Ninth International Conference on Permafrost ... - IARC Research

Bathymetric Mapping of Lakes in the Western Arctic Coastal Plain, AlaskaBarry WinstonUniversity of Cincinnati, Cincinnati, USAKenneth HinkelDepartment of Geography, University of Cincinnati, Cincinnati, USARichard BeckDepartment of Geography, University of Cincinnati, Cincinnati, USAIntroductionThe Arctic Coastal Plain (ACP) of northern Alaskais characterized by thousands of lakes developed atopcontinuous permafrost. This region can be subdivided intothe Younger Outer Coastal Plain (YOCP), the Outer CoastalPlain (OCP), and the Inner Coastal Plain (ICP) (Hinkel etal. 2005), which are demarcated by ancient shorelines at23–29 m. a.s.l. and 7 m a.s.l. (Hopkins 1973, Péwé 1975).These subregions can be further differentiated by surficialsediments that reflect the effects of recent geomorphicprocesses, with fine-grained marine sediments associatedwith the OCP and YOCP and eolian sand characteristic ofthe ICP (O’Sullivan 1961, Williams et al. 1978, Williams1983). Lakes are significantly different among these threesubregions with respect to lake surface area, depth, and lakedensity (Hinkel et al. 2005).Many lakes on the ACP have depths of less than 2 m(Carson & Hussey 1962). Such lakes are shallow enoughthat water freezes to the lake bottom during winter and iscontinuous with the underlying permafrost. However, in thecase of deeper lakes, water at depth remains unfrozen. Withwater temperature above freezing, the underlying permafrostbegins to thaw, thereby creating a thaw bulb. The thaw bulbexpands and the substrate slowly subsides as it loses volumewhen pore ice is converted to water. As the lake growslarger and deeper over time, the thaw bulb beneath the lakeexpands, the underlying permafrost thaws, and the groundsubsides further. It is possible for the thaw bulb to reach themaximum depth of permafrost (>300 m) in large, deep lakes,resulting in the formation of an open talik (French 2007).Several models for lakes on the ACP suggest that thesefeatures develop, expand, and drain as part of a cyclicalprocess (Cabot 1947, Carson & Hussey 1962, Everett1980, Billings & Peterson 1980). However, recent researchsuggests that lake evolution is more complex and may not beapplicable across the entire ACP (Jorgenson & Shur 2007).The goal of this study is to gather basic bathymetric andsedimentological information from these small, deep lakes.They appear to be fundamentally different from the wellstudiedlakes of the OCP that were used to develop the thawlakecyclical model.Study Area and MethodologyThe lakes reported on here are all with 32 km of ourbase camp at 70°0′N and 153°5′W. They are located withinthe ICP and are characterized by deep central basins andprominent shelves composed of sand deposits. Bathymetricdata were collected along transects from 17 lakes during latesummer 2007 to determine general basin depth, slope, andmorphological characteristics.Depth measurements from the lakes were recorded with anEagle SeaCharter 502c DF iGPS sonar mounted to the sternof the vessel. The sonar was equipped with a 50/200 kHzdual-frequency transducer that was positioned approximately15 cm below the water surface. The GPS was calibratedto UTM Zone 5 North, NAD-1983 and had a locationalaccuracy better than 3 m. The data were collected by makinga series of transect passes across the lakes, recording bothdepth and location once every second. The distance betweeneach sample was estimated during post-processing of theGPS locations. The slope was calculated between each pairof samples using the depth measurements.Results and ConclusionsOut of the 17 lakes sampled, 7 lakes contained deep basinsthat exceed a depth of 10 m; the maximum recorded depth inone lake was 19 m. These 7 lakes contained shallow shelvesthat declined into deep basins over short distances. However,the deep basins were not always located in the center of thelake, and several lakes contained multiple basins or exhibiteddiscontinuous basins.The shelves were very shallow (0.3–3 m), with dunesand ripples apparent in the sandy sediment. The basin floorswere relatively flat (1–2°), and the shelf-basin transition areaaveraged 30°. However, for the 2 transects shown in Figures1 and 2, a slope of 54° was estimated in Lake A, and a slopeof 44° was determined in Lake B.The sandy lake bottom sediments should have an angleof repose no greater than 35° (Stegner & Wesfreid 1999).However, in the case of the 2 observed lakes, the slope wassignificantly greater. This suggests that the shelf sedimentsare bonded by permafrost, since unconsolidated sand cannotmaintain the observed slope.It is not known if the permafrost is syngenetic, andaggraded upward with sediment deposition along the lakemargin. The association of the deepest lake basins (~18 m)with oversteepened slopes suggests thaw bulb developmentand subsidence. However, thaw consolidation is typicallynot very effective in sandy sediments unless significantlyenriched in ice. Jorgenson & Shur (2007) indicate that nearsurfacesediment cores from this region of the ACP havenot evidenced ice oversaturation. Subsequent fieldwork onlakes in the ACP will focus on mapping lake bathymetry,347