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1 A Laurentide outburst flooding event during the Last Interglacial Period Supplementary Material Data Data has been archived at the Pangaea Database ( Methodology Samples were wet sieved at 63µm and dried in an oven at 50 C prior to weighing. Twenty specimens of G. bulloides were picked from the µm size fraction for each data point. Particular care was taken not to pick any shells that appeared to contain any detrital sediment, as the detrital carbonate within the red layer would contaminate the isotopic ratio. Foraminifera shells were crushed and soaked in a solution of 3% hydrogen peroxide for 30 minutes in individual vials. Acetone was added and the samples placed in an ultra-sonic bath for 10 seconds, after which the liquid was carefully decanted to remove any contaminants. The samples were dried in an oven at 50 C overnight. Isotopic analysis of foram samples was performed on a VG SIRA mass spectrometer with a Multicarb system. Analytical precision is estimated to be ±0.08. Bulk carbonate isotope measurements were carried out on unwashed and unsieved sediment. Bulk oxygen isotope values were measured on a Thermo Finnegan MAT253 mass spectrometer with a Gas Bench II using a continuous flow system with an analytical precision of ±0.8. Oxygen isotope measurements are given as δ 18 O, calculated as; δ 18 O = ((( 18 O/ 16 O) Sample /( 18 O/ 16 O) Standard )-1)*1000, using the VPDB standard. Weight percent CaCO 3 was measured by acidification of bulk sediment samples using an AutoMateFX carbonate preparation system and evolved CO2 was measured using a UIC (Coulometrics) 5011 CO 2 coulometer. Analytical precision is estimated to be ±1% by measurement of a carbonate standard. Point counting of IRD was carried out on the >150mm size fraction. Samples were sieved then split into subsamples of at least 300 grains and were counted using a binocular optical microscope. Core scanning XRF measurements were carried out at the MARUM institute in Bremen using an Avaatech XRF scanner equipped with a Canberra X-PIPS Silicon Drift Detector (SDD; Model SXD 15C ) with 150eV X-ray resolution, the Canberra Digital Spectrum Analyzer DAS 1000 and an Oxford Instruments 100W Neptune Rh X-ray tube. The core surface was carefully scraped clean horizontally using a glass slide and covered with a 4 µm thin SPEX CertiPrep Ultralene foil to avoid contamination and minimize desiccation. Each section was measured twice: once at 1 ma and 30 kv using a lead filter in order to measure Sr; NATURE GEOSCIENCE 1

