TITLE: Geochemical Constraints on the Formation of Authigenic Clay Minerals During Reverse Weathering

Location: Ford ES&T Building, Room L1114; Online with Microsoft Teams Meeting (http://bitly.ws/PrLV, Meeting ID: 295 412 478 941; Passcode: Rgu7X5)

Time: 9:00 AM-11:30 AM EST, August 18th, 2023

Committee members: Dr. Yuanzhi Tang (Advisor), Dr. Martial Taillefert, Dr. Ellery Ingall, Dr. Chris Reinhard, Dr. Jeffrey Krause

Abstract: The formation of authigenic clay minerals from biogenic silica (bSi) in marine sediments, termed reverse weathering (RW), describes the formation of cation-rich aluminosilicate clay phases at the expense of bSi and detrital lithogenic minerals with concurrent sequestration of cations (e.g., Fe2+, Mg2+, and K+), consuming alkalinity and releasing CO2. The RW process acts as a globally significant sink for Si and other elements such as Fe. It is an important process for regulating ocean pH and atmospheric CO2 over geologic time scales. However, the formation mechanism, kinetics, and phases of authigenic clay during RW has long puzzled the geochemical community and much remains unknown, owing to the intrinsic challenges to trace and characterize authigenic clay formation during RW. This dissertation develops a framework of understanding for authigenic clay formation in the RW process by systematically investigating the potential geochemical triggers, identifying the product phases, and unrevealing the reaction rates and mechanisms under well-controlled laboratory conditions. Dissolved Fe2+ and Al were identified as triggers for authigenic clay formation from bSi. The formation mechanism was identified to occur through (1) dissolution of Fe and Al minerals and bSi for dissolved Fe2+, Al, and dissolved Si (DSi) inputs, and (2) reprecipitation of these species via homogeneous or heterogenous precipitation and transformation. Authigenic clay formation rates, mechanism, and steps were constrained by initial DSi concentration, dissolved Fe2+ and Al concentration, and the presence of substrates. The authigenic phase type and crystallinity are constrained by reactant type and concentration (e.g., solid Al mineral phase for sufficient Al supply or limited sporadic dissolved Al input spikes, dissolved Fe2+ concentration), the presence of substrates, and reaction time. The observed authigenic phase includes Fe-bearing mica, berthierine, chamosite, and Fe-Al layered double hydroxide, allophane-like Al-Si gel precursor phase, hisingerite-like Fe(II)-Si gel precursor phase, and Fe-smectite. The results shed light on authigenic clay formation during RW and provide bases to refine Earth system models and quantify the impacts of RW in regulating global biogeochemical cycles and sinks, CO2 dynamics, and global climate.