RECONSTRUCTING THE SECULAR EVOLUTION OF LITHIUM ISOTOPE COMPOSITION OF SEAWATER FROM MARINE HALITE

The lithium isotopic composition of seawater (δ⁷Li_SW) has emerged as a promising tracer for reconstructing global terrestrial silicate weathering and marine reverse weathering. Both terrestrial chemical weathering and marine reverse weathering are hypothesized to play important roles in regulating Earth’s geologic carbon cycle and climate [1,2]. Testing these hypothesized mechanisms requires establishing accurate records of ancient δ⁷Li_sw. Recent records of δ⁷Li_sw derived from foraminifera [2], brachiopods [3,4], and shallow marine carbonates [5] show an increase of ∼8–9‰ over the past 60 Myr. However, laboratory experiments [6] and studies of drill cores from modern carbonate platforms [7] suggest that reconstructing past seawater δ⁷Li values from skeletal and non-skeletal carbonates is complicated by vital effects, diagenesis, and mineralogy (e.g., calcite vs. aragonite). Thus, other archives are needed to determine whether δ⁷Li values measured in carbonates indeed reflect secular changes in δ⁷Li_sw. Here, we present lithium isotope ratio measurements of fluid inclusions in marine halites collected from modern salinas (solar evaporation ponds) and large suite of Neoproterozoic and Phanerozoic evaporite basins. These fluid inclusions were previously used to document the major, minor, and trace element composition of paleoseawater, including lithium concentration [8]. Lithium isotope ratios measured in ancient marine halites provide an independent archive of δ⁷Li_sw, extending the record of the δ⁷Li_sw to 550 Myr.

[1] Walker J. C. G., Hays P. B., Kasting J. F. (1981) J. Geophys. Res. 86, 9776–9782.

[2] Misra S., Froelich P. N. (2012) Science 335, 818–826.

[3] Washington K. E., West A. J., Kalderon-Asael B., Katchinoff J. A., Stevenson E. I., Planavsky N. J. (2020) Geology, 48, 1058–1061.

[4] Gaspers N., Magna T., Jurikova H., Henkel D., Eisenhauer A., Azmy K., Tomašovych A. (2021) Chem. Geol. 586, 120566.

[5] Kalderon-Asael B., Katchinoff J. A. R., Planavsky N. J., Hood A. V. S., Dellinger M., Bellefroid E. J., Pogge von Strandmann P. A. et al. (2021) Nature, 595, 394–398.

[6] Vigier N., Rollion-Bard C., Levenson Y., Erez J. (2015) C.R. Geosci. 347, 43–51.

[7] Murphy J. G., Ahm A. C., Swart P.K., Higgins J.A. (2022) Geochim. Cosmochim. Acta 337, 140–154.

[8] Weldeghebriel, M. F., Lowenstein, T. K., Xia, Z., & Li, W. (2023). Geochim. Cosmochim. Acta 346, 165-179.