News & Events

PhD defense: David Harning

Wednesday, March 13, 2019, 12:00PM - 2:00PM


David John Harning


SEEL 303 (Sustainability, Energy and Environment Laboratory, East Campus)


Refining the climate, glacier, and volcanic history of Iceland during the Holocene


Iceland’s position at the confluence of major oceanic and atmospheric fronts results in a highly sensitive climate evident in both instrumental and paleo records. However, open questions still remain regarding the pre-instrumental evolution of climate, glacier, and volcanic activity at this hemispherically relevant location. This dissertation capitalizes on and merges a range of analytical techniques in an effort to refine our understanding of Icelandic climate variability, glacier extent, and tephrochronology during the Holocene epoch, with a focus on Northwest Iceland. In order to provide robust age control in our records, this research required the development of a tephrochronological framework for West Iceland, a region that lacks the otherwise widely-dispersed rhyolitic marker tephras. Glacier proxies (threshold lake sediment records and emerging dead vegetation from receding ice margins) provide firm constraints on the Holocene activity of Drangajökull, an ice cap in northwest Iceland, and high-resolution lake sediment proxy records (TOC, δ13C, C/N and biogenic silica) collected adjacent to the glacier elucidate the concomitant climate. Furthermore, we explore two lipid biomarker paleothermometers (alkenones and branched glycerol dialkyl glycerol tetraether, GDGTs) in one of these lakes and its catchment soils for the first time in Iceland to quantify the evolution of Holocene summer temperature.

Similar to other Icelandic ice cap histories, our records collectively illustrate that a warm early Holocene (2 to 5 oC above modern) likely resulted in the complete demise of Drangajökull shortly after 9 ka. Subsequent to peak early Holocene summer warmth, lake sediment climate proxies indicate punctuated declines in algal productivity and increases in terrestrial soil erosion alongside steadily decreasing northern hemisphere (NH) summer insolation. As summers continued to cool, Drangajökull re-nucleated by ~2.3 ka and episodically expanded to its maximum dimension during the Little Ice Age (0.7-0.1 ka), when summer temperatures are estimated to be ~1 oC below modern. Triggers for cold anomalies are linked to variable combinations of freshwater pulses from waning Pleistocene ice sheets, low total solar irradiance, explosive and effusive volcanism, and internal modes of climate variability, with cooling likely sustained by ocean/sea-ice feedbacks.

In addition to the lake record, GDGTs were also applied in two other settings: a Holocene soil archive in central Iceland and in the marine realm along the North Iceland Shelf. For the latter, we also developed an Icelandic GDGT-temperature calibration based on marine surface sediment that highlights the reduced uncertainty (± 0.4 oC) achievable for local rather than global calibrations (e.g., ± 4.0 oC). Local calibrations are particularly important for areas where the temperature relationship of GDGTs deviates from the overall linear correlation observed in global calibrations (i.e., cold and warm regions), such as Iceland. Although clearly reflected in the maximum dimensions of Drangajökull, the Little Ice Age cooling is obscured in all lake, soil and marine organic geochemical records investigated in this dissertation. For the former two, the erosion of older soils, nutrients and relic GDGTs likely compromise the records and imply warmth. On the other hand, the development of thick sea ice inferred from highly branched isoprenoid biomarkers on the North Iceland Shelf insulated the subsurface waters during the peak Little Ice Age, likely preventing the ventilation of heat from below the surface layer to the atmosphere. This dissertation provides critical and nuanced observations necessary for evaluating modeling simulations aiming to forecast the poorly constrained climate of the coming century.