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"The retreat of large tidewater- and lake-calving glaciers, as well as nearby land-based glaciers, in southeastern Alaska during the middle to late Holocene was primarily triggered by increases in summer temperature. Shakes, LeConte, Patterson, and Baird glaciers, located along the southwestern margin of the Stikine Icefield in southeastern Alaska, experienced two or three major periods of advance and retreat during this period. Historical, stratigraphic, and dendrochronological evidence suggests that these periods of advance culminated approximately 3,500-3,300, 2,700-2,200, 1,100-900, and 220-110 years ago in the study area. Comparison with previously published regional records from glaciers located along the coast of northwestern North America suggests a general synchrony in the timing of ice advance across the region. Regional intervals of ice maxima date approximately 3,000-1,900,1,500-900, and 250-100 years ago, and encompass three of the main periods of advance represented in the study area. To determine a regional cause of glacier synchrony, glacier chronologies were compared to local and regional climate and climate proxies. Summer temperature fluctuations in the study area for the past four centuries were derived from tree-ring-width time-series from Crystal Mountain near Petersburg. Previously published precipitation and summer temperature values, inferred from palynological studies, provide a record of climate change for the last 10,000 Years. Throughout southeastern Alaska, periods of glacier retreat for both calving glaciers and land-based glaciers tend to correlate with periods of warming summer temperature. The collective data imply that the geologic record left by calving glaciers, like that left by 1and-based glaciers. has the potential to serve as an important climate-proxy record in a region where few such records have been studied. Furthermore, such a relationship helps further to quantify calving glacier dynamics and improve prediction of calving-glacier response to human-induced global warming"--Abstract.
Previous research examining sedimentation rates and varve thickness at Iceberg Lake, a glacier-dammed proglacial lake in southern Alaska, revealed a 1,500 year record of summer temperatures, but the record is complicated. To better understand basinwide glaciolacustrine response to late Holocene climate variability, I returened to Iceberg Lake and collected five cores from two areas using a vibracore system. I used high resolution DMT core scans to identify and describe eight stratigraphic facies. Based on eight AMS 14C dates and several stratigraphic correlations, I described ~1000 years of basinwide stratigraphy. Deposition at both areas as dominated by fine-grained varves, but the distal area uniquely contained coarser deposits from ~1250-1650 AD. These results suggest an early onset of the Little Ice Age (LIA) that I attributed to thickening and consequent incursion of the impounding glacier margin, and the associated lateral moraine, into the lake. The basinwide impact of this event illustrates the potentially significant spatial and temporal variability of lacustrine sedimentary processes in dynamic glacial landscapes and emphasizes the importance of using a multi core approach when documenting that variability over time.
A chronology of glaciation spanning from the late Pleistocene through the late Holocene is constructed for Fish Lake valley in the central Alaska Range using 10Be surface exposure dating and lichenometry. Ice initially retreated from its Last Glacial Maximum terminal position -22 ka and remained close to its maximum ice extent as late as 16.5 ka. Glaciers retreated well into the cirque region by 15-14 ka, before a late Pleistocene re-advance. Culminating by 11.6 ka, limited evidence indicates a late Pleistocene re-advance in Fish Lake valley may be coincident with the North Atlantic Younger Dryas event (12.9 - 11.6 kya). This adds to the growing amount of evidence in Alaska suggesting that glaciers were advancing in response to Younger Dryas related cooling. Late Holocene moraines pre-dating the Little Ice Age (LIA; -AD 1250-1850) are preserved immediately in front of LIA moraines and a minimum age of 3.3-3.0 ka on one moraine marks the oldest Neoglacial moraine preserved in the valley agreeing with regional glacier advances. Subsequent advances occurred in the middle to late First Millennium AD and pulses of LIA glaciation are recorded at AD 1290, 1640, 1860 and 1910. Preservation of the pre-LIA late Holocene moraines suggests that the magnitude of several late Holocene glacial advances were similar. This finding is consistent with moraine records in the Brooks Range, but contradictory to findings in southern Alaska. The innermost moraine crest, lichenometrically dated to AD 1930, indicates that the current glacier terminus has retread approximately 1.3 km to its AD 2007 position. Comparisons of 10 Be and lichenometric ages on late Holocene moraines show remarkable similarity and add confidence for using the regional lichenometric growth curve to date surficial deposits. Results indicate that 10 Be surface exposure dating can be used to constrain the timing of Holocene glacier fluctuations in Alaska; more so when supplemented with well constrained lichenometric ages.
