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 Pleistocene

Abstract

Site 927 was cored during Leg 154 of the Ocean Drilling Program at a depth of 3315 m on the northeast ßank 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 scientiÞc 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 m m), and the d 18 O 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 signiÞcantly 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 reßected 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 d 18 O deÞned glacial/interglacial cycles. The four most abundant species (Globigerinoides ruber, G. sacculifer, Globigerinita glutinata, and Neogloboquadrina dutertrei) exhibit high frequency ßuctuations (1/f on the order of 10 4 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). Th...

The temporal dimension of marine speciation

“…8c) while at other times the same sites differed almost completely in community composition. These results echo those for a foraminiferal community where populations in the tropical North Atlantic were only rarely more than 85% similar to modern populations between 400 and 900 thousand years (Cullen and Curry 1997). This modest level of community repeatability suggests that the ecological landscape seen by incipient species can deviate significantly from modern configurations.…”

Carbonate preservation patterns at the Ceará Rise – Evidence for the Pliocene super conveyor

“…2). However, the intervals of enhanced carbonate dissolution in the early Pliocene are less intense when compared to findings for the late Pliocene to Pleistocene records (Bickert et al, 1997;Cullen and Curry, 1997;Gröger et al, 2003b).…”

“…These are controlled by a combination of (1) carbonate dissolution in response to changes in deep-water circulation after Northern Hemisphere Glaciation and (2) dilution due to variable terrigenous supply from the Amazon River (Bickert et al, 1997;Cullen and Curry, 1997;Curry and Cullen, 1997;Tiedemann and Franz, 1997).…”

Glacial variability over the last two million years: an extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression

“…References and statistics for each sediment core used in this study are given in Bickert et al (1997) and Curry and Cullen (1997) B 3.7 2.2 4 N 43 W ODP927 Cullen and Curry (1997), and Curry and Cullen (1997) B,P 4.5 3.2,2.2 6 N 43 W ODP980 Flower (1999), McManus et al (1999McManus et al ( , 2002, and Oppo et al (1998Oppo et al ( , 2001 Ruddiman et al (1989), Raymo et al (1989), andShackleton (1995). Increasing the number of agecontrol points decreases agemodel uncertainty and better preserves the structure of the d 18 O variability when records are averaged (Huybers, 2004).…”