Vertical evolution of petrographic and organic geochemical parameters in Las Dueñas mire (Cantabrian Coast, North Spain)

Abstract

Las Dueñas mire has developed on a flat siliceous substrate over the Holocene. The sedimentary cover has a thickness of around 50 cm and the mire comprises a surface area of 27 ha. A core sample obtained using a manual probe was split into sub-samples of around 3–4 cm thickness for petrographic and organic geochemical analysis. The fen originated by minerogenesis and contains a high concentration of mineral matter throughout its profile which decreases towards the top. The present day vegetation is dominated by Sphagnum . A variety of transparent textinite with intense fluorescence is evidence of the low maturity of the profile, which is also confirmed by the presence of low reflecting huminite of the A variety (Hr ~ 0.15%). The fluorescence spectra of sporinite and cutinite are similar and, compared with those of transparent textinite, they have a lower intensity and are shifted to higher wavelengths. The low maturity of the organic matter is also confirmed by the presence of unstable ββ hopane configurations, the abundance of hopene, and the presence of functionalized hopanoids, steroids and other triterpenoids. The evolution of the geochemical parameters in the peat profile indicates the presence of more humid conditions in the top 4 cm and at a depth of 11–15 cm. These periods are characterized by a high Sphagnum /higher plant contribution as deduced from the n- alkane ratios. The low reflectance of huminite at the depth of 11–15 cm could also support less oxidizing condition. The sediment at a depth of 22 cm where peaty matter starts to accumulate has been dated as 9603 cal BC, whereas the plant fraction at the same depth dates back to a maximum of 200 years. Sediment reworking derived from radiocarbon dates indicates that this fen is not suitable for palaeoenvironmental studies. ►Organic petrology and geochemistry of Las Dueñas mire (North Spain) is included. ►The mire is at a coastal cliff location. It is a low maturity high ash profile (49 cm depth). ►The sediment age is around 9500 cal BC whereas the plant material dates back 200 years. ►Geochemical parameters indicate more humid conditions at the top of the profile