New insights into source of long chain n-alkanes in cave stalagmites

Long-chain (＞C₂₁) n-alkanes are one of the most widely utilized biomarkers in geological archives. The ubiquitous occurrence of these compounds exhibiting a pronounced odd-over-even carbon predominance across diverse geological settings suggests that they are predominantly derived from higher plants. Here we identified an unusual “pyramid-like” pattern of n-alkane distributions in globally distributed cave stalagmites, characterized by a maximum at C₂₆ and equivalent odd/even chain-length abundances, suggesting non-vegetative leaf-wax sources. To elucidate these unconventional biomarker signatures, we conducted systematic investigations of n-alkanes in coupled soil-dripwater-stalagmite systems from two karst caves (Shennong and Furong Cave) in the southern China. Notably, the drip waters and stalagmites exhibit consistent “pyramid-like” profile of n-alkanes, which contrasts sharply with overlying soils displaying typical pattern of the strong odd-over-even carbon predominance. These results indicate that the stalagmite n-alkanes are likely derived predominantly from in situ cave rather than surface vegetation inputs. Furthermore, our dual compound-specific isotope analyses (δD and δ13C) of n-alkanes obtained from enrichment cultures of cave drip waters (–83‰ to –41‰ for δD; –30‰ to –27‰ for δ13C) shows a characteristic signal of heterotrophic microbes, especially with high D enrichment. This provides more accurate assessment that these cave-specific n-alkanes originate mainly from heterotrophic microbial communities. Our study provides fundamental knowledge for applying n-alkane-based proxies in cave stalagmites as a potential biogeochemical tool to understand past microbial carbon cycling and ecology.