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Benthic Foraminiferal Living Depths, Stable Isotopes, and Taxonomy Offshore South Georgia, Southern Ocean: Implications for Calcification Depths

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Benthic Foraminiferal Living Depths, Stable Isotopes, and Taxonomy Offshore South Georgia, Southern Ocean: Implications for Calcification Depths J. Micropalaeontology, 37, 25–71, 2018 https://doi.org/10.5194/jm-37-25-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. “Live” (stained) benthic foraminiferal living depths, stable isotopes, and taxonomy offshore South Georgia, Southern Ocean: implications for calcification depths Rowan Dejardin1, Sev Kender2,3, Claire S. Allen4, Melanie J. Leng1,5, George E. A. Swann1, and Victoria L. Peck4 1Centre for Environmental Geochemistry, School of Geography, University of Nottingham, University Park, Nottingham, NG7 2RD, UK 2Camborne School of Mines, University of Exeter, Penryn, Cornwall TR10 9FE, UK 3British Geological Survey, Keyworth, Nottingham NG12 5GG, UK 4British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK 5NERC Isotope Geosciences Facilities, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK Correspondence: Rowan Dejardin ([email protected]) Published: 5 January 2018 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 re- construct 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 relation- ship 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 13 18 in deep-water settings, we find no significant intra-species variability in either δ Cforam or δ Oforam with sed- iment 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”, “in- faunal” 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. 1 Introduction sets. In part this offset is accounted for by “vital effects”, inter-species differences in the fractionation of stable iso- Benthic foraminifera live both on (epifaunal) and beneath topes due to a range of biological factors (see review in (infaunal) the sediment–seawater interface, and their assem- Ravelo and Hillaire-Marcel, 2007). Another consideration, blages and stable isotope and trace element compositions are adopted in numerous studies, is that epifaunal and infau- widely utilized as tools in the reconstruction of past oceano- nal inter-species isotopic offsets reflect, in part, pore wa- graphic conditions (see review in Jorissen et al., 2007). Sta- ter chemistry at the preferred depth habitat of each species ble isotopes of all species typically exhibit inter-species off- within the sediment (e.g. Loubere et al., 1995; McCorkle et Published by Copernicus Publications on behalf of The Micropalaeontological Society. 26 R. Dejardin et al.: Implications for calcification depths al., 1997; Theodor et al., 2016). The carbon isotope composi- ferred depths and that their δ13C composition is reflective of 13 13 13 tion of pore water dissolved inorganic carbon (δ CDIC) has pore water δ CDIC (i.e. decreasing δ CDIC with increasing been shown to become lighter (lower δ13C) with sediment calcification depth), we measured the living depths and iso- depth (e.g. McCorkle et al., 1985), due to the remineraliza- tope composition of three as yet unstudied species (Astronon- tion (by oxic respiration or denitrification) of organic carbon. ion echolsi, Cassidulinoides porrectus, and Buccella sp. 1) 13 The difference between bottom and pore water δ CDIC has from the South Georgia shelf, South Atlantic. We also report been shown to increase with bottom water oxygenation (Mc- the depth ranges of all “living” (Rose Bengal stained) benthic Corkle and Emerson, 1988). Assuming that infaunal species foraminifera. To underpin these and future analyses, we car- calcify at preferred depths within the sediment hypotheti- ried out a detailed taxonomic analysis of benthic foraminifera 13 cally allows δ CDIC gradients to be reconstructed through from six surface samples, two box-core downcore sequences, the isotope analysis of paired epifaunal and deep-infaunal and two gravity cores, the first such study focused on South taxa (e.g. Cibicidoides wuellerstorfi and Globobulimina spp.; Georgia since Earland (1933). Hoogakker et al., 2015). Similarly, the difference in δ13C be- tween epifaunal and shallow infaunal species (e.g. Uvigerina spp.) has been used to reconstruct the intensity of organic 1.1 Previous benthic foraminiferal studies matter remineralization, a proxy for the flux of organic mat- ter to the sea floor (Zahn et al., 1986; McCorkle and Emer- Earland (1933) conducted a survey of the benthic son, 1988; Schilman et al., 2003). Additionally, Elderfield et foraminifera of South Georgia, utilizing surface sediment al. (2012) used the presence of a constant offset between the samples collected from around the island as part of the Dis- δ13C data from paired epifaunal (Cibicidoides wuellerstorfi) covery expeditions. Since this study, there has been no fur- and infaunal (Uvigerina spp.) foraminifera to negate the sug- ther taxonomic work focussed on South Georgia, but a num- gestion that a negative δ13C excursion during Marine Iso- ber of benthic foraminiferal assemblage studies have been tope Stage 22 was due to productivity changes and to support conducted at other Southern Ocean and Antarctic locations. their hypothesis that the δ13C excursion was instead related The Antarctic Peninsula is one of the better-studied areas to global ocean change. in the region, with foraminiferal research here commencing These applications rely on the previously stated assump- in the first half of the 20th century (Earland, 1934; Cush- tion that benthic foraminifera inhabit species-specific depth man, 1945), followed by a number of brief US Antarctic habitats and, more importantly, calcify at that depth, thereby Program taxonomic reports (e.g. Lipps et al., 1972). In a 13 recording the pore water δ CDIC gradient. This assumption more comprehensive study of the Antarctic Peninsula, Ish- is held despite the wide range of sediment depths that indi- man and Domack (1994) analysed a large number of sur- vidual infaunal species have been observed to live at (e.g. face sediment samples from three areas on the western side McCorkle et al., 1997). Furthermore, isotope analysis of of the peninsula (Bransfield Strait, Marguerite Bay, and “live” (Rose Bengal stained) foraminifera reveals a narrow Palmer Archipelago). A detailed assessment of the benthic range of isotope values for individuals of the same species foraminiferal assemblages of Admiralty Bay, King George despite them having been recovered from a wide range of Island, and the South Shetland Islands has been conducted by depths beneath the sediment surface (McCorkle et al., 1990, Majewski (2005, 2010), again on surface sediment samples. 1997; Schmiedl et al., 2004; Fontanier et al., 2006b, 2008; On the eastern side of the Antarctic Peninsula, Majewski Theodor et al., 2016). A narrow intra-species range of δ13C and Anderson (2009) examined the palaeoclimatic implica- composition of “live” specimens recovered from a range of tions of Holocene foraminiferal assemblages from the Firth sediment depths argues against individuals having calcified of Tay. Further east, Anderson (1975) identified 160 species at the depths from which they were recovered (since pore wa- in surface samples distributed across the Weddell Sea, whilst 13 ter δ CDIC gradients are not reflected in the foraminifera). Mackensen et al. (1990) focussed on surface samples from More recent studies raise further questions about the valid- the sea’s eastern corner. ity of the assumed calcification depths of infaunal species. Herb (1971) described the variation in benthic faunas For example, it has been assumed that Mg = Ca data from across the Drake Passage, illustrating the differences be- infaunal foraminifera are relatively insulated from the ef- tween the shelf environments of southern South America 2− fect of bottom water [CO3 ] changes (Elderfield et al., 2006, and Antarctica. Mackensen et al. (1993) analysed a simi- 2010). However, Weldeab et al. (2016) have shown that in- lar transect of surface samples through the subantarctic zone faunal Globobulimina spp. are sensitive to changes in bot- between 0 and 10◦ E. Useful studies from Southern Ocean 2− tom water [CO3 ], implying that calcification may not occur localities more distant from South Georgia include Majew- exclusively beneath
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