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

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 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 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 (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.

Unique benthic foraminiferal communities (stained) in diverse environments of sub-Antarctic fjords, South Georgia

“…7). Although the lower distribution limits appeared significantly shallower than on the shelf north of SG (Dejardin et al, 2018), they were comparable to those from similar fjord settings in the South Shetlands (Majewski, 2010) and Spitsbergen (Fossile et al, 2020). Moreover, the 63−125 μm fraction of the SG-21 sub-core revealed a strong subsurface peak in the abundance of Stainforthia fusiformis (Fig.…”

“…The data of Earland (1933) cannot be quantitatively compared with the present study due to different methodologies, but they do show that all major taxa defining the FAs were present in SG during the 1920s (see also Dejardin et al, 2018).…”

“…The clear distribution pattern of this minute, predominantly infaunal species agrees with its known occupancy of infaunal microhabitats, as reported, for example, by Alve (1994), and confirms that the Van Veen grab samples were relatively undisturbed. Shannon diversity index calculated for >63-μm data was in the range 1.30 to 2.89, encompassing the values of 2.29 and 2.36 recorded more offshore by Dejardin et al (2018). Nevertheless, it is important to note that our surface sampling targeted mainly epibenthic species and that the following discussion is based on the >125 μm data, thereby underestimating the presence of minute, infaunal species, such as S. fusiformis.…”

“…These publications established 49 new foraminiferal taxa, making SG one of the best known areas for taxonomy of this group in the Southern Ocean. Apart from a recent paper based on samples from the outer shelf and upper slope (238-354 m water depth) north of SG (Dejardin et al, 2018), and an earlier study that included several sites at similar upper bathyal depths to the south of SG (Echols, 1971), nothing further has been published on foraminiferal distribution and taxonomy in this region. However, a number of foraminiferal studies have focused on surrounding shelf areas, including the Falkland Islands and their adjacent shelf (Heron-Allen and Earland, 1932), fjords and channels of Patagonia (Violanti et al, 2000;Hromic et al, 2006), the South Shetland Islands (Finger and Lipps, 1981;Gray et al, 2003;Majewski, 2005Majewski, , 2010Majewski et al, 2007;Rodriguez et al, 2010), and the Antarctic Peninsula (Ishman and Domack, 1994;Ishman and Szymcek, 2003;Murray and Pudsey, 2004;Majewski et al, 2016).…”

Molecular and Morphological Diversity of Monothalamous Foraminifera from South Georgia and the Falkland Islands: Description of Four New Species

“…A gap of more than eighty years separates these first surveys from recent taxonomic studies conducted on South Georgia foraminiferal fauna by Dejardin et al, (2018). This recent morphology-based analysis included 58 genera and 60 species of hard-shelled calcareous and agglutinated foraminifera.…”