Deep subsurface microbial life in impact-altered Late Paleozoic granitoid rocks from the Chicxulub impact crater

Abstract

In 2016, IODP-ICDP Expedition 364 recovered an 829-meter-long core within the peak ring of the Chicxulub impact crater (Yucatán, Mexico), allowing us to investigate the post-impact recovery of the heat-sterilized deep continental microbial biosphere at the impact site. We recently reported increased cell biomass in the impact suevite, which was deposited within the first few hours of the Cenozoic, and that the overall microbial communities differed significantly between the suevite and the other main core lithologies (i.e., the granitic basement and the overlying Early Eocene marine sediments; Cockell et al., 2021). However, only seven rock intervals were previously analyzed from the geologically heterogenic and impact-deformed 587-m-long granitic core section below the suevite interval. Here, we used 16S rRNA gene profiling to study the microbial community composition in 45 intervals including (a) 31 impact-shocked granites, (b) 7 non-granitic rocks (i.e., consisting of suevite and impact melt rocks intercalated into the granites during crater formation and strongly serpentinized pre-impact sub-volcanic, ultramafic basanite/dolerite), and (c) 7 cross-cut mineral veins of anhydride and silica. Most recovered microbial taxa resemble those found in hydrothermal systems. Spearman correlation analysis confirmed that the borehole temperature, which gradually increased from 47 to 69°C with core depth, significantly shaped a subset of the vertically stratified modern microbial community composition in the granitic basement rocks. However, bacterial communities differed significantly between the impoverished shattered granites and nutrient-enriched non-granite rocks, even though both lithologies were at similar depths and temperatures. Furthermore, Spearman analysis revealed a strong correlation between the microbial communities and bioavailable chemical compounds and suggests the presence of chemolithoautotrophs, which most likely still play an active role in metal and sulfur cycling. These results indicate that post-impact microbial niche separation has also occurred in the granitic basement lithologies, as previously shown for the newly formed lithologies. Moreover, our data suggest that the impact-induced geochemical boundaries continue to shape the modern-day deep biosphere in the granitic basement underlying the Chicxulub crater.

Keywords: Chicxulub impact crater; biogeochemical cycling; continental deep biosphere; impact-altered granitoid basement; niche separation.