Al Werner

Mild Little Ice Age and unprecedented recent warmth in an 1800 year lake sediment record from Svalbard

The Arctic region is subject to a great amplitude of climate variability and is currently undergoing large-scale changes due in part to anthropogenic global warming. Accurate projections of future change depend on anticipating the response of the Arctic climate system to forcing, and understanding how the response to human forcing will interact with natural climate variations. The Svalbard Archipelago occupies an important location for studying patterns and causes of Arctic climate variability; however, available paleoclimate records from Svalbard are of restricted use due to limitations of existing climate proxies. Here we present a sub-decadal- to multidecadal-scale record of summer temperature for the past 1800 yr from lake sediments of Kongressvatnet on West Spitsbergen, Svalbard, based on the first instrumental calibration of the alkenone paleothermometer. The age model for the High Arctic lake sediments is based on 210 Pb, plutonium activity, and the first application of tephrochronology to lake sediments in this region. We find that the summer warmth of the past 50 yr recorded in both the instrumental and alkenone records was unmatched in West Spitsbergen in the course of the past 1800 yr, including during the Medieval Climate Anomaly, and that summers during the Little Ice Age (LIA) of the 18 th and 19 th centuries on Svalbard were not particularly cold, even though glaciers occupied their maximum Holocene extent. Our results suggest that increased wintertime precipitation, rather than cold temperatures, was responsible for LIA glaciations on Svalbard and that increased heat transport into the Arctic via the West Spitsbergen Current began ca. A.D. 1600.

Glacier readvance during the late glacial (Younger Dryas?) in the Ahklun Mountains, southwestern Alaska

An expansion of alpine glaciers during the latest Pleistocene produced an extraordinarily well defined end moraine system in the Ahklun Mountains, southwestern Alaska. These moraines, deposited during the Mount Waskey advance, are several kilometers beyond modern glacier termini, and ∼80 km upvalley of the late Wisconsin Ahklun Mountains ice cap terminal moraine. Eleven cosmogenic 10 Be and 26 Al exposure ages on moraine boulders, combined with radiocarbon ages from a lake core upvalley of a moraine deposited during the Mount Waskey advance, suggest that the advance culminated between 12.4 and 11.0 ka, sometime during, or shortly following, the Younger Dryas event (ca. 12.9–11.6 ka). We believe that the Mount Waskey advance was a consequence of cooling during the Younger Dryas. These data further strengthen emerging evidence for Younger Dryas–age cooling of the North Pacific region.

Holocene cirque glacier activity in western Spitsbergen, Svalbard: sediment records from proglacial Linnévatnet

Holocene glacial variations of a presently ice-free cirque in western Spitsbergen are inferred from sediment records from the proglacial lake Linnévatnet. Glaciation is indicated in the core records by increased intensity of laminae derived from the cirque inflow and by increase in sediment accumulation rate. Radiocarbon ages on terrestrial plant macrofossils limit the onset of glaciation in the cirque to within the last six centuries. No evidence for earlier glaciation is recognized in the core records, indicating that the threshold for glaciation in the cirque was not reached during prior Neoglacial advances. Local climate events favouring glacier expan sion were either of greater magnitude or of longer duration during the ‘Little Ice Age’ than at any other time during the Holocene.

Holocene glacier fluctuations, Waskey Lake, northeastern Ahklun Mountains, southwestern Alaska

Lake sediments from Waskey Lake, Ahklun Mountains, southwestern Alaska were studied to decipher the history of upvalley glacier fluctuations during the Holocene. Several indicators of glacier activity were measured including: magnetic susceptibility, organic-matter content, grain-size distribution, bulk-sediment mineralogy and diatom assemblages. Seven radiocarbon ages on macrofossils, along with cross-checks by tephrochronology, provide the chronology of the cores. The results from core WL-1 indicate that glaciers lingered near Waskey Lake until 9100 cal. yr BP, perhaps under conditions of high winter accumulation. Peak organic-matter content occurs at 7400 cal. yr BP, when precipitation might have shifted to summer. The onset of Neoglaciation occurred 3100 cal. yr BP, and glaciers reached their maximum extenñ700 cal. yr BP. This chronology is consistent with the lichenometrically dated moraines from the glacier forefields. Although the ages are tentative, the youngest and most widespread group of moraines was deposited sometime between 650 and 200 cal. yr BP (during the‘Little Ice Age’). Since then, glaciers in the Waskey Lake area have shrunk bñ50% and equilibrium-line altitudes (ELA) have risen by 35± 22 m. This rise in ELA is much less than the 100 to 200 m rise observed elsewhere in Alaska and indicates considerable spatial variability in late-Holocene climatic change.

AMS-radiocarbon dating of organic-poor lake sediment, an example from Linnévatnet, Spitsbergen, Svalbard

AMS-radiocarbon dating of specific organic fractions is used to evaluate sources of errors in dating of organic-poor lake sediment from Linnévatnet, a proglacial arctic lake. Dates on un differentiated (bulk) organic matter are much too old because of contamination from detrital coal. Attempts to remove coal from other organic matter were only partially successful, a consequence of the wide grain-size distribution of the coal. Even if coal contamination is fully removed, the bulk of the remaining organic matter is of aquatic origin and is unsuitable for 14C dating because the lake waters are depleted in 14 C due to dissolution of carbonate minerals. Terrestrial plant macrofossils provide the only reliable material to date the lake sediment; however, this material is present in sufficient quantity for AMS-radiocarbon dating only in cores proximal to the main inlet streams. Paired dates on terrestrial plants and aquatic insects from the same core level and paired dates on modern aquatic and terrestrial vegetation indicate differences in carbon reservoir activities of 1000 and 3000 years. In hard-water lakes, coring is recommended in inlet-proximal areas where inwashed terrestrial vegetation is most concen trated. Chronologies established in proximal cores can be transferred to central-basin cores using litho or chemo-stratigraphic indices.

