14. VARIATIONS IN PLANKTONIC FORAMINIFER FAUNAS AND CARBONATE PRESERVATION AT SITE 927: EVIDENCE FOR CHANGING SURFACE WATER CONDITIONS IN THE WESTERN TROPICAL ATLANTIC OCEAN DURING THE MIDDLE PLEISTOCENE1

James L. Cullen2 and William B. Curry3

ABSTRACT

Site 927 was cored during Leg 154 of the Ocean Drilling Program at a depth of 3315 m on the northeast flank of the Ceara Rise beneath the pool of warm, nutrient-depleted waters of the western tropical Atlantic Ocean. The site was triple advanced hydraulic piston cored and a composite section was constructed by the shipboard scientific party. We sampled the composite section between 23 and 43 meters composite depth (mcd) at 10-cm spacing (200 samples) and produced records of changing surface water conditions and carbonate preservation between 500 and 1000 ka (time scale of Bickert et al., Chapter 15, this volume). Our record of changing surface water conditions is based on downcore variations in the relative abundances of planktonic foraminifers examined within the context of changes in foraminifer preservation, calcium carbonate content, coarse fraction content (>63 Ám), and the content of size-controlled specimens of Globigerinoides sacculifer.

Site 927 sediments exhibit systematic cyclic downcore changes in carbonate content and other indicators of carbonate preservation with periods on the order of 40 k.y. Variations in percent resistant species (RSP) and percent carbonate downcore are significantly less correlated to percent whole foraminifers than they are to percent whole foraminifers in modern tropical Atlantic and Ceara Rise sediments. This suggests that changes in carbonate preservation at Site 927 are best reflected by downcore variations in percent whole foraminifers and that both percent carbonate and percent RSP exhibit variability not exclusively related to changes in preservation. Many downcore percent whole foraminifer values, especially from samples deposited during glacial maxima, fall below values typical of well-preserved supra-lysoclinal modern tropical Atlantic sediments and identify intervals in Site 927 that were probably affected by differential dissolution.

The 15 most abundant species exhibit considerable variability downcore. Patterns of variability for important species are not highly correlated, nor do they seem to be related in a simple manner to defined glacial/interglacial cycles. The four most abundant species (Globigerinoides ruber, G. sacculifer, Globigerinita glutinata, and Neogloboquadrina dutertrei) exhibit high frequency fluctuations (1/f on the order of 104 yr) with amplitudes of 10%-20%. A number of important species (e.g., Globorotalia menardii, Pulleniatina obliquiloculata, Globigerina rubescens, and Globorotalia truncatulinoides [left coiling]) are absent or occur in trace amounts over long intervals that alternate with intervals of much higher average abundance and high frequency variability. A comparison of foraminifer faunal variation with carbonate preservation suggests that faunal variability is decoupled from changes in preservation. Thus, much of the faunal variability is likely associated with changes in the environmental conditions of Ceara Rise surface waters.

Downcore faunas were compared to modern faunas using the Modern Analog Technique (MAT). The MAT was also used to compare downcore faunas to modern Ceara Rise faunas. Foraminifer faunas were most similar to modern Ceara Rise faunas during isotope Stage 21 and, to a lesser extent, Stages 19 and 17. Finally, the MAT was used to estimate downcore cold-season and warm-season sea surface temperatures (SST). SST estimates show little variability and suggest that the significant faunal variability observed between 1000 and 500 ka is related to surface water changes other than SST. Comparison with foraminifer faunal variability in modern tropical Atlantic sediments suggests that the observed downcore species variability may be related to changes in the mixed layer depth and the amount of time the thermocline is in the photic zone during the annual cycle. In general, “mixed layer” species (G. ruber, G. glutinata, and G. sacculifer) are most abundant during interglacials, whereas “thermocline” species (G. menardii, P. obliquiloculata, N. dutertrei, and Globorotalia tumida) are most abundant during glacial intervals when conditions in the surface waters may have been more similar to conditions found farther east in the present-day tropical Atlantic where surface sediments are richer in “thermocline” species. These interpretations, however, cannot easily explain the numerous large-scale and abrupt downcore changes in relative abundances observed for many species.

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).
2Department of Geological Sciences, Salem State College, Salem, MA 01970, U.S.A. CULLEN@dgl.ssc.mass.edu
3Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.