Holocene accumulation and ice sheet dynamics in central West Antarctica

Abstract

[1] We derive depth-age relationships across the ice divide between the Ross and Amundsen Seas by tracking radar-detected layers from the Byrd ice core and a dated 105-m core near the divide. The depth-age relationships and an ice-flow model are used to establish histories of accumulation and ice sheet dynamics over the past 8000 years. Results show that accumulation was approximately 30% higher than today from 5000 to 3000 years ago. Antarctic climate variability today is dominated by periodic fluctuations in strength of the circumpolar vortex, which raises the possibility that the vortex was systematically weaker during the period of high accumulation. Accumulation today decreases almost linearly across the divide. It is unlikely that this pattern has changed through the Holocene. The radar-detected stratigraphy shows no evidence of the arched layers that are expected beneath a stable divide that is frozen to its bed, implying that the divide has also been migrating and/or the basal ice has been sliding through the Holocene. We cannot rule out the possibility of sliding because the basal ice is near its pressure melting point. Other evidence indicates that divide migration is likely. The Ross Sea sector is now near steady state, but it had a strong negative imbalance 200 years ago when Kamb Ice Stream was active. In contrast, recent speedups of Pine Island and Thwaites Glaciers have likely caused the mass balance of the Amundsen Sea sector to become negative. The divide is likely migrating toward the Ross Sea today.

Recent thinning and migration of the Western Divide, central West Antarctica

“…[12] Although the accumulation rate 3000 years ago was about 30% higher than present, its gradient across the divide has not changed for at least 5000 years [Neumann et al, 2008]; the pattern of thickness change is a result of asymmetric dynamical thinning on either side of the divide rather than changes in the accumulation-rate gradient.…”

“…Case II is for constant g = 0.85. Case I is our preferred distribution, which is based on results from Neumann et al [2008]. Points 1 -5 are used for calculations shown in Table 1; points 1 -3 and 5 coincide with locations of survey poles.…”

“…Here we assume g(x, y) varies as a function of distance from the divide and calculateQ for three different g-distributions. Our preferred g-distribution (case I in Figure 2d) is based on results from Neumann et al [2008] that indicate (1) the location of the Western Divide has not been stable but rather has moved around for at least 5000 years and (2) the bed is likely frozen on the Amundsen Sea side of the divide but possibly melting (and sliding) on the Ross Sea side.…”

“…[4] We used high-frequency radar to track continuous internal layers and spatially extrapolate the depth-age relationship from a 105-m core (ITASE 00-1) collected near the divide [Neumann et al, 2008]. The depth-age relationship extends back to AD 1651 [Dixon et al, 2004]; the ice-equivalent accumulation rate at the core site (averaged over the 349 years between AD 1651 and 2000) is 0.25 m a [Banta et al, 2008].…”

Antarctic contribution to meltwater pulse 1A from reduced Southern Ocean overturning

“…On this basis, outputs from each simulation were assessed against the following temporal and spatial constraints (in order of importance, based on area covered): (1) lateral extent at LGM reached almost to the continental shelf edge around the entire continent 18 ; (2) ice surface elevations in interior East Antarctica lowered by 100-150 m at LGM 19,54 ; (3) ice surface elevations in West Antarctica rose by 200-400 m at LGM 55,56 ; (4) grounded ice in the Ross Sea was fed from both East and West Antarctica, and was confluent around 180 longitude 57 ; (5) retreat from LGM extent began earliest in the Antarctic Peninsula 58 ; (6) retreat of the East Antarctic Ice Sheet started earliest in the Amery Trough, and later elsewhere 59 ; (7) 61 . Finally, the geometry and surface velocities of grounded ice at the end of the run were compared with values from the most recent continent-wide syntheses of present-day ice thickness and velocity 12,62 .…”