Holocene relative sea-level history of Franz Josef Land, Russia Steven L. Forman* Department of Geological Sciences, University of Illinois, Chicago, Illinois 60607-7059 Richard Weihe Byrd Polar Research Center and Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210-1002 David Lubinski Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0450 Gennady Tarasov Sergey Korsun Murmansk Marine Biological Institute, 17 Vladimirskaya Street, Murmansk, Russia 183023 Gennady Matishov } ABSTRACT INTRODUCTION One of the largest uncertainties in ice-volume changes during the The pattern of postglacial emergence for many areas in the Northern late Quaternary Period is the extent of ice sheets over the Barents and Hemisphere is pivotal in assessing the distribution of past ice-sheet loads Kara seas. Field research on central and eastern Franz Josef Land, and deglacial history. Observations on postglacial emergence are also Russia, provide new observations on postglacial emergence and important for developing a better understanding of the glacio-isostatic deglaciation that further constrain the magnitude and timing of late adjustment process and the constraining properties of the underlying solid Weichselian glaciations. Radiocarbon dating of driftage from raised- Earth (Cathles, 1975; Peltier, 1974). Recent refinements in models of marine sequences place deglacial unloading prior to 9.4 ka. At a num- mantle viscoelastic structure and an improved understanding of the extent ber of localities within 1 to 2 km of the present glacier margin, in situ and chronology of the Laurentide and Fennoscandinavian ice sheets pro- shells from raised-marine sediments yield 14C ages between 9.7 and vide a basis for estimating variations in ice-sheet thickness during the last 8.4 yr B.P., evidence that outlet glaciers were at or behind present deglaciation (Peltier, 1994; Lambeck, 1995). These earth rheological margins by the early Holocene Period. models accommodate site-specific relative sea-level and global eustatic The altitude of the marine limit on Franz Josef Land ranges records (Fairbanks, 1989), providing new insight into the balance between from 49 to 20 m above sea level (asl) and is low compared to eastern glacier cover and changes in global sea-level in the past approximately Svalbard (110 to 60 m asl). The age of the marine limit ranges from 20 000 yr (Tushingham and Peltier, 1991; Peltier, 1994). ³10.4 ka to ca. 6.0 ka and exhibits greater diachrony where emer- One of the largest uncertainties in ice volume changes in the late Qua- gence is <35 m. A low initial rate of emergence reflects glacio-isosta- ternary Period is the timing and geometry of glaciers over the shelf seas tic compensation offset by a eustatic rise in sea level, and perhaps an bordering the Arctic Ocean. Reconstructions of late Weichselian ice-sheet additional component from renewed water loading of the Barents Sea extent in the Barents Sea region range from a contiguous marine-based ice after deglaciation. The presence of raised beaches at 1 to 4 m asl that sheet over much of the European arctic (e.g., Peltier, 1994; Lambeck, are 1 to 2 ka indicates that uplift is incomplete. An exponential 1995), to smaller, coalescent ice caps based on arctic archipelagos (e.g., extrapolation of uplift data for Franz Josef Land and eastern Sval- Siegert and Dowdeswell, 1995; Lambeck, 1995). This disparity in ice bard yields current maximum uplift estimates of 0.7 to 1.6 mm/yr, an sheet reconstructions reflects the paucity of field observations that con- inferred <5.5 m of uplift remaining. These values are at least 50% to strain the extent, thickness, and timing of late Quaternary glacial events in 80% lower than the inferred uplift residual for a modeled ice sheet in northern Eurasia. the Barents and Kara seas. The discrepancy indicates that further re- Critical field observations needed to constrain the magnitude and dis- finement is needed for this ice-sheet model. tribution of past glacier loads and the timing of deglaciation are the height The compilation of emergence data from Franz Josef Land, and age of raised-beach deposits. Raised beaches were initially recog- Svalbard, and Novaya Zemlya confine maximum glacio-isostatic nized on Franz Josef Land, Russia, during early geologic exploration compensation to the Barents Sea; comparatively minor ice sheet (Koettlitz, 1898). The first systematic studies of postglacial emergence on loads were over Novaya Zemlya and the southeastern Barents Sea. Franz Josef Land were undertaken by Dibner (1965) and Grosswald Emergence isobases since 5 and 9 ka descend northward across Franz (1973). They identified extensive raised-beach sequences on Hooker, Josef Land, indicating a diminishing glacio-isostatic response into the Hayes, Alexandra, and other islands in the Franz Josef Land archipelago. Arctic Ocean. Most notably, they collected five driftwood samples from raised beaches for 14C dating, providing the first age constraints (ca. 6000 yr B.P.) on deglaciation and emergence of Franz