Post-glacial relative sea-level history of northwestern Spitsbergen, Svalbard
STEVEN L. FORMAN
Center for Geochronological Research Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0450
GSA Bulletin (1990) 102 (11): 1580-1590
Abstract
The late Weichselian and Holocene raised-beach sequences on northwestern Spitsbergen provide a detailed record of sea-level oscillations during the last deglacial hemicycle. Initial emergence commenced at ≥13 ka, coincident with local deglaciation, and until 10.5 ka was characterized by a relative slow rate of emergence (1.5 to 5 m/ka) interrupted by minor stillstands or transgressions associated with possible glacier re-advances. The orientation of paleo-spits and the paucity of whale remains at the late Weichselian marinelimit indicate that the prevailing westerly fetch was dampened by a semi-permanent sea-ice cover, allowing long shore-drift to predominate. A period of accelerated emergence (15 to 30 m/ka) between 10.5 and 9 ka is correlative with rapid deglaciation of fiords. The middle and late Holocene is characterized by two brief transgressions: one between 6.5 and 5.0 ka that did not exceed 7 m asl and a present, probable slow rise in sea level that commenced 2 to 1 ka.
The strandline tilts in northwestern Spitsbergen, and the pattern and rate of emergence on Svalbard indicate that the maximum glacier loads during the late Weichselian were centered on Nordaustlandet, and on the central and eastern part of the archipelago. Northwestern and southwestern Spitsbergen were marginal to glacial loading and were deglaciated early (≥13 ka) which is presently incompatible with maximum ice sheet extent and the timing and position of deglacial margins recently reconstructed for the Barents Sea and Svalbard. Mountain ranges and large fiords probably confined major outlet glaciers to the central part of the archipelago, preventing inundation of much of western Spitsbergen by an inferred large ice sheet based in eastern Svalbard and the Barents Sea.
GeoRef Subject: absolute age ,carbon , geomorphology , C-14geochronology, Cenozoic, Arctic regiong, lacial geology, Holocene, Pleistocene, Svalbard, isotopesupper, Pleistocene, sediments, Weichselian, peat, radioactive isotopes, Spitsbergen, Quaternary.
INTRODUCTION
During the past decade, the Svalbard archipelago and the adjacent Barents Sea Shelf have been recognized as important source areas for late Weichselian ice sheets in the Northern Hemisphere (Boulton, 1979; Danilov, 1980; Denton and Hughes, 1981; Elverhai and Solheim, 1983; Grosswald, 1980; Vorren and Kristoffersen, 1986; Vorren and others, 1988). Sedimentologic analysis of cores from the Barents Shelf indicates that much of this area was covered by a grounded ice sheet (Elverhai and Solheim, 1983). Marine geologic studies on the shelf, west and north of Spitsbergen, identified moraine morphologies to the depth of 250 m and 15 to 75 km offshore; the age of these features, however, remains unknown (Liestdl, 1972; Ohta, 1982). Glacial geologic investigations on the northwestern forelands of Spitsbergen show that the expansion of late Weichselian glaciers was relatively diminutive, advancing no more than 15 km beyond their present position (Forman, 1989). In contrast, Isfjorden and Van Mijenfjorden, major fiords in the central part of the archipelago, supported sizable outlet glaciers which were probably fed from an ice sheet based in eastern Svalbard and the Northern Barents Sea (Mangerud and others, 1987; Svendsen, J.-I., 1988, personal commun.)
The forelands of western Spitsbergen contain multiple generations of raised-beach sequences that were deposited during episodic glacialisostatic sea-level events in the late Pleistocene (Forman and Miller, 1984). The lowest and best preserved sequence was formed with glacialisostatic unloading during the late Weichselian and Holocene. These youngest raised-beach deposits are spatially ubiquitous and offer a high-fidelity record of timing and rate of sealevel change during the last deglacial hemicycle. This paper presents new radiocarbon dates on whalebone, shell, and peat from localities on northwestern Spitsbergen that further constrain the timing of high sea-level events during the late Weichselian and Holocene. Analysis of three new relative sea-level curves from this area in conjunction with previously reported emergence data provide a first-order approximation of the nature of glacial loading during the late Weichselian on Svalbard.
Geological Society of America Bulletin, v. 102, p. 1580-1590, 14 figs., 1 table, November 1990.
METHODS
The elevation of raised beaches and finds of whalebone, shell, or peat were determined by a Wallace and Tiernan altimeter (precision ±1 m) with the high-tide mark as the calibration datum. The tidal range on western Spitsbergen is 1 to 2 m, with the dominant fetch from the west, building storm beaches up to 5 m above present sea level (asl).
All whalebone radiocarbon ages are on bone collagen, which is exceptionally well preserved in the cold environment of Svalbard (C. Sullivan, 1984, personal commun.). Isotopic and reservoir corrections on 14C ages are identical to those reported in Forman and others (1987), Forman (1989), and Miller and others (1989). Radiocarbon ages on whalebone and shell are corrected for fractionation of carbon isotopes by normalizing to a S13C of -25°/oo- The oceanic reservoir effect is corrected by subtracting 425 yr from 13C corrected ages on shells and 300 yr from whalebone ages (Olsson, 1980).
