Holocene environmental changes inferred from pollen record of Nordenskiöld Land alluvium sequences (West Spitsbergen Island): new data and review

The central part of West Spitsbergen, Nordenskiöld Land, is characterized by comparably small extension of glaciers, high landscape diversity and the long-term development of river valleys. In doing so the number of objects suitable for paleobotanical, in particular, palynological research is limited. Holocene climate and vegetation in numerous studies were reconstructed by using palaeobotanical data from lake sediments and peat sequences. Fluvial sediments are widespread and include both terrigenous and organic deposits, but studies focussing on alluvium archives are rare. Such records relate to Coles and Gröndalen valleys.

During the researches of the Russian Arctic expedition in the Svalbard archipelago in 2019, the outcrop of marine sediments overlain by an alluvial stratum (with a general thickness of 4.2 m) was found and studied on the right slope of Semmeldalen valley (16 m a.s.l.). The sediments are represented by sand and silt with Mytilus edulis shells in situ (0.2 m), which are covered by gravel-pebble material (2.0 m), followed by stratum of interlayered silt, sand, loam with plant remains lenses and layers (2.0 m). The laboratory studies included radiocarbon dating and pollen analysis. Radiocarbon dating results show that the studied deposits were formed in the period from 9300 to 3500 cal BP.

According to pollen data, six stages of vegetation and climate changes were distinguished.  The first stage - about 9300 - 9000 cal. BP corresponds to the stage of sedimentation in a shallow sea bay under relatively favourable environmental conditions. The deposits contain rare microfossils of poorly preserved shrub forms. The almost total absence of Quaternary pollen and a spore in the second stage - gravel-pebble sequence - reflects a high rate of sedimentation in the river mouth during sea regression.  About 8700 cal BP (stage 3) the subshrub-sedge tundra developed in a relatively warm and humid climate. Following (about 8300 cal BP) it changed by the willow-sedge tundra (stage 4). The low content of microfossils at this stage is evidence of an increase in river runoff and, probably, an increase in the amount of atmospheric precipitation. Most probably study records contain a hiatus in sedimentation between 8000 - 4000 cal BP. The fifth stage is the increase in pollen amount and development of the willow-motley-grass tundra. The sixth stage reflects modern vegetation - willow-grass tundra.

The obtained dates and lithology description allow us to make a preliminary conclusion that a sharp decrease in sea level occurred about 9000 cal BP, thereby determining a radical restructuring of the natural environment of the study area. Preliminary results compared with published data show that there are local differences in valley development and environmental conditions changes in Central Svalbard during the Holocene.