Palaeoenvironmental conditions for the natural vulcanization of the Eocene “monkeyhair” laticifers from Geiseltal, Germany, as elucidated by Raman spectroscopy

Abstract

AbstractThe evolutionary history of latex, a widespread chemical defense against insect herbivores, is not fully understood, yet a more detailed understanding of the fossil record of latex could help answer important evolutionary questions. This is, however, hampered by the difficulty of recognizing fossil latex and our still incomplete comprehension of the processes preserving latex. The best-studied fossil latex comes from the middle Eocene Geiseltal lignites in Germany, where fibrous laticifer mats, called “monkeyhair,” are preserved in the severely degraded remains of some ancient trees in the brown coals. Laticifers are specialized elongate cells that carry latex throughout the plant. In previous studies, researchers have hypothesized that these fossil laticifers are preserved through natural low temperature vulcanization of rubber within the latex. Here, we report the results of Raman spectroscopic study on Geiseltal laticifers to identify the vulcanization of natural rubber and on spatially associated carbonaceous material to test various Raman carbon geothermometers for their accuracy for low-thermal-maturity samples. Raman spectra of the fossil laticifers are virtually identical to that of rubber (cis-1,4-polyisoprene) with additional bands demonstrating sulfur vulcanization. Raman spectra from the surrounding lignite and existing Raman-based carbon thermometers, currently calibrated down to about 100 °C, clearly indicate that these samples were never exposed to temperatures higher than the surrounding lignite. These results directly validate the previous hypothesis of fossilization through natural vulcanization. Moreover, this work demonstrates that Raman spectroscopy is a rapid, non-destructive method for reliably identifying and characterizing fossil latex and that further development and calibration of the carbon thermometer may allow quantitative temperature measurements for low-thermal-maturity carbonaceous material.