Late Quaternary foraminiferal assemblages from western Ross Sea (Antarctica) in relation to the main glacial and marine lithofacies

Abstract

Investigations on foraminifers from Upper Pleistocene–Holocene sediments were carried out on twelve cores from the western Ross Sea continental margin (Drygalski, Joides, North Victoria Land Basins) as part of a “Progetto Nazionale di Ricerche in Antartide” (P.N.R.A.) multidisciplinary project. Data on the foraminiferal frequency, species diversity, tests abundance and their state of preservation were presented as a synthesis of 404 core samples to establish their relationships with the main glacial and marine lithofacies of this area. A total of 126 benthic species, pertaining to 73 genera have been identified; just few taxa, such as Cibicides spp., Globocassidulina spp., Trifarina angulosa and Miliammina spp. being the most ubiquitous and in some cases the dominant species of these paleoenvironments. Two variants of Neogloboquadrina pachyderma, including thin and thick-shelled forms have been recovered. We propose to use these results to provide the degree of glacial control during the Last Glacial Maximum and the following Holocene retreat of the ice sheets. High test fragmentation, low diversity and density tests reflect higher glacial influence of the ice sheet in the Drygalski Basin, whereas the decreasing percentage of fragmentation and a relative increase of density and diversity in Drygalski, Joides and North Victoria Land Basins indicate the paleoenvironmental passage from the ice sheet to the ice shelf condition. The ice shelf retreat is well evidenced in the Joides Basin by a succession of levels barren of foraminifers alternating with high-density levels, rich in T. angulosa, followed by a total disappearance of the calcareous foraminifers. Open-marine settings indicative of lower glacial influence and increased corrosiveness of the water masses is testified by the Miliammina foraminiferal assemblage during the Holocene in Drygalski and Joides Basins cores. On the contrary, rich and abundant benthic and planktonic assemblages characterize the Holocene paleoenvironment of the North Victoria Land area, indicating that the water masses were less corrosive with respect to the other areas. In addition to the glacial reworking of the tests, and the dissolution due to the corrosive water mass conditions, the volcaniclastic sediments recovered in the North Victoria Land Basin cores also affected the condition of test preservation. In volcaniclastic sediments, older than about 20 ka BP, the foraminifers concentration tends to zero and, when present, their tests are highly damaged or completely broken.