Oxygen isotopes of detrital carbonate in Heinrich Layers: Implications for reconstructing surface water δ18O

We measured oxygen isotopes of individual detrital carbonate grains in Heinrich layers at three North Atlantic sites, including Core HU90-013-028 from the Labrador slope, IODP Site U1302/03 at Orphan Knoll, and IODP Site U1308 (re-occupation of DSDP Site 609) in the eastern North Atlantic. Oxygen isotopic values of individual detrital carbonate grains from all Heinrich events at all sites average -5.55 ±1.46 permil (1 σ, n=166). The δ18O of bulk carbonate at Site U1308 records the proportion of detrital to biogenic carbonate and bulk δ18O decreases to -5 permil during Heinrich (H) events 1, 2, 4 and 5 relative to a background value of +1 to 2 permil for biogenic carbonate. Bulk δ18O also decreases during H3 and H6 but only attains values of -1 permil. Bulk carbonate δ18O is a sensitive proxy for detecting detrital carbonate and may be useful for identifying Heinrich layers in cores within and outside the North Atlantic ice-rafted detritus belt that lack conspicuous evidence of Heinrich layers in coarse-grained lithic proxies. Because the δ18O of detrital and bulk carbonate contained in Heinrich layers have very low values, any fine-grained detrital carbonate not removed from the inner test chambers of foraminfera will lower δ18O, perhaps leading to an overestimation of the influence of low-δ18O meltwater during Heinrich events. We assessed this potential problem at IODP Site U1308 by rigorously cleaning specimens of Neogloboquadrina pachyderma (sinistral) by breaking each specimen open to reveal internal chambers, immersing tests in an ultrasonic bath, and repicking clean test fragments prior to isotopic analysis. Broken tests clearly reveal the presence of some fine-grained detrital carbonate filling internal chambers. Comparison of δ18O results from Site U1308 with those obtained by Bond et al. (1992) at Site 609 indicate that for most Heinrich layers, the magnitude of the δ18O decrease is reduced, but not eliminated, when thoroughly cleaned specimens of N. pachyderma are analyzed. Benthic foraminifera may be similarly susceptible to contamination by detrital carbonate. Radiocarbon dates of foraminifera from Heinrich layers may be similarly affected by the presence of detrital carbonate with infinite radiocarbon age. Accurate assessment of meltwater influence during Heinrich events is important for providing boundary conditions for numerical models ("hosing experiments") that seek to evaluate the sensitivity of deepwater circulation to freshwater forcing. We recommend rigorous cleaning and screening procedures for foraminifera used for isotope and radiocarbon analysis in North Atlantic Heinrich layers to eliminate contamination by fine-grained detrital carbonate.