A sedimentary insight into the coastal and environmental evolution of Hollendarbukta, Isfjorden: Early Holocene to present

Abstract

Dramatic physical and biological shifts are occurring in the Arctic in response to rising surface air temperatures. Sedimentary coastal environments are particularly sensitive to these changes as they form at the interface between the terrestrial and shallow marine realm. The need to investigate sedimentary archives from coastal environments is crucial, to understand how they have responded during periods of abrupt climate change and elucidate how they may evolve in the future. This study investigates the raised marine sediments exposed within Hollendarbukta, Isfjorden, to document sedimentological and geomorphic evidence for the coastal and environmental evolution of the study site from the Early Holocene to present.

The data collected from four outcrops along Hollendardalen and Bogebekken is compiled to propose a three-phase evolutionary model of the coastal environment. The initial stage of development (c. 11.0-9.4 cal. ka BP) is recorded at Hollendardalen by the progradation of an embayed fjord side delta. The coarsening upwards sequence of sediments contains five sub- environments that progressively shallow and include: prodelta, lower delta slope, upper delta slope, delta front beach and backshore deposits. In a distal shoreline setting of the embayed fjord side delta, a glacimarine bottomset outcrops along the raised marine terrace in Bogebekken. Hypopycnal sediment plumes, surge-type turbidity currents and periods of ice- rafting governed sedimentation during this phase of delta progradation. The results align with a rapidly falling relative sea level during the Early Holocene acting as a governing regime control, alongside a strongly seasonal climate and dominant fluvial regime modulating a high sediment influx of fines and freshwater to the shallow marine realm.

The second stage of evolution occurred between c. 9.4-7.9 cal. ka BP. High sedimentation during the Early Holocene promoted rapid filling of the coastal basin and caused the transition from an embayed fjord side environment to a higher energy, linear coastline. This corresponds with the Holocene Thermal Maximum where thermophilous Arctica islandica molluscs in the marine terrace at Bogebekken, indicate that the shallow marine environment was subject to warmer than present Atlantic Water intrusion. This study suggests that shallowing of the coastal basin during shoreline progradation, was concurrent with a transition to a higher energy linear shoreline, that promoted favourable conditions for Atlantic Water intrusion into the shallow marine environment at the study site. Whilst warmer and improved environmental conditions allowed Arctica islandica to thrive, the shoreline was likely too high energy and too shallow for foraminifera to inhabit the area.

The final stage of coastal evolution occurred from c. 7.9 cal. ka BP to present. Progradation of the beachface and backshore continued and approached the present-day shoreline at Hollendarbukta. In conjunction with slowing glacio-isostatic uplift during the Middle to Late Holocene, the braided fluvial system of Hollendarelva and Bogebekken migrated throughout the entire valley. Uplift was significant enough for the braided river to cannibalise the raised marine sediment and incise c. 8-13 m marine terraces within Hollendarbukta. Ultimately, resulting in the present-day system characterised by an extensive braidplain that feeds into the wave dominated, micro-tidal and coarse-grained shoreline at Hollendarbukta.