In a previous post on PAGES2K Arctic, I pointed out that they had used the Hvitarvatn, Iceland series (PAGES2K version shown below), upside-down to the interpretation of the original authors (Miller et al), who had interpreted thick varves as evidence of the Little Ice Age. A few days ago, Miller and coauthors archived a variety of series from Hvitarvatn, prompting me to review this data.
It turns out that Hvitarvatn is a very interesting lake sediment site for a variety of reasons. It is much more securely dated than the Baffin Island sites; it has been carefully described geologically; a variety of proxies have been calculated for the location; some proxies are available through the Holocene, not just the last 1000 years or so; and, there are a variety of other well-dated proxies near Iceland.
In geophysics, specialists work out from high-quality data to lower quality data, rather than throwing all of the data into a jumble (as paleoclimatologists do). In today’s post, I’m going to parse the Hvitarvatn data, which I will later use to parse the Baffin Island and Ellesmere Island varve data, all of which has become widely used since Kaufman et al 2009 (including PAGES2K, Ljungqvist and most recently Shi et al 2013).
Varve chronologies are determined by simply counting varves – which seems straightforward enough. In Iceland, a series of well-dated volcanic tephra permit a cross-check of varve counting. When reconciled to tephra, one of the varve chronologies was over 100 years short by the medieval period. The possibility of similar errors in varve chronologies in Baffin Island and elsewhere needs to be kept firmly in mind.
Varve Thickness and Glacier Advance
A number of canonical varve sites are from proglacial lakes (e.g. Big Round, Donard and Ayr in Baffin Island; Iceberg Lake in Alaska). Like these other sites, Hvitarvatn is a proglacial lake in the watershed of the Langjökull ice cap. Miller et al observe that varve thickness is controlled by the glacier:
Varve thickness is controlled by the rate of glacial erosion and efficiency of subglacial discharge from the adjacent Langjökull ice cap.
They observe that the Langjokull ice cap had receded during the Holocene optimum and had only advanced to the lake during the last millennium. They dated the start of the Little Ice Age to ~1250AD. They dated a first phase of glacier advance between 1250 and 1500AD, with a second phase commencing ~1750AD and ending only around 1900AD. Miller et al report that, within the entire Holocene, ice-rafted debris occurred only during this second phase, especially during the 19th century.
…The largest perturbation began ca 1250 AD, signaling the onset of the Little Ice Age and the termination of three centuries of relative warmth during Medieval times. Consistent deposition of ice-rafted debris in Hvítárvatn is restricted to the last 250 years, demonstrating that Langjökull only advanced into Hvítárvatn during the coldest centuries of the Little Ice Age, beginning in the mid eighteenth century. This advance represents the glacial maximum for at least the last 3 ka, and likely since regional deglaciation 10 ka.
The two outlet glaciers terminating in Hvitarvatn, Norjurjokull and Suojurjokull, advance slowly into the lake, occupying their maximum lake area in the late 19th century, and retreat comparatively rapidly in the mid- to late 20th century.
we place the start of the LIA in the highlands in the mid-thirteenth century, when Langjökull began a series of two long periods of expansion, with high stands at ca 1500 AD and in the nineteenth century.
This study presents the first continuous record of ice cap extent for the entire Holocene and clearly demonstrates that the LIA contained the most extensive glacial advance of the Neoglacial interval. The strong multi-proxy signal at Hvítárvatn implies that the LIA was the coldest period of the last 8 ka and suggests that is unlikely for any non-surging Iceland glacier to have reached dimensions significantly larger than its LIA maximum at any time during the Holocene.
The figure below shows varve thickness and ice-rafted debris data from Hvitarvatn.
Ice-rafted debris is interpreted as evidence that the glacier had advanced into the lake. It is completely absent from the record through the Holocene Optimum and is rare until the second phase of the Little Ice Age, with maximum intensity in the 19th century with local maxima in 1940 and 1890, before declining rapidly with the 20th century recession of the glacier.
The varve record begins about 3000 BP. Varves thicken quite dramatically commencing about 1250AD, which Miller et al interpret as the start of the Little Ice Age. They interpret 1500-1760AD as a sort of standstill, with, as noted above, thick varves in the second phase (19th century) coinciding with ice-rafted debris. Varves continue to be thick during glacier recession in the 20th century, continuing to be thick into the warm 1930s.
During the Holocene Thermal Maximum (HTM), the recession of the glacier meant that sediments were not varved. Miller et al utilized biogenic silica and information on diatoms to reconstruct HTM warmth, as shown below.
The Hvitarvatn data seems to provide rather secure information glacier advance and retreat, a topic of considerable importance in the older (pre-IPCC) paleoclimate literature (e.g. Matthes 1940).
Bradley and Jones 1993, an article which (rather than Mann et al 1998) did much to initiate the present programme of “multiproxy” studies, referred back to those older literature (Matthes 1940) in its introduction. They argued that records of glacier advance and retreat were compromised by the absence of continuous records and advocated instead reliance only on precisely-dated continuous records (tree rings and ice cores, but, in practice, mostly tree rings). This programme subsequently dominated the field, in part because of IPCC highlighting such studies.
At several sites (Soper Lake and Donard Lake, Baffin Island; Lower Murray Lake, Ellesmere Island), varve thickness has been said to be positively correlated with summer temperature. (Though no significant correlation between varves and temperature has been reported at similar sites: East Lake, Melville Island; Ayr Lake, Baffin Island).
PAGES2K appears to have incorporated Hvitarvatn varve thickness data into their dataset (with thick varves denoting warmth) on the basis of this practice at other Arctic sites. However, the original authors clearly interpreted thick varves as evidencing the Little Ice Age (the existence of which in Iceland is established by numerous indicators.) The PAGES2K version is clearly upside-down to this interpretation.
In an accompanying post, future post, I will compare Hvitarvatn varve thickness to varve thicknesses in the five “standard” varve thickness series used in Shi et al 2013 (and many other recent posts.)