Reconstructing the redox environment of the Cambrian Harkless Formation, Nevada, USA: Navigating the effects of iron-rich clay minerals on the iron speciation redox proxy

Understanding the oxygenation of ancient oceans is critical for contextualizing evolutionary events throughout Earth's history. While there are many geochemical proxies available for evaluating paleoredox state, iron speciation is one of the most robust and widely applied recorders of local bottom water redox. The proxy uses a series of sequential extractions which are operationally designed to target redox active iron in different mineral groups, including carbonates, oxides, magnetite, and sulfides. However, clay minerals that contain iron are not specifically targeted but can be important reservoirs of redox active iron. These clay minerals are abundant during many times in Earth's history, so it is critical to understand how they are extracted during iron speciation and their impact on paleoenvironmental interpretations. Here we apply the iron speciation proxy in combination with other geochemical proxies (FeT/Al, T /Al, Mn enrichments) and mineralogical analyses to determine local bottom water redox conditions during deposition of the early Cambrian Harkless Formation exposed near Gold Point, Nevada, USA. The Harkless Formation contains one of the last occurrences of archaeocyathan reefs before their extinction, thus studying the underlying strata can provide information about the environmental conditions which led to reef development. Iron speciation results indicate oxic to potentially anoxic conditions in the water column. The presence of FeT/Al T /Al enrichments, Mn depletion, and the Fe-rich redox sensitive clay minerals chamosite and glauconite suggest intermittent anoxia in the water column and porewaters during some intervals of Harkless deposition. Further, by combining iron speciation extractions with petrographic and x-ray diffraction analyses, we demonstrate that glauconite and chamosite were likely formed at or below the sediment water interface and are also not consistently or fully removed by the standard sequential iron reagents, as noted by previous work. The presence of these authigenic Fe-bearing clay minerals overall results in an underestimation of the proportion of redox active iron during deposition and early sediment diagenesis that may lead to incorrect environmental characterization. We therefore advocate for caution and careful examination of the mineralogy of samples when applying the iron speciation proxy, particularly when authigenic and/or early diagenetic iron- bearing clay minerals are suspected to be present. Based upon these findings, we suggest that the Harkless Formation experienced variable local bottom water redox conditions ranging from oxic to anoxic during deposition. Changes in the relative abundances of chamosite and glauconite may also indicate an increase in oxygenation up section, suggesting that local water column oxygenation played a role in archaeocyathan reef development.