X‐ray fluorescence (XRF) core scanning and X‐ray computed tomography data were measured every 1 mm to study the structure of Heinrich Event 1 during the last deglaciation at International Ocean Discovery Program Site U1308. Heinrich Layer 1 comprises two distinct layers of ice‐rafted detritus (IRD), which are rich in detrital carbonate (DC) and poor in foraminifera. 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 that they are two distinct events, but the calendar ages depend 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 containing detrital carbonate. Heinrich Event 1.1 was the larger of the two events and began at ~16.2 ka (15.5–17.1 ka) when the polar North Atlantic was already cold and Atlantic Meridional Overturning Circulation (AMOC) weakened. The younger peak (H1.2) at ~15.1 ka (14.3 to 15.9 ka) was a weaker event than H1.1 that was accompanied by minor cooling. Our results support a complex history for Heinrich Stadial 1 (HS1) with reduction in AMOC during the early part (~20–16.2 ka) possibly driven by melting of European ice sheets, whereas the Laurentide Ice Sheet assumed a greater role during the latter half (~16.2–14.7 ka).

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(Fig. 1). The red layer is ∼10 cm thick at Site U1305 and ∼20 cm thick at U1302 (Figs 2-4). The base of the layer is very sharp at both sites, suggesting rapid onset of deposition with little bioturbation. At Site U1305, the top of the layer is well defined, whereas at Site U1302 the top of the layer is more diffuse with a distinct tail, probably the result of mixing by bioturbation. The bright red colour of the layer is distinctive and marked by anomalously high values of a * (red-green colour parameter) at both sites. A peak in the first derivative of the reflectance spectrum at 565 nm suggests that the red colour is primarily imparted by haematite in the sediment 6 (see Supplementary Information).Core scanning X-ray fluorescence (XRF) measurements show high values of Ca/Sr in the red layer interval at both sites (Figs 3 and 4), indicating an increase in the proportion of strontium-poor detrital carbonate relative to strontium-rich biogenic carbonate 7 . The peak carbonate content of the sediment within the layer is ∼30% at Site U1305 and ∼40% at Site U1302. At Site U1305, the carbonate peak corresponds to a significant increase above background levels whereas at Site U1302 the increase is less pronounced, most likely owing to relatively higher deposition rates of synsedimentary biogenic carbonate during the interval.Bulk carbonate δ 18 O shows a strong decrease to values of about −5 (versus Vienna PeeDee Belemnite) within the red layer at both sites, which is typical of detrital carbonate from Hudson Bay 8 . Organic biomarkers provide further evidence of the source of the red layer. At both sites the red layer contains high abundances of a suite of organic compounds normally absent in recent sediments. These petrogenic compounds include carotenoid-derived aromatic hydrocarbons (for example, isorenieratane and palaerenieratane), aromatic steroids and secohopanoids (see Supplementary Information). The relative abundances of these compounds are approximately three times higher at Site U1302 than at Site

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