The Holocene environmental changes in boreal fen peatland of northern Mongolia reconstructed from diatom assemblages

Abstract

The Holocene hydrological changes of fen peatland in northern Mongolia were reconstructed using diatom fossils with an aid of Sphagnum leaf fossils. Analyses were conducted on three cores taken across the basin and reconstructed water environments were compared to find basin-wide hydrological changes and climate anomalies. The peatland has been covered by sedges during most of its 10 thousand years history, but all cores showed common peatland hydrosere processes from fluvial environment in the early Holocene period, shallow marsh to present acidic fen peatland. The establishment of fen peatland at 6.8–6.4 cal ka BP was marked synchronously by the shift of diatom and Sphagnum taxa assemblages and this period was inferred as the onset of the mid-Holocene dry climate which is widely observed around Mongolia. The dry environment lasted until around 2.8 cal ka BP with a temporal wetter condition at 4.4–3.5 cal ka BP. Short term diatom trend shifts indicated temporal wetter environments at 8.7–8.4, 8.0–7.6, 2.4–2.1, 1.2–0.5 cal ka BP and a dry environment at 0.4–0.2 cal ka BP. These periods of hydrology changes were correlative with other studies in Mongolia and East Siberia, so they would represent climate-induced hydrological changes, and those regions might share the same response to long term insolation changes or global climate anomalies. However, disagreements of climate data even within northern Mongolian region suggest large influence of regional geography and the balance of evapotranspiration on climate behaviors as shown by recent dendrological paleoenvironment researches. Coherent differences of main taxa compositions between three cores were attributed to core site-specific local hydrology mainly in terms of relative wetness from wettest core site accompanied by abundant planktonic diatom taxa to driest site with acidic peat indicator taxa. Timings and manners of temporal diatom trend shifts were also largely different between cores suggesting significant difference in each core site sensitivity to basin-wide hydrology changes, confirming the necessity of using multiple cores and proxies in peatland sediments to detect climate-induced environmental changes.