Holocene Climate Evolution and Glacier Fluctuations Inferred from Proglacial Lake Sediments at Hvítárvatn, Central Iceland

Larsen, Darren Jon

Graduate Thesis Or Dissertation

Holocene Climate Evolution and Glacier Fluctuations Inferred from Proglacial Lake Sediments at Hvítárvatn, Central Iceland Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/08612n67z

Abstract

Understanding the character and expression of past climate changes in response to discrete forcing mechanisms is prerequisite to a critical assessment of modern climate variability and to anticipating future changes to Earth’s climate system. In this dissertation, Holocene paleoclimate and glacier fluctuations in central Iceland are reconstructed using the sediment archive preserved in proglacial lake Hvítárvatn. Sediment cores retrieved from >35 locations throughout Hvítárvatn’s main basin are used in conjunction with seismic reflection profiles and multibeam bathymetric data to construct a detailed record of environmental conditions and the changing dimensions of the adjacent Langjökull ice cap (925 km²) for the past 10.2 ka. A multiple-core varve chronology, determined by sedimentological analyses and supported by tephrostratigraphy, provides exceptional chronologic control of the lake sediment and contains annual resolution for the past 3 ka. Age uncertainty for the composite varve chronology is ±6 years for the past 1.2 ka and increases to approximately ±100 years at the Hekla 3 tephra layer (~3 ka). Older sediments are placed in a secure geochronology by splicing the varve chronology with a paleomagnetic secular variation derived chronology, which contains an estimated age uncertainty of 100-200 years.

Spatial and temporal changes in varve thickness reflect variations in the production and delivery of erosional products related to Langjökull fluctuations. A suite of environmental and glacier proxies, including sediment accumulation rate, bulk density, magnetic susceptibility, grain size, ice-rafted debris (IRD), sediment organic matter, biogenic silica, and diatom abundance, reveal a dynamic Holocene terrestrial climate with abrupt and high magnitude changes in Langjökull size and landscape stability. Following regional deglaciation of the main Iceland Ice Sheet by 10.2 ka, a first-order trend of early warmth transitioning toward cooler summers and

Langjökull expansion is punctuated by notable periods of non-linear and rapid changes in ice cap dimensions and catchment stability. Two perturbations at ~8.5 and ~8.2 ka have similar durations of ~200 years and are interpreted to represent discrete pulses of a major environmental perturbation between 8.7 and 7.9 ka, which includes the “8 ka event” and which led to widespread landscape destabilization and possible glacier growth. Subsequently, all sedimentary proxies indicate high within-lake primary productivity and the absence of glacier ice in the catchment during the Holocene Thermal Maximum (HTM), between 7.9 and 5.5 ka. Neoglaciation is recorded as a step-like transition toward cooler summers, landscape destabilization, and the inception and expansion of Langjökull beginning ~5.5 ka, with notable increases in ice cap size and landscape instability at 4.2, 3.0, 1.4, and 0.7 ka. Langjökull’s maximum Holocene extent occurred during the Little Ice Age (LIA), the most recent and severe climate anomaly of the Neoglacial period. Sediment yield calculated from decadal average varve thickness patterns recorded at six core sites reveals two main phases of Langjökull growth during the LIA, ca. 1400 to 1550 AD and ca. 1680 to 1890 AD. These advances are separated by a persistent interval of ice retreat between 1550 and 1680 AD, suggesting that a substantial period of warming interrupted LIA cold. The pattern of Icelandic glacier activity contrasts with that of European glaciers but shows strong similarities to reconstructed changes in North Atlantic oceanographic conditions, indicating differing regional responses to coupled ocean-atmosphere-sea ice variations.

Lateral moraines and lake bathymetry mark the limit of Langjökull’s second and ultimate LIA advance, and its age is recorded by IRD measured in sediment cores collected from five locations. Significant IRD deposition is restricted to between 1750 and 1930 AD, demonstrating that only during this interval was Langjökull sufficiently expanded such that outlet glaciers Nordurjökull and Sudurjökull advanced into the lake and maintained calving margins. Sediment stratigraphy and physical characteristics indicate that Nordurjökull advanced into the basin stably behind a moraine shoal and remained at or near maximum extension for most of the 19^th century. In contrast, Sudurjökull experienced a quasi-periodic series of eight surges in 1828, 1838, 1855, 1866, 1885, 1905, 1917, and 1929 AD, each of which resulted in fragmentation of the glacier terminus and advances of up to ~1.6 km in less than 2 years. Rapid retreat of the glacier front back to the grounding line occurred through collapse of the fragmented terminus, which began immediately after each surge advance and was accomplished in 1-3 years.

This thesis develops the first well-dated, continuous, high-resolution Holocene climate and glacier record from central Iceland. The main contributions of this work are: definitive evidence demonstrating that some of Iceland’s large ice caps disappeared during the HTM, secure dating of the onset of Neoglaciation in Iceland, firm evidence for abrupt climate change in Iceland during the Holocene, a continuous record revealing the pattern of LIA glacier advances, and the first firm evidence demonstrating surging behavior of Sudurjökull.

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