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doi:10.2204/iodp.proc.340.203.2015

Methods

IODP Expedition 340 successfully collected cores from three sites (U1396, U1395, and U1394) offshore of Montserrat in the Lesser Antilles, Caribbean Sea (Fig. F1). Sites U1395 and U1394 lie within the Bouillante–Montserrat Graben, ~15 and ~33 km from the island, respectively, and in the path of major submarine landslides (Trofimovs et al., 2013). Sedimentation at these two sites has been considerably affected by the input of volcanic material and resedimentation by density current deposits (Expedition 340 Scientists, 2013). Here, we present a ~300 ky holoplanktonic gastropod record from Sites U1395 and U1394 that corresponds to ~166 and ~44 m hole depth, respectively. Material analyzed was collected from undisturbed hemipelagic sediments only and the data are presented with gaps where disturbed material interrupts the record.

The holoplanktonic gastropod content of Site U1396 is not presented here because the site does not preserve a record of holoplanktonic gastropods between ~43 and 250 ky (Wall-Palmer et al., 2014a), despite being positioned out of the main path of density current deposits. This absence was attributed to winnowing by bottom water currents that are thought to have removed the delicate shells. Previously cored Site CAR-MON 2 (Fig. F1) records a rich fossil record of holoplanktonic gastropods (Le Friant et al., 2008; Wall-Palmer et al., 2013, 2014b), despite being only ~15 km southwest of Site U1396. Therefore, Site CAR-MON 2 was used as a comparison site, rather than Site U1396. All drilling and coring sites are above the aragonite lysocline, reducing any effects of postdepositional dissolution upon the aragonitic gastropod shells.

Core sampling

Holes U1394A, U1394B, and U1395B were sampled at ~50 cm intervals (10, 2, and 6 ky, respectively) from 0 to 5.9, 91.8 to 166.1, and 0 to 43.6 meters below seafloor (mbsf), respectively. All sampling was carried out at the Gulf Coast Core Repository in College Station, Texas (USA). Samples were taken from hemipelagic sediment only to avoid the effects of reworking and volcanic input, which can dissolve holoplanktonic gastropod shells (Wall-Palmer et al., 2011). Sample processing was carried out at Plymouth University in the United Kingdom. Samples were dried in an oven at 40°C for 24 h to facilitate the subsequent removal of clay particles. Samples were then rehydrated and washed over a 63 µm sieve. Both the <63 and >63 µm fractions were collected, filtered, and dried in an oven at 40°C for 24 h.

Microfossil analysis

For each sample, just more than 300 (or until the sample was exhausted) holoplanktonic gastropod specimens were counted and identified from the >150 µm size fraction. Only whole specimens were counted to avoid the distortion produced by several fragments of the same specimen. Pteropod species identification was made using the keys published by Bé and Gilmer (1977) and Janssen (2012). Heteropod species identification was made using Seapy (1990), Seapy et al. (2003), the Tree of Life web project (tolweb.org/Atlantidae), and Janssen (2012). Counts of holoplanktonic gastropods are expressed as a percentage (relative abundance) of the total number of specimens per sample.

A large number of the heteropod specimens were juveniles, which complicates their species identification. Therefore, the species Atlanta peronii, Atlanta gaudichaudi, Atlanta rosea, Atlanta lesueurii, and Atlanta oligogyra are presented here as the A. peronii group because their juvenile shells could not be differentiated.

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