土壤正构烷烃及其单体同位素指示 有机碳空间变化及来源*

Leaf wax ra-alkanes are commonly used as proxies to reconstruct past climatic and environmental changes and to trace organic matter sources. However, in the inland orogenic belts where topography and moisture transport are complex, little is known on the relationships between the abundance distribution of ra-alkanes and their carbon and hydrogen isotope ratios(S D _alkand S ¹³C _alk)with environmental parameters. Specifically, whether they can reveal the spatial variation and sources of organic carbon remains unclear. Here, we focus on the upper Min Jiang basin situated within the Longmen Mountain region along the eastern margin of the Tibetan Plateau. This tectonically area is distinguished by vigorous tectonic movements, elevated erosion rates, and substantial sediment discharge. We analyzed the total organic carbon(TOC) contents and bulk isotope ratios (S ¹³C _org) , concentrations and distribution characteristics of ra-alkanes and their compound-specific S D _alkand S ¹³C _alkat different elevations along the Min Jiang, different elevation transects along the mountains and soil depths in the upper Min Jiang basin. The results showed that ra-alkane parameters such as carbon preference index (CPI) , average chain length (ACL) , SD _alk, and S ¹³C _alkin the surface soils of the mountain transects exhibited significant linear correlations with altitude ( CPI; r ²= 0 . 6 1 , p<0. 01, n = 26; ACL;r² = 0 . 5 6 , p<0. 01, n = 26; <5DC alk -.r ² = 0. 8 1 , p<0. 01, n = 26; 8 Cc _alk: r² = 0. 5 1 , p<0. 01 , n = 22 ) . The 8 Corg demonstrated an initial decline followed by an increase with ascending elevation, whereas the TOC contents exhibit an inverse trend relative to 8 C . In contrast to the mountain transects, surface soil indicators along the Min Jiang, with the exception of the <5Dalk which displays a significant linear correlation with altitude ( r ² = 0 . 5 1 , p <0. 01) , reveal nonlinear responses to altitudinal variations in other metrics such as CPI, ACL, TOC contents, 8 ¹³C _org, and 8 Calk. This pattern underscores the integrated effects of vegetation types, aridity, temperature, and high-altitude inputs. Within the soil profiles, the concentrations of ra-alkanes, ACL, CPI, and TOC contents diminish with increasing soil depth, whereas 8 C and 8 Calk showed the opposite trends. Notably, the concentrations of ra-alkanes, TOC contents, and S ¹³C _org display pronounced variability in the surface soil layers(0~ 30 cm) and more subdued trends in the deeper soil layers(30~83 cm). The variation of <5Dalk across different soil strata is more intricate. The observed trends in soil profile indicators predominantly reflect the degradation of organic matter, potentially modulated by historical fluctuations in climate, hydrology, vegetation, and soil erosion. Our results suggest that the <5Dalk is a good proxy for tracing the elevation-based sources of organic carbon, whereas CPI, ACL, and 8 Calk can also indicate spatial variations in organic carbon. The joint use of multiple indicators is effective for tracing the sources of organic carbon. It is important to note that in geologically complex mountainous areas, significant differences exist between deep and surface soil ra-alkanes and their isotopes. Consequently, when using sedimentary biomolecular tracers for material source tracing, the impact of soil erosion depth should be considered.