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doi:10.2204/iodp.proc.340.204.2016

Results

This section summarizes the stratigraphy, composition, and dating at each of the three sites (shown in “Appendix A” and “Appendix B”). Raw data are included in “Supplementary data tables.”

Unit identification

Tephra fall and some volcaniclastic deposits are associated with episodes of island building, whereas bioclastic turbidites, mixed turbidites, and some volcaniclastic turbidites are associated with collapse events from Montserrat.

Unit types were defined using grain size and componentry criteria, as described previously. Grain size data can be found in Table 204_ST01.CSV, grain size data analyses in Table 204_ST02.CSV, and componentry data can be found in Table 204_ST03.CSV. Hole U1394B includes around 17 bioclastic turbidites, 24 mixed turbidites, 55 volcaniclastic turbidites, and 32 tephra fall layers. Hole U1395B contains 18 bioclastic turbidites, 27 mixed turbidites, 43 volcaniclastic turbidites, and 52 tephra fall layers. The uppermost 7 m of Hole U1396C contains 1 bioclastic turbidite, 1 mixed turbidite, 9 volcaniclastic turbidites, and 13 tephra fall layers.

Lead isotopes were used to identify the source of fallout units, although only a few of the fallout layers could be sampled for logistical reasons. Two tephra units in Hole U1396C were found to have a Guadeloupe signature (²⁰⁸Pb/²⁰⁴Pb > 38.85; ²⁰⁶Pb/²⁰⁴Pb > 19.1; Fig. F4; Table T2); thus, most visible tephra units present in Holes U1394B, U1395B, and U1396C are likely to originate from Montserrat.

Dating of units

We outline the available age control for the three sites in “Appendix A.” This age control was obtained using a combination of AMS radiocarbon dating, oxygen isotope stratigraphies, biostratigraphic zones, and paleomagnetic stratigraphies.

AMS radiocarbon dating

Two AMS radiocarbon dates were obtained from Hole U1395B at 2.3–2.5 mbsf (Table T1; “Appendix A”). AMS dates of 10.4 and 12.7 ka assisted correlations to layers dated by Trofimovs et al. (2013) (Fig. F2). Two additional samples located further down Hole U1395B were found to be too old (>43.5 ka) for radiocarbon dating.

Correlation to units dated previously

Turbidites in the upper 3–5 m of Sites U1394 and U1395 can be correlated to turbidites in adjacent vibrocores, whose ages are well constrained by numerous radiocarbon dates (Trofimovs et al., 2013). These correlations are shown in Figures F2 and F3 and allow turbidites in the upper parts of Holes U1394B and U1395B to be dated more precisely. Below recent turbidites from the 1995 eruption are turbidites at ~2, ~3, and 6 ka (Trofimovs et al., 2013). The 6 ka turbidite is relatively thick at Site U1395 (Fig. F3). The 2 and 6 ka events are thought to originate from collapses of English’s Crater on land, whereas the 3 ka turbidite may be the result of a more local carbonate platform failure (Trofimovs et al., 2013). The 6 ka turbidite is underlain by thick bioclastic-rich turbidites that were emplaced at 11.5–14 ka and are most likely associated with landslide Deposit 1 (Fig. F3) (Trofimovs et al., 2013). Radiocarbon dating shows that the 11.5–14 ka turbidity current was particularly erosive. Mafic-rich turbidites dated to ~110 ka by Trofimovs et al. (2013) may also be found at Sites U1394 and U1395 (Fig. F3).

Oxygen isotope stratigraphies

Oxygen isotope stratigraphies were developed for Sites U1394, U1395, and U1396 (“Appendix A”). Raw data are available in Table 204_ST04.CSV. Each site will be described in turn below.

Site U1394

The oxygen isotope record at Site U1394 is incomplete because of poor core recovery. Hole U1394A is largely incomplete, and Hole U1395B has minimal core recovery between 15 and 60 mbsf. Seismic surveys also identify a chaotic section between 15 and 95 mbsf (M. Vardy, pers. comm., 2015), suggesting the sediments in this 15–95 mbsf interval may not be in situ (“Appendix A”).

