The onset of neoglaciation in Iceland and the 4.2 ka event

Abstract

Abstract. Strong similarities in Holocene climate reconstructions derived from multiple proxies (BSi, TOC – total organic carbon, δ13C, C∕N, MS – magnetic susceptibility, δ15N) preserved in sediments from both glacial and non-glacial lakes across Iceland indicate a relatively warm early to mid Holocene from 10 to 6 ka, overprinted with cold excursions presumably related to meltwater impact on North Atlantic circulation until 7.9 ka. Sediment in lakes from glacial catchments indicates their catchments were ice-free during this interval. Statistical treatment of the high-resolution multi-proxy paleoclimate lake records shows that despite great variability in catchment characteristics, the sediment records document more or less synchronous abrupt, cold departures as opposed to the smoothly decreasing trend in Northern Hemisphere summer insolation. Although all lake records document a decline in summer temperature through the Holocene consistent with the regular decline in summer insolation, the onset of significant summer cooling occurs ∼5 ka at high-elevation interior sites but is variably later at sites closer to the coast, suggesting that proximity to the sea may modulate the impact from decreasing summer insolation. The timing of glacier inception during the mid Holocene is determined by the descent of the equilibrium line altitude (ELA), which is dominated by the evolution of summer temperature as summer insolation declined as well as changes in sea surface temperature for coastal glacial systems. The glacial response to the ELA decline is also highly dependent on the local topography. The initial ∼5 ka nucleation of Langjökull in the highlands of Iceland defines the onset of neoglaciation in Iceland. Subsequently, a stepwise expansion of both Langjökull and northeast Vatnajökull occurred between 4.5 and 4.0 ka, with a second abrupt expansion ∼3 ka. Due to its coastal setting and lower topographic threshold, the initial appearance of Drangajökull in the NW of Iceland was delayed until ∼2.3 ka. All lake records reflect abrupt summer temperature and catchment disturbance at ∼4.5 ka, statistically indistinguishable from the global 4.2 ka event, and a second widespread abrupt disturbance at 3.0 ka, similar to the stepwise expansion of Langjökull and northeast Vatnajökull. Both are intervals characterized by large explosive volcanism and tephra distribution in Iceland resulting in intensified local soil erosion. The most widespread increase in glacier advance, landscape instability, and soil erosion occurred shortly after 2 ka, likely due to a complex combination of increased impact from volcanic tephra deposition, cooling climate, and increased sea ice off the coast of Iceland. All lake records indicate a strong decline in temperature ∼1.5 ka, which culminated during the Little Ice Age (1250–1850 CE) when the glaciers reached their maximum Holocene dimensions.

Subdividing the Holocene Series/Epoch: formalization of stages/ages and subseries/subepochs, and designation of GSSPs and auxiliary stratotypes

“…The event appears to be global in nature, occurring in proxy records across seven continents from North America and northern Europe, through the Mediterranean, Middle East (Weiss, , 2017a) and India (Berkelhammer et al ., ; Kathayat et al ., ) to China (Cai et al ., ; Zhang et al ., ) and Australia (Denniston et al ., ); and across Africa (Chase et al ., ; Ruan et al ., ), Andean–Patagonian South America (Schimpf et al ., ), and Antarctica (Peck et al ., ). In the North Atlantic and western Canada, the event is reflected in atmospheric and oceanic cooling (Gkinis et al ., ; Orme et al ., ; Zhang et al ., ) and so‐called “neoglaciation” with glacier readvances (Balascio et al ., ; Geirsdóttir et al ., ; Menounos et al ., ), while in the mid‐ and low latitudes across both hemispheres it is marked by aridification (Booth et al ., ; Wanner et al ., ). Extensive megadrought prevailed across mid‐latitude North America from Idaho to Massachussets, “with median moisture levels reaching a minimum from 4.2 to 3.9 ka” (Shuman and Marsicek, , p. 42).…”

“…2015; Geirsdóttir et al, 2019;Menounos et al, 2008), while in the mid-and low latitudes across both hemispheres it is marked by aridification (Booth et al, 2005;Wanner et al, 2015). Extensive megadrought prevailed across mid-latitude North America from Idaho to Massachussets, "with median moisture levels reaching a minimum from 4.2 to 3.9 ka" (Shuman and Marsicek, 2016, p. 42).…”

Is there evidence for a 4.2 ka BP event in the northern North Atlantic region?

“…Next, we consider studies in the western part of the North Atlantic, north of Iceland on the Icelandic Shelf, and further to the west near Denmark Strait. Here, many studies have examined, inter alia, foraminiferal assemblages, coccoliths, dinoflagellate cysts, and sea ice biomarkers and ice-rafted debris (IRD) reflecting the transport of material in the cold East Greenland Current (e.g., Andrews et al, 1997;Jennings et al, 2002;Giraudeau et al, 2004;Solignac et al, 2006;Sicre et al, 2008;Justwan et al, 2008;Perner et al, 2015;Moossen et al, 2015;Cabedo-Sanz et al, 2016;Kolling et al, 2017). In this region, the warmest conditions occurred around 6.0 ± 1.5 ka BP (the timing depending on location); these conditions were associated with minimal input of IRD, reflecting the recession of tidewater glaciers onto land along the eastern coast of Greenland, and a weak East Greenland Current, with minimal stratification of the water column at that time as the flux of warmer, more saline Irminger Current water increased (Justwan et al, 2008;Jennings et al, 2011;Werner et al, 2014;Telesiński et al, 2014;Perner et al, 2016).…”

“…There is currently no evidence for a more widespread glacial advance at 4.2 ka BP. Given that cooling was persistent over the last 5000 years and the elevational threshold for glacier- ization is close to mountain tops across the region (declining in elevation poleward), it is understandable that different locations would have experienced the onset of neoglaciation at different times (Geirsdóttir et al, 2019). However, as the equilibrium line altitude (ELA) continued to lower over the last 3-4 millennia, glaciers that had greatly diminished in size or disappeared entirely during the warmest period of the Holocene were eventually regenerated, with the exact timing varying across the region.…”

“…Here we review sedimentary records from the northern North Atlantic (north of 60 • N) with a specific focus on whether there is evidence for an "event" around 4.2 ka BP. We do not focus on records from Iceland as these have been reviewed separately by Geirsdóttir et al (2019).…”