A 33,000 year record of environmental change from Arolik Lake, Ahklun Mountains, Alaska, USA

A continuous record of lacustrine sedimentation capturing the entire full-glacial period was obtained from Arolik Lake in the Ahklun Mountains, southwestern Alaska. Fluctuations in magnetic susceptibility (MS), grain size, organic-matter (OM) content, C/N ratios, δ13C, and biogenic silica (BSi) record marked environmental changes within the lake and its watershed during the last ∼33 cal ka. Age control is provided by 31 14C ages on plant macrofossils in four cores between 5.2 and 8.6 m long. Major stratigraphic units are traceable throughout the lake subbottom in acoustical profiles, and provisional ages are derived for six prominent tephra beds, which are correlated among the cores. During the interstadial interval between ∼33 and 30 cal ka, OM and BSi contents are relatively high with values similar to those of the Pleistocene–Holocene transition, suggesting a similar level of aquatic productivity. During the glacial interval that followed (∼30–15 cal ka), OM and BSi decrease in parallel with declining summer insolation. OM and BSi values remain relatively uniform compared with the higher variability before and after this interval, and they show no major shifts that might correlate with climate fluctuations evidenced by the local moraine record, nor with other global climate changes. The glacial interval includes a clay-rich unit with a depauperate diatom assemblage that records the meltwater spillover of an ice-dammed lake. The meltwater pulse, and therefore the maximum extent of ice attained by a major outlet glacier of the Ahklun Mountain ice cap, lasted from ∼24 to 22 cal ka. The Pleistocene–Holocene transition (∼15–11 cal ka) exhibits the most prominent shifts in OM and BSi, but rapid and dramatic fluctuations in OM and BSi continue throughout the Holocene, indicating pronounced paleoenvrionmental changes.

Indirect Growth Curves Remain the Best Choice for Lichenometry: Evidence from Directly Measured Growth Rates from Svalbard

Abstract Directly measured growth rates of two lichens (Pseudephebe minuscula and Rhizocarpon sections Rhizocarpon and Superficiale) from Svalbard made over a two-decade interval (1984–2007) are presented. Growth rates were determined by measuring the change in area of the lichen thalli from digital images and converting area to diameter. Pseudephebe diameter growth rates ranged from 0.2 to 1.5 mm yr−1 and Rhizocarpon grew 0.05 to 0.30 mm yr−1. Growth rates of both are a function of thalli size—growth rates increase with increasing thallus size up to 70 mm diameter for Pseudephebe and 30 mm diameter for Rhizocarpon. While these directly measured growth rate results are consistent with other recent directly measured lichen growth studies, they are not consistent with indirectly determined age-size curves that show a negative correlation between size and growth rate (i.e., rapid “great growth” followed by slower “linear growth”). We explore several reasons to explain the apparent discrepancy between directly measured and indirectly determined growth rates, including climate change, increased nutrient fluxes, and population sampling differences between the two methods. We argue that indirectly determined growth curves, which integrate the effects of changing growing conditions over time, remain the best basis for lichenometric dating.

Fluvial suspended sediment yields over hours to millennia in the High Arctic at proglacial Lake Linnévatnet, Svalbard

Sediment yield can be a sensitive indicator of catchment dynamics and environmental change. For a glacierized catchment in the High Arctic, we compiled and analyzed diverse sediment transfer data, spanning a wide range of temporal scales, to quantify catchment yields and explore landscape response to past and ongoing hydroclimatic variability. The dataset integrates rates of lake sedimentation from correlated varve records and repeated annual and seasonal sediment traps, augmented by multi-year lake and fluvial monitoring. Consistent spatial patterns of deposition enabled reconstruction of catchment yields from varve- and trap-based fluxes. We used hydroclimatic data and multivariate modeling to examine annual controls of sediment delivery over almost a century, and to examine shorter-term controls of sediment transfer during peak glacier melt. Particle-size analyses, especially for annual sediment traps, were used to further infer sediment transfer mechanisms and timing. Through the Medieval Warm Period and Little Ice Age, there were no apparent multi-century trends in lake sedimentation rates, which were over three times greater than those during the mid-Holocene when glaciers were diminished. Twentieth-century sedimentation rates were greater than those of previous millennia, with a mid-century step increase in mean yield from 240 to 425 Mg km-2 yr-1. Annual yields through the twentieth century showed significant positive relations with spring/summer temperature, rainfall, and peak discharge conditions. This finding is significant for the future of sediment transfer at Linnevatnet, and perhaps more broadly in the Arctic, where continued increases in temperature and rainfall are projected. For 2004-2010, annual yields ranged from 294 to 1330 Mg km-2 yr-1. Sediment trap volumes and particle-size variations indicate that recent annual yields were largely dominated by spring to early summer transfer of relatively coarse-grained sediment. Fluvial monitoring showed daily to hourly sediment transfer to be related to current and prior discharge, diurnal hysteresis, air temperature, and precipitation.