Due to cryosphere-albedo feedbacks mechanisms, climate change is amplified in the Arctic, making it sensitive to changes in temperature. Alpine glaciers grow and retreat depending on climate, and are excellent recorders of past climate fluctuations. By analyzing the landforms and sediment deposited by glaciers, high-resolution climate chronologies can be constructed and past glacier fluctuations can be inferred. 10Be ages and physical properties of lake sediment are used here to reconstruct Late Pleistocene and Holocene glacier activity from Alapah River valley and Shainin Lake in the north-central Brooks Range. 10Be ages from moraine boulders in Alapah River valley in the north-central Brooks Range were used to reconstruct the maximum glacier extent during the LGM. After eliminating outliers, the 10Be ages from a terminal moraine deposit in the Alapah River valley indicate that the local LGM culminated at 21. 0 ℗ł 0. 8 ka. This new 10Be chronology is the first to firmly constrain the timing of the local LGM in the Brooks Range, and is in agreement with LGM moraine records from other sites in Alaska and the globe. Two 10Be ages from boulders located on bedrock 14 km upvalley from the Itkillik II terminal moraine give an age of deglaciation in Alapah River valley of 18. 2 ℗ł 0. 8 ka. This indicates rapid retreat after the LGM and shows that deglaciation is synchronous with sites in Alaska but was initiated earlier than the age of 17 ka previously proposed for onset of LGM deglaciation in the western US. Physical and geochemical properties of lake sediment from a proglacial lake in Alapah River valley, Shainin Lake, were analyzed to investigate any glacial signals recorded in the lake sediment. Age-depth models for each core were established using 14C ages and analytical methods included magnetic susceptibility, wet bulk density (WBD), scanning X-Ray fluorescence (ITRAX) and visible scanning reflectance spectroscopy. The WBD record from Shainin Lake may serve as a proxy for glacial history of Alapah and Kayak Creek valleys. If interpreted correctly, glacial activity increased from 12,700 to ~10,000 cal yr, decreased from ~10,000 to ~5700 cal yr BP, then increased from ~5700 cal yr BP to the present. This indicates that there is evidence for early Holocene glacial activity, the retreating or stagnating glaciers in the middle Holocene until ~5700 cal yr BP, followed by expanding ice.
Arctic nearshore environments proximal to large rivers, like Simpson Lagoon, Alaska, potentially contain high-resolution sediment archives that can be utilized to reconstruct paleoclimate variability over the late Holocene. The ongoing, rapid environmental changes recently observed in the Arctic highlight the need for high-resolution records of pre-industrial climate change in this climatically sensitive region; such records are fundamental for understanding recent anthropogenic changes in the context of natural variability. This dissertation utilizes a suite of geochemical and sedimentological proxies in combination with age-constrained, shallow acoustic reflection data to demonstrate that these underutilized coastal sediment archives are capable of generating high-resolution paleoclimate records on par with other terrestrial climate archives (i.e. lake sediments, ice cores, tree rings) and provides the first ~1650-year long record of climate variability from the inner shelf of the Alaskan Beaufort Sea. An analysis of sedimentation patterns within Simpson Lagoon using CHIRP seismic data and radioisotope geochronology reveals that sediment infilling in Simpson Lagoon began ~3500 y BP, creating a primary depocenter with mm y−1 sediment accumulation in western Simpson Lagoon. The interbedded sediments suggest that major sediment reworking from ice processes, a common occurrence in Arctic shelf environments, does not disrupt the sediment archive contained within the lagoon. Quantitative reconstructions of surface air temperature are obtained using the brGDGT-derived MBT'/CBT paleothermometer. A comprehensive study of lagoon and river sediments and catchment soils demonstrate that brGDGTs are primarily soil-derived, and yield reconstructed temperatures consistent with instrumental summer temperature observations from Alaska's North Slope. Temperature reconstructions from Simpson Lagoon also show similarities with regional and pan-Arctic climate records over the last few millennia, with evidence of temperature departures correlative with noted climate events (i.e., Little Ice Age, Medieval Climate Anomaly). In addition, temporal variability in sediment sourcing to the lagoon, determined using a multi-proxy approach (i.e., granulometry, elemental analysis, clay mineralogy), broadly corresponds with temperature fluctuations, indicating relative increases in fluvial sediment discharge during colder intervals and decreased river discharge/increased coastal erosion during warmer periods. This paleoclimate variability may be driven by variations in solar output and/or shifts in the regional ocean-atmosphere circulation patterns (e.g., the Aleutian Low).
