Application of elemental and isotopic ratios of bulk sediment and detrital carbonate to identify and source Heinrich events in IODP Exp 303/306 sites.

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Heinrich events, named after Hartmut Heinrich who first described them in 1988, are layers of ice rafted debris (IRD) found in North Atlantic sediment cores dating to the last glaciation. They are believed to have formed by massive discharges of ice bergs during surging of the Laurentide Ice Sheet in the region off Hudson Strait in northern Canada. The ice bergs produced by these events traveled across the entire North Atlantic between ~40 and 55 oN, melted, and dropped sediment onto the seafloor (known as 'ice rafted debris'). The diagnostic feature of Heinrich events is they contain detrital carbonate (limestone and dolomite) derived from large lower Paleozoic basins in northern Canada and northwest Greenland. The melting icebergs produced meltwater that decreased the salinity of North Atlantic surface waters and increased surface water stratification and water column stability. In turn, this increased sea ice formation and diminished rates of deep-water production, resulting in large decreases in temperatures over Greenland and Europe. Several mechanisms have been proposed to explain these remarkable events, but none have met with universal acceptance. Although Heinrich events have been studied for the last glacial cycle, little is known about their occurrence in previous glacial periods of the Pleistocene. Such studies have been hampered by the availability of long continuous cores with high sedimentation rates from the North Atlantic IRD belt. To address this shortcoming, Integrated Ocean Drilling Program (IODP) Expedition 303 recovered long cores from multiple locations in the North Atlantic with high sedimentation rates (5 to 20 cm/kyr). To study these long records, we are faced with the prospect of having to make tens of thousands of measurements at multiple sites to identify Heinrich and other types of IRD events. Such an effort would be impractical in terms of both time and expense for the hundreds of meters of section collected during IODP Exp. 303/306. Instead, our approach has been to utilize automated, non-destructive core logging and scanning techniques to identify Heinrich-like events in previous glacial periods. For example, we have found that some Heinrich events are marked by peaks in Ca/Sr ratio measured by core scanning x-ray fluorescence (XRF), an analytical techniques that measures semi-quantitive elemental data along the surface of the core at high spatial resolution. Here we propose to test and calibrate the XRF elemental ratios and develop new techniques (oxygen isotopes of bulk sediment) for recognizing Heinrich events in North Atlantic sediments using cores recovered by IODP Exp. 303/306. These methods should have widespread application for tracing Heinrich events throughout the North Atlantic IRD belt between ~40 and 55 N. In addition to identifying Heinrich and other IRD events, it is necessary to determine the source of the IRD to characterize the dynamics of the specific ice sheets that have advanced, surged, or collapsed. It is often possible to fingerprint the bedrock over which an ice sheet has moved by analyzing geochemically the IRD in marine sediment cores. Most studies have focused on the silicate fraction of IRD in Heinrich events to determine provenance, but the detrital carbonate grains on which Heinrich (H) events are defined have received less attention. Here we propose to test new methods (using oxygen, strontium, and lead isotopes) to identify the source of detrital carbonate mineral grains contained in IRD layers in North Atlantic sediments. These methods will then be applied to identify and source detrital carbonate layers (Heinrich-like events) in older glacial periods of the Pleistocene to help constrain the origin and cause of these remarkable events in the North Atlantic.