DOSECC’s GLAD200 drilling system was used to target three Icelandic lakes, Hestvatn (HST), Hvítárvatn (HVT), and Haukadalsvatn (HAK), each with 10 to 12 ka years of sediment fill, averaging 1 to 2 m sediment per thousand years. From these collections, one PhD (J Black, Colorado HVT), and two MSc (H Hannesdottir, Iceland HST and Gudrun Eva Johannsdottir, tephra geochemistry) theses have been completed. Two additional theses (D Larson, Colorado HVT, PhD and K Olafsdottir, Iceland HAK, MSc) are in progress.
1) During the Holocene thermal maximum (9 to 5 ka) summer temperatures were up to 3 °C warmer than late 20th Century averages, and Iceland’s large ice caps were either absent (Langjokull), or greatly reduced (Vatnajokull).
2) Neoglaciation set in about 5 ka with the maximum snowline lowering during the Little Ice Age.
3) Summer temperature depression during the Little Ice Age was up to 1.5 °C below late 20th Century averages, producing the most expanded local glacier limits since regional deglaciation.
4) Laminations in the HVT sediment cores have been confirmed to be annual varves. Varve thicknesses have been determined in two cores back to 870 AD, and in one additional core back 3 ka BP, with confirmation from 5 historic tephras and one 14C-dated tephra (H3). Varve thickness provides proxies for glacier size (summer temperature) and precipitation at annual resolution over this time interval. Spectral analyses of these signals (Olafsdottir) is in progress.
5) A geochemical characterization of all major tephra deposited over the past 10 ka is nearing completion. Lake records provide a far more complete record of explosive volcanism in Iceland than does any other archive, especially for tephras older than about 6 ka.
Figure 1 – GLAD200 drilling platform being used to core Hvítárvatn.