"Live" (stained) benthic foraminiferal living depths, stable isotopes, and taxonomy offshore South Georgia, Southern Ocean: Implications for calcification depths
Journal of Micropalaeontology
© Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License.
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 deepwater 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 300m 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 (δ 13 C and δ 18 O) 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 δ 13 C foram or δ 18 O foram 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.
This study forms part of a doctoral project carried out by Rowan Dejardin, funded by the Centre for Environmental Geochemistry (University of Nottingham and the British Geological Survey) and supported by BGS-University Funding Initiative (BUFI). We thank the officers, crew, and scientific party on board the RRS James Clark Ross during scientific cruises JR257 and JC15002. Participation in cruise JR15002 by Rowan Dejardin was funded by the British Antarctic Survey Collaborative Gearing Scheme. We thank Hilary Sloane and Jack Lacey (BGS) for analytical assistance and Tony Milodowski and Gren Turner for their assistance generating the SEM images. We also thank the editor, Laia Alegret, one anonymous reviewer, andWojciech Majewski for their insightful comments, which helped to strengthen this paper. This project was supported by NERC Isotope Geoscience Facility grant IP/1495/1114 (RCUK).
This is the final version of the article. Available from Geological Society via the DOI in this record.
Vol. 37, pp. 25 - 71