Bulk and compound-specific δ13C and n-alkane indices in a palustrine intermontane record for assessing environmental changes over the past 320 ka: the Padul Basin (Southwestern Mediterranean realm)

Here we provide valuable information about the palaeoenvironmental evolution of Southwestern Mediterranean region during the last ca. 320 ka through a biomarker-based study of the longest continuous continental Quaternary record in the Iberian Peninsula. The n-alkane content and delta C-13 values of these lipids were measured in 300 samples taken from the uppermost 55 m of the Padul Basin (PB) record. The delta C-13 signal of long-chain n-alkanes was a reliable proxy for C-4/C-3 terrestrial vegetation composition in the basin, as emergent macrophytes made a minor contribution to these homologues. In contrast, the delta C-13 values of C-23 and C-25 alkanes reflected mainly phases of increasing water level of the lacustrine/palustrine water body since aquatic macrophytes contain a large proportion of these compounds. Low delta C-13 values were attributed to a marked contribution of plants using the C-3 photosynthetic pathway. Intervals with the lowest delta C-13 values were attributed to an important input of angiosperms, although they could also be explained by changing environmental conditions or environmental stress, as large shifts in delta C-13 occurred in long-chain homologues typically abundant in terrestrial plants. Shifts in delta C-13 of medium-chain homologues reflected limited CO2 availability induced by water temperature, salinity, pH, enhanced productivity, low atmospheric pCO2, or stagnant barriers, rather than the abundance of aquatic macrophytes. Our results also suggest enhanced isotopic fractionation during lipid synthesis by aquatic macrophytes within MIS 7 and the Holocene, leading to increased delta C-13 values of bulk OM and of long-chain n-alkanes. Hence, the delta C-13 logs were ideal for studying the contribution of aquatic macrophytes to the lipid and isotopic composition of sediments and for the reconstruction of palaeoenvironmental conditions. These results confirmed that C-4 plants had a low presence in the PB. Comparison with biomarker analysis and pollen data of the PB and other records of the Southwestern Mediterranean revealed that delta C-13 values of bulk OM and of long-chain n-alkanes reflected global climatic oscillations during MIS 7 and the episodes Heinrich Events 3, 2, 1 and Younger Dryas.

Automated summary

By analyzing the Padul Basin record, this study found that the delta C-13 values of long-chain n-alkanes are reliable indicators of C-4/C-3 terrestrial vegetation composition in swamps, while the delta C-13 values of C-23 and C-25 alkanes mainly reflect phases of increased water level in lacustrine/palustrine water bodies. Long-chain homologues are abundant in terrestrial plants, and the delta C-13 shifts of medium-chain homologues are primarily limited by CO2 availability.