Molecular evidence for recent land use change from a swampy environment to a pond (Lorraine,France)

Evidence of a changing environment in the catchment area of a small pond (Lansquenet, Lorraine, France) during the last millennium was provided by coupling palynology data and organic ratios terrestrial to aquatic ratio: TAR_(HC); the C₂₉ vs. C₂₇ sterols, C₂₉/C_27(ST); average chain length, ACL and aquatic/macrophytes ratio, P_aq]. The depositional and alteration conditions of organic matter (OM) were provided by the 5α(H)-stanols/Δ⁵-sterols ratio. This study shows that quite simple organic ratios clearly allow identification of the different evolution steps of a watershed, from a swamp to the progressive settlement of a pond. During the swampy period, the C/N_(at) (ca. 11.1), TAR_(HC) (ca. 36.8)_, C₂₉/C_27(ST) (ca. 8.7) and P_aq (<0.3) show a high contribution of allochthonous OM associated with the presence of particular aquatic and mesohygrophilous species. A woody level dated to the XIIIth century reflects the period of the settlement of the pond in a more oxic environment 5α(H)-stanol/Δ⁵sterol ratio <0.2]. Then, the progressive increase in aquatic input and the development of submerged and floating macrophytes are evidenced by low C/N_(at) (<7.6), TAR_(HC) (ca. 2.7), C₂₉/C_27(ST) (ca. 2) and high P_aq (ca. 0.5), suggesting an increase in the water level and eutrophication of the pond. A second oxic event 5α(H)-stanols/Δ⁵sterols ratio averaging 0.16], associated with extensive OM degradation, corresponds to the most recent dredging in 2002 and reflects an important disturbance of sediments due to pond management.