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How long have dry valleys been dry? (Olympus Range Till ~ 14 Ma (Interp…
How long have dry valleys been dry?
Beacon valley ashfall
~ 8 Ma
Polygon formation
Periglacial envmt w freeze-thaw action (expansion-contraction)
Cracks = filled by ashfall deposits in snowfall
Ash dated ~ 8 Ma
Ash sits atop ice ∴ ice free for atleast 8 Ma
Olympus Range Till
~ 14 Ma
Till = associated w moraine (cirque glacier)
Displays transition from clay ---> gravel
Suggests thermal regime ∆
Ash @ transition dated ~14 Ma
∴ Moraine preseved for ~ 14 Ma
Interp
Permanent shift thermal regime of local glaciers
Wet --> cold based regimes over 14 Ma
∴ ↓ M.A.T. ≈ 25-30°C
Royal Society Range
Mt Dromedary
Volcanic eruption cone sits beside glacier deposits
∴ glacier deposits there since ~ 12 Ma
Arena Valley
Upper Pliocene moraine overlies in-sity ash deposit
∴ Ice advanced (~3 Ma) over ash avalanche (~11 Ma)
Ice did not disturb ash ∴ must be cold (< 0°C) for over 11 Ma
Marchant (1996) implications:
Air temps < 0°C for > 11 Ma
Coombs Hill Ferrar Dolerite
Margerison 2005
Ferrar dolerite glacial erratic
on surface long time
weathering pits
Apparent He-3 age ~ 10-14 Ma
14 ma accounting for erosion
10 Ma assuming no erosion
∴ Infer v. low erosion rates (0.03-0.06 m/Ma) (lowest in world)
Summary
Labyrinth
catastrophic flood/ landscape change
Sirius till
extensive warm-based ice (erosive) before 14 Ma
stable, cold-based ice sheet since 14 Ma
Arena Valley
Extremely low erosion rates since 14 Ma
Air temps < 0 °C for > 11 Ma
Olympus Range till ~14 Ma
transition warm -> cold based
Ages Summary
Beacon valley 8 Ma
Olympus Range till ~14 Ma
Arena Valley ~11.28 Ma
Antarctic IS Summary
Two steps cooling
Initial Glaciation 34 Ma
14 Ma
Ice sheet expands to cont shelf edge
Retreat & stabilises to present extent
Sudden cooling
terrestrial >8°C
ocean > 6°C
Two episodes glaciation
35 - 15 Ma
33 glacial-interglacial cycles
orbitally driven
progressive cooling
13.6 Ma - present
Permanent AIS
Controls its own climate (high polar)
Subject only to sealevel forcing