Radiocarbon age offsets in different-sized carbonate components of deep-sea sediments

We compared accelerator mass spectrometry (AMS) (14)C ages of large (>150 mu m) pelagic foraminifera with radiometric bulk carbonate (14)C ages in two northeastern Atlantic cores. The foraminiferal ages are consistently older than those of the bulk sediment (by +0.76 ka in Core 11881 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 foraminiferal and bulk sediment age/depth relations (3.0 cm ka(-1) in Core 11881 and 5.9 cm ka(-1) in Core 11886). Consideration of various factors that might produce such offsets leads us to believe that they are not artifacts, but were most probably caused by differential bioturbation of the different size-fractions in the sediment surface mixed layer. The importance of this finding is that many paleoceanographic records, 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.