The role of clay minerals in the preservation of Precambrian organic-walled microfossils

Precambrian organic-walled microfossils (OWMs) are primarily preserved in mudstones and shales that are low in total organic carbon (TOC). Recent work suggests that high TOC may hinder OWM preservation, perhaps because it interferes with chemical interactions involving certain clay minerals that inhibit the decay of microorganisms. To test if clay mineralogy controls OWM preservation, and if TOC moderates the effect of clay minerals, we compared OWM preservational quality (measured by pitting on fossil surfaces and the deterioration of wall margins) to TOC, total clay, and specific clay mineral concentrations in 78 shale samples from 11 lithologic units ranging in age from ca. 1650 to 650 million years ago. We found that the probability of finding well-preserved microfossils positively correlates with total clay concentrations and confirmed that it negatively correlates with TOC concentrations. However, we found no evidence that TOC influences the effect of clay mineral concentrations on OWM preservation, supporting an independent role of both factors on preservation. Within the total clay fraction, well-preserved microfossils are more likely to occur in shales with high illite concentrations and low berthierine/chamosite concentrations; however, the magnitude of their effect on preservation is small. Therefore, there is little evidence that bulk clay chemistry is important in OWM preservation. Instead, we propose that OWM preservation is largely regulated by physical properties that isolate organic remains from microbial degradation such as food scarcity (low TOC) and low sediment permeability (high total clay content): low TOC increases the diffusive distances between potential carbon sources and heterotrophic microbes (or their degradative enzymes), while high clay concentrations reduce sediment pore space, thereby limiting the diffusion of oxidants and degradative enzymes to the sites of decay.

Aqueously altered igneous rocks sampled on the floor of Jezero crater, Mars

The Perseverance rover landed in Jezero crater, Mars, to investigate ancient lake and river deposits. We report observations of the crater floor, below the crater's sedimentary delta, finding the floor consists of igneous rocks altered by water. The lowest exposed unit, informally named Séítah, is a coarsely crystalline olivine-rich rock, which accumulated at the base of a magma body. Fe-Mg carbonates along grain boundaries indicate reactions with CO₂-rich water, under water-poor conditions. Overlying Séítah is a unit informally named Máaz, which we interpret as lava flows or the chemical complement to Séítah in a layered igneous body. Voids in these rocks contain sulfates and perchlorates, likely introduced by later near-surface brine evaporation. Core samples of these rocks were stored aboard Perseverance for potential return to Earth.

An olivine cumulate outcrop on the floor of Jezero crater, Mars

The geological units on the floor of Jezero crater, Mars, are part of a wider regional stratigraphy of olivine-rich rocks, which extends well beyond the crater. We investigate the petrology of olivine and carbonate-bearing rocks of the Séítah formation in the floor of Jezero. Using multispectral images and x-ray fluorescence data, acquired by the Perseverance rover, we performed a petrographic analysis of the Bastide and Brac outcrops within this unit. We find that these outcrops are composed of igneous rock, moderately altered by aqueous fluid. The igneous rocks are mainly made of coarse-grained olivine, similar to some Martian meteorites. We interpret them as an olivine cumulate, formed by settling and enrichment of olivine through multi-stage cooling of a thick magma body.