Oxygen and carbon isotopes of detrital carbonate in North Atlantic Heinrich Events
Introduction
Heinrich (1988) first described the occurrence of layers rich in ice-rafted detritus (IRD) and poor in foraminifera in sediment cores from the area of the Dreizack seamount in the eastern North Atlantic. Six so-called Heinrich events were originally recognized (“H1” through “H6”) and two additional events were later proposed (e.g. H0 and H5a; Andrews et al., 1995, Bond and Lotti, 1995, Stoner et al., 1996, Rashid et al., 2003).
Heinrich layers are thought to have formed by massive discharges of icebergs during surging of the Laurentide Ice Sheet in the region off Hudson Strait (HS) in northeastern Canada (Broecker et al., 1992). The icebergs produced by these surges traveled across the North Atlantic between ∼ 40 and 55 °N, melted, and dropped their IRD onto the seafloor.
A diagnostic feature of Heinrich layers is that they contain detrital carbonate (limestone and dolomite) derived from lower Paleozoic basins of northeastern Canada (Broecker et al., 1992, Andrews, 1998, Bond et al., 1999, Andrews and Tedesco, 1992, Hemming, 2004 and references therein). Most studies have focused on the silicate fraction of IRD in Heinrich layers to determine provenance, but the detrital carbonate grains on which Heinrich events are defined have received less attention. Here we measured oxygen and carbon isotopic ratios of individual detrital carbonate grains from Heinrich layers at three sites in the North Atlantic, 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. The objective was to characterize the oxygen and carbon isotope composition of detrital carbonate in Heinrich layers and determine whether the isotopic signature can be used to trace the occurrence of HS detrital carbonate in North Atlantic sediment cores.
Section snippets
Site locations and identification of Heinrich Layers
Piston Core HU 90-013-028 was recovered from the Labrador Slope off Hudson Strait (Fig. 1). Rashid et al. (2003) tentatively identified detrital carbonate layers in this core and correlated them to Heinrich layers in the open North Atlantic. The site provides an ice-proximal location from which to sample detrital carbonate derived from Hudson Strait during the last glaciation.
IODP Sites U1302 and U1303 were drilled on Orphan Knoll and are separated by only 5.68 km (Fig. 1). Piston Cores
Methods
Stable isotopes were measured on single detrital carbonate grains from each of the Heinrich layers. Detrital carbonate grains were picked from the > 150 μm fraction and leached in a weak acid. Each detrital carbonate grain was ground and between 20 and 60 μg of powdered sample was loaded into glass vessels and reacted with three drops of H3PO4 for 15 min using a Finnigan MAT Kiel III carbonate preparation device.
Isotope ratios were measured online by a Finnigan MAT 252 mass spectrometer.
Results
Between 6 and 10 grains of detrital carbonate were measured individually for oxygen and carbon isotopes from each of six Heinrich layers at three North Atlantic sites (Appendix A). The mean and standard deviation of oxygen isotope values of detrital carbonate grains from all six Heinrich layers is − 5.6‰ ± 1.5‰ (1σ; n = 166) (Fig. 2a). The mean δ18O for any single Heinrich layer is not significantly different from the other five nor is the mean different among the three sites studied. The mean δ18O
Oxygen isotopes of detrital carbonate in Heinrich layers
Few measurements of stable isotope values of detrital or bulk carbonate have been reported for the North Atlantic. Balsam and Williams (1993) measured oxygen and carbon isotopes of the sediment fine fraction in eight cores from the western North Atlantic. Cores closest to the North American margin had the lowest values of fine-fraction δ18O reaching − 8‰ during the last glaciation. They interpreted these low values to indicate a source of diagenetically altered carbonate rock derived from
Conclusions
Oxygen isotopic values of individual detrital carbonate grains in North Atlantic Heinrich layers average − 5.6‰ ± 1.5‰ (1σ, n = 166), reflecting values of dolomitic limestone derived from lower Paleozoic basins in northeastern Canada. Because the δ18O of detrital carbonate is significantly lower than biogenic carbonate, the δ18O of bulk carbonate in North Atlantic sediments is sensitive to variations in the proportion of detrital-biogenic carbonate. Oxygen isotopes of bulk or fine-fraction carbonate
Acknowledgments
We thank C. Hillaire-Marcel for generously making samples available from core HU-90-013-28 off Hudson Strait. L. Keigwin and an anonymous reviewer provided valuable input that refined the presentation and interpretation of the results. This research used samples provided by the IODP. Funding was provided by grants from the Joint Oceanographic Institutions — United States Science Support Program and the National Environmental Research Council (NE/G001448/1).
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