Timing and Potential Causes of 19th-Century Glacier Advances in Coastal Alaska Based on Tree-Ring Dating and Historical Accounts

The Little Ice Age (LIA), ca. CE 1250–1850, was a cold period of global extent, with the nature and timing of reduced temperatures varying by region. The Gulf of Alaska (GOA) is a key location to study the climatic drivers of glacier fluctuations during the LIA because dendrochronological techniques can provide precise ages of ice advances and retreats. Here, we use dendrochronology to date the most recent advance of La Perouse Glacier in the Fairweather Range of Southeast Alaska. After maintaining a relatively contracted state since at least CE 1200, La Perouse advanced to its maximum LIA position between CE 1850 and 1895. Like many other glaciers bordering the GOA, the La Perouse Glacier reached this maximum position relatively late in the LIA compared with glaciers in other regions. This is curious because reconstructions of paleoclimate in the GOA region indicate the 19th century was not the coldest period of the LIA. Using newly available paleoclimate data, we hypothesize that a combination of moderately cool summers accompanying the Dalton Solar Minimum and exceptionally snowy winters associated with a strengthened Aleutian Low could have caused these relatively late LIA advances. Such a scenario implies that winter climate processes, which are heavily influenced by ocean-atmospheric variability in the North Pacific region, have modulated these coastal glaciers’ sensitivity to shifts in summer temperatures

Radiocarbon age-offsets in an arctic lake reveal the long-term response of permafrost carbon to climate change

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Biogeosciences 119 (2014): 1630–1651, doi:10.1002/2014JG002688.Continued warming of the Arctic may cause permafrost to thaw and speed the decomposition of large stores of soil organic carbon (OC), thereby accentuating global warming. However, it is unclear if recent warming has raised the current rates of permafrost OC release to anomalous levels or to what extent soil carbon release is sensitive to climate forcing. Here we use a time series of radiocarbon age-offsets (14C) between the bulk lake sediment and plant macrofossils deposited in an arctic lake as an archive for soil and permafrost OC release over the last 14,500 years. The lake traps and archives OC imported from the watershed and allows us to test whether prior warming events stimulated old carbon release and heightened age-offsets. Today, the age-offset (2 ka; thousand of calibrated years before A.D. 1950) and the depositional rate of ancient OC from the watershed into the lake are relatively low and similar to those during the Younger Dryas cold interval (occurring 12.9–11.7 ka). In contrast, age-offsets were higher (3.0–5.0 ka) when summer air temperatures were warmer than present during the Holocene Thermal Maximum (11.7–9.0 ka) and Bølling-Allerød periods (14.5–12.9 ka). During these warm times, permafrost thaw contributed to ancient OC depositional rates that were ~10 times greater than today. Although permafrost OC was vulnerable to climate warming in the past, we suggest surface soil organic horizons and peat are presently limiting summer thaw and carbon release. As a result, the temperature threshold to trigger widespread permafrost OC release is higher than during previous warming events.National Science Foundation. Grant Number: ARC-09021692015-02-2

Ice-age megafauna in Arctic Alaska: extinction, invasion, survival

Radical restructuring of the terrestrial, large mammal fauna living in arctic Alaska occurred between 14,000 and 10,000 years ago at the end of the last ice age. Steppe bison, horse, and woolly mammoth became extinct, moose and humans invaded, while muskox and caribou persisted. The ice age mega fauna was more diverse in species and possibly contained 6x more individual animals than live in the region today. Mega faunal biomass during the last ice age may have been 30x greater than present. Horse was the dominant species in terms of number of individuals. Lions, short-faced bears, wolves, and possibly grizzly bears comprised the predator/scavenger guild. The youngest mammoth so far discovered lived ca 13,800 years ago, while horses and bison persisted on the North Slope until at least 12,500 years ago during the Younger Dry as cold interval. The first people arrived on the North Slope ca 13,500 years ago. Bone-isotope measurements and foot-loading characteristics suggest mega faunal niches were segregated along a moisture gradient, with the surviving species (muskox and caribou) utilizing the warmer and moister portions of the vegetation mosaic. As the ice age ended, the moisture gradient shifted and eliminated habitats utilized by the dry land, grazing species (bison, horse, mammoth). The proximate cause for this change was regional paludification, the spread of organic soil horizons and peat. End-Pleistocene extinctions in arctic Alaska represent local, not global extinctions since the mega faunal species lost there persisted to later times elsewhere. Hunting seems unlikely as the cause of these extinctions, but it cannot be ruled out as the final blow to mega faunal populations that were already functionally extinct by the time humans arrived in the region

