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doi:10.2204/iodp.proc.340.203.2015

Introduction

Holoplanktonic gastropods are uniquely adapted to live their entire lives within the plankton. They can reach densities of up to 10,000/m3, constituting an important component of the ocean food web at all latitudes (Fabry et al., 2008). There are two groups of holoplanktonic gastropods: pteropods (Thecosomata and Gymnosomata) and heteropods (Pterotracheoidea). Within these groups there are shelled, partially shelled, and shell-less species, although shell-less species still have a shell at the larval stage. Both pteropod and heteropod shells are small, delicate, and formed of aragonite and are, consequently, particularly susceptible to dissolution (Fabry, 1990) and mechanical damage. Their shells are, however, frequently preserved in shallow warm-water regions that are supersaturated with respect to aragonite. Offshore of Montserrat, in an area of exceptional calcium carbonate preservation, the fossil record of holoplanktonic gastropod shells within the sediments is rich and accurately reflects the living population at the time of deposition (Wall-Palmer et al., 2014b). Moreover, any sediment transport, such as density current deposits that are common in this area (Watt et al., 2012; Trofimovs et al., 2013; Cassidy et al., 2014), fragments the delicate shells, meaning that, where shells are present, their stratigraphy is unlikely to have been affected by reworking. All species of holoplanktonic gastropods presented here are extant, and living examples can be found in the oceans today.

Holoplanktonic gastropods are sensitive to environmental changes (Maas et al., 2012), so their fossil record can be used as an archive of past ocean surface conditions (Wall-Palmer et al., 2013, 2014b). Changes in dominant holoplanktonic gastropod species are often temperature driven, especially where temperature has a large range over time (e.g., during the Late Pleistocene in the Mediterranean Sea [Sbaffi et al., 2001; Wall-Palmer et al., 2014b]). In the Lesser Antilles, however, where temperature variations are smaller over time, changes in species abundance can be more difficult to interpret. Furthermore, there are a number of holoplanktonic gastropods, in particular the heteropods, for which we know very little about their environmental preferences, despite being extant. This makes it difficult to use holoplanktonic gastropods as stratigraphic marker species in such areas and obstructs their potential as paleoceanographic indicators.

In this study, we present the downcore distribution of all holoplanktonic gastropod shells found at Integrated Ocean Drilling Program (IODP) Sites U1395 and U1394. Although their analysis has not allowed detailed interpretations of the paleoceanography of this area, a number of species associations have been identified. It is anticipated that these data will help to stimulate more widespread study of holoplanktonic gastropods in marine sediment cores, as well as contributing new ecological information that can be applied to living populations, in particular the heteropods, whose environmental requirements are currently poorly known.