Parker E. Calkin

Tree-ring-dated 'Little Ice Age' histories of maritime glaciers from western Prince William Sound, Alaska

Tree-ring studies at 13 glacier forefields in western Prince William Sound show‘Little Ice Age’ glacial fluctuations were strongly synchronous on decadal timescales. Cross-dated glacially overrun trees at eight sites indicate ice margins advanced in the early (late twelfth through thirteenth centuries) and middle (seventeenth to early eighteenth centuries)‘Little Ice Age’. Tree-ring dates of 22 moraines at 13 glaciers show two main periods of stabilization. The earlier of these, in the first decades of the eighteenth century, overlaps with the second period of glaciers overrunning trees and marks culmination of this middle‘Little Ice Age’ expansion. Stabilization of moraines on nine of the study forefields in the latter part of the nineteenth century delineates a third interval of‘Little Ice Age’ glacial advance. The detailed‘Little Ice Age’ record from land-terminating glaciers in western Prince William Sound is consistent on a timescale of decades with four other tree-ring-dated glacial histories from across the northern Gulf of Alaska. This coastal northeastern Pacific glacial record reveals the structure of the‘Little Ice Age’ in the region and provides a strong basis for comparison with other proxy climate records spanning the past 1000 years.

Late Holocene, High-Resolution Glacial Chronologies and Climate, Kenai Mountains, Alaska

Recent retreat of outlet glaciers from the Harding and Grewingk-Yalik Icefields has revealed a vast array of deposits on the eastern and western flanks of the Kenai Mountains that records multiple glacier advances into coastal forests during late Holocene time. Treering dating, together with radiocarbon and lichenometric analyses, allows for the reconstruction of these glacial fluctuations to decadal precision over the past two thousand years. The records of fluctuations are derived from 16 land-terminating and seven tidewater glaciers in three fjord systems, as well as two cirque glaciers. Three major intervals of Holocene glacier expansions are evident; they occurred about 3600 yr B.P., 600 A.D., and during the Little Ice Age, from 1300 to 1850 A.D. The earliest expansion beyond present ice margins is known only from the McCarty tidewater glacier. The 600 A.D. event involved the simultaneous advance of land-terminating and tidewater glaciers. During the Little Ice Age, however, tidewater glaciers were advancing several centuries prior to their land-terminating neighbors. Those land-terminating glaciers on the western mountain flank retreated from their Little Ice Age maxima as much as two centuries before those on the eastern mountain flank. Land-terminating tongues on the eastern, more maritime, mountain flank have shown more sensitivity to variations in winter precipitation during the Little Ice Age and within recent decades than the more continental glaciers on the western flank that are affected more by summer temperatures. The glacial and climatic records suggest that advances of the ice tongues from about 1420 to 1460 A.D., between 1640 and 1670 A.D., at about 1750 A.D., and from 1880 to 1910 A.D. reflected times of increased winter precipitation. Advances between 1440 to 1460 A.D., from 1650 to 1710 A.D., and from 1830 to 1860 A.D. followed intervals of lower summer temperatures.

Expansion of alpine glaciers in Pacific North America in the first millennium A.D

Radiocarbon ages and lichen-dated moraines from 17 glaciers in coastal and near- coastal British Columbia and Alaska document a widespread glacier advance during the first millennium A.D. Glaciers at several sites began advancing ca. A.D. 200-300 based on radiocarbon-dated overridden forests. The advance is centered on A.D. 400-700, when glaciers along an ;2000 km transect of the Pacific North American cordillera overrode forests, impounded lakes, and deposited moraines. The synchroneity of this glacier ad- vance and inferred cooling over a large area suggest a regional climate forcing and, to- gether with other proxy evidence for late Holocene environmental change during the Me- dieval Warm Period and Little Ice Age, provide support for millennial-scale climate variability in the North Pacific region.

