David M. Hopkins

Arctic oceanic climate in late cenozoic time.

Faunal and lithologic evidence is used to reconstruct paleoceanographic events over the last 4.5 million years. The inception of perennial sea-ice cover is dated at about 0.7 million years.

Sea Level History in Beringia During the Past 250,000 Years

This paper attempts to relate current knowledge of sea-level history in Beringia to the Broecker-van Donk “Termination” concept of climatic and sea-level history. The Einahnuhtan transgression is thought to represent Termination III, which according to Broecker and van Donk, took place about 225,000 y.a. The Kotzebuan transgression is thought to represent a positive fluctuation that modulated the generally falling sea level during the ensuing 100,000 yr. Sea level probably fell to about −135 m in the Bering Sea area during the maximum phase of the penultimate glaciation. The two Pelukian shorelines probably represent Termination II (about 125,000 yr BP in the Broecker-van Donk chronology) and one of the two positive fluctuations that modulated the generally falling sea level of early Wisconsinan time, about 105,000 and 80,000 y.a. according to Broecker and van Donk. Another positive modulation brought sea level to at least −20 m, about 30,000 y.a. Sea level evidently fell to between −90 and −100 m during the late Wisconsinan regression, but a substantial part of the outer Bering shelf remained submerged. Submerged shoreline features at −38m, −30 m, −24 to −20 m, and −12 to −10 m represent stillstands or slight regressions that modulated Termination I, the late Wisconsinan, and early Holocene recovery of sea level.

Dated wood from Alaska and the Yukon: Implications for forest refugia in Beringia

Abstract Postulations on the existence of forest refugia in parts of Beringia during the last glacial have been, in large part, based on ambiguous evidence. Existing data on radiocarbon-dated and identified fossil wood and macrofossils from Alaska and northwest Canada are synthesized here and are augmented by results of palynological studies in an effort to show the persistence of some, and total extinction of other, tree and large shrub species. Possible dispersal routes taken by species that reinvaded Beringia in postglacial times are also reconstructed from the fossil record. Macrofossil and pollen evidence, when combined with climatic factors, makes cottonwood a good candidate for survival during the last glacial. Larch and aspen are also candidates, though the evidence for them is less positive. Pollen and macrofossils of alder are very scarce in deposits of the last glacial age, and if it survived at all, it was probably in very isolated vegetatively reproducing clones. Shrub birch may have been present in Beringia, but tree birch probably was reintroduced during the Holocene. Spruce also appears to have been absent in Alaska from about 30,000 to 11,500 yr ago and probably reinvaded Beringia from a refugium south of the Laurentide ice sheet.

Evidence for an Early Recent Warm Interval in Northwestern Alaska

A warm interval that began at least 10,000 years ago and lasted until at least 8300 years ago is recorded in the coastal tundra covered area of northwestern Alaska by the presence of fossil wood of tree size or tree species, fossil beaver-gnawed wood found beyond the modern range of beaver, evidence of ice-wedge melting, buried soils, and soils that extend below the top of modern permafrost. Dating of the warm interval is based on eight radiocarbon dates. Although these do not provide tight control for either the beginning or the end, they permit the interpretation that the warm event began at the start of the worldwide, postglacial warming and that it ended at the time of the Anivik Lake glacial readvance in the Brooks Range. If this is correct, the early Recent warm interval and the “postglacial thermal maximum” recognized by Livingstone in the Brooks Range were separated by a period of cooler climate. Deposits 7200 and 3600 years old also record moments when the climate was warmer than at present in coastal northwestern Alaska. Although these may record a continuation of the early Recent warm period, it seems more likely that they represent later and separate brief intervals of warmer climate. A postglacial thermal maximum between 6000 and 3000 years ago is recorded by pollen profiles in the Brooks Range, but is not clearly recorded in the coastal areas of northwestern Alaska. We suggest that as sea level rose to near its present position, the accompanying maritime climate lowered summer temperatures in this coastal area during the time at which areas farther inland were experiencing the high temperatures of the postglacial thermal maximum.

