Cenozoic Island Dolostones — Cole A. McCormick

Overview

Modern examples of low-temperature dolomite formation are rare, which makes Cenozoic “island dolostones” — like those on Grand Cayman and Cayman Brac (British West Indies) — one of the best natural laboratories available for testing how carbonate geochemical proxies hold up during dolomitization. These platforms are geologically young, have never been deeply buried, and the conditions responsible for dolomitization can be reasonably inferred.

A well-documented dolomitization front on Grand Cayman allows for these proxies to be directly tested. Using carbonate clumped isotopes (Δ₄₇), dolostones along the periphery of the island formed at a temperature of 19.1 ± 1.4°C from a fluid with δ¹⁸O_FLUID = 0.2 ± 0.5‰ VSMOW, indistinguishable from normal seawater. Petrographically, the dolomite crystals are pristine. By any standard, this is about as close to textbook, low-temperature seawater dolomitization as the geological record offers.

Then we measured uranium isotopes (δ²³⁸U) in that same dolomitization front. Despite pristine petrography and an unambiguous seawater-temperature and δ¹⁸O_FLUID signature, δ²³⁸U in the dolostones is offset from both modern seawater and the coeval limestone: A clear diagenetic overprint. Cerium anomalies (Ce/Ce*), measured on the same samples, show no such offset at all. Two proxies, same rocks, same fluid history: One survives, one doesn't. Nothing in the texture or the clumped-isotope temperature would have warned us which.

Reconstructing ancient seawater chemistry from dolomitized rocks therefore requires a diagenetic correction factor for δ²³⁸U, calibrated to the specific depositional and diagenetic setting, rather than one fixed value applied everywhere. A proxy that fails under otherwise ideal, texturally pristine conditions is a reminder that every geochemical archive needs an independent check like this one before its signal is taken at face value.

Key Figures

Core photograph and thin-section photomicrographs of dolomitized Cayman Formation facies, showing recognizable skeletal grains including bivalves, coralline algae, and large benthic foraminifera despite complete dolomitization. — Core photograph (A) and thin-section photomicrographs (B–C) of dolomitized Cayman Formation facies. Original skeletal grains remain clearly recognizable despite complete replacement by dolomite, including bivalves (bi), large benthic foraminifera (lf), coralline algae (ca), and *Amphistegina* (A). Fig. 9, McCormick & Jones (2021), *Marine Geology*, 436, 106470.

Additional core photograph and thin-section photomicrographs of dolomitized Cayman Formation facies, showing preserved coral, algal, and foraminiferal grains. — Additional core (E) and thin-section (F–G) photographs of the same dolomitized facies, showing preserved *Halimeda* (Ha), *Stylophora* coral (St), rhodoliths (rh), and coralline algae (ca), evidence of fabric-retentive dolomitization that leaves the original depositional texture intact. Fig. 9, McCormick & Jones (2021), *Marine Geology*, 436, 106470.

SEM images of dolomite crystals from the interior and periphery of Grand Cayman, showing clean, well-ordered crystal faces with minimal secondary alteration. — The same story at much higher magnification. SEM petrography of Grand Cayman dolomite, interior (a–b) and periphery (c–d), shows pristine crystal faces and minimal secondary alteration from hand sample down to the micron scale. Fig. 4, McCormick et al. (2026), Carbonate clumped isotopes (Δ₄₇) in Cenozoic ‘island dolostones’.

Plot of fluid oxygen isotope composition versus clumped isotope temperature, showing Cayman Islands periphery samples plotting within the normal seawater range alongside other global island dolostone localities. — Clumped-isotope temperature (TΔ₄₇) versus diagenetic fluid composition (δ¹⁸O_FLUID): The Cayman “periphery” samples (dark blue diamonds) cluster at low temperature with near-zero δ¹⁸O_FLUID, a normal-seawater signature distinct from the warmer, evaporated “interior” samples (orange). Fig. 7, McCormick et al. (2026), Carbonate clumped isotopes (Δ₄₇) in Cenozoic ‘island dolostones’.

Two stacked crossplots: cerium anomaly and uranium concentration, each plotted against uranium isotope ratio, showing dolostone and limestone samples in distinct color-coded bands. — Ce/Ce* (upper) survives dolomitization; U/(Ca+Mg) and δ²³⁸U (lower) are overprinted. Tan markers/band: Dolostone (±95% CI). Blue: Limestone (±95% CI). Dashed line: Modern seawater. Fig. 2C–D, McCormick et al. (In Prep), Decoupled behaviour of cerium anomalies and uranium isotopes during dolomitization.

Selected Publications

McCormick, C.A., Jones, B., Hardisty, D.S., Lau, K.V. (In Prep). Decoupled behaviour of cerium anomalies and uranium isotopes during dolomitization.

McCormick, C.A., Jones, B., Olsen, E.K., Ingalls, M. (2026). Carbonate clumped isotopes (Δ₄₇) in Cenozoic ‘island dolostones’: Dolomitization driven by modified seawater at Earth-surface conditions. Geochemistry, Geophysics, Geosystems, 27, e2026GC013142.

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McCormick, C.A., Jones, B. (2021). On the efficacy and limitations of isolated carbonate platforms as “oceanic dipsticks” to reconstruct subsidence histories, a case study from the Paleogene to Neogene strata on Grand Cayman and Cayman Brac, BWI. Marine Geology, 436, 106470.

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