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

• Published in: Journal of Micropalaeontology
• Main Authors: Dejardin, Rowan, Kender, Sev, Allen, Claire S., Leng, Melanie J., Swann, George E. A., Peck, Victoria L.
• Format: Text
• Language: English
• Published: 2018
• Subjects: Southern Ocean; Antarc*; Antarctica
• Online Access: https://doi.org/10.5194/jm-37-25-2018; https://jm.copernicus.org/articles/37/25/2018/

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 <q>living</q> (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 <q>live</q> 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 <q>living</q>, <q>infaunal</q> 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.