Holocene climate change in south-central Iceland: A multi-proxy lacustrine record from glacial lake Hvítárvatn
This dissertation presents a 10 ka record of Holocene climate change from south-central Iceland. A continuous, high resolution, multi-proxy study was completed on sediments from Hvítárvatn, a glacier-dominated lake located on the eastern margin of Langjökull, Iceland's second largest ice cap. Langjökull's Holocene ice cap extent was reconstructed through the analysis of diatom assemblages preserved in Hvítárvatn sediments, together with physical and chemical proxies (magnetic susceptibility, TOC, biogenic silica, density, sedimentation rate, grain size, ice rafted debris). The erosive power of Langjökull and the efficient delivery system of these glacial sediments into Hvítárvatn are the dominant factors affecting the nature and rate of sedimentation and the nutrient and light availability in the lake. Any perturbations to the lake system, such as large fluctuations of the ice cap margin, resulted in a swift response in the physical, chemical, and biological properties of the lake.
There are two phases of early to middle Holocene warmth in south-central Iceland in the last 10.2 ka, from <10.2–8.7 ka ago and coincident with peak summer solar insolation at that latitude at 11 ka ago, and peak warmth from 7.35–5.5 ka ago which lags peak insolation by several millennia. Langjökull, Iceland's second largest ice cap, largely disappeared during these two phases indicating temperatures were warmer than today. The early to middle Holocene warmth was interrupted by a cold period from 8.7–7.35 ka ago. The ice cap regrew to a small extent during this cold phase. The 8.2 ka event was encompassed by this much longer period of cooling and did not appear as a distinct event in this record. The onset of Neoglaciation was at 5.5 ka ago, with significant glacier regrowth resulting in a dominance of glacio-fluvial sedimentation in the lake by ∼4.2 ka ago. The Langjökull Holocene glacial maximum since 10.2 ka ago was during the LIA (1550–1920 AD), with the coldest conditions occurring during the AD 1800s. Langjökull is currently retreating in response to warming conditions which began in the latest 1800s AD.
No strong climate related temporal frequencies were detected in the 10 ka record (but there is some indication of a ∼200 yr cycle that may reflect one of the widely seen solar cycles). There is little variance detected in Hvítárvatn during the early to middle Holocene. Decadal to centennial-scale variability increases significantly beginning in the Neoglacial (ca. 4.2 ka ago), reflecting a major shift in boundary conditions in the North Atlantic to a more variable mode during the latter half of the Holocene. Multi-decadal to centennial-scale variance further increases 2 ka ago. The centennial-scale variability detected in the 10 ka record indicates a centennial-scale response time of Langjökull to climate perturbations. Langjökull is highly sensitive to climate perturbations and the Hvítárvatn sediments are a sensitive recorder of climate change during the last 10 ka.
The Hvítárvatn climate record presented here provides an important record from the North Atlantic, adding to the growing body of literature detailing the spatial heterogeneity of responses to climate forcing in different regions of the Arctic and North Atlantic.