Discovery of a large 2.4 Ma Plinian eruption of Basse-Terre, Guadeloupe, from the marine sediment record

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1 GSA Data Repository Discovery of a large 2.4 Ma Plinian eruption of Basse-Terre, Guadeloupe, from the marine sediment record Martin R. Palmer et al. Methods Geochemistry mg of volcanic grains were handpicked from the >63 m fraction of sieved tephra samples and powdered in an agate mortar. Trace element concentrations were measured by a VG Plasmaquad PQ2+ ICP-MS multi-element analyzer, with accuracy and precision better than 3% and ±3.5% (RSD), respectively (Supplementary Table DR1). Pb isotope analyses were carried out on ~ mg of handpicked grains (depending on Pb concentration), using the method described by Cassidy et al. (2012). Pb isotope ratios were measured on a MC- ICPMS (Neptune) (Supplementary Table DR1), using the double spike technique to correct for instrumental bias (Taylor et al., 2015). NBS 981 standard gave the following results: ± (2SD) for 206 Pb/ 204 Pb, ± for 207 Pb/ 204 Pb and ± for 208 Pb/ 204 Pb during the course of the analyses reported here. The small sample sizes available preclude the use of XRF analysis, so the major elements were also determined by a VG Plasmaquad PQ2+ ICP-MS multi-element analyzer on 5 bulk samples that were leached with 6N HCl (to remove biogenic carbonate) (Supplementary Table DR2). SiO 2 concentrations were determined by subtracting the sum of the other major elements from 100%. While this method is less accurate than traditional techniques, the inter-element relationships are consistent with trends observed for subaerial exposures on Guadeloupe (Samper et al., 2007; 2009). Grain characteristics analysis

2 Grain size analyses were carried out on ~1 cm 3 of bulk samples that were added to 30 ml of Calgon solution (as a dispersant), and run through a Malvern Mastersizer 2000 particle size analyzer to measure grain sizes in the μm size interval. The data were entered into the Gradistat program (Blott & Pye, 2001) to determine the sorting value (Folk & Ward, 1957). The samples were then wet sieved into a >100 μm fraction and dried. This fraction was photographed under a stereo microscope using an integrated digital camera (Leica EZ4D). Images were processed and analyzed using ImageJ software, and the area, perimeter and aspect ratio of grains were measured according the procedure outlined by Cassidy et al. (2014). Samples from T 2.36 show a range in angularity and elongation, but except for the uppermost sample (6H2 94/95), they all fall within the primary tephra fallout field that was calibrated against know fallout and flow deposits from Montserrat (Suppl. Data Fig. DR1). Age assignment The sediments recovered from Holes U1396A and U1396C were primarily correlated using shipboard magnetic susceptibility (MS) which was acquired at 2.5 cm intervals prior to the core being split on the Whole Round Multi Sensor Logger (WRMSL). To independently corroborate MS correlations, and assist decisions if MS correlation became ambiguous, density data measured through Gamma Ray Attenuation on the WRMSL at 2.5cm intervals and Natural Gamma Ray (NGR) radiation data measured at 10cm intervals on the NGR system were also used for correlation. Using this approach, the 82 cm mark in U1396A 6H4 (6H4-82) and 101 cm mark in U1396C 6H2 (6H2-101) both lie at a composite core depth (CCSF-A) of m below seafloor. This CCSF-A depth is bracketed by the C2r.1n (B) Reunion polarity zone (2.148 Ma) at a mid-point CCSF-A of m and the C2r.2r (B) Gauss/Matuyama polarity zone (2.581 Ma) at a mid-point CCSF-A of m (Hatfield, 2015). If sedimentation rates are assumed to be linear, this depth corresponds to a

3 depositional age of 2.36 Ma. The uncertainties in this age arise from the presence of any hiatuses in sedimentation, variations in the background hemipelagic sedimentation rate, and the number and thickness of tephra layers above and below T There was no evidence during shipboard core logging of any hiatuses in this interval (Le Friant et al., 2013). Biostratigraphic and 18 O stratigraphical analysis of the upper 7 m (past 250 kyr) of sediments from U1396C showed that background sedimentation rates varied from 2.8 to 9.4 cm ka -1 (Wall-Palmer et al., 2014). The period of rapid sedimentation was confined to an interval of ~0.5 m (~50 kyr) in the uppermost part of the core, and the hemipelagic sedimentation rates were relatively constant (~2.8±0.5 cm ka -1 ) through the rest of the upper 7 m. Overall, however, there is no clear evidence to suggest the putative age for T 2.36 is biased towards younger or older ages by variations in the hemipelagic sedimentation rate. Visual shipboard core logging and magnetic susceptibility data (Le Friant et al., 2013) (as a proxy for volcanogenic component of the sediments; Cassidy et al., 2014) shows there were similar levels of tephra deposition in both the lower and upper portions of the section bound by the two polarity zones. Hence the true depositional age of T 2.36 is unlikely to be significantly biased to older or younger ages than 2.36 Ma based on the assumption of linear sedimentation rates. Additional References Blott, S.J., and Pye, K., GRADISTAT: a grain size distribution and statistics package for the analysis of unconsolidated sediments: Earth Surface Processes and Landforms, v. 26, p , doi: /esp.261. Carey, S.N., and Sigurdsson, H., A model of volcanogenic sedimentation in marginal basins Marginal Basin Geology, in Kokelaar, B.P., and Howells, M.F., eds., Volcanic and

4 Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins: Journal of the Geological Society of London Special Publication, v. 16. Folk, R.L., and Ward, W.C., Brazos River bar: studying the significance of grain-size parameters: Journal of Sedimentary Petrology, v. 27, p Mulder, T., and Alexander, J., The physical character of subaqueous sedimentary density flows and their deposits: Sedimentology, v. 48, p , doi: /j x. Sparks, R.S.J., et al., Magma production and growth of the lava dome of the Soufrière Hills Volcano, Montserrat, West Indies: November 1995 to December 1997: Geophysics Research Letters, v. 25, p , doi: /98GL Taylor, R.N., Ishizuka, O., Michalik, A., Milton, J.A., and Croudace, I.W., Evaluating the precision of Pb isotope measurement by mass spectrometry. Journal of Analytical Atomic Spectroscopy, 30, , doi: /c4ja00279b.

5 Supplementary Table DR1. Sample La Sm 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 7/4 U1396C 6H2 (ppm) (ppm) 94/95 WP ± ± ± /100 WP ± ± ± /102 WP /102 VJ ± ± ± /107 WP /107 VJ ± ± ± /114 NVJ /114 VJ ± ± ± WP = white pumice, VJ = vesicular juvenile, NVJ = non-vesicular juvenile. See Cassidy et al. (2014) for definition of classifications. Supplementary Table DR2. Sample U1396C 6H2 SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O 94/ / / / /

6 6 Sorting 4 2 A Elongation (aspect ratio/area) 6 B Sorting Angularity (perimeter/area) Elongation C Angularity (perimeter/area) Figure DR1. Grain size characteristics of tephra samples from Site 86 U1396C. Green and red symbols are data from Cassidy et al. (2014) defining characteristics 87 of primary fallout tephra and reworked tephra, respectively. Upper T2.36 (light blue) refers to 88 samples 94/95 and 99/100 from U1396C 2H2, and lower T2.36 (dark blue) refers to samples /102, 106/107 and 113/114 from U1396C 2H2.