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. Currently, there is disagreement over what processes control 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.
A total of 639 primary fluid inclusions in 65 halites from 13 Mesozoic and Cenozoic marine evaporite basins, well-documented for major ion chemistry of seawater, were analyzed to obtain [Li+] in paleoseawater using LA-ICP-MS.
Over the past 150 Myr, [Li+] in seawater declined from 180 to 27 µmolal. The decrease in [Li+] parallels: (1) the increase in Mg2+/Ca2+ ratio from a minimum of 1.2 in the Cretaceous to the modern value of 5.2; (2) the change from calcite seas in the Jurassic/Cretaceous to the aragonite seas of the late Cenozoic with the rise of Mg2+/Ca2+; (3) the change from KCl potash evaporites of the Mesozoic to the MgSO4 type of the Cenozoic as seawater [Mg2+] and [SO42-] rose and [Ca2+] decreased; (4) the increase in 87Sr/86Sr from the Late Jurassic minimum of 0.7071 to the maximum today of 0.7092; (5) the increase of δ7Li from 22‰ at 60 Ma to 31‰ today; and (6) the change from the greenhouse climates of the Mesozoic-Early Cenozoic to the icehouse of the late Cenozoic accompanied by lower atmospheric CO2.
A 150 Myr forward model for [Li+] was produced to test fluctuations in ocean crust production and hydrothermal fluid flux control on the Li cycling in seawater. The forward model involves 10 Kyr time steps and variable cycling of hydrothermal fluids through the axial portion of the MOR system, variable rates of low-temperature weathering of seafloor basalts, and constant river flux. 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 65% of the 8–9‰ increase of δ7Li in seawater from 60–0 Ma. The Li 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.
(Hosts: John Higgins & Elizabeth Niespolo)
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