Environmental history of the German Lower Rhine Embayment during the Middle Miocene as reflected by carbon isotopes in brown coal

Stable carbon isotope investigations have been carried out on Miocene brown coal from the Garzweiler Seam of the German Lower Rhine Embayment. Materia l studied included fossil wood from seven different taxa, and brown coal ma trix. Isotope results from macrofossil analysis show variations of more tha n 6 parts per thousand within individual samples and reveal a general isoto pic difference between angiosperm and gymnosperm wood specimens. According to mean carbon isotope values found for gymnosperms, angiosperms and brown coal matrix (-23.3 parts per thousand, -26.0 parts per thousand and -25.8 p arts per thousand), the peat-forming vegetation of the Garzweiler Seam was dominated by angiosperm taxa. Results from brown coal matrix establish a co ntinuous high-resolution depth profile of carbon isotope variations during the late Middle Miocene. They show a significant and characteristic isotope pattern with distinct medium- and short-term cycles (high-frequency variat ions) in the two main units of Garzweiler Seam (locally split into three un its). The medium-term delta(13)C variations are most likely caused by varyi ng proportions of gymnosperms within the peat-forming vegetation while high -frequency oscillations seem to be a direct signal of environmental changes . A long-term decline of carbon isotope values observed within the complete Garzweiler Seam from base to top is presumably due to a cooling trend in t he Miocene. (C) 1999 Elsevier Science B.V. All rights reserved.