxes of the most abundant species were calculated, with emphasis on the four most common species—Nuttallides umbonifera, Globocassidulina subglobosa, Epistominella exigua, and Alabaminella weddellensis. There are two patterns in relative abundance of these species with time: the first group of species (N. umbonifera and G. subglobosa) shows abundance variations that differ from site to site (i.e., with depth). The other group (E. exigua and A. weddellensis) shows variations in relative abundance that are similar at all three sites and thus independent of depth.

In the first group, G. subglobosa increased in relative abundance at the shallowest Site 926 from 2.4 to 1 Ma, but decreased at the two deeper sites. In contrast, N. umbonifera, a marker species for corrosive bottom waters and specifically for Antarctic Bottom Water (AABW), shows large fluctuations in relative abundance, but overall increased in relative abundance at the two deeper sites and decreased at the shallower site from 2.1 Ma to the present. Its relative abundance shows three peaks during the last 0.9 m.y., which were coeval at the two deeper sites. These large fluctuations in abundance of N. umbonifera suggest that production of AABW fluctuated in intensity, with increases during interglacial periods. AABW formation increased overall from about 3 Ma, and the amplitude of the fluctuations increased from 0.9 Ma to present. The differences between sites are interpreted as showing that the deeper Sites 928 and 929 were commonly within AABW, whereas the shallower site was not.

The species in the second group, E. exigua and A. weddellensis, occur in the recent oceans commonly in oceanic regions where spring blooms lead to seasonal deposition of phytodetrital material. At the Ceara Rise sites, they show variations in relative abundance that are similar at all three sites from 4.5 to 1.2 Ma, confirming that these species react to environmental factors that are not depth related, and thus possibly to surface productivity. The two species, however, do not covary exactly: at 2.6 Ma, for example, E. exigua decreased in relative abundance whereas A. weddellensis increased.

1Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
22-5-1 Akebono-machi, Department of Geology, Kochi Univeristy, Kochi 780, Japan. yasuda@cc.kochi-u.ac.jp