Hydrogen isotopic composition of plant leaf wax in response to soil moisture in an arid ecosystem of the northeast Qinghai-Tibetan Plateau, China

The hydrogen isotopic composition of plant leaf wax (delta D-wax) is used as an important tool for paleohydrologic reconstruction. However, the understanding of the relative importance of environmental and biological factors in determining delta D-wax values still remains incomplete. To identify the effects of soil moisture and plant physiology on delta D-wax values in an arid ecosystem, and to explore the implication of these values for paleoclimatic reconstruction, we measured delta D values of soil water (delta D-water) and delta D-wax values in surface soils along two distance transects extending from the lakeshore to wetland to dryland around Lake Qinghai and Lake Gahai on the northeast Qinghai-Tibetan Plateau. The results showed that the delta D-water values were negatively correlated with soil water content (SWC) (R-2=0.9166), and ranged from -67 parts per thousand to -46 parts per thousand with changes in SWC from 6.2% to 42.1% in the arid areas of the Gangcha (GCh) and Gahai (GH) transects. This indicated that evaporative D-enrichment in soil water was sensitive to soil moisture in an arid ecosystem. Although the shift from grasses to shrubs with increasing aridity occurred in the arid area of the GH transect, the delta D-wax values in surface soils from the arid areas of the two transects still showed a negative correlation with SWC (R-2=0.6835), which may be due to the controls of primary evaporative D-enrichment in the soil water and additional transpirational D-enrichment in the leaf water on the delta D-wax values. Our preliminary research suggested that delta D-wax values can potentially be applied as a paleo-humidity indicator on the northeast Qinghai-Tibetan Plateau.</p

Distributions and hydrogen isotopic compositionsof plant leaf wax from Orinus kokonorica along ageneral aridity gradient around Lake Qinghai,China

To quantitatively analyze the response of distributions and hydrogen isotopic compositions (&delta;D) of plant leaf wax to moisture, and to better understand their implications for paleoclimatic reconstruction, we measured average chain length (ACL) and &delta;D values of n-alkanes and n-fatty acids (n-FAs) from Orinus kokonorica, a typical and representative plant in Lake Qinghai area, along a distance transect extending from lakeshore to wetland to dryland in the arid ecosystem. The results showed that the ACL values of n-alkanes and n-FAs were negatively correlated with soil water content (SWC) with R2=0.593 and R2=0.924, respectively. This is as a result of plant&rsquo;s response to water loss with more abundance in long-chain n-alkyl lipids under increasing aridity by analyzing relationships between the molecular ratios of long-chain n-alkyl lipids (n-alkanes and n-FAs) from O. kokonorica and SWC. The &delta;D values of C29 n-alkane and C28 n-FA were also negatively correlated with SWC with R2=0.778 and R2=0.760, respectively, which may due to enhanced D-enrichment in leaf water by evapotranspiration (soil water evaporation and leaf water transpiration) with increasing aridity. Our results demonstrated that moisture exerts a significant control on the ACL and &delta;D values from O. kokonorica in an arid ecosystem. This preliminary study on a modern single plant (O. kokonorica) sets a foundation for comprehending these values as quantitative proxies for paleo-humidity reconstruction.</p

Major increase in winter and spring precipitation during the Little Ice Age in the westerly dominated northern Qinghai-Tibetan Plateau

There have been numerous attempts to use long-chain alkenones (LCAs) in saline lakes for paleotemperature and paleosalinity reconstructions. However, LCAs in saline lakes are often produced by multiple haptophyte species, which may confound data interpretations. Here we analyzed LCAs in a finely laminated, high sedimentation rate core from the hypersaline Lake Gahai in the northern Qinghai-Tibetan Plateau and compared our results with regional instrumental records. We find that LCA unsaturation ratios display a stepwise jump during the instrumental period, most likely originating from a sudden shift in the dominant alkenone producers. In contrast, the percentage of the C37:4 alkenone (%C37:4) displays strong correlations with spring and combined winter-spring precipitation amount (R2 = 0.83 and R2 = 0.81, respectively). We hypothesize that high winter-spring precipitation leads to greater freshening of lake surface water immediately after spring melting, promoting greater production of LCAs with relatively high %C37:4 values by the early blooming haptophyte species. Extending the instrumental calibration downcore allows us to quantitatively reconstruct regional spring and winter-spring precipitation for the past millennium. We find a major increase in winter-spring precipitation during the Little Ice Age (LIA), especially during the phases of solar minima. Our finding provides novel quantitative support to the previous studies suggesting relatively wet conditions during the LIA in the westerly dominated regions of central Asia and northern Tibetan Plateau and infers a substantial increase in regional winter-spring precipitation should the predicted grand solar minimum in the forthcoming decades become a reality.</p