Lake studies , Ahklun Mountains, southwestern Alaska
Lakes preserve a continuous record of late Quaternary paleoenvironmental change within the uplands of the Ahklun Mountains. Lake cores recovered beginning in 1998 provide evidence for the timing of late Quaternary glacier fluctuations, and pollen, diatom, and sedimentological analyses from the same cores provide information on the paleoenvironmental and paleoclimatic changes associated with the glacial fluctuations.
Our longest record comes from Arolik Lake in the southwestern Ahklun Mountains, which lies just beyond the limit of late Wisconsin ice. Three sediment cores from the lake, and seismic stratigraphic profiles, show clear evidence for the maximum extent of late Wisconsin glacier ice. A clay-rich sediment interval containing a sparse diatom assemblage dominated by benthic taxa records the meltwater spillover of an ice-dammed lake that was impounded by an outlet glacier in the Goodnews River valley. On the basis of 12 AMS 14C ages on macrofossils, alternative age models for one of the cores (AL4) show that the meltwater pulse, and therefore the interval of maximum ice extent, began ~24 cal ka and ended soon after, but no later than ~20 cal ka. Glaciers across interior Alaska appear to have reached their greatest extent a few thousand years prior to the maximum build up of global ice volume. The pollen assemblage from one of the cores is dominated by a protracted herb zone with minor variability during the late Pleistocene.
High-resolution analyses of the Arolik Lake cores reveal cyclic variations in effective moisture, aquatic productivity, and terrestrial vegetation during the Holocene. These variations occurred with periodicities similar to those of solar activity and appeared to be coherent with time series of the cosmogenic nuclides 14C and 10Be as well as North Atlantic drift ice. These results imply that weak solar variations induced pronounced cyclic changes in northern high-latitude environments. They also provide evidence that centennial-scale shifts in the Holocene climate were similar between the subpolar regions of the North Atlantic and North Pacific possibly because of sun-ocean-climate linkages.
In addition to Arolik Lake, we have multiple lake cores from four other lakes in the Ahklun Mountains that encompass the late glacial and Holocene intervals, including, Nimgun and Little Swift lakes, located in the western and northern parts of the Ahklun Mountains, respectively, and separated by ~90 km. The base of one core from Little Swift Lake penetrated mud with grain-size, diatom, and geochemical evidence for ice-proximal sedimentation. An alpine glacier upvalley of Little Swift Lake retreated sometime following 11 14C ka, as recorded in lake core LS-A by abrupt fining of grain size, increase in organic matter content, and reduction in cold, fresh-water diatoms. Glacier retreat was accompanied by significant changes in vegetation, most notably the dramatic expansion of Poaceae, which doubled in relative abundance. Although the glacier retreat might reflect the overall post-glacial climatic amelioration, coincident vegetation changes record a distinct reversal to cooler or drier conditions, or both. If cooling occurred, then the glacier could only have retreated in response to increased aridity, especially during the winter accumulation season. We conclude, therefore, based on the combination of glacier retreat and paleovegetation changes, that climate at Little Swift Lake undoubtedly became drier, and possibly also cooler, during the Younger Dryas. Poaceae did not retract until ~9 14C ka, nearly 1000 yr after the termination of the North Atlantic Younger Dryas. Contraction of Poaceae and rapid expansion of mesic taxa at this time may have resulted from an increase in moisture availability due to rapid transgression of the Bering Sea coastline. The core chronology can be compared with a 36Cl age of 13.0 ± 2.5 ka on a prominent moraine 1.3 km upvalley from Little Swift Lake, and 5.1 ± 0.8 36Cl ka on a readvance moraine in the upper reaches of the basin.
Recent efforts have focused on extracting a record of Holocene glacier fluctuations from lakes located downvalley of active glaciers. The glaciers emanate from a few of the highest summits of the Ahklun Mountains that are underlain by granitic rocks and otherwise surrounded by metasedimentary rocks. Fluctuations in meltwater from these glaciers provide a distinctive mineralogical and geochemical signature to their ice-distal lakes. Cores from Waskey Lake show variability in magnetic susceptibility, organic matter, and grain size that we interpret as upvalley glacier fluctuations, and appear to be represented geomorphically by moraine sequences flanking existing glaciers. The data suggest that neoglaciation commenced just prior to 3.6 ka, and that the Little Ice Age advances were less extensive than earlier neoglacial expansions.
References
Axford, Y.L. and Kaufman, D.S., 2004, Late glacial and Holocene vegetation and glacier fluctuations at Little Swift Lake, southwestern Alaska: Arctic, Antarctic, and Alpine Research 36, 139-146.
Levy, L.B., Kaufman, D.S., and Werner, A., 2004, Holocene glacier fluctuations, Waskey Lake, northeastern Ahklun Mountains, southwestern Alaska: The Holocene 14, 185-193.
Kaufman, D.S., Hu, F.S., Briner, J.P., Werner, A., Finney, B.P., Gregory-Eave, I., 2003, A ~33,000 year record of environmental change from Arolik Lake, Ahklun Mountains, Alaska: Paleolimnology 30, 343-362.
Hu, F.S., Kaufman, D., Yoneji, S., Nelson, D., Shemesh, A., Huang, Y., Tian, J., Bond, G., Clegg, B., Brown, T., 2003, Cyclic variation and solar forcing of Holocene climate in subarctic Alaska: Science 301, 1890-1892.
Hu, F.S., Lee, B.Y., Kaufman, D.S., Soneji, S., Nelson, D.M., and Henne, P.D., 2002, Response of tundra ecosystem in southwestern Alaska to Younger-Dryas climatic oscillation: Global Change Biology 8, 1156-1163.
