In situ foraminifera in grounding zone diamict: a working hypothesis

Abstract

The ice-proximal diamict sediment deposited on the foreset of a grounding zone wedge in Glomar Challenger Basin on the eastern Ross Sea continental shelf yielded a low abundance assemblage of foraminifera at two piston core sites. We found 302 small well-preserved specimens representing 18 species of benthic foraminifera from 825 ml of sediment. Only three poorly preserved specimens of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral) were found. Our combined analyses of preservation state, assemblage composition and stable isotopes suggest that the benthic foraminifera may be in situ. This possibility is of interest to palaeoclimatologists who use ice-proximal sediments on the Antarctic continental shelves to radiocarbon date the post-glacial retreat history.

Foraminiferal Patterns in Deglacial Sediment in the Western Ross Sea, Antarctica: Life Near Grounding Lines

“…Our results confirm that proximal glacimarine facies have the potential to bear in situ foraminifera when collected from foresets and toes of GZWs in the western Ross Sea, supporting previous studies in the eastern Ross Sea (Bart & Cone, 2012; Bart et al, 2016; Bart et al, 2018; Majewski et al, 2018). However, abundant, post‐LGM foraminifera were encountered only in the four cores (KC17, KC30, KC48, and KC49; Table 2 and Figure 7).…”

“…It is less dominant in core KC17 from Pennell Trough, especially in intervals lacking a coarser (>63 μm) fraction (Figure 7). A similar assemblage dominated by G. subglobosa was described by Melis and Salvi (2009) from proximal glacimarine sediments of the western Ross Sea and in Glomar Challenger Basin in the eastern Ross Sea (Bart et al, 2016).…”

“…The most widespread calcareous FA in the western Ross Sea is the G. subglobosa FA. Accessory taxa are primarily Epistominella spp., G. biora , Cibicides spp., Nonionella spp., T. earlandi (see also Data Set S2 and Supporting Information S1), and, in KC17, agglutinated Textularia sp., the same species as in Figure 2r of Bart et al (2016). This FA is mainly associated with proximal glacimarine deposits (Figure 7).…”

A centuries-long delay between a paleo-ice-shelf collapse and grounding-line retreat in the Whales Deep Basin, eastern Ross Sea, Antarctica

“…Where possible, we used monospecific samples comprised of benthic G. biora and T. earlandi (and in one instance planktonic N. pachyderma ) that were the most numerous and fairly massive (Table 1 ). The good to pristine preservation of these foraminifera, along with the lack of clearly reworked specimens, that could be identified either by different coloration, sediment filling 33 and/or presence of extinct species 34 , indicates that practically all specimens could be considered as being in situ 22 , 35 . Rare specimens showing signs of etching, found especially among Cibicides or foraminiferal specimens in cores from the middle continental shelf, although most likely also being in situ , were not used for radiocarbon analysis.…”

“…Bart et al . 35 showed that calcareous foraminifera from GZWs can be used for radiocarbon dating of marine diamicts from the eastern Ross Sea if they are selected with considerable care.…”