Search

Stable carbon isotopes of long-chain n-alkanes in lake sediments have been used for reconstructing past environmental changes. Gas chromatography isotope ratio mass spectrometry coupled to a combustion unit is the commonly applied technique for compound-specific carbon isotope analysis. This paper shows that sulfur in the lacustrine sediment can cause severe interference during the mass spectrometric measurement, leading to significant biases in δ13C of n-alkane. The δ13C deviation could be resulted from additional 12C33S+, 13C32S+ at mass-to-charge ratio (m/z) 45, and 12C34S+, H2C32S+at m/z 46, respectively. Such elemental sulphur interference can be eliminated by the activated copper in the pretreatment procedure. [ABSTRACT FROM AUTHOR]

[Display omitted] • δ13C of CO 2 in lake water was in situ measured from three Tibetan Plateau lakes. • δ13C of CO 2 in lake water at different water depths show small variations. • δ13C of CO 2 /HCO 3 – cannot fully explain δ13C difference of measured aquatic plants. The carbon isotope values (δ13C) of aquatic plants are of great significance for recognizing carbon cycles and tracing environmental changes in lakes. Yet, the influence of δ13C values of their utilizing carbon sources (especially in the case of CO 2 separated from the HCO 3 – in water) on their δ13C differences have never been evaluated, which is mainly due to the lack of CO 2 δ13C investigations in lake water. In this study, we used a newly developed fast-response automated gas equilibrator for real-time concentration and δ13C determination of dissolved CO 2 in the surface and bottom lake water of Lake Tuosu, Lake Qinghai and Lake Keluke, China. Moreover, we determined the corresponding δ13C values of total organic carbon (TOC) of a submerged plant (Potamogeton) and an alga (Cladophora), as well as the δ13C values of dissolved inorganic carbon (DIC) in bottom lake water. The results showed that the CO 2 δ13C values of bottom and surface water in these three lakes did not show significant differences and the δ13C values of CO 2 at different water depths also did not show significant differences. However, Potamogeton had higher TOC δ13C values (average 14–16‰) than Cladophora in these lakes. Although Potamogeton and Cladophora could utilize HCO 3 – as an additional carbon source whose δ13C values were more positive (ca. 7–8‰) than CO 2 , this effect on their δ13C values was no >4‰ after evaluation. Thus, our results show that the δ13C values of carbon sources (including CO 2 and HCO 3 –) are not the main factors in determining the TOC δ13C difference between Potamogeton and Cladophora , which could be primarily related to the biosynthetic fractionation for different aquatic plant species. Our results benefit further understanding and interpretations of δ13C variations in carbon cycles and paleoenvironment reconstructions for lakes. [ABSTRACT FROM AUTHOR]

The expansion of plants using the C4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the Cenozoic Era. A significant late Miocene expansion is well documented. However, the existence and cause of subsequent expansions are still not clear, owing in part to the lack of long, continuous climate-vegetation records. Here we present two high-resolution carbon isotope time series, covering the past 7 m.y., derived from eolian deposits on the Chinese Loess Plateau. The data indicate three intervals of significant C4 plant expansions within the semiarid monsoonal region of East Asia (ca. 2.9-2.7 Ma, 1.3-0.9 Ma, and 0.6 Ma-present). These expansions covary with strengthened East Asian summer monsoon circulation. We conclude that in East Asia, large-scale late Miocene C4/C3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation. [ABSTRACT FROM AUTHOR]