The onset of Neoglaciation in Iceland and the 4.2 ka event

Abstract

Abstract. Strong similarities in Holocene climate reconstructions derived from multiple\nproxies (BSi, TOC – total organic carbon, δ13C , C∕N , MS – magnetic susceptibility, δ15N )\npreserved in sediments from both glacial and non-glacial lakes across Iceland\nindicate a relatively warm early to mid Holocene from 10 to 6\u2009ka,\noverprinted with cold excursions presumably related to meltwater impact on\nNorth Atlantic circulation until 7.9\u2009ka. Sediment in lakes from glacial\ncatchments indicates their catchments were ice-free during this interval.\nStatistical treatment of the high-resolution multi-proxy paleoclimate lake\nrecords shows that despite great variability in catchment characteristics,\nthe sediment records document more or less synchronous abrupt, cold\ndepartures as opposed to the smoothly decreasing trend in Northern Hemisphere\nsummer insolation. Although all lake records document a decline in summer\ntemperature through the Holocene consistent with the regular decline in\nsummer insolation, the onset of significant summer cooling\xa0occurs ∼5 \u2009ka at high-elevation interior sites but is variably later at sites\ncloser to the coast, suggesting that proximity to the sea may modulate the impact\nfrom decreasing summer insolation.\xa0The timing of glacier inception during the\nmid Holocene is determined by the descent of the equilibrium line altitude\n(ELA), which is dominated by the evolution of summer temperature as summer\ninsolation declined as well as changes in sea surface temperature for coastal\nglacial systems. The glacial response to the ELA decline is also highly\ndependent on the local topography. The initial ∼5 \u2009ka nucleation of\nLangjokull in the highlands of Iceland defines the onset of neoglaciation\nin Iceland. Subsequently, a stepwise expansion of both Langjokull and\nnortheast Vatnajokull occurred between 4.5 and 4.0\u2009ka, with a second\nabrupt expansion ∼3 \u2009ka. Due to its coastal setting and lower\ntopographic threshold, the initial appearance of Drangajokull in the NW\nof Iceland was delayed until ∼2.3 \u2009ka. All lake records reflect abrupt\nsummer temperature and catchment disturbance at ∼4.5 \u2009ka, statistically\nindistinguishable from the global 4.2\u2009ka event, and a second widespread\nabrupt disturbance at 3.0\u2009ka, similar to the stepwise expansion of\nLangjokull and northeast Vatnajokull. Both are intervals\ncharacterized by large explosive volcanism and tephra distribution in Iceland\nresulting in intensified local soil erosion. The most widespread increase in glacier advance, landscape\ninstability, and soil erosion occurred shortly after 2\u2009ka, likely due to a\ncomplex combination of increased impact from volcanic tephra deposition,\ncooling climate, and increased sea ice off the coast of Iceland. All lake\nrecords indicate a strong decline in temperature ∼1.5 \u2009ka, which\nculminated during the Little Ice Age (1250–1850 CE) when the glaciers\nreached their maximum Holocene dimensions.