Seafloor weathering and the oxygen isotope ratio in seawater: Insight from whole-rock δ18O and carbonate δ18O and Δ47 from the Troodos ophiolite
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Abstract The controls on, and history of, the oxygen isotope ratio in seawater continue to be debated after many decades of research with the lack of consensus in large part reflecting uncertainty in the role of hydrothermal exchange between seawater and the oceanic crust. We have investigated this using new carbonate Δ 47 and δ 18 O data, and whole-rock O-isotope data, for samples from the lava section of the Troodos ophiolite. Carbonate data confirm that fluid-to-rock ratios in the upper lavas during off-axis hydrothermal circulation are generally sufficiently large that both the fluid δ 18 O and temperature are similar to those of bottom water. However, some samples require more complicated interpretations that could reflect changes in the rate of calcite formation. Whole-rock data indicate that O-isotope exchange in the lavas is directly linked to the major element exchange that leads to alkalinity production (i.e., CO2 consumption) and both are dependent on bottom water temperature. This means that the O-isotopic composition of seawater is linked to the long-term C-cycle. The data are used to parameterise a simple model of the evolution of the O-isotopic composition of seawater driven by changes in solid earth CO2 degassing. Alkalinity balance links the total extent of weathering of the continents and seafloor, which are sinks for high δ 18 O material, to CO2 degassing rate and surface temperature. The modelling suggests that if solid earth CO2 degassing and the rate of formation of oceanic crust are linked, the O-isotopic composition of the ocean (including any ice sheets) is unlikely to have varied more than ±1‰ over the Phanerozoic.
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Carbonates formed close to isotopic equilibrium are only expected to be found in environments with very low supersaturation states, such as recrystallized carbonates from deep-sea sediments or carbonates associated with low-temperature hydrothermal alteration of young oceanic crust.
“Carbonates formed close to isotopic equilibrium are only expected to be found in environments with very low supersaturation states, such as for example recrystallized carbonates from deep-sea sediments54, or carbonates associated with low-temperature hydrothermal alteration of young oceanic crust55.”
From Most Earth-surface calcites precipitate out of isotopic equilibrium (2019) · cites this paper as [55]
Section 4.1 reports that eight Troodos carbonates are most simply interpreted as having grown slowly in isotopic equilibrium during low-temperature alteration of oceanic crust. It does not establish that such carbonates occur only at very low supersaturation; it also suggests that calcite formed when the crust was youngest may have grown rapidly and out of equilibrium. The deep-sea sediment example is attached to a different citation, not this source. (LLM, conf=0.84)
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References 61 · Cited by 26
Research works printed but not linked: 1
- [21]21. McArthur, J. M., & Howarth, R. J. (2004). Strontium Isotope Stratigraphy. In F. M. Gradstein, J. G. Ogg, & A. G. Smith (Eds.), A Geological Time Scale (pp. 96-105). Cambridge: Cambridge University Press.
- Authors
- Coogan, L. A., Daëron, M., & Gillis, K. M.
- Affiliations
- University of Victoria (CA)
- Centre National de la Recherche Scientifique (FR)
- Corresponding author
- Coogan, L.A.
- Venue
- Earth and Planetary Science Letters
- Publisher
- Elsevier BV
- Published
- 2019-02
- Volume
- 508
- Pages
- 41-50
- ISSN
- 0012-821X (ISSN-L)
- OpenAlex
- W2907771304
- Semantic Scholar
- e299ed5f869f717def85c07c2d5ec510db31bed9
- Cited by
- 26
- References
- 61
- Citations by year
- 2026: 2 · 2025: 1 · 2024: 3 · 2023: 1 · 2022: 2 · 2021: 5 · 2020: 10 · 2019: 2
- FWCI
- 3.61
- Type
- journal article
- Language
- en
- Open access
- green
- hal.science
- Licence
- Locations
- Earth and Planetary Science Letters (publishedVersion)
- HAL (submittedVersion, other-oa)
- UVic’s Research and Learning Repository (University of Victoria) (submittedVersion)
- Funders
- NSERC (5098, 155396)
- Topics
- Physical Sciences › Earth and Planetary Sciences › Paleontology › Paleontology and Stratigraphy of Fossils
- Physical Sciences › Earth and Planetary Sciences › Geophysics › Geological and Geochemical Analysis
- Physical Sciences › Earth and Planetary Sciences › Geochemistry and Petrology › Geochemistry and Elemental Analysis
- Keywords
- seafloor weathering, Troodos ophiolite, carbonate oxygen isotopes, CO2 degassing, carbon cycle, oceanic crust, calcite formation, bottom water temperature
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