RADIOCARBON AGE OFFSETS IN DIFFERENT-SIZED CARBONATE COMPONENTS OF DEEP-SEA SEDIMENTS

We compared accelerator mass spectrometry (AMS) C-14 ages of large (>1 50 mu m) pelagic foraminifera with radiometric bulk carbonate C-14 age s in two northeastern Atlantic cores. The foraminiferal ages are consi stently older than those of the bulk sediment (by +0.76 ka in Core 118 81 and by +1.1 ka in Core 11886), whereas corresponding fine (<5 mu m) fraction ages are similar to those of the bulk sediment carbonate. We calculated near-identical sediment accumulation rates from both the f oraminiferal and bulk sediment age/depth relations (3.0 cm ka(-1) in C ore 11881 and 5.9 cm ka(-1) in Core 11886). Consideration of various f actors that might produce such offsets leads us to believe that they a re not artifacts, but were most probably caused by differential biotur bation of the different size-fractions in the sediment surface mixed l ayer. The importance of this finding is that many paleoceanographic re cords, such as the oxygen isotope record, also derive from analyses of large foraminifera, so that these records must be offset in time from the bulk of the sediments that they characterize.