Constraining the Lithium Seawater Mass and Isotope Budget: Diagenetic Processes Through Marine Pore Waters

Abstract

Silicate weathering consumes CO2 and controls cation fluxes to the ocean, thus playing a critical role in modulating long-term seawater chemistry and climate. There are very few markers of seawater chemistry whose value has changed over time as a function of the uplift – weathering – subduction cycle. Lithium, being one such proxy has been extensively utilized as a geochemical tracer whose long-term evolution in seawater is a function of Urey’s tectonic cycle. Marine pore-waters are an excellent archive to study the sedimentary processes affecting seawater chemistry. Thus, marine pore water Li concentration and isotopes are utilized in elucidating numerous under-constrained diagenetic processes such as marine clay formation, clay transformation, carbonate diagenesis, subduction of slab, and clay dewatering. To constrain the above processes through Li isotopes, we have developed a method for separation of Li from matrix elements using a single-step column chromatography technique and precise measurement of Li isotope ratios using inhouse ICP-QQQ. Some of the features of the developed method are high column recovery 101.0 ± 1.2 % (n = 20), low cumulative Li blank (<0.6 pg) and crustal element blanks (<1.5 ng), high Na tolerance (up to 100:1 of Na:Li), low mass requirement (<0.15 ng per analysis), and a sub-permil precision (±0.6 ‰, 2s). Utilizing the above method, we analysed pore water samples from IODP Leg 339 (Mediterranean outflow) and Leg 379 (Amundsen Sea, Southern Ocean) for Li isotopes to deduce the processes occurring at these sites and its implications on Li seawater budget. At the pore water-sediment interface of continental margins, authigenic alumino-silicate clay (smectite) formation also termed as Reverse Weathering, removes Li from seawater/pore water. The reverse weathering process preferentially uptakes 6Li over 7Li. Thus, pore waters are depleted in Li compared to seawater ([Li]PW < [Li]SW), and the pore water Li isotopic composition is more enriched in 7Li than seawater (δ7LiPW > δ7LiSW). This process occurs in shallow sediment depths where smectite is the dominant clay and illitization is not commenced. However, most pore water profiles exhibit higher Li concentrations and lighter isotopic compositions indicating clay transformation process. During clay transformation i.e., smectite to illite transformation, owing at high pressure and temperature (150-200 Celsius) during burial, isotopically light Li is released from the clays. This release of isotopically light Li increases the pore water Li concentration while driving it isotopically light. During sediment subduction, a significant fraction of this clay bound isotopically light Li is released as a part of clay dewatering. This thesis investigates IODP Leg 339 pore water samples with clay dewatering evidence (recorded by δ18O of pore waters) and IODP Leg 379 pore water samples in silica-rich environment with high Li concentration and lighter isotopic composition relative to seawater helps us to constrain Li seawater mass budget further. A preliminary set of equations that govern flux of an element between the marine sediments and seawater following the general diagenetic equation (GDE) is also developed in the present work. These equations incorporate the effects of diffusion, advection and reaction kinetics in the sediments and thus, govern the transfer of Li within the sediment column. Flux calculations for 5 IODP sites and 12 from the literature have been included in the work, and the implications of these flux calculations have been discussed in detail. A thorough development of this model will lead to establishing a mass and isotope budget in seawater which will be applicable across elements and processes. This fundamental study of Li seawater chemistry brings us a step closer in understanding the complex dynamics of ocean systems.