Remote sensing-based statistical approach for defining drained lake basins in a continuous Permafrost region, North Slope of Alaska

Lake formation and drainage are pervasive phenomena in permafrost regions. Drained lake basins (DLBs) are often the most common landforms in lowland permafrost regions in the Arctic (50% to 75% of the landscape). However, detailed assessments of DLB distribution and abundance are limited. In this study, we present a novel and scalable remote sensing-based approach to identifying DLBs in lowland permafrost regions, using the North Slope of Alaska as a case study. We validated this first North Slope-wide DLB data product against several previously published sub-regional scale datasets and manually classified points. The study area covered \u3e71,000 km2, including a \u3e39,000 km2 area not previously covered in existing DLB datasets. Our approach used Landsat-8 multispectral imagery and ArcticDEM data to derive a pixel-by-pixel statistical assessment of likelihood of DLB occurrence in sub-regions with different permafrost and periglacial landscape conditions, as well as to quantify aerial coverage of DLBs on the North Slope of Alaska. The results were consistent with previously published regional DLB datasets (up to 87% agreement) and showed high agreement with manually classified random points (64.4–95.5% for DLB and 83.2– 95.4% for non-DLB areas). Validation of the remote sensing-based statistical approach on the North Slope of Alaska indicated that it may be possible to extend this methodology to conduct a comprehensive assessment of DLBs in pan-Arctic lowland permafrost regions. Better resolution of the spatial distribution of DLBs in lowland permafrost regions is important for quantitative studies on landscape diversity, wildlife habitat, permafrost, hydrology, geotechnical conditions, and high-lat-itude carbon cycling

Yellow-cedar blue intensity tree ring chronologies as records of climate, Juneau, Alaska, USA

This work was supported by the National Science Foundation’s Paleoclimatic Perspectives on Climatic Change (P2C2) Program grant nos. AGS 1159430, AGS 1502186, AGS 1502150, and PLR 15-04134 and by the Keck Geology Consortium funded by The National Science Foundation under Grant No. (NSF-REU #1358987).This is the first study to generate and analyze the climate signal in Blue Intensity (BI) tree-ring chronologies from Alaskan yellow-cedar (Callitropsis nootkatensis D. Don; Oerst. ex D.P. Little). The latewood BI chronology shows a much stronger temperature sensitivity than ring-widths (RW), and thus can provide information on past climate. The well-replicated BI chronology exhibits a positive January-August average maximum temperature signal for 1900-1975, after which it loses temperature sensitivity following the 1976/77 shift in northeast Pacific climate. The positive temperature response appears to recover and remains strong for the most recent decades although the coming years will continue to test this observation. This temporary loss of temperature sensitivity from about 1976 to 1999 is not evident in RW or in a change in forest health, but is consistent with prior work linking cedar decline to warming. A confounding factor is the uncertain influence of a shift in color variation from the heartwood/sapwood boundary. Future expansion of the yellow-cedar BI network and further investigation of the influence of the heartwood/sapwood transitions in the BI signal will lead to a better understanding of the utility of this species as a climate proxy.PostprintPeer reviewe

A Spatiotemporal Assessment of Extreme Cold in Northwestern North America Following the Unidentified 1809 CE Volcanic Eruption

Two large volcanic eruptions contributed to extreme cold temperatures during the early 1800s, one of the coldest phases of the Little Ice Age. While impacts from the massive 1815 Tambora eruption in Indonesia are relatively well-documented, much less is known regarding an unidentified volcanic event around 1809. Here, we describe the spatial extent, duration, and magnitude of cold conditions following this eruption in northwestern North America using a high-resolution network of tree-ring records that capture past warm-season temperature variability. Extreme and persistent cold temperatures were centered around the Gulf of Alaska, the adjacent Wrangell-St Elias Mountains, and the southern Yukon, while cold anomalies diminished with distance from this core region. This distinct spatial pattern of temperature anomalies suggests that a weak Aleutian Low and conditions similar to a negative phase of the Pacific Decadal Oscillation could have contributed to regional cold extremes after the 1809 eruption

