“Live” (stained) benthic foraminiferal living depths, stable isotopes, and taxonomy offshore South Georgia, Southern Ocean: implications for calcification depths

Rowan Dejardin,Victoria L Peck,George E A Swann,Melanie J Leng,Claire S Allen,Sev Kender

doi:10.5194/jm-37-25-2018

Abstract

Abstract. It is widely held that benthic foraminifera exhibit species-specific calcification depth preferences, with their tests recording sediment pore water chemistry at that depth (i.e. stable isotope and trace metal compositions). This assumed depth-habitat-specific pore water chemistry relationship has been used to reconstruct various palaeoenvironmental parameters, such as bottom water oxygenation. However, many deep-water foraminiferal studies show wide intra-species variation in sediment living depth but relatively narrow intra-species variation in stable isotope composition. To investigate this depth-habitat–stable-isotope relationship on the shelf, we analysed depth distribution and stable isotopes of living (Rose Bengal stained) benthic foraminifera from two box cores collected on the South Georgia shelf (ranging from 250 to 300 m water depth). We provide a comprehensive taxonomic analysis of the benthic fauna, comprising 79 taxonomic groupings. The fauna shows close affinities with shelf assemblages from around Antarctica. We find live specimens of a number of calcareous species from a range of depths in the sediment column. Stable isotope ratios (δ13C and δ18O) were measured on stained specimens of three species, Astrononion echolsi, Cassidulinoides porrectus, and Buccella sp. 1, at 1 cm depth intervals within the downcore sediment sequences. In agreement with studies in deep-water settings, we find no significant intra-species variability in either δ13Cforam or δ18Oforam with sediment living depth on the South Georgia shelf. Our findings add to the growing evidence that infaunal benthic foraminiferal species calcify at a fixed depth. Given the wide range of depths at which we find living, infaunal species, we speculate that they may actually calcify predominantly at the sediment–seawater interface, where carbonate ion concentration and organic carbon availability is at a maximum.

Highlights

Benthic foraminifera live both on and beneath the sediment–seawater interface, and their assemblages and stable isotope and trace element compositions are widely utilized as tools in the reconstruction of past oceanographic conditions
We show that the diversity of benthic foraminifera does not vary greatly at our study sites across the South Georgia shelf and that there are similarities between the assemblages recorded here and those found on the Antarctic Peninsula and elsewhere on the Antarctica shelf, among the more abundant taxa
We find that for most of the taxa in this study, there is no strong preference for a specific living depth, except for a small number of species that exhibit a preference for an epifaunal/shallow infaunal microhabitat (e.g. Labrospira scitula and Glaphyrammina rostrata)

Summary

Introduction

Benthic foraminifera live both on (epifaunal) and beneath (infaunal) the sediment–seawater interface, and their assemblages and stable isotope and trace element compositions are widely utilized as tools in the reconstruction of past oceanographic conditions (see review in Jorissen et al, 2007). Stable isotopes of all species typically exhibit inter-species offsets In part this offset is accounted for by “vital effects”, inter-species differences in the fractionation of stable isotopes due to a range of biological factors (see review in Ravelo and Hillaire-Marcel, 2007). Another consideration, adopted in numerous studies, is that epifaunal and infaunal inter-species isotopic offsets reflect, in part, pore water chemistry at the preferred depth habitat of each species within the sediment Elderfield et al (2012) used the presence of a constant offset between the δ13C data from paired epifaunal (Cibicidoides wuellerstorfi) and infaunal (Uvigerina spp.) foraminifera to negate the suggestion that a negative δ13C excursion during Marine Isotope Stage 22 was due to productivity changes and to support their hypothesis that the δ13C excursion was instead related to global ocean change

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