PP44A-01
Sediments Exposed by Drainage of a Collapsing Glacier-Dammed Lake Show That Contemporary Summer Temperatures and Glacier Retreat Exceed the Medieval Warm Period in Southern Alaska
In the mountains of southcentral Alaska, recent and widespread glacier retreat is well-documented, but few instrumental or proxy records of temperature are available to place recent changes in a long-term context. The Medieval Warm Period in particular, is poorly documented because subsequent Little Ice Age glacier advances destroyed much of the existing sedimentary record. In a rare exception, sudden and unexpected catastrophic drainage of a previously stable glacier-dammed lake recently revealed lacustrine stratigraphy that spans over 1500 years. Located near the Bagley Icefield in Wrangell-St. Elias National Park and Preserve, Iceberg Lake first drained in A.D. 1999 and has not regained a stable shoreline since that time. Rapid incision of the exposed lakebed provided subaerial exposure of annual laminations (varves, confirmed by radiogenic evidence) that record continuous sediment deposition from A.D. 442 to A.D. 1998. We present a recalculated master chronology of varve thickness that combines measurements from several sites within the former lake. Varve thickness in this chronology is positively correlated with northern hemisphere temperature trends and also with a local, ~600 year long tree ring width chronology. Varve thickness increases in warm summers because of higher melt, runoff, and sediment transport, and also because shrinkage of the glacier dam allows shoreline regression that concentrates sediment in the smaller lake. Relative to the entire record, varve thicknesses and implied summer temperatures were lowest around A.D. 600, high between A.D. 1000 and A.D. 1300, low between A.D. 1500 and A.D 1850, and highest in the late 20th century. Combined with stratigraphic evidence that contemporary jokulhlaups are unprecedented since at least A.D. 442, this record suggests that late 20th century warming was more intense, and accompanied by more extensive glacier retreat, than the Medieval Warm Period or any other time in the last 1500 years. We emphasize that the chronology presented here does not include the entire sedimentary history of the lake. Deeper sediments unexposed by the subaerial exposures we examined may extend this record of summer temperatures back to the onset of significant glaciation in this region. Traditional coring techniques could capture this record before ongoing erosion of the dry lakebed exports it to the Gulf of Alaska.
PP44A-02
Late Holocene Temperature Reconstruction Inferred from a Varved Glacial Lake in the Central Brooks Range, Alaska, USA
Climate models and recent observations suggest that the Arctic is warming more rabidly than any other region on Earth. However, few long-term temperature records exist that can place this recent warming trend in a larger context. Varved glacial lake sediments provide one such archive at annual resolution, provided that a relationship between varve thickness and near-surface temperature can be established. We present a paleotemperature record inferred from varve thicknesses measurements of cores collected from Blue Lake in the Central Brooks Range of Alaska, USA. Three distinct climatic periods are observed in the Blue Lake sediment record: 1) A period of thinner varves persists between ~1200-1900 AD, with a minimum thicknesses occurring between 1660 - 1900 AD, 2) after 1900 AD a warming trend is marked by increased varve thickness that continues to the present, 3) thicker varves prior to 1200 AD may indicate an interval of warmth equal to or greater than that of the 20th century. Our results suggest that 20th century warming in the Central Brooks Range is anomalous within the context of at least the last ~800 years, but periods prior to 1200 AD may have experienced warmth equal to or greater than the present.
PP44A-03 INVITED
Glacier Changes and Inferred Temperature Variability in Alaska for the Past Two Thousand Years
Comparisons of temperature sensitive tree-ring records with histories from land-terminating, non-surging glaciers for the last two millennia from Alaska identify summer temperature as a primary driver of glacier changes. Uniform retreat of glaciers across Alaska attests to the sensitivity of this proxy to warming, whereas examination of the paleorecord places these contemporary changes into a longer-term perspective. Two major intervals in the Alaskan chronology of glaciation, during the First Millennium AD (FMA) and again during the Little Ice Age (LIA), are evident as broad times of cooling and ice expansion. These two intervals are respectively followed by ice retreat coincident with the Medieval Warm Period (MWP) and contemporary warming, and together correspond with millennial-scale variations recognized in other Alaskan proxy records. The FMA advance appears to be of similar extent as the subsequent LIA expansions indicating a uniformity of forcing over the past two millennia. This general millennial-scale history of temperature variability can be compared with the behavior of tidewater glacier margins over the same two thousand year interval. It is clear over this time that tidewater expansions were underway in the FMA, during the height of the MWP and persisted through the LIA. That some tidewater glaciers advanced during the MWP and retreated during portions of the LIA reflects the control of non-climatic factors on tidewater glacier fluctuations. Understanding these non-climatic factors and the fate of Alaskan glaciers is vital as their contribution to global sea level rise in the coming decades to centuries is of major importance, and as the economic and ecological impacts of deglaciated Alaskan mountain ranges becomes a reality.
PP44A-04
Late Holocene Vegetation and Climate Change From the Central and Western Canadian Arctic Inferred From Fossil Pollen Data
Two sediment cores from the central and western Canadian Arctic were used to document landscape-scale vegetation and climate changes spanning the last ~2500 years. Both cores were dated by Pb-210 and C-14 techniques. Fossil pollen was enumerated at continuous 1 cm intervals (each centimeter representing a period of ~70 years), permitting centennial-scale changes to be placed into a long-term context. The pollen percentages are dominated by Cyperaceae and show relatively uniform values throughout the cores. Quantitative climate reconstructions, based on the percentage values, are similarly stable. However, the influx of locally- and regionally-derived pollen grains increases over the last ~150 years, suggesting that higher primary production and summer temperatures occurred over this time. The pollen results from these cores are consistent with other high-resolution (~25 year) lake sediment proxy data (BSi and LOI) from the region. Despite this, a comparison of these data with several Holocene-length pollen records from the same region indicates that the changes that characterized the last 2000 years were relatively minor compared to those of the early Holocene.
