Terrigenous or not? δ13C reveals the origin of the retene and dehydroabietic acid methyl ester

Retene and dehydroabietic acid methyl ester (DAME) are biomarkers linked mostly to terrigenous organic matter. However, their aquatic origin from algae and/or cyanobacteria has been discussed in the past few years. The complex origin of retene and DAME can lead to misinterpretation during biomarker interpretation, which can be minimalised with the support of isotopic analyses. This study contains the results of thirty-two compound-specific isotope analyses (CSIA) of δ 13 C performed on retene and forty-six CSIA analyses of δ 13 C performed on DAME. Obtained δ 13 C results were compared with simple biomarker results to tie isotopic data with the origin of organic matter. Pure aquatic retene and DAME were present within the Middle Cambrian and Early Ordovician samples, as the first appearance of plants was noted in the Middle Ordovician. CSIA was performed on a wide spectrum of the Cambrian-Silurian, Late Triassic-Middle Jurassic, and Neogene samples. Cambrian-Silurian strata include aquatic-origin retene and DAME, as evidenced by their comparatively low carbon isotopic signature. The average δ 13 C for retene and DAME within Cambrian-Silurian strata was equal to −31.7 ‰ and − 30.8 ‰, respectively. The Late Triassic-Middle Jurassic strata were dominated by terrigenous organic matter, resulting in higher δ 13 C values in comparison with the Cambrian-Silurian strata. The average δ 13 C for retene and DAME within the Late Triassic–Middle Jurassic strata was equal to −25.2 ‰ and − 26.3 ‰, respectively. The average δ 13 C for retene and DAME within the Neogene strata was equal to −27.3 ‰ in both cases, as these strata also contain a significant input of terrigenous organic matter. Some Neogene samples containing strong aquatic organic matter admixture are characterised by a lower δ 13 C for retene and/or DAME, oscillating around −28.0 ‰ to −30.0 ‰ for both compounds. Despite possible terrigenous input within some Silurian samples, the δ 13 C for retene and DAME still shows values typical for aquatic origin. These results suggest that retene and DAME within the Silurian strata should not be linked with non-vascular plants or that, due to their affinity with algae and cyanobacteria, the early plants were characterised by a relatively low δ 13 C composition, characteristic for phytoplankton organic matter. δ 13 C of retene and DAME were compared with δ 13 C of C 16 -C 30 n -alkanes and stigmastane, which supported the thesis about isotopic differences in aquatic and terrigenous retene and DAME. • Retene and dehydroabietic acid methyl ester (DAME) were found in pre-Devonian rocks. • Retene and DAME are biomarkers linked mostly to conifers. • Minor retene and DAME sources were found within algae and cyanobacteria. • δ 13 C characteristics for terrigenous and aquatic organic matter were shown. • Terrigenous retene and DAME have higher δ 13 C than in the case of aquatic origin.