Betula Pollen Influx as an Indicator of Holocene Climate Change in Northwest Iceland

Abstract:

Downy birch (Betula pubescens Ehrh.) forest is the natural climax vegetation in Iceland and reconstructions of Holocene forest dynamics can potentially provide important information about climate and environmental change in the North Atlantic.

Pollen and macrofossil analysis have been carried out on a lake sediment core from Northwest Iceland, covering the period from the end of Younger Dryas to ca. 3000 cal. yr. BP.

Only the most recent Icelandic pollen studies have included Betula pollen influx (pollen grains deposited per cm² per year). Comparison between the Betula pollen percentages and Betula pollen influx in the core show a different trend between the two methods of data presentation, particularly after birch forest has been established.

Both methods show expansion of dwarf birch (Betula nana) dominated shrub heath during the early Holocene. The appearance of downy birch (Betula pubescens) woodland and subsequent decline is also represented by both methods and is supported by the macrofossil record.

The expansion of birch forest during the Holocene Thermal Maximum (HTM) is seen from high influx values of more than 2400 Betula grains cm^-2 year^-1 and Betula percentages above 80%. The influx decreases significantly at ca. 6600 cal. yr. BP and drops to less than 2000 Betula grains cm^-2 year^-1, decreasing upwards in the core. Betula percentage values become more variable and drop slightly at the same time, but not as prominently as the influx values.

The terrestrial response recorded in the Betula pollen influx corresponds to changes apparent in marine cores from Húnaflói bay, and signifies a sensitivity of the Holocene terrestrial ecosystem to climate fluctuations. Betula pollen influx is potentially a sensitive proxy for temperature changes in Iceland and should be included in future palynological studies in order to improve the interpretation of Holocene vegetation and climate changes.

The study is funded by the Eimskip University fund, University of Iceland Research Fund, Landsvirkjun’s Energy Research Fund, and the Icelandic Centre for Research (grant no.141842-051).