SEAFLOOR HYDROTHERMAL SYSTEMS CONTROL SEAWATER CHEMISTRY: EVIDENCE FROM FLUID INCLUSIONS IN HALITE

Long-term changes in the major ion and isotopic composition of seawater coincide with icehouse-greenhouse climate fluctuations, calcite-aragonite seas, and sea level changes. However, there is disagreement over what processes controlled the changes in ocean chemistry. This study uses a new record of Li concentration in paleoseawater to explore how temporal variations in the flux of MOR hydrothermal brines, the largest source of Li to seawater, and reverse weathering of seafloor basalts (important sink) control the oceanic Li cycle on multimillion-year time scales.

Here we present a 350-million-year record of seawater lithium concentrations [Li⁺]_sw from direct measurement of primary fluid inclusions in marine halite using combined LA-ICP-MS and cryo SEM-EDS. We also present a 150 Myr forward model of [Li⁺]_sw.

From 350-0 Ma, the lithium concentration of seawater oscillated systematically, parallel to secular variations of sea level, greenhouse-icehouse climates, and major ion chemistry such as the Mg²⁺/Ca²⁺ ratio. Highest seawater Li occurred during the Cretaceous, up to one order of magnitude higher than modern [Li⁺]_sw, which coincides with low seawater Mg²⁺/Ca²⁺ ratios, high atmospheric CO₂, and Mesozoic-Early Cenozoic Greenhouse climates. Such high Li concentrations require high MOR hydrothermal activity. Conversely, Permian and Cenozoic (35-0 Ma) seawater had relatively low Li, consistent with high Mg²⁺/Ca²⁺ ratios, low atmospheric CO₂, and late Paleozoic and Cenozoic icehouse periods.

The forward model involves 10 Kyr time steps and variable cycling of hydrothermal fluids through the axial portion of the MOR system and variable rates of low-temperature weathering of seafloor basalts. The model agrees well with paleoseawater fluid inclusion data for Li. The same model parameters, with variable Li isotope fractionation of off-axis oceanic crust, are used to successfully model the 9‰ increase of δ⁷Li in seawater from 60-0 Ma. Our data and modeling suggest that seafloor hydrothermal systems exerted the dominant control on the [Li⁺] and δ⁷Li composition of Phanerozoic seawater. These data will be further used to test the long-term relationships between seafloor MOR activity, the carbon cycle, and climate.