PP44A-05
Warm Times and Cold Times During the Last 2000 Years Reconstructed from Icelandic Lake and Marine Sediments
Holocene lacustrine records and their synchronization with available marine records from around Iceland provide compelling evidence for the alternating influences of the warm Irminger Current, and the cold East Greenland Current during Holocene times. Here we present data for the past 2000 years from two lakes in Iceland; Haukadalsvatn, a non-glacial lake in northwest Iceland; and Hvitarvatn, a glacial lake east of Langjokull glacier in Central Iceland. Physical and chemical proxies (magnetic susceptibility, TOC, biogenic silica, sedimentation rate, ice rafted debris) in the lake sediments were analyzed at subdecadal resolution. Overall, the interval from 2000 to 800 BP is characterized by a covariant pattern of biogenic silica and TOC, indicating relatively stable climate. A moderate Medieval Warm Period shows up in our records between 1200 and 800 BP. About 800 BP, a sharp increase in TOC coincides with a decline in biogenic silica. This decoupled pattern between the two proxies intensifies after 600 BP, ending about 100 BP. The δ13C of total organic carbon in Haukadalsvatn sediments during this interval (800 to 100 BP) indicates that carbon in the lake sediments was dominantly from terrestrial sources, most likely brought into the lake by soil erosion during storm events. The timing of most intense soil erosion coincides with historically documented sea-ice intensity off the coast of Iceland during this time. The ice rafted debris record from Hvitarvatn indicates that the height of the Little Ice Age (LIA) was between 1750 - 1850 AD. Glacier simulations constrained by our proxies from Hvitarvatn suggest Langjokull attained its maximum LIA volume around 1840 AD with a second advance around 1890 AD; the magnitude of glacier advance suggests summer temperature depression 1-2°C lower than present. The two outlet glaciers terminating in Hvitarvatn, Nordurjokull and Sudurjokull, advanced slowly into the lake, occupying their maximum lake area (4-6 km2) in the late 19th century, and retreated comparatively rapidly in the mid- to late 20th century. Newly obtained climate proxy records from the shelf southwest and northwest of Iceland also reflect these centennial scale climate fluctuations, and a similar picture is emerging from lake-sediment records in north Iceland (Axford et al., this volume). Coherency between the lacustrine records, sea-ice variability, and records of marine productivity suggests a strong maritime influence on the terrestrial climate of Iceland.
PP44A-06
Marine Paleoclimate Records of the Last 1500 Years off Northern Norway: An Elevated Arctic Warming?
Instrumental records have highlighted a warming trend over the last few decades in the high northern latitudes. Current observations as well as climate modelling experiments demonstrate an amplification of the global warming signal in the polar region. This study aims to put this warming into context with natural climate change by providing longer term paleoclimate proxy records of the arctic climate signal. For this purpose we are using benthic foraminiferal derived oxygen isotope records from short marine sediment cores from the northern Norwegian continental margin. The results are presented from five cores from open ocean (western Barents Sea), shelf sea (Andfjorden) and fjord (Malangen, Vestfjorden and Sagfjorden) settings thus providing a north-to-south transect of paleoclimate records within this sub Arctic to Arctic region. Radiocarbon and Pb210 dating demonstrate a series of high- resolution (annual to decadal) timescales spanning approximately the last 500 to 2000 years. A clearly pronounced Little Ice Age (LIA) cooling period (ca. 1250-1900) is evident within all cores of ca. 1oC variation, although a slight temporal offset exists between the records. The subsequent warming trend of > 1oC is also offset temporally between the records, however, the marked warming of the early 20th century is in agreement in all cores. Particular focus is placed on this clear 20th century warming signal and statistical comparisons are made to both the historical global temperature record and northern hemisphere temperature records. Two periods of rapid warming in the middle (ca. 1940's) and late 20th century (post 1980's) are observed in the instrumental, model and proxy data. Preliminary results suggest the Norwegian Arctic records may confirm the observed global warming trend, while further comparison studies will be required to quantify the proposed elevated response in northern latitudes.
PP44A-07 INVITED
Simulation of Arctic climate response to high- and low- latitude volcanic eruptions during the late 13th Century: Possible early onset of the Little Ice Age
In some parts of the Northern Hemisphere high latitudes, the transition from relatively warm climatic conditions during medieval time (sometimes termed "Medieval Warm Period" or "Medieval Climatic Anomaly") to the first episode of clearly cooler conditions associated with the "Little Ice Age" occurred in the second half of the 13th Century. As part of a synthesis project of the last 2000 years of Arctic climate variability derived from high- resolution lake records, we are evaluating the synchroneity of this important climatic transition across the northern American to northern European sector and studying potential mechanisms responsible for the change. One strong candidate is volcanic forcing. Several of the recent estimates of the temporal evolution of volcanic forcing suggest that the late 13th Century was among the most volcanically perturbed half-century of the last 2000 years. Here, we discuss simulations of such volcanic perturbations for the 13th Century performed with a moderate resolution version of the Community Climate System Model version 3 (a fully coupled atmosphere, ocean, land surface and sea-ice model). Because none of the 13th century eruptions have been clearly attributed to specific volcanoes, we performed two sets of ensemble simulations, one assuming a sequence of tropical eruptions as the source of the volcanic sulfate spikes in the polar ice records, and one assuming multiple high- latitude eruptions. We address the spatial-temporal patterns of Northern Hemisphere climate response, including the contribution of both the direct radiative perturbation and dynamical responses such as changes in the Northern Annular Mode and how they contrast between the experiments with different source of volcanic perturbations.