Carbonate Archives of Seawater Lithium: Element and Isotope Ratios, Diagenesis, and the History of Seawater Chemistry

Abstract

Records of the element and isotopic composition of ancient seawater provide insight into the long-term evolution of the Earth system. The lithium isotopic composition of seawater (ẟ7Lisw), preserved in the lithium isotopic composition of carbonate fossils and shallow-marine carbonate sediments have emerged as important archives for investigating the geologic controls of seawater chemistry, the global carbon cycle, and Earth’s climate on million-year timescales. However, inferring past seawater ẟ7Li from carbonate fossils and sediment is complicated by analytical challenges and the effects of mineralogy and diagenesis. This dissertation investigates each of those challenges and provides a pathway to producing accurate records of past seawater ẟ7Li. Measurements of carbonate-associated Li requires careful leaching of bulk samples to avoid silicate-associated Li contamination, followed by complete separation of Li from matrix elements for isotope-ratio analysis. Chapter 2 presents the development of a high-throughput method suitable for measuring ẟ7Li in seawater, carbonate fossils, and bulk carbonate sediment. Well-preserved fossils can provide robust records of past seawater chemistry if the effects of biomineralization (i.e., vital effects) and mineralogy are considered. Chapter 3 presents a new record of seawater ẟ7Li values and Li/Ca ratios from well-preserved fossil corals extending back to the Triassic. Secular variation in these records is strongly correlated with seawater 87Sr/86Sr and Mg/Ca. Taken together, these records support the hypothesis that the primary control of seawater chemistry on Phanerozoic timescales is seafloor hydrothermalism, as opposed to continental weathering. Such fossil archives are limited by the rarity of well-preserved fossils. To "ll-in and extend the record of seawater ẟ7Li throughout Earth history, we must turn to shallow-water marine carbonates. The chemistry of bulk-carbonate marine sediment contains convolved information about ancient seawater, mineralogy, and the diagenetic history of the rock. Chapters 4 and 5 investigate the influence of marine and meteoric diagenesis, respectively, on the lithium isotopic composition of carbonate sediment. Using paired measurements of element and isotope ratios and a numerical model of diagenesis, we present pathways for characterizing diagenesis and inferring past seawater ẟ7Li values.