Microbial Sulfur Isotope Fractionation in the Chicxulub Hydrothermal System

Abstract

Target lithologies and post-impact hydrothermal mineral assemblages in a new 1.3 km deep core from the peak ring of the Chicxulub impact crater indicate sulfate reduction was a potential energy source for a microbial ecosystem (Kring et al., 2020). That sulfate was metabolized is confirmed here by microscopic pyrite framboids with δ³⁴S values of -5 to -35 ‰ and ΔS_{sulfate-sulfide} values between pyrite and source sulfate of 25 to 54 ‰, which are indicative of biologic fractionation rather than inorganic fractionation processes. These data indicate the Chicxulub impact crater and its hydrothermal system hosted a subsurface microbial community in porous permeable niches within the crater's peak ring.

Keywords: Chicxulub.; Hydrothermal; Impact crater; Origin of life.

FIG. 1.

Location of the Chicxulub peak-ring crater beneath the northern edge of the Yucatán Peninsula, México. Samples analyzed in the current study were recovered from the International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364 Site M0077 borehole. Other samples described in the paper were recovered from boreholes Yucatán-2 (Y-2), Yucatán-5a (Y-5a), Yucatán-6 (Y-6), and Yaxcopoil-1 (Yax-1). Background: NASA image produced by MODIS satellite observations in October 2004.

FIG. 2.

Images of sulfide. Regions analyzed for sulfur isotopes in (A) 0077-40R-2, (B) 0077-46R-1, (C) 0077-63R-2, and (D) 0077-297R-1. (E) Pyrite framboids within analcime, 0077-63R-1. (F) Pyrite framboid adjacent to calcite, 0077-63R-1. (G) Pyrite framboid exposed by abrasion within analcime and with potential biofilm, 63R-1. Optical microscopic images (A–D), backscattered electron image (E), and secondary electron images (F, G). Labeled bars provide scale of each image. Anl = analcime; Cal = calcite; Dac = dachiardite; Py = pyrite.

FIG. 3.

Sulfur isotope compositions for sulfide from four hydrothermally altered core samples recovered by IODP-ICDP Expedition 364 in order of relative depth in the peak ring. Forty-one sulfide analyses were made in suevite within peak ring granitoid rocks (0077-297R-1-93-95, pink squares) and three overlying suevitic breccias (0077-63R-2-69.5-72, yellow squares; 0077-46R-1-46-52, green squares; 0077-40R-2-105-107, blue squares). Analytical uncertainty is smaller than the sizes of symbols. Expedition 364 sulfur isotopes in 22 samples of post-impact sediments (open squares) and their stratigraphic age are from Schaefer et al. (2020). For comparison, analyses of sulfate samples in bedrock of the northern Yucatán (Claypool et al., ; Koeberl 1993), in Chicxulub ejecta on the Yucatán (Koeberl 1993), and in two previous core samples (Y-6 and Yax-1) recovered from the crater (Strauss and Deutsch, 2003) are provided (gray squares). Sulfide isotope compositions (‰, V-CDT) of the pyrite framboids are extremely fractionated from those of target sulfate compositions.

FIG. 4.

Images of sulfate (anhydrite). (A) Clast of target bedrock anhydrite (Anh) in polymict impact breccia, Y6-N14. (B) Quartz (Qtz)-anhydrite vein in polymict impact breccia, Y6-N19. (C) Close-up of anhydrite in quartz-anhydrite vein, Y6-N19. (D–E) Cavity-filling blooms or sprays of secondary anhydrite filling vesicles in altered impact melt fragments in polymict impact breccia, Y6-N14. Optical microscope images with crossed-nicols. Labeled bars provide scale of each image.