High-resolution Holocene record based on detailed tephrochronology from Torfdalsvatn, north Iceland, reveals natural and anthropogenic impacts on terrestrial and aquatic environments

Abstract

Open questions remain around the Holocene variability of climate in Iceland, including the relative impacts of natural and anthropogenic factors on Late Holocene vegetation change and soil erosion. The lacustrine sediment record from Torfdalsvatn, north Iceland, is the longest known in Iceland (∼ 12 000 cal yr BP), and along with its high sedimentation rate, it provides an opportunity to develop high-resolution quantitative records that address these challenges. In this study, we use two sediment cores from Torfdalsvatn to construct a detailed age model derived from marker tephra layers, paleomagnetic secular variation, and radiocarbon (20 age control points). We then apply this robust age constraint to support a complete tephrochronology (> 2200 grains analyzed in 33 tephra horizons) and sub-centennial geochemical (MS, TOC, C / N, δ13C, and BSi) and algal pigment records (chlorins, lutein, diatoxanthin, and canthaxanthin). Along with previously published proxy records from the same lake, these records demonstrate generally stable terrestrial and aquatic conditions during the Early and Middle Holocene, except for punctuated disturbances linked to major tephra fall events. During the Late Holocene, there is strong evidence for naturally driven algal productivity decline beginning around 1800 cal yr BP. These changes closely follow regional Late Holocene cooling driven by decreases in Northern Hemisphere summer insolation and the expansion of sea-ice-laden Polar Water around Iceland. Then at 880 cal yr BP, ∼ 200 years after the presumed time of human settlement, a second shift in the record begins and is characterized by a strong uptick in landscape instability and possibly soil erosion. Collectively, the Torfdalsvatn record highlights the resilience of low-elevation, low-relief catchments to the pre-settlement soil erosion in Iceland, despite a steadily cooling background climate. The precisely dated, high-resolution tephra and paleoenvironmental record from this site can serve as a regional template for lowland regions of north Iceland.