Constraining the Response of Glaciers and Ice Caps to the Range of Holocene Climates in Iceland Through Lacustrine Studies

The climate of Iceland is largely determined by the position of the Polar Front; subtle changes in this position are expected to leave dramatic imprints on the terrestrial environment of Iceland. To take advantage of this sensitivity to North Atlantic circulation changes we used the GLAD 200 coring system to recover long (20-30 m), continuous sediment cores from three deep lakes lying on a transect from south to northwest Iceland in summer 2003. Our previous seismic surveys of the three lakes revealed 20 to 50 m of sediment fill, with distinct seismic units, reflecting different sediment processes and influx. The lakes were strategically located to capture both the early deglacial interval, and a full, high-resolution Holocene record. Cores from Hestvatn (the southernmost lake) and Haukadalsvatn (the northernmost lake), both low elevation lakes, capture the earliest phase of deglaciation and isostatic rebound (beginning perhaps 14 cal ka), with marine sediments at the base, overlain by high-resolution Holocene lacustrine records (14-15 m of sediment over the last 10 cal ka). In contrast, Hvitarvatn lies in a glacier-dominated setting in the high mountains of interior Iceland. Deglaciated about 10 cal ka, and with more than 25 m of sediment fill, it provides the best opportunity to evaluate the status of Iceland's large ice caps during the Holocene, and the timing and magnitude of Neoglacial advances. To create a chronology for the sediment cores and correlation between lakes we use diagnostic Icelandic marker tephra, particularly the Saksunarvatn ash (10 ka), and widespread Hekla tephra layers e.g., H5 (6 ka), H4, (4 ka) and H1 (0.9 ka). The sediments in Hvitarvatn exhibit all the characteristics of clastic varves. Aquatic macrofossils provide suitable material for AMS 14C dating. In glacially dominated lakes, varve thickness is regulated by the intensity of summer melt, and as each clastic varve represents one year, these are also being used to develop an absolute chronology. Our first Magnetic Susceptibility (MS) data show that we have recovered continuous, high-resolution sediment from all 3 lakes, which can be correlated based on the magnetic signature and identified tephra layers. A distinct change occurs in the MS signal between the basal marine sediment in the two low elevation lakes and the overlying lacustrine sediments pinpointing the change in sediment environments. Inclination and declination records are being studied and will provide a secure means of synchronizing the lacustrine records through the Holocene, and with high-resolution marine records from the adjacent Iceland shelf. Because of Iceland's location, our records will help define modes of Arctic and North Atlantic variability, and better constrain the response of glaciers and ice caps to the range of Holocene climates.