Quantitative assessment of precipitation seasonality and summer surface wetness using ombrotrophic sediments from an Arctic Norwegian peatland

a r t i c l e i n f o Seasonality of precipitation is an important yet elusive climate parameter in paleoclimatological reconstructions. This parameter can be inferred qualitatively from pollen and other paleoecological methods, but is difficult to assess quantitatively. Here, we have assessed seasonality of precipitation and summer surface wetness using compound specific hydrogen and carbon isotope ratios of vascular plant leaf waxes and Sphagnum biomarkers extracted from the sediments of an ombrotrophic peatland, Bøstad Bog, Nordland, Norway. Our reconstructed precipitation seasonality and surface wetness are consistent with regional vegetation reconstructions. During the early Holocene, 11.5–7.5 ka, Fennoscandia experienced a cool, moist climate. The middle Holocene, 7.5–5.5 ka, was warm and dry, transitioning towards cooler and wetter conditions from the mid-Holocene to the present. Changes in seasonality of precipitation during the Holocene show significant coherence with changes in sea surface temperature in the Norwegian Sea, with higher SST corresponding to greater percentage of winter precipitation. Both high SST in the Norwegian Sea and increased moisture delivery to northern Europe during winter are correlated with a strong gradient between the subpolar low and subtropical high over the North Atlantic (positive North Atlantic Oscillation). Introduction Seasonality of precipitation has a profound impact on agriculture, natural vegetation, and regional moisture balance. It is an integral part of the climate system that can be assessed using climate models, but paleoclimate datasets to test model output of precipitation seasonality are rare. Changes in the amount of summer versus winter precipitation also have tremendous impacts on vegetation and lake levels (Shuman and Donnelly, 2006). Therefore, knowledge of precipitation seasonality is also crucial for interpreting past records of plant ecology and lake level. Many methods are available to assess total precipitation or overall moisture balance in the past. For example, records of lake level, diatom assemblages and mineralogy from lake sediments (Fritz, 1996; Shuman and Donnelly, 2006), and testate amoebae assemblages (Booth, 2002) and n-alkane distributions from peat bogs (Nichols et al., 2006) can provide total water balance information. However, existing proxies from these paleoclimate archives do not provide a sufficient level of quantification for precipitation seasonality on the continents. Because it is isolated from groundwater, the water level in an ombrotrophic peatland is extremely sensitive to changes in moisture balance (i.e., precipitation–evaporation) (e.g.: Barber et al., 2000). The response to this change in water level is manifested at the surface by a change, …