Stratigraphy, Extent, and Paleoclimatic Implications of Middle Pleistocene Glacial/Interglacial conditions, Coastal Northwest Alaska

Extensive glaciation of Kotzebue Sound approximately 350 ka BP (marine oxygen isotope stage 10) was initiated during the waning stages of one of the warmest interglacial intervals in the last one million years (marine oxygen isotope stage 11; 410 ka). Coastal bluffs surrounding Kotzebue Sound, Hotham Inlet, and Selawik Lake provide excellent exposures of interglacial marine sediments overlain by prodeltaic glaciomarine sediments, representing the onset of extensive glaciation in the western Brooks Range when sea level may have been 23 meters above present along the coast of western Alaska. Glaciofluvial outwash and subglacial diamicts overlying the glaciomarine unit were deposited as eustatic sea level progressively dropped and glacial ice advanced into Kotzebue Sound, forming Baldwin Peninsula, an extensive terminal push-moraine. Weathering limits and ice marginal features (kame terraces) indicate that valley glaciers terminating in Kotzebue Sound were relatively thin and characterized by low basal shear stress. Relative age estimates for these events are based upon amino acid geochronology of enclosed molluscs, paleomagnetic data, geomorphic expression, and tephrachronology (presence of Old Crow tephra and an unknown tephra). Subsequent glaciations of the local mountains and western Brooks Range during marine isotope stages 8, 6, 4, and 2, were of limited extent and restricted to the mountain fronts.

Loess, thaw lake deposits, and in situ peats that discontinuously overlie the middle Pleistocene glacial sequence record terrestrial conditions since oxygen isotope stage 10. Infinite 14C age estimates on wood and peat, coupled with new discoveries of the Old Crow tephra (c. 140 ka) in two disjunct thaw lake sections have aided the identification of sequences recording the last interglacial.

Circumstances leading to extensive glaciation throughout western Alaska during Stage 10 were enhanced, we believe, by a lengthy period of low winter latitudinal insolation gradients (the difference between December 60° N and 30° N insolation receipts) during the middle Pleistocene interval (350-450 ka). Exceptionally warm oceanographic conditions over the North Pacific and Bering Sea coupled with reduced winter and summer sea ice conditions during the waning phase of interglacial stage 11 allowed the advection of moisture to extensively glacierize western Alaska. Regional aridity and limited glaciation since that time are the result of the progressive expansion of the North Pacific Subarctic Gyre causing southward displacement of the Subtropical Gyre and increased Bering Sea ice cover.

Resulting Publications

Master' Thesis work of Matt Huston in:
Huston, M.M., Brigham-Grette, J., and Hopkins, D.M., 1990, Paleogeographic significance of middle Pleistocene glaciomarine deposits on Baldwin Peninsula, Northwest Alaska, Annals of Glaciology, v. 1, p. 111-114.

Steve Roof's PhD work:
Roof, S. R., 1995, Sedimentology, Stratigraphy and Paleoclimatic significance of Middle Pleistocene Marine, Glaciomarine, and Glacial Deposits in the Kotzebue Sound Region, Northwestern Alaska, Dept of Geosciences, UMass Dissertation. 169 pages.

Roof, S.R., Comparison of isoleucine epimerization and Leaching potential in the molluskan genera Astarte, Macoma, and Mya. Geochimica et Cosmochimica, in press.

Vladimir S. Pushkar, Steven R. Roof, Marina V. Cherepanova, David M. Hopkins, Victor F. Ivanov, Julie Brigham-Grette, Paleogeographic and Paleoclimatic Significance of Diatoms from Middle Pleistocene Marine and Glaciomarine Deposits on Baldwin Peninsula, Northwestern Alaska (in review).

Abtracts Presented at Meetings

• Brigham-Grette, J. and Hopkins, D.M., 1991, The first paleomagnetic evidence from the Cape Deceit : a critical late Pliocene/early Pleistocene mammal, plant macrofossil, and beetle locality in central Beringia,. Geological Society of America Abstracts with Programs, v. 23(5), p. A99.
• Mulvey, K.B., and Brigham-Grette, J., 1994, Paleoecological Reconstruction of last interglacial (Substage 5e?) peat layers from the Kotzebue Sound area, northwestern Alaska, 24th Arctic Workshop Abstracts Volume, INSTAAR, University of Colorado, Boulder, p.141-142.
• Roof, S.R., 1994, Sedimentology and Stratigraphy of Middle Pleistocene interglacial to glacial transition, Baldwin Peninsula, Alaska, International Conference on Arctic Margins, Magadan, Sept. 6-10, in press.
• Roof, S.R. and Brigham-Grette, J., 1992, A new Old Crow Tephra locality on Baldwin Peninsula, Northwestern Alaska, 22nd Annual Arctic Workshop, Boulder, CO, Mar. 5-7, Abtract Vol., p. 120-121.
• Roof, S.R., and Brigham-Grette, J., 1992, Paleoclimatic Significance of Middle Pleistocene glacial deposits in the Kotzebue Sound Region, Northwest coastal Alaska, Geological Society of America Abstracts with Programs, v.24,n.7, A314.
• Roof, S.R., and Brigham-Grette, J., 1994, Are glaciations in the Beringian Arctic restricted by Middle Latitude ice volume?, 24th Arctic Workshop Abstracts Volume, INSTAAR, University of Colorado, Boulder, p.141-142.p. 79-80.
• Roof, S.R., and Brigham-Grette, J., 1994, Strong Autun latitudinal insolation gradient and high sea level provided favorable conditions for middle Pleistocene glaciation in central Beringia, AAAS Arctic Science Conference, Abstracts volume, Anchorage/Vladivostok, Aug. 25-Sept. 2.

Notes on PHYSICAL COLLECTIONS

Tephra
Two different tephras have been identified from the field area. Samples were sent to Jim Beget, University of Alaska-Fairbanks, for geochemical work and comparison with his Alaskan tephra database. They are:

Tephra Name	Approximate Age	Comment
Old Crow Tephra	149 ka BP	establishes three new localities
Aniakchak Tephra	3600 yrs BP	now most northern locality known

Bones
Vertebrate remains include several mammoth teeth, cetacean and walrus bones. One exceptionally well-preserved adult mammoth tooth was donated to the National Park Service in Kotzebue for public display.

Shells
Over 200 shells from the collections have been processed for amino acid geochronology.

Sediment
18 samples were processes and analyzed for ostracodes by Dr. Ellie Brouwers, USGS-Denver.

Macrofossils and Beetles
Under contract, 23 samples were processed and analyzed by Dr. Robert Nelson, Colby College.

Pollen
Under contract, 40 samples were processed and analyzed by Dr. Pat Anderson, University of Washington.