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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. [ABSTRACT FROM AUTHOR]

Iron speciation is one of the most widely applied proxies used to reconstruct oxygen levels and redox conditions in past aqueous environments. The iron speciation proxy estimates proportions of different reactive iron species in fine‐grained sedimentary rocks, which are mapped to redox conditions based on empirical calibrations from modern sediments. It is based on a standardized extraction technique of sequentially applying acetate, hydroxlamine‐HCl, dithionite, and oxalate solutions to a powdered sample in order to dissolve iron phases and quantify the amount of iron carried by carbonates, "easily reducible" oxyhydroxides, ferric iron (oxyhydr)oxides, and magnetite, respectively. Although tested on pure minerals and mixtures, assessments of whether this sequential extraction process accurately dissolves the targeted minerals in natural sediments and sedimentary rocks are lacking. In our study, residues from each sequential extraction step were analyzed using rock magnetic and X‐ray diffraction experiments to identify and quantify the iron‐bearing minerals that were dissolved. The dithionite extraction robustly removes the targeted mineralogy as magnetic data show it to solubilize nearly all of the goethite. However, magnetic quantification of magnetite was orders of magnitude less than the iron measured in the oxalate extraction; X‐ray diffraction data suggest that dissolution of iron‐bearing clays, specifically berthierine/chamosite, could explain this disparity. Our data compilation shows higher values of iron from the oxalate extraction in Precambrian sedimentary rock samples, suggesting a significant temporal shift in iron cycling. Recognition of heterogeneity in chemical extraction efficiency and targeting is vital for holistic multiproxy interpretation of past oxygen levels and communication between disciplines. Plain Language Summary: Sequential chemical extractions, where a series of solutions are applied to a powdered rock sample to selectively dissolve certain phases, are heavily utilized throughout Earth Science research. These methodologies provide a tool for estimating different reactive forms of an element; understanding how these pools change over time in a given environment allows us to better understand cycling of the element by biological, chemical, and geologic processes on the Earth's surface. In this study, we focus on a sequential chemical extraction method that measures the element iron, the most abundant transition metal in Earth's crust. Although heavily utilized for understanding nutrient cycling and ancient oxygen levels, the method is largely untested using actual rock samples that contain a mixture of minerals of different shapes and sizes. Such tests are needed to evaluate whether the extractions are accurately and completely dissolving the targeted minerals. We utilized magnetic and X‐ray diffraction methods that can sensitively measure iron minerals within natural samples. We found that some of the extractions worked as expected, but others did not, dissolving additional unexpected mineral types and/or slowly dissolving minerals across multiple extractions. Key Points: Magnetic and X‐ray diffraction analyses on natural samples corroborate the efficiency of certain chemical extractions, such as dithioniteThe majority of iron in the oxalate extraction is not dissolved from magnetite, but instead comes from iron‐bearing claysRecognition of the heterogeneity in chemical extraction efficiency and targeting is vital for studies of past and present iron cycling [ABSTRACT FROM AUTHOR]