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Climate of the Past An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Oct 2018

Research article | 05 Oct 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).

The onset of Neoglaciation in Iceland and the 4.2 ka event

Áslaug Geirsdóttir1, Gifford H. Miller1,2, John T. Andrews2, David J. Harning1,2, Leif S. Anderson1,3, and Thor Thordarson1 Áslaug Geirsdóttir et al.
  • 1Faculty of Earth Science, University of Iceland
  • 2NSTAAR/Department of Geological Sciences, University of Colorado Boulder
  • 3Department of Earth Sciences, Simon Fraser University, CANADA

Abstract. Strong similarities in Holocene climate reconstructions derived from multiple proxies (BSi, TOC, δ13C, C/N, MS, δ15N) preserved in sediments from both glacial and non-glacial lakes across Iceland indicate a relatively warm early-to-mid Holocene from 10 to 6ka, overprinted with cold excursions presumably related to meltwater impact on North Atlantic circulation until 7.9ka. Sediment in lakes from glacial catchments indicates their catchments were ice-free during this interval. Statistical treatment of the high-resolution multiproxy paleoclimate lake records shows that despite great variability in catchment characteristics, the 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 ~5ka in high-elevation interior sites, but is variably later in sites closer to the coast, suggesting some combination of changing ocean currents and sea ice modulate the impact from decreasing summer insolation. The timing of glacier inception during the mid-Holocene is determined by the decent 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 glacial systems particularly in coastal settings. The glacial response to the ELA decline is also highly dependent on the local topography. The initial nucleation of Langjökull in the highlands of Iceland starting by ca 5ka, was followed by a stepwise expansion of both Langjökull and northeast Vatnajökull between 4.5 and 4.0ka, with a second abrupt expansion ca. 3ka. However, the initial appearance of Drangajökull in the NW of Iceland was delayed until after 2.5ka. All lake records reflect abrupt summer temperature and catchment disturbance at about 4.5ka, statistically indistinguishable from the ~4.2ka event with a second widespread abrupt disturbance centered on 3.0ka. Both are intervals of large explosive volcanism on Iceland. The most widespread increase in glacier advance, landscape instability, and soil erosion occurred shortly after 2ka, likely due to a complex combination of increased impact from volcanic activity, cooling climate, and increased sea ice off the coast of Iceland. All lake records indicate a strong decline in temperature ~1.5ka, culminating during the Little Ice Age between 1300 and 1900CE when most glaciers reached their maximum dimensions.

Áslaug Geirsdóttir et al.
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Áslaug Geirsdóttir et al.
Áslaug Geirsdóttir et al.
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Short summary
Compositing climate proxies in sediment from 7 Iceland lakes documents abrupt summer cooling between 4.5 and 4.0 ka, statistically indistinguishable from the 4.2 ka. Although the decline in summer insolation was an important factor, a combination of superposed changes in ocean circulation and explosive Icelandic volcanism were likely responsible for the abrupt perturbation recorded by our proxies. Lake and catchment proxies recovered to a colder equilibrium state following the perturbation.
Compositing climate proxies in sediment from 7 Iceland lakes documents abrupt summer cooling...