Lichenometry as Applied to Moraines in Alaska, U.S.A., and Kamchatka, Russia

Abstract A selective review of lichenometry as used to date Holocene moraines in five diverse regions of Alaska and in southeastern Kamchatka suggests that growth curves for this North Pacific area may be improved by attention to several factors. These included lichen identification, control point number and distribution, radiocarbon calibration, alternative curve models, and compatibility of lichen growth rate with climate. Support for control points presented for Kamchatka and published for Alaska areas will benefit from supplementary control at and beyond the break from the great growth curve segments of the last centuries. With regard to alternative—linear, logarithmic, and composite curve—models drawn for the published lichenometric data, the composite (logarithmic and linear composite models) appear the best fit for the Brooks Range and Wrangell–St. Elias areas of slow growth and continental interior climates. Calibration of 14C ages make minor changes in well-controlled curves, but differences may be marked where a single age supports the long-term portion of growth curves. Lichen subgenus Rhizocarpon section Geographicum and section Alpicola should, and usually can be, differentiated in North Pacific areas. Nevertheless, growth curves that may represent both yellow-green Rhizocarpons (e.g., central Brooks Range and southeastern Kamchatka) appear to allow derivation of reasonable surface dating where the taxa distribution is similar to that of the curves. Chronologies of glaciation based on lichenometry of moraines over the last millennium in these two areas across the Bering Sea are strikingly similar to each other and to more precisely dated tree-ring-based glacial chronologies in southern Alaska.

Chronology of Holocene glaciation, central Brooks Range, Alaska

The central Brooks Range was glacierized in the highest, north-facing cirques during late-middle to late Holocene (Neoglacial) time. This Neoglaciation involved at least 5 major cirque-glacier expansions of similar magnitude, as based on lichenometric mapping of more than 50 glaciers and radiocarbon dates directly associated with 5 moraines. Initial stabilization of debris-covered glaciers took place by early Holocene time, but evidently no moraines formed during this interval. Few morainal ridges are preserved that date from the older expansions, but they have been lichenometrically dated (± 20% age reliability) to 3 separate intervals: 4400, 3500, and 2900 yr B.P. Twelve morainal complexes have ridges indicative of glacial advance that lichenometrically date at 1800 ± 500 yr B.P. Relict lichens that are now emerging undisturbed from beneath a receding glacier toe imply that this time was one of prolonged recession in at least some parts of the Brooks Range, however. Radiocarbon analysis of dead moss at this same site dates a subsequent Neoglacial advance across the cirque floor at 1120 ± 180 yr B.P. Our data suggest that for the past ∼1,100 yr, cirque glaciers have been continuously in more extended positions than they are today. During this glaciologically favorable interval, the last two major advances occurred at 800 ± 150 and 390 ± 90 yr B.P. (A.D. 1410–1600). Glaciers across the central Brooks Range stayed close to their maxima until A.D. 1640–1750. Historical photographs and lichenometry show that retreat was most rapid after A.D. 1870 and decelerated after the mid-1900s. Recession from this most recent Neoglacial maximum has amounted to 150–700 m and continues at present. The cirque glacier advances were accompanied by equilibrium-line altitude (ELA) depressions of 100 to 200 m below levels maintained in the late 1970s. Environmental lapse rate estimates and 1977–1981 glaciologic-meteorologic measurements suggest that summer air temperatures accompanying Neoglacial maxima were, respectively, 1 °C or 3 to 4 °C cooler than those of the late 1970s. Across the central Brooks Range, the Neoglacial maxima ELAs rise northeastward and northward from 2 to 5 m km −1 .

A 1119-year tree-ring-width chronology from western Prince William Sound, southern Alaska

Living and subfossil trees from glacier forefields are used to develop a 1119-year-long tree-ringwidth chronology. Strong cross-dating among ring-width series from sites up to 60 km apart and an analysis of sample homogeneity support combination of all samples into a single, regional composite chronology. Comparison with instrumental climate data indicates May through July temperatures of the growth year are the primary control on ring-widths. Multidecadal-length warm periods in western Prince William Sound during the past 800 years were centred on AD 1300, 1440 and possibly 1820. Multidecadal-length cool periods were centred on AD 1400, 1660 and 1870. This is the first tree-ring chronology from the Gulf of Alaska region to extend into the first millennium AD.

