Sedimentary processes and paleoenvironments in St. Jonsfjorden, western Spitsbergen

Abstract

Multibeam (swath) bathymetry, high-resolution two-dimensional seismic (chirp) profiles and four gravity cores have been analyzed to reconstruct the Late Weichselian and Holocene sedimentary environments and glacier dynamics in St Jonsfjorden, central western Spitsbergen (Svalbard). The sediment cores comprise stratified glacimarine sediments with varying amounts of ice rafted debris. Three cores terminate in dense diamict units; in the outer fjord (unit 956-1) this is interpreted as being of an ice rafted origin while in the inner fjord (units 953-1 and 954-1) this is interpreted as ice-contact till. Fast-flowing, grounded ice draining the Svalbard-Barents Sea Ice Sheet during the Last Glacial is suggested to have formed flutes, drumlins and glacial lineations that are, in part, controlled by a complex underlying bedrock basement. Clusters of regularly spaced recessional moraines, interpreted to have been deposited annually, on the north shore of the outer fjord indicate that ice retreated stepwise during the last deglaciation. Thick packages of seismoacoustically stratified sediments in the outer fjord likely bury any indication of grounded ice in this area. However, the presence of glacial lineations in Forlandsundet is interpreted as evidence that ice flowed out of St Jonsfjorden and south through Forlandsundet during the Weichselian. The mouth of the fjord was deglaciated by 12,625 cal BP; the ice margin had retreated inward to a bend in the fjord’s orientation (11.2 km from the fjord mouth) by 8,960 +/- 70 cal BC and to 15 km from the fjord mouth – the location of a broad, shallow ridge – prior to 7,840 +/- 90 (Forman 1989, re-calibrated herein), exposing open water in the inner fjord and providing a suitable habitat for marine microfauna. During this ice retreat, sediments may have been deposited on the floor of the inner fjord as five transverse zones of elevated topography, which were subsequently reworked during later Holocene advances of the Osbornebreen-Konowbreen glacier complex that occupied the fjord head. During the Little Ice Age, and possibly also before, ice flowed to but not past a broad shallow ridge expected to be at least partially made of bedrock. Reworked foraminiferal tests from a gravity core (HH12-954) taken east of this ridge’s crest provide an AMS date of 2030 +/- 113 cal BP, indicating that glacial advance reached the ridge crest after this point and is herein interpreted as a neoglacial advance which lasted until the Little Ice Age. Annual recessional moraines in the inner fjord indicate that the post-Little Ice Age deglaciation of St Jonsfjorden may have begun between 1840 and 1860, and that it occurred at a relatively constant rate since (average: 50 m/year); this rate is only slightly higher than the rate at which Osbornebreen has retreated on the period 1909-1986, known from historical photographs (average: 46 m/year). Lead decay-derived sedimentation rate estimates from a gravity core in the inner fjord, 2.2 km east of the ridge crest, indicate that the stratified glacimarine sediments of unit 953-3 are likely to represent sedimentation after the 1850s, supporting this estimate.