Stable isotope (δ13C, δ15N, δ18O) record of soils in Buryatia, southern Siberia: Implications for biogeochemical and paleoclimatic interpretations

Abstract

Stable isotope analyses of soils significantly contribute to a better understanding of biogeochemical and paleoenvironmental processes. This paper presents and discusses stable carbon, nitrogen and oxygen isotope (δ13C, δ15N and δ18O) data of representative soils of Buryatia, southern Siberia.In undisturbed soils (kastanozem, phaeozem, gleysol, podzol, leptosol) δ13C and δ15N values of soil organic matter (SOM) increase from the organic layers (O-layers) to the mineral topsoils (Ah-horizons) and subsoils. This primarily indicates advanced C and N-mineralization in lower soil horizons and reflects the input of fresh litter in the O-layers. Combined with numerical ages, these data indicate that during the last ca. 40 ka no C4 grasses prevailed in semiarid regions of Buryatia. Disturbed soils (ploughed chernozem and an anthrosol) do not show with depth increasing δ13C and δ15N values but are characterized by fluctuating depth profiles. High δ15N values in the anthrosol (up to 10.5‰) point to an open N cycle. In a middle and late Holocene aeolian paleosol record (Burdukovo), δ18O of the hemicellulose-biomarkers arabinose and xylose show minima in the dark humic buried A horizons and maxima in the greyish silty fine sand layers. This reflects the alternation of more arid and more humid conditions. This interpretation is also in agreement with higher grain size ratios (20–63 μm)/(2–6 μm) in the greyish fine sand layers, documenting increased wind strength and dust supply.SOM δ13C and δ15N values of O-layers and Ah-horizons were furthermore investigated along an altitudinal transect in the Khamar Daban Mts (from 470 m to 1800 m a.s.l.). Generally, δ15N values decrease with increasing altitude giving evidence of more closed N cycles at higher altitudes, which are characterized by lower temperatures. As an exception, δ15N of the Ah-horizons above 1500 m reveals conspicuously positive values, presumably because of frequent burning of the dwarf pine belt with high N losses due to volatilization. δ13C shows no significant altitudinal trend above 750 m a.s.l., but more negative values below 750 m a.s.l. This may be attributed to frequent temperature inversions close to Lake Baikal. Similarly, this inversion inducing fog and higher humidity close to Lake Baikal could also explain why δ18O of the hemicellulose-biomarkers arabinose and xylose do not reflect the ‘altitude-effect’ as it could be expected from δ18O of precipitation.