The environmental context of carbonaceous compressions and implications for organism preservation 1.40 Ga and 0.63 Ga

Abstract

Fossil life in the Precambrian is commonly preserved as carbonaceous compressions, where Pre-Cryogenian carbonaceous compressions mainly occur in organic-poor rocks. Black-shale-preserved carbonaceous compressions have become more common since the Ediacaran Period. To explore the depositional context of carbonaceous compressions, micro X-ray fluorescence spectroscopy (μ-XRF) was used to image elemental distributions in carbonaceous compressions and surrounding rocks in Mesoproterozoic and Ediacaran-aged rocks. The carbonaceous compressions preserved in the Xiamaling green silty mudstones (1.40 Ga) and Lantian black shales (0.63 Ga) were selected and scanned, respectively. The behavior of redox-sensitive elements (Mo, U, V, Zn, Cu) in the Xiamaling and Lantian rocks and carbonaceous compressions suggest that the overlying water conditions during their sedimentation were not sulfidic. However, the co-enrichments of Fe and S in Xiamaling and Lantian carbonaceous compressions indicate pyrite permineralization. The Lantian carbonaceous compressions have a higher pyrite permineralization degree and a lower organism decomposition degree than those of the Xiamaling carbonaceous compressions. We speculate that these differences are related to a higher sulfate reduction rate and a lower dissimilatory iron reduction rate in the sediments hosting the Lantian carbonaceous compressions. Such conditions might be achieved through rapid burial and encircling by clay minerals. Overall, our results shed new light on understanding the preservation processes of the Precambrian life and the influence of environmental factors.