Trace element geochemistry in the earliest terrestrial ecosystem, the Rhynie Chert

The symbiotic partnership of plants and fungi was a critical means of nutrient uptake during colonization of the terrestrial surface. The Lower Devonian Rhynie Chert shows evidence for extensive phosphorus mobilization in plant debris that was pervasively colonized by fungi. Sandy sediment entrapped with fungi-rich phytodebris contains grains of the phosphate mineral monazite which exhibit alteration to highly porous and leached surfaces. Mixed manganese-iron oxide
precipitates contain up to 2 % P2O5. The mobilization of Mn, Fe and P are all features of mycorrhizal nutrient concentration. However, the ecosystem was also exposed to toxic elements from hot spring hydrothermal activity. The oxide precipitates include titanium and iron-titanium oxide which sequestered potentially toxic tungsten and antimony. Abundant pyrite framboids in the Rhynie Chert indicate that plant decomposition included microbial sulphate reduction. This caused the removal of some of the arsenic from the groundwaters into the pyrite, which reduced toxicity while leaving enough for putative arsenic metabolism. These relationships show the mineral component of the ecosystem modified the geochemistry of ambient waters.