Stable isotope variations of diatoms as an indicator of Holocene climate change in Late Hovsgol, Mongolia

Abstract

A 82.5cm-long gravity core (HS-15) was retrieved from the central part of Late Hovsgol, Mongolia. The core sediments are mainly composed of structureless and finely laminated diatomaceous mud. Five AMS radiocarbon ages for organic matter were obtained from the core sediment. The age of the sediments near the surface (3~4 cm in core depth) is estimated to be about 1,100 yr BP. Thus the reservoir effect of 734 years can be extrapolated. According to age-depth relationship, the sedimentation rate ranges from 8.3 to 15.7 cm/kyr with the higher rate in the upper part. Oxygen, carbon and nitrogen isotopes of diatoms are also analyzed. The core is divided into two parts with the transitional boundary between the core depth of 16 and 24 cm based on stable isotope contents of diatoms. In the lower part (ca.1500~6500 cal.yrBP), the d18O values are gradually enriched by ca. 0.5‰ whereas the d13C and d15N are depleted. The higher d18O data may indicate d18O enrichment of the lake water and the depletion of d13C and d15N should reflect the decrease of diatom productivity, which resulted from the decrease in nutrients influx. These isotope trends may imply drier climate with lower precipitation/evaporation ratio aournd this region. In the upper part (ca.0~1000 cal.yrBP), the d18O is rapidaly depleted by ~1.5‰ whereas the d15N is enriched. It can imply the abrupt increase in regional precipitation/evaporation ratio and runoff to the lake. As a result, the d18O of the lake water became depleted and the diatom productivity increased.