A Pliocene flora and insect fauna from the Bering Strait region

Abstract A flood-plain forest has been preserved beneath a lava flow that invaded the Inmachuk River Valley in the northern part of the Seward Peninsula, Alaska, during the Pliocene Epoch. The fossil flora is of great biogeographic interest because of its position (Fig. 1) in a tundra region about 250 km east of Bering Strait, 75 km south of the Arctic Circle, and 65 km west of the northwestern limit of spruce-birch forest. It provides insight into the history of the development of the circumpolar boreal forest (taiga). A rich arthropod fauna casts light on the phylogeny of several modern insect genera and on the origin of modern tundra faunas. A potassium-argon analysis of the overlying basaltic lava provides our first radiometric age estimate (5.7±0.2 million years) for the Clamgulchian Stage, a Late Tertiary time-stratigraphic unit based on fossil plants and widely recognized in Alaska ( Wolfe and Hopkins 1967) and northeastern Siberia.

Geomagnetic Polarity Epochs: Pribilof Islands, Alaska

A comparison was made between geologic, radiometric, and paleomagnetic methods of dating and stratigraphic correlation on the Pribilof Islands of Alaska. Age relationships based on geomorphology and paleontology agree with those determined using potassium-argon dating and paleomagnetic reversals, permitting the following synthesis of the geologic and geomagnetic history of the Pribilofs: The two islands formed during two pulses of volcanism separated by about 1 m.y. St. George Island was formed during the earlier pulse, which lasted from 2.2 to at least 1.6 m.y. ago. During this short interval of 0.6 m.y. the geomagnetic field changed polarity from an initial reversed direction to a normal direction and then back again to a reversed direction. The normal flows, which are both underlain and overlain by reversed flows, have ages between 2.0 and 1.8 m.y. They were extruded during the Olduvai normal event, a relatively brief interval of worldwide normal geomagnetic polarity within the otherwise reversed Matuyama polarity epoch, which lasted from 2.5 to 0.7 m.y. ago. A second pulse of volcanism, which began about 1 m.y. after the first, produced St. Paul Island. This pulse has continued for the past one third m.y., an interval of normal polarity. Fossiliferous marine sedimentary rocks deposited during the Beringian marine transgression are covered on St. George Island by a reversely magnetized pillow lava 2.14 ± 0.08 m.y. old. Fossiliferous marine sedimentary rocks deposited during a transgression younger than the Anvilian and older than the Kotzebuan transgressions are bracketed at the Einahnuhto Bluffs, St. Paul, between normally magnetized lava flows and are less than 0.4 and more than 0.1 m.y. old.

Geologic history of the continental margin of North America in the Bering Sea

Abstract The North American continental margin beneath the Bering Sea is nearly 1,300 km long and extends from Alaska to eastern Siberia. The margin is a canyon-scarred 3,200–3,400-m high escarpment separating one of the worlds largest epicontinental seas (the shallow Bering Sea) and the Aleutian Basin (the deep-water Bering Sea), a marginal oceanic basin distinguished by having its southern boundary formed by the Aleutian Ridge. Three geomorphic provinces can be recognized: a southeastern province characterized by a gentle continental slope (lacking V-shaped canyons) and an outlying continental borderland (formed by Umnak Plateau); a central province distinguished by a steep canyon-scarred slope, and a northwestern province having a gentler and, apparently, less eroded continental slope. Continuous seismic reflection profiles show that the margin is constructed of three major structural-stratigraphic units: (1) an acoustic basement underlying the outer shelf and upper slope; (2) an overlying main layered sequence; and (3) a stratified rise unit underlying and forming the continental rise at the base of the slope. The existing margin evolved with downbowing and faulting of the acoustic basement, an older margin probably of Late Mesozoic age, consisting in part of well-indurated siltstone and mudstone, in Early Tertiary time. Concomitant with subsidence as much as 1,500 m of main-layered-sequence strata were draped over the basement. Intense canyon cutting, presumed to have been caused by the rapid deposition of unstable masses of riverborn sediment over the outer shelf and upper slope, is thought to have begun in Late Tertiary and Quaternary time. Concurrent with canyon cutting, submarine fans, consisting of turbidites forming the rise unit, accrued at the base of the continental slope. Subsidence of the continental margin during the Tertiary may be related to foundering (“oceanization”) of a continental block to form the Aleutian Basin, or to simple isostatic depression of a former segment of the North Pacific oceanic floor in response to sediment infilling north of the Aleutian Ridge.