Site U1394 is located within 8 km of 18 shallow vibrocores described by Trofimovs et al. (2013) (Fig. F1) as part of an extensive core survey that correlates units across the Boulliant-Montserrat Graben and to the southwest of Montserrat (Trofimovs et al., 2013; Cassidy et al., 2013). Units within the upper 15 m of stratigraphy from Site U1394 can be correlated with units described by Trofimovs et al., 2013. These include the 2–1.5, 6, 14, 74–59, and 103–99 ka turbidites described by Trofimovs et al. (2013), which can be correlated to units within Hole U1394B (Fig. F3).

Below 15 mbsf it is difficult to identify marine isotope stages in Hole U1394B, as the core is incomplete. The first occurrence of the nannofossil species Emiliania huxleyi (250 ka) is found at Sites U1395 and U1396 near the marine isotope Stage (MIS) 7/8 boundary (“Appendix B”). In Hole U1394B the first occurrence of E. huxleyi is located at 133 mbsf, coinciding with an increase in δ¹⁸O at ~133 mbsf. Thus, the increase in δ¹⁸O in Hole U1394B at ~133 mbsf has been correlated with the MIS 7/8 boundary (Fig. F5; “Appendix B”).

Site U1395

Site U1395 is located near the shallow vibrocore JR123-12V described by Trofimovs et al. (2013) (Fig. F1). Units in the upper 15 m of Site U1395 can be correlated with units in the shallow vibrocores described by Trofimovs et al. (2013) and Cassidy et al. (2013, 2014a). The 2–1.5, 6, 14, 74–59, and 110–103 ka turbidites from Trofimovs et al. (2013) can be correlated with units in Hole U1395B (Figs. F2, F3).

Some of the units show evidence of erosion (e.g., the 14 ka turbidity current event is suggested to have removed ~30 ky of underlying hemipelagic material; Cassidy et al., 2013). Despite erosion, the oxygen isotope record from Hole U1395B correlates well to Core JR 123-12-V and to other shallow vibrocores described by Trofimovs et al. (2013) (Figs. F2, F3). The Hole U1395B isotope record also correlates well to the more complete oxygen isotope record of CAR-MON2 (Fig. F3) and with the standard global oxygen isotope curve (Imbrie et al., 1984; Prell et al., 1986; Martinson et al., 1987).

Site U1396

Oxygen isotope stratigraphy of Hole U1396C is mostly complete, as there are fewer erosive events. Site U1396 is located near the site of CAR-MON2 (Le Friant et al., 2008), and the oxygen isotopes correlate well (Fig. F3). Only the uppermost unit, where MIS 2 is absent, shows any evidence of significant erosion (Wall-Palmer et al., 2014) (Fig. F5).

Differences between the data presented here and previously published data can be explained by the use of different methodologies. For example, oxygen isotope analysis of CAR-MON2 was carried out on homogenized <63 µm material, which contains both planktonic and benthic organisms, whereas analysis of Hole U1396C was carried out on a single species of planktonic foraminifer.

Biostratigraphy

The first occurrence datum of the nannofossil species Emiliania huxleyi (250 ka) was identified at all three sites (“Appendix B”), confirming the position of the boundary between MIS 7 and 8. This datum was particularly useful in Hole U1394B, where erosion and poor core recovery between 16 and 60 mbsf made it difficult to identify particular isotope stages. The zonation of G. menardii was also used in confirming the stratigraphy of and the correlations between Holes U1396C, U1395B, and U1394B (“Appendix B”; Table 204_ST04.CSV). Zones U–Z were identified at Sites U1395 and U1394; however, only Zones U–Y were identified at Site U1396 because of erosion close to the surface.

Paleomagnetic stratigraphies

Shipboard NRM identifies no paleomagnetic reversals in Hole U1394B, suggesting that all of the core material was deposited in the last 781 ky. However, it should be noted that some of the material in Hole U1394B comprises landslide Deposit 2, and this material may have originated elsewhere.

Two paleomagnetic reversals occur in Hole U1395B at 781 and 988 ka. They are found at depths of 68.5–71 and 89.08 mbsf, respectively (“Appendix B”). The exact location of the 781 ka reversal is obscured by a thick volcaniclastic turbidite. At the base of Site U1395, declination begins to change, indicating that a 1072 ka reversal may appear at the base of Hole U1395B. For the sake of this study the boundary of the 1072 ka reversal was placed at 115.45 mbsf, at the base of a volcaniclastic turbidite.

A detailed paleomagnetic stratigraphy was obtained for Site U1396 that stretched back to ~4.5 Ma and comprises 16 reversals (“Appendix B”).

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