STXM IMAGING OF TRACES OF LIFE IN METAMORPHIC ROCKS

The fossil record contains fundamentally important 'biogeochemical' signals. However, the quality of this record is far from perfect: during biodegradation, fossilization and subsequent burial processes, the original chemical, structural and isotopic signatures of biogenic organic molecules and biominerals are inevitably altered. The poorly understood magnitude of such degradations ultimately prevents rigorous molecular and isotopic investigations of ancient life as well as precise estimations of the biases induced by fossilization processes in paleoenvironmental reconstructions. As a result, fossilization and burial-related processes are generally seen as detrimental to molecular fossil preservation.

Here, we report STXM and TEM observations of morphologically preserved plant micro- and macrofossils found in metasedimentary rocks from the French Alps and from New Zealand. Both of these fossil-rich metasediments have experienced deep burial and intense blueschist-facies metamorphism as shown by their mineral assemblage. Using XANES spectroscopy, the geochemical nature of the organic molecules composing the metamorphic fossils has been documented at the submicrometer scale. The results demonstrate that even high-grade metamorphic rocks may retain, to some extent, morphologically and geochemically recognizable traces of life. In addition, we evidence that nanoscale investigations may provide, in some instances, essential insights to deconvolve metamorphic from diagenetic imprints, thereby offering crucial information regarding early prograde geochemical conditions.

To better constrain the impact of fossilization processes on original ‘biogeochemical’ signals, we experimentally investigated the fate of chemical and isotopic signatures of biodegradation-resistant biomolecules during a simulated geological history. In addition to evidencing the differential evolutions of different organic precursors during carbonification and early steps of graphitization, our results provide a calibration for the spectroscopic evolution of reference biocompounds with increasing temperatures. Altogether, the present results constitute a new step towards better constraining the fate of natural organic matter during burial.