Onset of Hudson Strait'' Heinrich events in the eastern North Atlantic at the end of the middle Pleistocene transition (∼640 ka)?

Heinrich events are well documented for the last glaciation, but little is known about their occurrence in older glacial periods of the Pleistocene. Here we report scanning XRF and bulk carbonate delta O-18 results from Integrated Ocean Drilling Program Site U1308 (reoccupation of Deep Sea Drilling Project Site 609) that are used to develop proxy records of ice-rafted detritus (IRD) for the last similar to 1.4 Ma. Ca/Sr is used as an indicator of IRD layers that are rich in detrital carbonate (i.e., Heinrich layers), whereas Si/Sr reflects layers that are poor in biogenic carbonate and relatively rich in detrital silicate minerals. A pronounced change occurred in the composition and frequency of IRD at similar to 640 ka during marine isotope stage (MIS) 16, coinciding with the end of the middle Pleistocene transition. At this time, Hudson Strait'' Heinrich layers suddenly appeared in the sedimentary record of Site U1308, and the dominant period of the Si/Sr proxy shifted from 41 ka prior to 640 ka to 100 ka afterward. The onset of Heinrich layers during MIS 16 represents either the initiation of surging of the Laurentide Ice Sheet (LIS) off Hudson Strait or the first time icebergs produced by this process survived the transport to Site U1308. We speculate that ice volume (i.e., thickness) and duration surpassed a critical threshold during MIS 16 and activated the dynamical processes responsible for LIS instability in the region of Hudson Strait. We also observe a strong coupling between IRD proxies and benthic delta C-13 variation at Site U1308 throughout the Pleistocene, supporting a link between iceberg discharge and weakening of thermohaline circulation in the North Atlantic.