EAPS

Seafloor Hydrothermal Systems Control Seawater Chemistry: Evidence from Fluid Inclusions in Halite

The major ion chemistry (Mg2+, Ca2+, Na+, SO42-, Cl-, HCO3-) and isotopic composition (δ7Li, δ11B, 87Sr/86Sr) of seawater have varied in the Phanerozoic. Changes in the Mg2+ and Ca2+ concentration of ancient seawater influenced the evolution of marine shell-building organisms because the Mg/Ca ratio of seawater controls which carbonate, calcite or aragonite, is favored to precipitate. Secular changes in the Ca2+, Mg2+, and SO42- concentration in Phanerozoic seawater controlled the mineralogy of marine potash evaporites such that MgSO4 evaporites occur in phase with aragonite seas and KCl evaporites occur with calcite seas. Long-term changes in the major ion chemistry of seawater also parallel sea level fluctuations, icehouse-greenhouse climates, and variations in atmospheric CO2.


Fluid inclusions in marine halite are direct samples of ancient evaporated seawater. Chemical analyses of seawater fluid inclusions were used to document the major ion chemistry of the Phanerozoic oceans, but there is disagreement over which global-scale processes were most directly responsible for the observed variations. First order controls on the temporal changes in seawater chemistry include variations in the rates of: (1) seafloor spreading and midocean ridge hydrothermal discharge, (2) continental weathering, for example, Cenozoic uplift of the Himalayas, (3) deposition of marine carbonates and dolomitization, and (4) low-temperature weathering of seafloor basalt and incorporation into marine sediments.

Recent analytical advances using Laser ablation ICP MS now allow quantitative measurement of minor and trace elements in fluid inclusions in halite. Lithium, for example, has undergone significant variations in concentration in Phanerozoic seawater. Li is a conservative element that is not incorporated to any significant degree in minerals precipitated from seawater, such as calcite, aragonite, gypsum or halite, so there is no Li+ loss as seawater evolves during progressive evaporation. Li today is largely supplied to the oceans by midocean ridge hydrothermal brines (~70% of modern Li+ inflow to the oceans) at concentrations three orders of magnitude higher than river water sources of Li, so past variations in the Li+ concentration of seawater may be related to fluctuations in midocean ridge hydrothermal activity. Here, we present a 350 My record of seawater Li+ concentrations from direct measurement of primary fluid inclusions in marine halite and show from modeling that seafloor hydrothermal systems have controlled the Li+ composition of seawater over geologic time. The modeling results apply to other elements whose past concentrations mirror that of Li+, most notably Ca2+ and Sr2+.