This dissertation contributes to our understanding of the use of the Li-isotopic composition of seawater as a tracer of the earth system with a focus on the role of low-temperature hydrothermal systems within the lava section of the ocean crust. Experiments were conducted to study the exchange coefficient (D(Li/Ca)) and isotopic fractionation factor (α; 1000ln(α)=Δ) for lithium between inorganic calcite and an aqueous solution as a function of solution chemistry. These experiments show that, under the conditions used, D(Li/Ca) negatively correlates with solution H+/Ca2+ ratio (and the solution pH) and Δ positively correlates with solution pH. The change in D(Li/Ca) with solution chemistry is interpreted as indicating that Li is incorporated into calcite as LiHCO3, and hence depends on solution H+/Ca2+. A series of diffusion experiments were performed to test whether changes in pH led to changes in the aqueous Li speciation that would lead to changes in the relative diffusivity of the two Li-isotopes, but no such changes were observed. It is proposed that the change in Δ with changing solution pH may either reflect a kinetic or equilibrium isotope fractionation associated with changing solution chemistry. These results have important implications for interpreting the Li content of calcite that has undergone any diagenetic modification. The Li-content and isotopic composition of rocks altered by low-temperature, off-axis hydrothermal systems in the upper oceanic crust were studied to better understand the role of these systems in controlling the Li-isotopic composition of seawater. Results of a detailed study of DSDP Holes 417A, 417D and 418A from 119 Myr Western North Atlantic Ocean basin show that the Li content of the lavas decreases with depth in the upper ~30 m below sediments while the Li-isotopic composition increases from a low value and then does not show systematic variation in deeper sections. No evidence was found to support a role for a change in mineralogy of alteration products to explain the observed variation in Li composition of lavas. There is also no evidence for the modification of the composition of hydrothermal fluid due to a mixture with sediment pore-fluid. Simple one-dimensional fluid flow and fluid-rock reaction models also cannot explain the observed variation. Instead, a model of free-circulation of seawater through the upper few tens of meters of the lavas, and leakage of modified fluid into the deeper portion of the lava pile seems to explain the observed variations best. To investigate the role of low-temperature off-axis hydrothermal systems on the Li content and isotopic composition of seawater from the Cretaceous to modern era, five DSDP/ODP holes with crustal age spanning from 13.6 to 95 Myr were studied in combination with results from previous studies. Results suggest that the average amount of Li added to the upper oceanic crust decreases, while its average Li-isotopic composition increases, from the Cretaceous to the modern. The simplest explanation for these variations may be a decrease in Li concentration and an increase in Li-isotopic composition, of seawater over this time interval. / Graduate

Manual collection and processing of seawater samples for dissolved gas analyses are technically challenging, time consuming and costly. Accordingly, in situ analysis techniques present attractive alternatives to conventional gas measurement procedures. To meet the demands of sustained, high-resolution chemical observations of the oceans, the University of South Florida and SRI International developed underwater mass spectrometer systems for quantitative measurements of dissolved gases and volatile organic compounds. This work describes the influence of variable in situ conditions on the performance of a membrane introduction mass spectrometer used for measurements in both the water column and sediment porewater. Laboratory experiments to simulate the effects of field conditions on the membrane were performed by varying sample flow rate, salinity, hydrostatic pressure, and chemistry. Data indicate that membrane permeability has a strong dependence on hydrostatic pressure, and a weak dependence on salinity. Under slow flow conditions bicarbonates in solution contributed to carbon dioxide instrument response as a result of carbon system equilibration processes in the boundary layer at the membrane interface. In addition, method development was undertaken to enable underwater sediment porewater analyses and quantitative (calibrated) measurements of total dissolved inorganic carbon (DIC). This work establishes the capability of membrane introduction mass spectrometry to measure two compatible variables (DIC and dissolved CO2) for comprehensive CO2-system characterizations. In addition to laboratory studies three types of field observation were obtained in this work. High-resolution vertical profiles of dissolved gases in the Gulf of Mexico were obtained through system calibration and characterization of the influence of hydrostatic pressure on the behavior of polydimethylsiloxane membranes. In the South Atlantic Bight, sediment porewater profiles of dissolved gases were repeatedly obtained over a 54 hr period. Data trends were in agreement with high remineralization rates facilitated by porewater advection. Finally, time-series underwater DIC measurements that were undertaken proved to be in good accord with results obtained using conventional techniques. These measurements constitute the first quantitative observations of dissolved gas ocean profiles, sediment porewater profiles, and DIC measurements by underwater mass spectrometry.