QUATERNARY RESEARCH 35, 214-290 (1991) 14C Offsets and Apparently Non-synchronous 6180 Stratigraphies between Nannofossil and Foraminiferal Pelagic Carbonates CHARLES K. PAULL,* SCOTT J. HILLS,~~ HANS R. THIERSTEIN,? GEORGES BONANI,~ AND WILL1 WoLFLI$ *University of North Carolina, Chapel Hill, North Carolina 27599-3315; fETH, Zentrum, Zurich, Switzerland 8092; #Chevron Oil Field Research Co., La Habra, California 90633-0446; and PETH, Honggerberg, Zurich, Switzerland 8093 Received March 3, 1990 Multiple stratigraphies of subtropical South Atlantic cores reveal significant stratigraphic dis- crepancies between the nannofossil and the foraminiferal carbonates. Variations in the strati- graphic signals carried in the nannofossil-dominated <38 pm fraction and foraminifera-dominated >38 km fraction were measured with detailed stratigraphies of 8i*O, 613C, 14C, grain size, per- centage carbonate, percentage aragonite, and taxonomic composition across the last deglaciation in INMD box core 111 No. 9. Three other cores (INMD box cores 113 and 115, and V 22-174) also from the shallow flanks of the mid-Atlantic ridge in the South Atlantic (lo”-17” S), contain similar stratigraphies indicating that these cores represent regional patterns. The onset of the deglacial a”0 shift in foraminiferal carbonate occurs 6 to 20 cm deeper than the 6’*0 shift in the nannofossil fraction. Nineteen accelerator mass spectrometer 14C dates of various fractions (<38 pm, 38-62 pm, 62-150 km, 150-250 pm, >355 km, and G. ruber) from INMD Box 111 show that the com- ponents within an individual core slice may differ by up to 4900 i4C years. Twelve traditional i4C dates (determined by beta counting) of the >38 and <38 p,rn fractions from six levels in INMD Box 111 No. 6 confirm the large offsets. The observed isotopic offsets are not explained by an individual process, and suggest that multiple causes have left a lumpy stratigraphic record. However, the variability in the data illustrates the difficulty in accurately measuring the chronology of deglaci- ation, reopens the question of which components of pelagic sediment best monitor surface water conditions, and complicates the direct interpretation of benthic-planktonic age differences in terms of ocean ventilation changes. Q 1991 University of Washington. INTRODUCTION The last deglaciation is associated with a major shift in the El80 values that are pre- served in the calcareous tests of foraminif- era found in deep-sea sediments (Emiliani, 1955). Isotopic measurements of fine car- bonates indicate similar trends in &I80 val- ues (Douglas and Savin, 1973; Anderson and Cole, 1975; Margolis et al., 1975; Berger et al., 1978; Letolle, 1979; Dudley et al., 1980; Scholle and Arthur, 1980; Ander- son and Steinmetz, 1981; Schiffelbein and Thierstein, 1981; Dudley and Nelson, 1989; Paul1 and Thierstein, 1990). These changes are believed to be modulated by global ice volume change (Shackleton, 1967; Shackle- ton and Opdyke, 1973) associated with de- glaciation. The deglaciation occurred about 11,000 years ago (Ericson et al., 1956; Bro- ecker et al., 1960; Peng et al., 1977; Berger et al., 1985; Duplessy et al., 1986; Shack- leton et al., 1988; Broecker et al., 1988a,b,c), although there is considerable scatter in the measured 14C ages. Calcareous nannoplankton (<38 km) and planktonic foraminifera (>38 urn) are the dominant carbonate components in well- preserved deep-sea sediments. In samples from above the lysocline, the ~38 pm frac- tion is largely composed of nannofossils (Paul1 et al., 1988), while the >38-p,rn frac- tion is largely composed of calcitic forami- nifera with secondary amounts of arago- nitic pteropods. The two size fractions are thus dominated by plant (i.e., phytoplank- ton) and animal (zooplankton) carbonates, respectively. Each fraction typically com- 274 OO33-5894/91 $3.00 Copyright 0 1991 by the University of Washington. All rights of reproduction in any form reserved.