Collaborative Research: Characterizing the role of reverse weathering reactions on marine Si and Li isotope mass balances

Silicon is the second most abundant element in the Earth’s crust and the reactions that it undergoes on the surface of our planet controls the biogeochemical cycles of many elements, including carbon. Most minerals in the Earth’s crust are composed of silicate minerals and the dissolution (or weathering) of these minerals on land can be driven by carbonic acid, which forms by carbon dioxide and reacting with water. The drawdown of atmospheric CO2, a greenhouse gas, via these forward weathering reactions can modulate the C cycle and regulate the temperature of the surface of our planet. At the same time, clays and silicate minerals can form in the ocean seabed (reverse weathering), mostly in tropical deltas, and these reactions produce carbon dioxide. The magnitude of both forward and reverse weathering reactions can be traced through the ratios of Si isotopes and the ratios of an associated product of Si mineral weathering, lithium, over different time scales provided we can differentiate the signatures of these distinct reactions. This project will study reverse weathering signatures within major sediment depocenters that can impact C cycling and the coupled cycles of Si and Li. It seeks to develop Si and Li isotope ratios in different biogeochemical reservoirs as proxies of modern reverse weathering reactions. Further, the project aims to develop models that provide a basis for reconstructing ancient C and coupled elemental cycling in the ocean. Measurements and model simulations are an important perspective from which to view modern changes and rates of change in the C cycle. Anthropogenic forcing of the C cycle is driving rapid changes in climate and ocean chemistry, for example, acidification. These changes and impacts are of major societal importance. In this regard, it is critical to check the dynamics of the C cycle over multiple time scales and understand the processes that control them. This project will provide support for two early career scientists, two graduate students, and five undergraduate students. The PIs will prioritize recruiting undergraduate and graduate students from underrepresented backgrounds, two-year community colleges, and the AGU Bridge program which will offer measurable socio-economic benefits to minoritized populations and improve STEM recruitment from two-year to four-year degree granting institutions. The project will also provide STEM learning experiences for more than one hundred local 4th and 5th grade students through a partnership with the Girls at the Museum Exploring Science (GAMES) program run by the CU Museum of Natural History. Stable isotope ratios of Li and Si in marine geological records are promising proxies of the many mechanisms through which the Si and C cycle are coupled on the surface Earth. Advances in instrumentation and technological innovation over the past ~20 years have allowed us to resolve isotope ratios of silicate mineral weathering products Li and Si with enough precision to render their compositions in marine geological records as integral tools to deciphering past and present biogeochemical cycling, weathering regimes, secondary mineral formation, and low and high temperature geochemical reactions across the surface Earth. These proxies are used with a presupposition of isotopic mass balance which may not be valid for all spatiotemporal scales (e.g., embayment scale versus global ocean, or glacial-interglacial cycles versus geological time scales of 106 years). Reverse weathering reactions, or the neoformation of alumino-silicate phases that consume alkalinity and produce CO2, in deltaic systems are the second largest estimated silica sink in the modern ocean and are unconstrained in isotope mass balance summaries. Archived samples of porewater and sedimentary reactive Si reservoirs from three major deltas (Amazon delta, French Guiana mobile mud belt, and the Gulf of Papua), the depocenters where the majority of reverse weathering reactions apparently occur in the modern Earth, using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) for Li and Si stable isotope ratios. We propose to constrain the isotopic composition of end-members under various sediment transport, geomorphic, and lithological regimes. End-member characterizations and constraints will be applied to a global inverse isotope mass balance model to assess whether Si marine summaries are in isotopic mass balance. The model will be calibrated against available Si isotopic compositions in geological records of biogenic Si through the Last Glacial Maximum. A new inverse isotope mass balance model will be constructed for modern Li marine summaries, a promising proxy for the abiological Si cycle, and also tested. Modeling results will allow us to probe the nuances of CO2 control by the silicate mineral weathering and reverse weathering reactions as relative importance of Si sources and sinks evolve through glacial extent and retreat. Sea level rise, salinization, and subsidence together may act to expand the global footprint of deltaic systems and consequently increase the area over which reverse weathering reactions are favored. Developing a conceptual framework for how these processes are linked to dissolved fluxes of constituents hosted in silicate minerals, CO2, and alkalinity fluxes in deltas is critical to understanding and modeling how this sink will evolve under changing environmental conditions and be incorporated into global biogeochemical cycles. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.