Landscape sensitivity to climate change in northern Alaska: lessons from the past

Thesis (Ph.D.) University of Alaska Fairbanks, 2016The climate is now changing rapidly at high-latitudes, and observing how the Arctic and sub-Arctic environment responded to prehistoric climate changes can hold valuable lessons as we adapt in the future. This dissertation presents four studies that use biogeochemical proxies to reconstruct environmental changes in northern Alaska over the last 40,000 years (40 ka). These records are used to infer how the environment responded to climate changes at different locations and over varying spatial and temporal scales. The first study presents a time series of stable oxygen isotopes contained in radiocarbon-dated (¹⁴C) willow wood to quantify the nature and rates of climate change on the North Slope of Alaska over the last 40 ka. The second study examines how past temperature fluctuations affected permafrost thaw and the release of ancient carbon over the last 14.5 ka by compiling ¹⁴C-age offsets in the sediment of a small lake in the Brooks Range foothills. In the third study, I document human-caused changes to boreal wildfire frequency near the city of Fairbanks to test whether the primeval forest type and permafrost in the surrounding watershed will be vulnerable to more frequent fires in the future. The fourth study examines how ice age (40-9 ka) climate changes impacted the activity of sand dunes, vegetation productivity, and the dynamics of permafrost recorded in a unique sedimentary exposure located near the Arctic Coastal Plain on Alaska’s North Slope. Overall, I present several new and interesting approaches and findings stemming from this work. Ancient willow isotopes show that between 17 and 8 ka, during the time when ice sheets were in retreat worldwide, temperatures fluctuated widely on the North Slope mostly in concert with those in Greenland. Most notably, rapid changes in temperature and moisture occurred during the initial deglacial warming (ca. 16 ka), and during the Younger Dryas cold period (12.9-11.7 ka). These climate trends were amplified on the North Slope by changes in sea-ice extent in adjacent seas, which also controlled the availability of local precipitation evaporated from these seas. However, these warming and cooling trends were occasionally dampened by the advent of more maritime climate accompanying sea-level rise during the early Holocene, and by the breakdown of the atmospheric circulation patterns created by continental ice sheets in North America during the last glacial maximum. Over the last 7 ka, a gradual, insolation-driven cooling trend ended in ca. AD 1850 when the exceptional rates of recent warming began that continue to today. I found that the vegetation, permafrost and sand dunes in Arctic Alaska were sensitive to external climate forcing, but their responses were moderated by strong, internal feedbacks, including the temperature-buffering effects that thick peat layers have on the underlying permafrost. Prior to peat buildup in the early Holocene, the timing of sedimentary transitions indicate permafrost and aeolian processes were highly responsive to the volatile climate during the last ice age, which included Greenland interstadials. This incessant ice age climate change, coupled with the complex biophysical landscape responses that are particular to the unglaciated Arctic, helped maintain the ecological mosaic of the Mammoth Steppe ecosystem. Prehistoric warming events triggered permafrost thaw and the release of ancient carbon during the Bølling-Allerød (14.5-12.9 ka) and early Holocene warm period (11.7-8.0 ka), and this release is likely to occur again given enough warming. In the boreal forest watershed near Fairbanks, Alaska, the current ecological regime has remained intact despite a three-fold increase in pre-settlement wildfires during the Fairbanks gold rush (1902-1940). Once continued warming surpasses the buffering effects of the current internal feedbacks of the North Slope and boreal forest and the threshold for change is reached, more dynamic aeolian and permafrost processes may again dominate as they did on the more unstable and diverse ice age landscape. Overall, the results of this work will be useful for understanding how climate and landscape change in northern Alaska will respond to global climate forcing in the future.Chapter 1. General Introduction -- Chapter 2. Oxygen isotopes from modern and ancient willows record postglacial climate changes and exceptional rates of recent warming in Arctic Alaska -- Chapter 3. Radiocarbon age-offsets in an Arctic lake reveal the long-term response of permafrost carbon to climate change -- Chapter 4. High-resolution records detect human-caused changes to the boreal forest wildfire regime in Interior Alaska -- Chapter 5. Sedimentary sequences from unglaciated Arctic Alaska describe climate forcing and landscape responses during the last ice age -- Chapter 6. General conclusions