Recurrent outburst floods and explosive volcanism during the Younger Dryas–Early Holocene deglaciation in south Iceland: evidence from a lacustrine record

Abstract

Lake sediment records give valuable insight into the dynamic events that characterized the last deglaciation in Iceland. Here, we focus on the well‐dated sediment record from Hestvatn, a low‐elevation lake in south Iceland, that features six graded bedding events deposited by outburst floods from glacial lakes dammed by the decaying Iceland Ice Sheet (IIS) in the time period of the Vedde Ash and the G10ka Series tephra. Using climate proxies preserved in the sediment cores, in conjunction with regional glacial geomorphology, we reconstruct the retreat of the IIS in south Iceland, from a marine‐based glacier during the Younger Dryas to a land‐based glacier during the Preboreal. As the ice sheet margin withdrew to the central highlands, ice‐dammed lakes formed along glacier margins. The ice‐dams were occasionally breached, generating large‐scale jökulhlaups (catastrophic outburst floods) that deposited thick turbidite sequences preserved in the sediment record of Hestvatn. The high concentration of volcanic material incorporated within deglacial sediments indicates that along with IIS retreat, subglacial volcanic activity may have helped initiate some of the jökulhlaups. Onset of more stable Holocene conditions was reached after the final turbidite at ~10 ka bp, when the IIS had withdrawn from most of the highlands of Iceland.

Delayed postglacial colonization ofBetulain Iceland and the circum North Atlantic

“…We note that the ∼12000 BP Vedde Ash has been previously identified in this record based on major oxide composition (Björck et al, 1992). However, the age of the Vedde Ash is stratigraphically too old for the radiocarbon-based chronology, suggesting that this tephra layer likely represents ash from a separate and subsequent volcanic eruption that produced the Vedde Ash, of which several possible correlations have been identified in Iceland and mainland Europe (e.g., Pilcher et al, 2005; Kristjánsdóttir et al, 2017; Matthews et al, 2011; Geirsdóttir et al, 2022). For the sed aDNA record, the Early Holocene portion of lake sediment core (Alsos et al, 2021) features two age control points, the so-called “Saksunarvatn Ash” (10267 ± 89 BP) and a graminoid radiocarbon age (6620 ± 120 BP, Alsos et al, 2021).…”

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

“…Surprisingly, the tephra record from Hestvatn, which is located close to many of the active volcanic centers in south Iceland only archives 38 distinct volcanic events (Fig. 6, Geirsdóttir et al, 2022). The records from west (Haukadalsvatn) and northwest Iceland (Vestfirðir) contain 37 and 27 tephra layers, respectively (Harning et al, 2018(Harning et al, , 2019.…”

“…These empirical records have analyzed physical properties for glacier history (Larsen et al 2011;Striberger et al, 2012;Harning et al, 2016aHarning et al, , 2016b, pollen, macrofossils, and sedimentary ancient DNA (sedaDNA) for plant community (Rundgren, 1995(Rundgren, , 1998Hallsdóttir and Caseldine, 2005;Gathorne-Hardy et al, 2009;Eddudóttir et al, 2015Eddudóttir et al, , 2016Alsos et al, 2021;Geirsdóttir et al, 2022;Harning et al, 2023), bulk geochemistry for soil erosion and diatom productivity (Geirsdóttir et al, 2009(Geirsdóttir et al, , 2013(Geirsdóttir et al, , 2019(Geirsdóttir et al, , 2020(Geirsdóttir et al, , 2022Larsen et al, 2012;Blair et al, 2015;Harning et al, 2018a;Tinganelli et al, 2018;Bates et al, 2021), lipid biomarkers for fire activity (Ardenghi et al, 2024), and chironomids and lipid biomarkers for quantitative temperature (Caseldine et al, 2003;Axford et al, 2007Axford et al, , 2009Langdon et al, 2010;Holmes et al, 2013;Harning et al, 2020;Richter et al, 2020). The Holocene Thermal Maximum (7900 to 5500 cal a BP, Caseldine et al, 2006;Geirsdóttir et al, 2013) has been a particular focus as it provides a potential analogue for future environmental change -current estimates suggest that summer temperatures were ~3 o C warmer than present (Flowers et al, 2008;Harning et al, 2020).…”

Delayed postglacial colonization of Betula in Iceland and the circum North Atlantic

“…We note that the ∼12000 BP Vedde Ash has been previously identified in this record based on major oxide composition (Björck et al, 1992). However, the age of the Vedde Ash is stratigraphically too old for the radiocarbon-based chronology, suggesting that this tephra layer likely represents ash from a separate and subsequent volcanic eruption that produced the Vedde Ash, of which several possible correlations have been identified in Iceland and mainland Europe (e.g., Pilcher et al, 2005;Kristjánsdóttir et al, 2017;Matthews et al, 2011;Geirsdóttir et al, 2022). For the sedaDNA record, the Early Holocene portion of lake sediment core (Alsos et al, 2021) features two age control points, the so-called "Saksunarvatn Ash" (10267 ± 89 BP) and a graminoid radiocarbon age (6620 ± 120 BP, Alsos et al, 2021).…”