NATURE AND DISTRIBUTION OF GLACIERS, NEOGLACIAL MORAINES, AND ROCK GLACIERS, EAST-CENTRAL BROOKS RANGE, ALASKA

The east-central Brooks Range was just high enough to support cirque glacierization during the middle to late Holocene; presently glaciers are shrinking. The 133 glaciers in the field area are all above 1500 m altitude, and those fronted by stable moraines occur on a trend surface rising from 1600 m south of the Continental Divide to 2000 m, 25 km farther to the north. Glaciers that extend into un

Holocene history of Hubbard Glacier in Yakutat Bay and Russell Fiord, southern Alaska

Stratigraphic and geomorphic data defined by radiocarbon ages, tree-ring dates, and historical observations provide evidence of three major Holocene expansions of Hubbard Glacier. Early in each advance the Hubbard Glacier margin blocked Russell Fiord to create Russell lake, raising base level and causing stream beds and fan deltas throughout the Russell drainage basin to aggrade. Each Hubbard Glacier expansion continued with an ice lobe advancing through Disenchantment and Yakutat Bays in the west, and an eastern lobe advancing into Russell Fiord. The earlier two Holocene expansions were, respectively, under way at 7690 and 5600 calibrated yr B.P., and each advance culminated more than 1 k.y. later. The late Holocene advance was under way by 3100 yr ago and reached ∼13 km farther south in Russell Fiord than the preceding two expansions. Late Holocene deglaciation of Yakutat and Disenchantment Bays was complete before A.D. 1791; Nunatak Glacier flowing from neves east of Russell Fiord became the primary ice source to the Russell Fiord lobe at or before this date. Ice retreat from the southern end of Russell Fiord began in the late eighteenth century and the penultimate Russell lake drained ca. A.D. 1860. The relatively slow advances and more rapid retreats of Hubbard Glacier are consistent with the model of the iceberg-calving glacier cycle. Hubbard Glacier is currently advancing and will likely reestablish Russell lake in the near future, affecting local fisheries. However, glacier lobes are unlikely to reach the area of the town of Yakutat, built on late Holocene glacial deposits, in the next 1 k.y.

DIRECT MEASUREMENT OF LICHEN GROWTH IN THE CENTRAL BROOKS RANGE, ALASKA, U.S.A., AND ITS APPLICATION TO LICHENOMETRIC DATING

The growth of 92 Rhizocarpon geographicum s.l. and 57 Alectoria minuscula Nyl. lichens was measured over 4 to 6 yr in the Atigun Pass region of the central Brooks Range, Alaska. Absolute growth rates of R. geographicum s.l. were inversely related to thallus diameter and ranged from 0.35 to near 0 mm yr-1. In contrast, A. minuscula growth rates were directly related to diameter and ranged from 0.14 to 2.01 mm yr-1. A growth curve derived from mean rates of R. geographicum s.l. growth is similar to the indirectly controlled lichenometric dating curve for this region, but cannot yet be used to modify ages assigned to Holocene deposits. The derived growth curve for A. minuscula closely resembles those from the eastern Canadian

Tree-ring analysis and Quaternary geology : Principles and recent applications

Abstract Variations in annual growth rings can be utilized to assign precise calendar dates to the study of Quaternary geologic or geomorphic events such as earthquakes, mass movements, glaciations and flooding. Living trees may provide the necessary absolute dates of events or minimum ages for surfaces. Crossdating among living trees and with subfossil wood can identify missing rings and extend tree-ring chronologies further back in time. These methods can lead to the accurate dating of tree rings in both living and dead trees that may have been affected by a geomorphic process. The dating of partial to complete ring growth, compression and release, as well as dating externally apparent responses, such as tree scarring or root sprouting, can often lead to the identification of the season and the year an event occurred. Ring-width patterns when compared with control chronologies can reveal the growth anomalies in witness trees that may be related to geomorphic events. Examples of the application of tree rings to Quaternary geology and geomorphology are varied and numerous. Root shearing, topping of trees, and missing rings tie a rupture along the San Andreas fault to the year 1812 A.D. Narrow tree rings linked to defoliation and tephra fallout differentiated distinct eruptions of Mt. St. Helens in 1480 and in 1482 A.D. Ring analysis has helped unravel glacial chronologies, and tree-ring linkages with climate have also verified that late Holocene ice advances in Argentina correlated with intervals of cool, wet summers. In another application, an 1818 A.D. Quebec landslide has been tied to the growing season, whereas slides dated to 1834 and 1846 A.D. occurred during the spring.