2 once at 0.25 ma and 10 kv in order to measure Ca. The irradiated surface was 12mm cross core and 5mm down core. The raw spectra produced were analyzed using the WINAXIL software package produced by Canberra Analytics using standard 30kv and 10kv processing models. The peak in Ca/Sr is caused by concurrent increases in Ca and decreases in Sr, suggesting an increase in total carbonate but a decrease in the proportion of biogenic carbonate (figure S1), in agreement with measurements of %carbonate. Colour Reflectance Diffuse color reflectance was measured every 2cm using a Minolta spectrophotometer mounted during IODP Expedition 303 (Expedition 303 Scientists, 2006). The color spectrum for each measurement ranges from 360 to 740 nm binned in 10 nm intervals. We used the CIELAB system where L* is the lightness varying between 0 and 100%, a* is the green (negative) to red (positive) axis, and b* is the blue (negative) to yellow (positive) axis. The first derivative of the colour reflectance spectrum can be used to identify the presence of hematite in sediment (Balsam and Deaton, 1991). The shipboard colour reflectance spectra have been analyzed for both sites. Figure S2 shows the first derivatives of the reflectance spectra for the interval of the red layer with the highest value of a* (The red-green colour parameter) and for background sediment. The main peak in the red layer spectra at 565nm is due to the presence of hematite, and is absent in the two spectra taken from background sediment. Biomarkers Heinrich layers in the North Atlantic and Labrador Sea contain a very distinct biomarker signature (Rosell-Melé et al., 1997; Rashid and Grosjean, 2006; Naafs et al., 2011). The most characteristic is the high abundance of so-called petrogenic compounds such as isorenieratane and palaerenieratane and their derivatives, triaromatic steroids, C-ring monoaromatic steroids, and D-ring monoaromatic 8,14- secohopanoids and an atypical dominance of mature αβ-hopanes and C 34 -hopanoids over the C 33 and C 35 homologues (Rashid and Grosjean, 2006). This highly specific biomarker distribution is incompatible with recent sediments and indicates the transport of ancient organic matter by glacial erosion of a organic rich source-rock formation from the Hudson area of eastern Canada with an Ordovician/Silurian age (Rosell-Melé et al., 1997; Rashid and Grosjean, 2006). As such, the presence of these petrogenic compounds in marine sediments can be used as specific organicgeochemical tracer for ice-surges draining the central part of the Laurentide ice sheet overlying the Hudson area (Rashid and Grosjean, 2006). 11 samples from Sites U1302 and U1305 across the red-layer were measured to determine whether the red-layer contained the same suite of petrogenic compounds found within Heinrich Layers and thus whether the source of the detrital material in the red layer was the Hudson Area. For this purpose between 6 and 13 gram of homogenized and freeze-dried sediment were extracted using 20 ml of dichloromethane (CH 2 CL 2 ) and Automated Solvent Extraction (ASE 200, DIONEX, 5 min. at 100 ºC and 1000 psi). The total extract was concentrated and separated into three fractions using liquid chromatography and successively eluting hexane (3 ml), 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION hexane/ch 2 CL 2 (1:1, 4 ml), and CH 2 CL 2 (4 ml) over a 5 cm long silica gel column, yielding a saturated hydrocarbon, aromatic, and ketone fraction. After drying under a nitrogen flow, the samples were re-dissolved in 500 µl hexane and analyzed using a LECO Pegasus III (LECO Corp., St. Joseph, MI), interfaced to an Agilent 6890 GC. The gas chromatograph (GC) was equipped with a 15 m x 0.18 mm i.d. Rtx-1MS (Restek Corp., USA) column (film thickness: 0.10µm) with an integrated 5 meter guard column. The GC oven was initially held at 60 C for 1 min, then heated at 50 C min -1 to 250 C and at 30 C min -1 to 310 C (held 2.5 min). The abundance of the various petrogenic compounds in the aromatic fraction was monitored using the characteristics ion fragments (Rashid and Grosjean, 2006) of m/z (iso- and palaerenieratane), 231 (triaromatic steroids), 253 (C-ring monoaromatic steroids), and 365 (D-ring monoaromatic 8,14-secohopanoids). Relative abundances were calculated semi quantitatively by normalizing the peak areas to the maximum area per gram sediment. The C 33 /C 34 hopanoid ratios were determined in both the saturated hydrocarbon and aromatic fraction using m/z 191 (17α(H),21β(H) hopanes and benzohopanes) and 365 (D-ring monoaromatic 8,14-secohopanoids). The results show that the red-layer is characterized by a high abundance of the petrogenic compounds such as palae- and isorenieratane, aromatic steroids, and secohopanoids (see Fig. S3). Outside this layer the abundance is low or below the detection limit. In addition, the C 33 /C 34 ratio of the hopanes and hopanoids within the red layer is below 1, reflecting the atypical dominance of the C 34 homologues (see Fig. S3). The ratios could only be determined in samples with sufficient petrogenic compounds. The complete biomarker signature of the red layer at both Site U1302 and U1305 is identical to that found in the Heinrich Layers in the Labrador Sea and North Atlantic (Rashid and Grosjean, 2006) and therefore indicates that the source of detrital material in the red-layer is the Hudson area of northern Canada, analogues to the Heinrich Layers. References Expedition 303 Scientists, Expedition 303 summary. In Channell, J.E.T., Kanamatsu, T., Sato, T., Stein, R., Alvarez Zarikian, C.A., Malone, M.J., and the Expedition 303/306 Scientists, Proc. IODP, 303/306, doi: /iodp.proc (2006). Deaton, B. & Balsam, W.L. Visible spectroscopy - A rapid method for determining hematite and goethite concentration in geological materials. Journal of Sedimentary Research (1991). Rosell-Mele, A., Maslin, M.A., Maxwell, J.R. & Schaeffer, P. Biomarker evidence for Heinrich events. Geochimica et Cosmochimica Acta 61, (1997). Rashid, H. & Grosjean, E. Detecting the source of Heinrich layers: An organic geochemical study. Paleoceanography 21, PA3014 (2006). NATURE GEOSCIENCE 3

4 Naafs, B.D., Hefter, J., Ferretti, P., Stein, R. & Haug, G.H. Sea surface temperatures did not control the first occurrence of Hudson Strait Heinrich Events during MIS 16. Paleoceanography 26, 1-10 (2011). Supplementary Figures Figure S1. Raw scanning XRF measurements of Ca and Sr from both sites. In both cases the red layer corresponds to an increase in Ca and a decrease in Sr. This shows that there is an increase in carbonate content but a decrease in the amount of biogenic carbonate. 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION Figure S2. First derivative of the colour reflectance spectra for four points from the two sites. The two lines with cross markers are from Site U1305, the lines with circle markers are from Site U1302. Dashed red lines indicate measurements of the red layer at the two sites, black lines indicate normal sediment. The two measurements made from within the red layer show a peak in the reflectance at 565nm that indicates the presence of hematite. This peak is absent from points outside of the red layer. NATURE GEOSCIENCE 5

6 Figure S3. Abundance of the petrogenic compounds at IODP Site U1302 (top) and U1305 (bottom). C 28 (S) C-ring monoaromatic steroid (green), C 29 and C 30 D-ring monoaromatic 8,14-secohopanoids (light blue), triaromatic steroid (orange), and palae- and isorenieratane (purple). In addition, in black the C 33 /C 34 ratios are shown for the D-ring monoaromatic 8,14-secohopanoids (filled circles), 17α(H), 21β(H) hopanes (open circles), and benzohopanes (filled triangles). The coloured bar indicates the position of the red layer. 6 NATURE GEOSCIENCE