Josef Land. Only in the past five *e-mail: [email protected] years, with improved access to the Russian arctic, are new assessments GSA Bulletin; September 1997; v. 109; no. 9; p. 1116–1133; 17 figures; 3 tables 1116 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/109/9/1116/3382763/i0016-7606-109-9-1116.pdf by guest on 30 September 2021 SEA-LEVEL HISTORY OF FRANZ JOSEF LAND emerging on the magnitude and timing of Quaternary glaciations in north- ern Eurasia (Glazovskiy et al., 1992; Näslund et al., 1994; Polyak and Solheim, 1994; Polyak et al., 1995; Forman et al., 1995, 1996; Lubinski et al., 1996). This contribution presents seven new postglacial emergence records and stratigraphic assessments on deglaciation for central and southern Franz Josef Land (Fig. 1); in combination with other recent re- sults (Glazovskiy et al., 1992; Näslund et al., 1994; Forman et al., 1995, 1996), these form a basis for an enhanced representation of the pattern of Holocene emergence in the northern Barents Sea. NEAR-SHORE CONDITIONS AND THE RAISED BEACH RECORD Our field research, in August 1992 and 1993, provides insight on the pattern of postglacial emergence and deglaciation for Hayes, Hall, Wilczek, Klagenfurt, Brady, Leigh Smith, Koldewey, Champ, and Wiener Neustadt islands, Franz Josef Land, Russia (Figs. 1 and 2). These studies concentrated on determining the elevational extent of raised-beach land- forms and providing a chronologic and stratigraphic assessment of raised- marine sequences. Radiocarbon dating of drift from raised-beach land- forms and mollusks from stratigraphic sections provide chronologic control to relate geomorphic and stratigraphic records. Geomorphic assessment is derived from field observations using 1:200 000, 1:100 000, and 1:50 000 scale maps; aerial photographs were not available for our field studies. The studied islands, in the central and the southern part of the archipel- ago, are bounded by sounds and fjords that have water depths of >250 m (Matishov et al., 1995). Most of the archipelago (85%) is covered by glac- iers, but all islands studied have low elevation (<100 m asl) forelands par- tially covered by raised-marine sediments. Most sounds and fjords in the archipelago are usually covered by sea ice for 9 to 10 months of the year (Denisov et al., 1993). Sea-ice conditions in the interisland channels dur- ing July and August are variable, ranging from open-water conditions to full sea-ice coverage. Gravel and boulder beaches dominate the present shore of central and eastern Franz Josef Land. Storm-beach gravels and sea-ice–pushed ridges often extend 2 to 3 m above the present high-tide level. The tidal range on Franz Josef Land is approximately 0.5 m (Denisov et al., 1993). All mea- Figure 1. Franz Josef Land in the west Eurasian arctic and loca- surements of altitude for raised beaches are determined in reference to the tion of studied islands within the archipelago. high-tide swash mark (m asl) by a Leitz digital altimeter with an analyti- cal precision of ±0.1 m. The altitude of a landform was measured multiple times by two different altimeters and these determinations usually agree RADIOCARBON DATING within ±1 m. The maximum estimated error in measuring altitude, in- cluding variable relief of a raised-marine landform, is ±2 m. Age constraint on marine inundation and deglaciation is provided by 14C The present altitude of raised beaches on Franz Josef Land reflects dating of driftwood, whalebone, walrus bone, and shell from raised-marine principally two competing processes; the postglacial rise in global sea sediments. Driftwood was preferentially collected from the raised-beach level and isostatic up-warping of the lithosphere with disintegration of the sequences because of its suitability for 14C dating. The outer rings of drift- last ice sheet that occupied the Barents Sea. Global sea level has been rel- wood logs were sampled to obtain 14C ages in close association with the atively stable in the past 6000 yr (Kidson, 1982); thus, raised beach sea-level depositional event (Table 1). If driftwood was not located, whale- elevation attained since the mid-Holocene Period reflects predominantly bone and walrus-skull bone were retrieved for 14C dating. Bones were sec- isostatic compensation. However, in areas where postglacial emergence tioned by saw and an internal, well-preserved dense part of the bone was was modest (<50 m), relatively brief (hundreds to thousands of years) ar- submitted for 14C dating. The collagen-dominated gelatin extract from each rests in sea level or transgressive-regressive events (<2 m) have been doc- bone was dated, which in previous studies has yielded accurate 14C ages umented, reflecting the interplay between eustasy, isostasy, and steric and (e.g., Forman, 1990; Forman et al., 1995). The apatite fraction for two sam- nonsteric changes in sea level (e.g., Hafsten, 1983; Svendsen and ples, a whalebone (GX-18313A) and a walrus-skull bone (GX-18306A), Mangerud, 1987; Forman, 1990, 1996 Fletcher et al., 1993).