The structure and origin of the large submarine canyons of the Bering Sea

Abstract Three exceptionally large and long submarine canyons — Bering, Pribilof, and Zhemchug — incise the continental slope underlying the southeastern Bering Sea. Bering Canyon, the worlds longest known slope valley, is approximately 400 km long and has a volume of 4,300 km 3 . The volume of Pribilof Canyon is 1,300 km 3 and that of Zhemchug is 8,500 km 3 ; Zhemchug Canyon may well be the worlds largest slope valley; most other large submarine canyons have volumes less than 500 km 3 . Pribilof and Zhemchug canyons are further distinguished by the headward bifurcation of their slope axes to form elongated trough-shaped basins behind the regionally projected position of the shelf edge. These troughs are superimposed over structural depressions formed by down-faulted basement rocks of Mesozoic and older ages. Prior to canyon cutting these depressions were filled with as much as 2,600 m of shallow-water diatomaceous, tuffaceous, and detrital sediments largely of Tertiary age. Deposition of these sediments took place concurrently with general margin subsidence of at least 2,000 m. The data and conclusions presented in this paper stress that the location, trend, and shape of the enormous submarine canyons cutting the Bering margin are structurally determined. However, axial cutting and headward erosion within the relatively unconsolidated Tertiary strata and the older, lithified basement rock is thought to have been caused by basinward-sliding masses of sediment; these unstable sediment bodies accumulated on the upper continental slope and outer shelf, probably near the mouths of major Alaskan rivers. Bering Canyon was periodically cut and filled by axial sedimentation during Late Tertiary and Quaternary time. Pribilof and Zhemchug canyons, however, are thought to have been excavated entirely during the Pleistocene. It is presumed that, during one or more periods of glacially lowered sea level, the Kuskokwim and Yukon rivers emptied into or near the heads of Pribilof and Zhemchug canyons. The enormous size and unusual shape of Zhemchug Canyon resulted from the breaching of the seaward wall of an outer-shelf basement depression and the subsequent removal of nearly 4,500 km 3 of Tertiary deposits filling it.

Canyon Creek: A late Pleistocene vertebrate locality in interior Alaska

The Canyon Creek vertebrate-fossil locality is an extensive road cut near Fairbanks that exposes sediments that range in age from early Wisconsin to late Holocene. Tanana River gravel at the base of the section evidently formed during the Delta Glaciation of the north-central Alaska Range. Younger layers and lenses of fluvial sand are interbedded with arkosic gravel from Canyon Creek that contains tephra as well as fossil bones of an interstadial fauna about 40,000 years old. Solifluction deposits containing ventifacts, wedge casts, and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of Donnelly Glaciation about 25,000–17,000 years ago. Overlying sheets of eolian sand are separated by a 9500-year-old paleosol that may correlate with a phase of early Holocene spruce expansion through central Alaska. The Pleistocene fauna from Canyon Creek consists of rodents (indicated by burrows), Mammuthus primigenius (woolly mammoth), Equus lambei (Yukon wild ass), Camelops hesternus (western camel), Bison sp. cf. B. crassicornis (large-horned bison), Ovis sp. cf.O. dalli (mountain sheep), Canis sp. cf. C. lupus (wolf), Lepus sp. cf. L. othus or L. arcticus (tundra hare), and Rangifer sp. (caribou). This assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites. Camelops evidently was present in eastern Beringia during the middle Wisconsin interstadial interval but may have disappeared during the following glacial episode. The stratigraphic section at Canyon Creek appears to demonstrate that the Delta Glaciation of the north-central Alaska Range is at least in part of early Wisconsin age and was separated from the succeeding Donnelly Glaciation by an interstadial rather than interglacial episode.

Cretaceous, Tertiary, and Early Pleistocene Rocks from the Continental Margin in the Bering Sea

Rocks dredged from the continental margin in eastern Bering Sea in and near the Pribilof Canyon indicate that the acoustic basement represents the upper surface of thoroughly lithified turbidite beds of graywacke and siltstone of Late Cretaceous age. The stratified sequence covering the acoustic basement is gently deformed and includes marine clastic and diatomaceous sediments ranging in age from middle or late Miocene through early Pleistocene. Dense argillite, siltstone, and calcareous sandstone of early Tertiary age in and near the Zhemchug Canyon probably represent an older part of this sequence. The main layered sequence accumulated above the acoustic basement in shallow water, and, because the older beds now lie as much as 1000 m below sea level, the continental margin must have undergone considerable subsidence during late Tertiary and Quaternary time. A rich pollen flora indicates that the shoreline lay only a few tens of kilometers away from the site of the Pribilof Canyon during late Miocene time.