Constraining Holocene 10Be production rates in Greenland

The absence of a production rate calibration experiment on Greenland has limited the ability to link ¹⁰Be exposure dating chronologies of ice‐margin change to independent records of rapid climate change. We use radiocarbon age control on Holocene glacial features near Jakobshavn Isbræ, western Greenland, to investigate ¹⁰Be production rates. The radiocarbon chronology is inconsistent with the ¹⁰Be age calculations based on the current globally averaged ¹⁰Be production rate calibration data set, but is consistent with the ¹⁰Be production rate calibration data set from north‐eastern North America, which includes a calibration site nearby on north‐eastern Baffin Island. Based on the best‐dated feature available from the Jakobshavn Isbræ forefield, we derive a ¹⁰Be production rate value of 3.98 ± 0.24 atoms g a^?1, using the ‘St’ scaling scheme, which overlaps with recently published reference ¹⁰Be production rates. We suggest that these ¹⁰Be production rate data, or the very similar data from north‐eastern North America, are used on Greenland. Copyright © 2011 John Wiley & Sons, Ltd.     

Supraglacial melt channel networks in the Jakobshavn Isbræ region during the 2007 melt season

Supraglacial channels are an important mechanism for surface water transport over the ablation zone of western Greenland. The first assessment of the spatio‐temporal distribution of surface melt channels and their relationship to supraglacial lakes over the Jakobshavn Isbræ region of Western Greenland was analysed using Landsat Enhanced Thematic Mapper Plus panchromatic images during the 2007 melt season. A total of 1188 melt channels were delineated and show an increase in the number of melt channels throughout the season, reaching a peak on 9 August. Water‐filled melt channels advanced to a maximum elevation of 1647 m on 9 August and attained a minimum average slope of 0.009 on 8 July. The ablation zone demonstrates two hydrologic modes, where crevasse and moulin terminating channels dominate at elevations <800 m and higher‐order channel networks >800 m. Development of higher‐order networks is interrupted by flow divergence due to partitioning of melt water into vertical infiltration through moulins and crevasse fields prevalent at lower elevations. Tributary and connector networks between 800 and 1200 m in elevation are correlated with fewer lake occurrences, lower surface velocities (~50 m a^?1), and ice flow dominated by internal deformation over basal sliding. High‐order channels are associated with lake basins that exceed melt water storage capacity. Evolution of channel networks is coupled to changes in melt water production, runoff, and ice dynamics with implication for the englacial and subglacial environments. © 2013 The Authors. Hydrological Processes Published by John Wiley & Sons, Ltd.     

Discrete displacement analysis for geographic linear features and the application to glacier termini

The extent of glacier terminus displacement is instrumental in investigations of natural or artificial geographic changes. Its importance to earth science and engineering is reflected in the considerable efforts that have been devoted to the development of several boundary displacement analysis methods. Among the methods, the buffering-based approach compares favorably with other approaches in objectivity and robustness. However, it does not consider the relative positions of boundaries, because its buffering operation cannot determine features' relative directions. This limitation incurs inaccurate calculation results – underestimation of mean shifts and overestimation of shape variations, especially when the two compared boundaries intersect. Discrete displacement analysis (DDA), an alternative method that considers given geographic objects as a set of a finite number of points, is proposed here. In a series of tests carried out, including Jakobshavn glacier's calving front, DDA was found to correctly calculate mean shift and shape variation even in cases where the conventional buffering-based method failed. Moreover, this approach is independent of the dimension of space in which it is implemented, and thus is expected to be utilized for analysis of 3D geographic object displacement.     

Seismic stratigraphy records the deglacial history of Jakobshavn Isbræ, West Greenland

Jakobshavn Isbræ is one of the largest ice streams in the Greenland Ice Sheet, presently draining c. 6.5% of the Inland Ice. Here we present high‐resolution Chirp and Sparker sub‐bottom profiles from a seismic survey conducted just outside of the Jakobshavn Isfjord, which provides detailed insight into the glacimarine sedimentary history of the Jakobshavn ice stream during the Holocene. We observe acoustically stratified and homogeneous sediments that drape an irregular substratum and were deposited between ～10 and c. 7.6k cal a BP. The stratified lower units are interpreted as the product of ice‐proximal glacimarine sedimentation deposited rapidly when the grounded ice margin was located close to depositional basins on topographic highs. The upper acoustically homogenous units reflect suspension settling of fine‐grained material and gravitational flows that were extruded from an increasingly unstable ice margin as the ice retreated into the fjord. Proximity to the ice margin and bedrock topography were the dominant controls on sediment accumulation during deglaciation although the 8.2‐ka cooling event probably influenced the position of the ice margin at the fjord mouth. The post‐glacial sedimentary record is characterized by glacimarine and hemipelagic rainout with an increased ice‐rafted detritus fraction that records sedimentation following ice stream retreat into Jakobshavn Isfjord sometime after c. 7.8k cal a BP. Copyright © 2011 John Wiley & Sons, Ltd.     

Giant iceberg plow marks at more than 1 km water depth offshore West Greenland

During a recent (2006) cruise of RV ‘Dana’ high resolution side-scan sonar and sub-bottom profiling was carried out on selected shelf and slope transects offshore Disko Bugt, central West Greenland. Available commercial seismic data from the margin indicate here irregular seabed morphology with erosional features locally extending down to c. 1100 m water depth. The newly acquired side-scan sonar data reveal the presence of giant iceberg plow marks extending (sub)parallel to the slope between about 800 and 1085 m water depth. The height difference between bottom of the scours and rim crest is up to 40 m. The largest plow marks are about 750 m wide. To date no observed modern icebergs including those from Antarctica have drafts in excess of c. 500 m. Taking into account maximum glacial sea level lowering of c. 120 m, the paleo-iceberg keel depth was at least 950 m. Due to the presence of the relatively shallow (< 700 m) sill of Davis Strait to the south, calving of these paleo-icebergs is thought to have occurred from an ice margin in the Baffin Bay region (Jakobshavn Isbræ paleo-ice stream?). The depth of occurrence (> 1000 m) and dimensions of the plow marks are concluded to be exceptional, and comparable to glacial scouring features reported from the Arctic Ocean.     

Ocean contributes to the melting of the Jakobshavn Glacier front

The Jakobshavn Glacier(JG)in Greenland is one of the most active glaciers in the world.It was close to balance before 1997 but this was followed by a sudden transition to rapid thinning.The reason for the change remains unclear.In this study,The NASA Pre-IceBridge ice thickness data are collected to monitor the melting of JG front.The surface elevation decreased by around 90 m from 1995 to 2002 on the floating front.A distributed energy balance model is developed to estimate the energy balance of JG front in the past 30 years(1986-2016).The results indicate that multi-year average energy fluxes absorbed by the floating front of JG from the ocean were about 500 Wm^-2 from 1986 to 2016.This is approximately two times of the energy fluxes from atmosphere during the same period.The energy fluxes from the ocean increased from 200 to 600 Wm^-2during the period from 1990 to 1998 while energy fluxes from the atmosphere remained stable at about 250 Wm^-2.These results demonstrate that ocean contributes more to the melting of the JG front,and suggest that bottom surface melting must have a profound influence on marine-terminating glacier dynamics.