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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 (≤12000 cal a BP) and along with its high sedimentation rate, 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 high-resolution age model derived from marker tephra layers, paleomagnetic secular variation, and radiocarbon. 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. 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 a 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 a 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 north Iceland.
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The onset of neoglaciation in Iceland and the 4.2 ka event
Abstract. Strong similarities in Holocene climate reconstructions derived from multipleproxies (BSi, TOC – total organic carbon, δ13C, C∕N, MS – magnetic susceptibility, δ15N)preserved in sediments from both glacial and non-glacial lakes across Icelandindicate a relatively warm early to mid Holocene from 10 to 6 ka,overprinted with cold excursions presumably related to meltwater impact onNorth Atlantic circulation until 7.9 ka. Sediment in lakes from glacialcatchments indicates their catchments were ice-free during this interval.Statistical treatment of the high-resolution multi-proxy paleoclimate lakerecords shows that despite great variability in catchment characteristics,the sediment records document more or less synchronous abrupt, colddepartures as opposed to the smoothly decreasing trend in Northern Hemispheresummer insolation. Although all lake records document a decline in summertemperature through the Holocene consistent with the regular decline insummer insolation, the onset of significant summer cooling occurs ∼5 ka at high-elevation interior sites but is variably later at sitescloser to the coast, suggesting that proximity to the sea may modulate the impactfrom decreasing summer insolation. The timing of glacier inception during themid Holocene is determined by the descent of the equilibrium line altitude(ELA), which is dominated by the evolution of summer temperature as summerinsolation declined as well as changes in sea surface temperature for coastalglacial systems. The glacial response to the ELA decline is also highlydependent on the local topography. The initial ∼5 ka nucleation ofLangjökull in the highlands of Iceland defines the onset of neoglaciationin Iceland. Subsequently, a stepwise expansion of both Langjökull andnortheast Vatnajökull occurred between 4.5 and 4.0 ka, with a secondabrupt expansion ∼3 ka. Due to its coastal setting and lowertopographic threshold, the initial appearance of Drangajökull in the NWof Iceland was delayed until ∼2.3 ka. All lake records reflect abruptsummer temperature and catchment disturbance at ∼4.5 ka, statisticallyindistinguishable from the global 4.2 ka event, and a second widespreadabrupt disturbance at 3.0 ka, similar to the stepwise expansion ofLangjökull and northeast Vatnajökull. Both are intervalscharacterized by large explosive volcanism and tephra distribution in Icelandresulting in intensified local soil erosion. The most widespread increase in glacier advance, landscapeinstability, and soil erosion occurred shortly after 2 ka, likely due to acomplex combination of increased impact from volcanic tephra deposition,cooling climate, and increased sea ice off the coast of Iceland. All lakerecords indicate a strong decline in temperature ∼1.5 ka, whichculminated during the Little Ice Age (1250–1850 CE) when the glaciersreached their maximum Holocene dimensions.
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- Award ID(s):
- 1836981
- PAR ID:
- 10554373
- Publisher / Repository:
- Climate of the Past
- Date Published:
- Journal Name:
- Climate of the Past
- Volume:
- 15
- Issue:
- 1
- ISSN:
- 1814-9332
- Page Range / eLocation ID:
- 25 to 40
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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 kabp, when the IIS had withdrawn from most of the highlands of Iceland.more » « less
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Abstract. Chronologies of glacier deposits in the Transantarctic Mountains provide important constraints on grounding-line retreat during the last deglaciation in the Ross Sea. However, between Beardmore Glacier and Ross Island – a distance of some 600 km – the existing chronologies are generally sparse and far from the modern grounding line, leaving the past dynamics of this vast region largely unconstrained. We present exposure ages of glacial deposits at three locations alongside the Darwin–Hatherton Glacier System – including within 10 km of the modern grounding line – that record several hundred meters of Late Pleistocene to Early Holocene thickening relative to present. As the ice sheet grounding line in the Ross Sea retreated, Hatherton Glacier thinned steadily from about 9 until about 3 ka. Our data are equivocal about the maximum thickness and Mid-Holocene to Early Holocene history at the mouth of Darwin Glacier, allowing for two conflicting deglaciation scenarios: (1) ∼500 m of thinning from 9 to 3 ka, similar to Hatherton Glacier, or (2) ∼950 m of thinning, with a rapid pulse of ∼600 m thinning at around 5 ka. We test these two scenarios using a 1.5-dimensional flowband model, forced by ice thickness changes at the mouth of Darwin Glacier and evaluated by fit to the chronology of deposits at Hatherton Glacier. The constraints from Hatherton Glacier are consistent with the interpretation that the mouth of Darwin Glacier thinned steadily by ∼500 m from 9 to 3 ka. Rapid pulses of thinning at the mouth of Darwin Glacier are ruled out by the data at Hatherton Glacier. This contrasts with some of the available records from the mouths of other outlet glaciers in the Transantarctic Mountains, many of which thinned by hundreds of meters over roughly a 1000-year period in the Early Holocene. The deglaciation histories of Darwin and Hatherton glaciers are best matched by a steady decrease in catchment area through the Holocene, suggesting that Byrd and/or Mulock glaciers may have captured roughly half of the catchment area of Darwin and Hatherton glaciers during the last deglaciation. An ensemble of three-dimensional ice sheet model simulations suggest that Darwin and Hatherton glaciers are strongly buttressed by convergent flow with ice from neighboring Byrd and Mulock glaciers, and by lateral drag past Minna Bluff, which could have led to a pattern of retreat distinct from other glaciers throughout the Transantarctic Mountains.more » « less
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Abrupt climate changes during the last deglaciation have been well preserved in proxy records across the globe. However, one long-standing puzzle is the apparent absence of the onset of the Heinrich Stadial 1 (HS1) cold event around 18 ka in Greenland ice core oxygen isotope δ 18 O records, inconsistent with other proxies. Here, combining proxy records with an isotope-enabled transient deglacial simulation, we propose that a substantial HS1 cooling onset did indeed occur over the Arctic in winter. However, this cooling signal in the depleted oxygen isotopic composition is completely compensated by the enrichment because of the loss of winter precipitation in response to sea ice expansion associated with AMOC slowdown during extreme glacial climate. In contrast, the Arctic summer warmed during HS1 and YD because of increased insolation and greenhouse gases, consistent with snowline reconstructions. Our work suggests that Greenland δ 18 O may substantially underestimate temperature variability during cold glacial conditions.more » « less
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The Holocene hydroclimate evolution and underlying mechanisms modulating the East Asian summer monsoon (EASM) remains controversial, especially in south eastern China. Here we present a multiproxy peat record of monsoon evolution from southeastern China covering the last 14 ka. Our new records show a relatively weaker EASM but wetter hydroclimate during the early (10 to 8 ka) and late Holocene (after 5.4 ka), while a stronger EASM and overall drier climate during the mid-Holocene (8 to 5.4 ka). In line with nearby speleothem records, our results reveal a dominant control of the northern-latitude ice-sheet meltwater forcing on millennial-scale East Asian hydroclimate variability during the last deglaciation and early Holocene. This dominant influence, however, likely waned once the global sea level had stabilized during the mid-to-late Holocene, giving way to other drivers of the monsoon and hydroclimate, including a combination of summer insolation and teleconnection patterns associated with vegetation-dust feedbacks.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/cp-15-25-2019
