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)
José E. Ortíz, Trinidad Torres, Antonio Delgado‐Huertas, Maruja Valle, Vicente Soler, Rafael Araújo, María R. Rivas, Ramón Julià, Yolanda Sánchez‐Palencia, Rogelio de la Vega
Abstract
Abstract 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 δ 13 C values of these lipids were measured in 300 samples taken from the uppermost 55 m of the Padul Basin (PB) record. The δ 13 C 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 δ 13 C 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 δ 13 C values were attributed to a marked contribution of plants using the C 3 photosynthetic pathway. Intervals with the lowest δ 13 C 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 δ 13 C occurred in long-chain homologues typically abundant in terrestrial plants. Shifts in δ 13 C of medium-chain homologues reflected limited CO 2 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 δ 13 C values of bulk OM and of long-chain n- alkanes. Hence, the δ 13 C 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 δ 13 C 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.