Anatomy of Heinrich Layer 1 and its Role in the Last Deglaciation

X-ray fluorescence (XRF) core scanning and X-ray computed tomography (CT) data were measured every 1-mm to study the structure of Heinrich Event 1 during the last deglaciation at IODP Site U1308. Heinrich Layer 1 comprises two distinct layers of ice-rafted detritus (IRD), which are rich in detrital carbonate (DC) and poor in foraminfera. Each DC layer consists of poorly-sorted, coarse-grained clasts of IRD embedded in a dense, fine-grained matrix of glacial rock flour that is partially cemented. The radiocarbon ages of foraminifera at the base of the two layers indicate a difference of 1400 14C years, suggesting they are two distinct events, but the calendar age depends upon assumptions made for surface reservoir ages. The double peak indicates at least two distinct stages of discharge of the ice streams that drained the Laurentide Ice Sheet through Hudson Strait during HE1 or, alternatively, the discharge of two independent ice streams. Heinrich Event 1 began at ~16.1 ka when the North Atlantic was already cold and Atlantic Meridional Overturning Circulation (AMOC) weakened. Strong warming ensued immediately following the older IRD peak (H1.1) as marked by a sharp decrease in the abundance of the polar species N. pachyderma (sin). Only a minor cooling accompanied the second peak (H1.2). Our results suggest a complex history for Heinrich Stadial 1 (HS1) with reduction in AMOC during the early part (~20-16.1 ka) possibly driven by melting of European ice sheets, whereas the Laurentide ice sheet assumed a greater influence during the latter half (~16.1-14.7 ka).