The Arctic is among the most sensitive locations to climate change, where feedback mechanisms involving the cryosphere result in climate amplification. Because of their sensitivity to summer temperature and winter precipitation, glaciers can be used as proxies for climate change and reconstructions of past glacier fluctuations provide details about paleoclimate. Here, a chronology of late Pleistocene deglaciation and Neoglacial growth is constructed for two valleys in the north-central Brooks Range, Arctic Alaska. Cosmogenic 10Be exposure dating was used on ice-sculpted valley-bottom bedrock outcrops and boulders from Holocene moraine crests. Both valleys show evidence of retreat from the range front ~16-15 ka, and retreat into individual cirques by ~14 ka. There is no evidence for a standstill or re-advance during Late Glacial (14-11 ka) time. Neoglaciation was underway during the middle Holocene, constrained by a moraine dated to 4. 6±0. 5 ka.^Using this moraine age, and another moraine dated at 2. 7±0. 2 ka, this project confirms the accuracy of the previously established lichen growth curve to estimate moraine ages. This project also confirms that glaciers during early Neoglaciation had equal or larger extents than during the Little Ice Age (1200-1900 AD). Sediments collected from a proglacial lake downvalley of modern cirque glaciers reveal episodic sediment deposition from which it is difficult to isolate a signal of glacier advance. Comparing the lake sediment data to the moraine chronology suggests that Upper Kurupa Lake, based on the measured proxies, does not record glacial advances. Several conditions within the lake's catchment likely obscures any glacial signal. Further, more detailed measurements on the lake sediment might reveal additional clues on glacier activity.^Despite the apparent lack of recording changes in glacial length, sediment characteristics suggest a period of stable deposition since 1300 AD, possibly attributed to cooling during the Little Ice Age.
Geologic mapping near Windy Creek, Katmai National Park, identified two sets of glacial deposits postdating late-Wisconsin Iliuk moraines and separated from them by volcaniclastic deposits laid down under ice-free conditions. Radiocarbon dating of organic material incorporated in the younger Katolinat till and in adjacent peat and lake sediments suggests that alpine glaciers on the northern Alaska Peninsula briefly expanded between ca. 8500 and 10,000 years B.P. Stratigraphic relationships and radiocarbon dates suggest an age for the older Ukak drift near the Pleistocene-Holocene boundary between ca. 10,000 and 12,000 years B.P. We suggest that rapid deglaciation following deposition of the Iliuk drift occurred ca. 13,000-12,000 years B.P. in response to large increases in global atmospheric greenhouse gas content, including C02. Short-term decreases in these concentrations, as recorded in polar ice cores, may be linked with brief periods of glacier expansion during the latest Pleistocene and early Holocene. A transient episode of low solar intensity may also have occurred during parts of the early Holocene. Rapid environmental changes and glacial fluctuations on the Alaska Peninsula may have been in response to transient changes in the concentration of atmospheric greenhouse gases and solar intensity.