Record of Earliest West Antarctic Ice Sheet Beneath Ross Sea?

Abstract:

Recent Global Climate Models, utilizing topography restored for rift-related subsidence and glacial erosion, produce a West Antarctic Ice Sheet for earliest Oligocene (~34 Ma) conditions. Additionally, global isotopic records indicate an Antarctic ice sheet larger than today’s at ~34 Ma. However, evidence for a pre-30 Ma major glaciation has been lacking in seismic stratigraphic studies of Ross Sea.

Utilizing deep scientific core holes and all available seismic reflection profiles, we investigated subsidence, sedimentation, and glacial erosion in the Ross Sea. These data image a smooth, tilted and laterally continuous (>200 km) unconformity interpreted as metamorphic basement eroded by waves and subaerial processes. At ~34 Ma, much of the central proto-Ross Sea’s ground surface was composed of this basement.

In the western Ross Sea, a pre-30 Ma trough >50 km-wide cuts downward as much as 2 km into acoustically reflective strata. These reflective strata are interpreted to be 80-55 Ma syn-rift sedimentary rocks cut by faults. The trough fill is unfaulted; therefore erosion postdates the rifting. Using the current depth of the top basement and the age of initial aggrading marine strata, post-~30 Ma to post-~25 Ma subsidence rates are easily calculated. Projecting these rates using tilts of 30 Ma and younger strata, Central Trough has subsided 2-3 km since 30 Ma, similar to published values for this trough. The unconformity flooring the trough currently is as deep as 6 km with shallower sills downflow. If backstripping and thermal modeling also indicate a reverse gradient of the trough axis at ~34 Ma and ~55 Ma, the erosion must have been due to ~34 Ma ice. In the event that the trough was eroded soon after cessation of rifting at 55 Ma, then total differential subsidence might explain both the current depth and reverse gradient. Projecting the known rates of sediment deposition between ~25 Ma and ~30 Ma, the trough fill can easily be post-~34 Ma. Therefore, one of the possible explanations is that the erosional trough comprising deep Central Trough may be evidence of the first advance of the West Antarctic Ice Sheet. However, since we have not yet done the modeling, we cannot yet rule out the trough being an early Cenozoic river valley or a submarine canyon.