RELATIVE SEA-LEVEL RECORD
Well-preserved and extensive late Weichselian and Holocene raised beaches occur below 50 m asl on northwestern Spitsbergen. The elevational limit of late Weichselian raised-beach deposits was determined, as in previous studies, from the degree of terrace dissection, the preservation of individual shorelines, and the extent of soil development (Forman and Miller, 1984;
Figure 1. Svalbard, study sites and reconstructed late Weichselian glacier limits, coastline geometry and contour (m) of the late Weichselian
marine limit for the Forlandsundet area, northwestern Spitsbergen. White numbers demarcate the elevation (m asl) of the late Weichselian
marine limit (figure modified from Forman, 1989)
Forman and others, 1987). Specifically, raised beaches that are laterally continuous for >200 m and exhibit 10- to 35-cm-thick soil B horizons with secondary silt accumulation (B1 horizon of Forman and Miller, 1984) were deposited during the late Weichselian/Holocene (15 to 5 ka). The elevational limit of late Weichselian deposits is termed the late Weichselian marine limit (LWML). The LWML is commonly demarcated by a broad constructional terrace which represents en echelon accreted storm beach gravels. Radiocarbon dates on biologic remains from raised beaches are used to reconstruct sea-level events. The raised-beach morphology and emergence history is presented for five separate strandflats of Mitrahalvaya, Sarsflyra, Kaffioyra, Daudmannsetyra, and southern Prins Karls Forland (Fig. 1).
Mitrahalveya Morphology. The LWML on the western side of Mitrahalvoya is marked by a wellpreserved compound spit remnant at 20 m asl (Fig. 2). The recurvature of individual strandlines on the spit indicates that it was accreted by waves mostly from the north. This paleo-spit was truncated by a regressional beach-ridge which crests at 15 m asl and was deposited by a paleo-fetch, mostly from the west. This conspicuous ridge has a broad surface (50 to 200 m wide), and relief up to 5 m; whalebones are rare on and above this feature. Between this terrace and 4 m asl, the surface is characterized by numerous, narrow (5 to 10 m) and low-amplitude (0.5 to 1.0 m) regressional strandlines with abundant whalebones. At 4 m asl, there is an abrupt change in terrace morphology to a bedrock surface with a thin veneer of gravels. Below 4 m asl, there is abundant whalebone and other drift-material associated with a 5-m-high, coarse clastic storm-beach kull bone from the paleo-spit surface yielded a 14C age of 13,100 ± 190 yr B.P. (Beta-10968) and was previously reported in Forman and others (1987). Mitrahalveya and other strandflats to the south are outside the late Weichselian glacial limit (Forman, 1989); thus the LWML is synchronous in the Forlandsundet area, and this radiocarbon age is a maximum age-estimate for the LWML. A whale bone collected from the upper broad terrace at 15 m asl dated to 10,450 ± 330 yr B.P. (lab. no. GX-10103). This age and the previous age on the LWML indicates that the emergence was relatively slow (1.5 m/ka) during the latest Weichselian (Fig. 3). Radiocarbon ages from 15 to 4 m asl ranging between 9.5 to 10 ka (Table 1) document a period of accelerated emergence (15 m/ka) during the earliest Holocene (Fig. 3). A whale vertebra from the modern flotsam at 4 m asl yielded the age of 165 ± 75 yr B.P. (no. GX-10771), indicating that sea level has been at least at its present position during the past century.
CONCLUSIONS
On northwestern Spitsbergen, post-glacial emergence commenced > 13 ka and was characterized by relatively slow initial rates of emergence (1.5 to 5 m/ka) with possible minor transgressions or stillstands associated with local glacier re-advances (Landvik and others, 1987). Spit morphologies and a paucity of whale remains at the late Weichselian marine-limit indicate somewhat persistent sea-ice coverage with paleo-wave directions along shore, from the north or south. The rate of emergence increased substantially between 10.5 to 9 ka (15 to 30 m/ka) coincident with final and rapid déglaciation of northwestern Spitsbergen (Forman, 1989). Raised-beach morphologies associated with this later regTessional phase indicate a paleo-fetch from the west, similar to present nearshore conditions.
Two transgressions occurred in the middle and late Holocene. The previously named Talavera Transgression (Forman and others, 1987) did not exceed 7 m asl on northwestern Spitsbergen and occurred between 6.5 to 5.0 ka. A second minor transgression has been underway since 2 to 1 ka, eroding older raised beach deposits, forming high-angle beaches.
The strandline tilt on northwestern Spitsbergen, and the pattern and rate of post-glacial emergence on Svalbard indicate that Nordaustlandet, islands in the Barents Sea, and central Spitsbergen sustained the greatest glacier loads during the late Weichselian. In contrast, northwestern and southwestern Spitsbergen were peripheral to glacial loading and were deglaciated early. The high mountains (>1,000 m) inland and along the west coast and the funneling of outlet glaciers into large fiord systems on Spitsbergen may have protected these areas from an advancing ice sheet from eastern Svalbard and the Barents Sea. The relative low LWML and rates of emergence in the southern area of the archipelago indicate either early dé- glaciation (>13 ka) and/or restricted glaciation of the Northern Barents Sea; neither is presently compatible with a reconstruction of a large ice sheet nor the inferred pattern of déglaciation in the Barents Sea during the late Weichselian (Vorren and others, 1988).
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