IRD evidence for Heinrich Events H1 and H2 on the NJ Margin

Recent seismic reflection studies suggest icebergs grounded on the late Pleistocene New Jersey margin (Goff and Austin, 2009) during Heinrich events H1 (~17 cal Ka), H2 (~23 cal Ka), H3 (~30 cal Ka) and H4 (~37 cal Ka). We tested this hypothesis by analyzing sediments on the upper NJ continental slope, near the area where icebergs were found. Ocean Drilling Program Site 1073A is located at 39°13.5214'N, 72°16.5461'W in 639 m water depth. The greater water depth increases the likelihood of preservation and improves the stratigraphic control. We obtained elemental measurements for Site 1073A Cores 1-6 using the X-ray Fluorescence (XRF) Core Scanner at the MARUM IODP core repository, University of Bremen. Cores were scanned at 10kV and 30kV to obtain a full suite of elements for analysis, with XRF data collected between 1 and 10 cm intervals. The values are reported in areas for the elements and were then converted to elemental ratios (Ca/Sr, Si/Sr, K/Al, and Si/Ti) for analysis. The origin of a Heinrich layer within an IRD belt can be identified by geochemical measures along with magnetic susceptibility and the presence of detrital materials that reflect its provenance (Hemming, 2004). Following analyses for Atlantic Ocean IRD by Hodell et al. (2008), higher Si/Sr values indicate abundant detrital silicates and low biogenic carbonate. Abundant detrital carbonate is identified by higher Ca/Sr values. K/Al may be used as a proxy for weathered matter being deposited and is a good indicator of terrigenous material (Yarincik, 2000). Si/Ti ratios may be used as a proxy for organic and siliceous productivity (Agnihotri, 2008). The elemental analysis was ground-truthed with grain size analysis at 10 cm intervals. Grain size analysis reveals large particles in a finer matrix at 123 cm and 284 cm. We interpret these as IRD. A linear sedimentation age model places the IRD around the time of Heinrich events H1 and H2. Age control is provided for the late Pleistocene principally by radiocarbon methods (McHugh and Olsson, 2002); however, the age model is not unequivocal so we present results for a few interpretations (linear sedimentation rates, and modeled). Nonetheless, the IRD is consistent with the timing of H1 and H2. While the IRD are obvious from the grain size analysis, we were not able to identify IRD from XRF elemental analyses. This may because the sediments are derived locally rather than transported from the higher latitudes, rendering IRD identification a function of grain size (anomalously large particles in a marine setting) rather than geochemistry. Alternatively, the particles were not close enough to the surface to impact the XRF response. This study provides further evidence for the presence of icebergs on the late Pleistocene NJ margin.