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A 2023 study reconstructs seawater lithium concentrations falling from about 184 μmol/kg of water around 150 million years ago to 27 μmol/kg today—an approximately sevenfold decline. Its geochemical-cycle model points chiefly to declining ocean-crust production and related hydrothermal activity at mid-ocean ridges and their flanks. The ancient values are estimates from geological evidence, not direct measurements of long-vanished seawater.
What the sevenfold decline means
Weldeghebriel and Lowenstein’s 2023 reconstruction estimates about 184 μmol/kg H₂O around 150 million years ago (Ma), compared with 27 μmol/kg H₂O today. The authors characterize the endpoint change as approximately sevenfold. It describes a change in lithium concentration in seawater, not a sevenfold change in every part of the ocean at every point in time.
The paper also gives a fitted long-term decline of about 1.2 μmolal per million years across roughly 150 million years. That is an average trend estimate, not a uniform rate for every geological interval.
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Ancient seawater cannot be sampled directly, so the study used a geological proxy: fluid inclusions—small pockets of trapped fluid—preserved in marine halite, or rock salt. The researchers measured lithium-to-magnesium (Li/Mg) ratios in those inclusions and combined them with published estimates of ancient seawater magnesium concentrations to calculate paleoseawater lithium concentrations.
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The conversion relies on the inclusion measurements and the paleomagnesium estimates; the resulting absolute lithium concentrations are reconstructed values. The authors describe lithium and magnesium as conservative ions for this calculation. The method therefore offers a record inferred from preserved geological material, rather than a direct chemical reading of seawater from the distant past. The full study is available through PubMed Central.
The record has highs and lows, not a perfectly smooth descent
The study reports substantial variation among intervals. Its values include high estimates in parts of the Jurassic and Cretaceous, followed by lower estimates in later periods:
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| Geological interval | Estimated seawater lithium concentration |
|---|---|
| Kimmeridgian–Tithonian, 155.7–145.5 Ma | 156–184 μmolal |
| Albian–Cenomanian, 112.2–93.5 Ma | 156–184 μmolal |
| Eocene, around 36 Ma | 68 ± 8 μmolal |
| Serravallian–Tortonian, about 14–11 Ma | 45–53 ± 3 μmolal |
| Messinian, about 6–5 Ma | 43 ± 3 μmolal |
These are reported estimates for particular intervals, not evidence that concentration moved steadily between each value. The authors’ headline comparison uses μmol/kg H₂O, while the intermediate figures are reported in μmolal.
Why the authors think concentrations declined
The proposed leading explanation comes from the study’s model of the lithium geochemical cycle. It finds that long-term decreases in ocean-crust production and in mid-ocean-ridge and ridge-flank hydrothermal fluxes chiefly controlled the reconstructed concentration decline. The model does not require a change in continental weathering fluxes to account for that trend.
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This is a model-based interpretation of the proxy record, not a direct measurement showing how much hydrothermal fluid flowed through ancient ridges. The distinction matters: the concentration estimates come from geological proxies and paleomagnesium values, while the proposed cause is inferred by modeling the lithium cycle. The original Science Advances paper describes both the reconstruction and the authors’ interpretation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Concentration is not the same as lithium isotopes
Seawater lithium concentration, written [Li⁺]SW, is the amount of dissolved lithium. Lithium isotopic composition, written δ⁷LiSW, describes the relative abundance of lithium isotopes. They are different measurements and should not be treated as interchangeable.
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The concentration record bears on discussion of the Cenozoic rise in δ⁷LiSW, but it does not settle why the isotope composition changed. Proposed explanations for the isotope trend include changes in continental weathering, marine clay processes, and isotopic fractionation at oceanic sinks. A 2026 Nature Communications article argues, using its own paleowater record and global lithium-cycle model, that continental silicate weathering from tectonically active mountains is unlikely to explain the observed isotope rise. That work addresses the broader isotope debate; it is not an independent replication of the 150-million-year concentration record.
How this trend fits the wider geological record
The lithium decline parallels a long-term increase in seawater magnesium-to-calcium ratio (Mg/Ca) and strontium isotope ratio (⁸⁷Sr/⁸⁶Sr), as well as changes in calcite-versus-aragonite seas, evaporite mineralogy, and greenhouse-to-icehouse climate. These are broad geological associations. Their coexistence does not show that falling lithium caused those other changes.
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