Transient Establishment of a Seasonal Sea-Ice Regime in the Late Pliocene, McMurdo Embayment, Ross Sea, Antarctica
Abstract
The marine sediment drillcores collected by the Antarctic Geological Drilling (ANDRILL) Program from sites beneath the McMurdo Ice Shelf (AND-1B) and in Southern McMurdo Sound (AND-2A) together represent the longest, most complete record to date of Neogene climatic evolution proximal to the Antarctic continent. Multiple extensive, well-preserved diatomite units distributed throughout the upper six hundred meters of the McMurdo Ice Shelf (MIS) core provide a unique perspective on the transition from the mid-Pliocene climatic optimum into modern cold-polar conditions. While the diatom assemblages preserved within these diatomites record a variety of paleoenvironmental conditions, the modern, fully-developed sea-ice community that characterizes the modern Ross Sea environment is absent from the record. Here, we focus on two discrete diatomite units deposited between ~3.3 and 3.0 million years ago. The base of the lower unit, at 292 mbsf, is marked by the radiation of a Fragilariopsis species complex that is previously undescribed from Antarctic sediments. This complex dominates the diatom assemblage throughout the late Pliocene, comprising up to 55 % of the community at 288 meters. By contrast, modern sea-ice marker species are poorly represented in most mid- to late Pliocene sediments at AND-1B. However a transient sea-ice event is recorded in the upper portion of the diatomite unit that spans 252-258 mbsf. The assemblage at the base of this diatomite is similar in character to the diatomite units above and below, however the proportion of F. curta, a taxon with well-established sea-ice affinity, increases gradually, reaching a transient but significant maximum at 253.11 mbsf. The mid-Pliocene climatic optimum is not recovered from the shallower Southern McMurdo Sound (SMS) site, which is comprised predominantly of diamictite and mudstone. However a diverse Pliocene assemblage between 44.5 and 48.3 mbsf in AND-2A appears equivalent to the AND-1B diatomite 164-180 mbsf. Preserved in five pulses incorporated into sandy, clast-poor diamictite, this assemblage includes numerous age diagnostic species but lacks a significant contribution from the sea-ice assemblage that dominates modern Ross Sea coastal environments. The 1138-meter AND-2B drillcore includes only two meters of true diatomite (from the Miocene), reflecting both the continental proximity and shallow paleodepth of the site. In this context, the episodic nature of Pliocene diatom preservation in AND-2B suggests that sedimentation in Southern McMurdo Sound was paced by an unusual oscillating environmental or depositional influence around 2.5 million years ago. The nearshore Southern McMurdo Sound section also provides an opportunity to place the dynamic McMurdo Ice Shelf record within a regional context.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.C21B0534R
- Keywords:
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- 0750 Sea ice (4540);
- 4944 Micropaleontology (0459;
- 3030);
- 9310 Antarctica (4207);
- 9605 Neogene