A Proto-Caribbean Record of the Weissert Event from Cuba

In the Sierra de los Órganos in western Cuba, the steep limestone mogotes preserve sediments that accumulated on the floor of the Proto-Caribbean about 135 million years ago, at a time when dinosaurs dominated terrestrial ecosystems. Thin dark, organic-rich bands alternate with paler grey limestones, forming a barcode-like pattern laid down by an ocean whose deeper waters were periodically starved of oxygen. These strata record one of the earliest major climate disturbances of the Cretaceous: an oceanic anoxic event.

In our recently published open-access article in Frontiers in Earth Science (here), we analyzed sediments from a quarry near the town of Pons, in the Sierra de los Órganos, to reconstruct how the early Proto-Caribbean responded when Earth’s carbon cycle shifted into a different state.
During the Valanginian (Early Cretaceous Period), greenhouse conditions were already in place, but something pushed the system further. Some scientists think in terms of volcanic outgassing, high CO₂, intensified weathering, and nutrient-rich runoff. The oceans stored this disturbance in the only way they can, by changing how they circulate, how they feed life, and how much oxygen reaches the depths. In many basins worldwide, that change shows up as intervals of dark, organic-rich sediments and a global shift in carbon isotopes. This coupled package of carbon-cycle disruption and marine deoxygenation is what we call the Weissert Event.

La Lata sits on what was then a marine slope along the margin of the young Proto-Caribbean Basin, between the evolving Americas and the Tethyan realm. Today it’s an active quarry. In the early Cretaceous it was a quiet pelagic seafloor, slowly accumulating the skeletal rain of coccolithophores, radiolarians, and tiny foraminifera. In that setting, any disturbance in oxygen, productivity, or sediment flux is recorded very efficiently.

We focused on just the lowermost ~4 meters of a ~30-meter section of the Pons Formation. At first glance it seems simple: comparatively thick, medium-gray limestones rich in carbonate are interbedded with much thinner, darker, carbonaceous marls and marly limestones. Under the microscope, however, this “barcode” resolves into five distinct microfacies, ranging from bioturbated, fossil-rich limestones to laminated, nearly barren, organic-rich levels with abundant sulphate minerals, like pyrite.

The first key result is how carbon cycle is distributed and sequestered within the sediments. The dark marls contain total organic carbon up to ~10-11 wt%, while the paler limestones usually sit around 1-3 wt%. Total inorganic carbon behaves in the opposite way: carbonate-rich in the thick gray beds, diluted where organic matter and clay concentrate. That alternation already hints at fluctuating conditions at the seafloor. These were intervals more favorable to the preservation of organic matter, separated by more “normal” background sedimentation.

Superimposed on that fabric is a clear negative excursion in δ¹³Corg of about 1.7‰. That excursion is not random in time. Biostratigraphic markers, especially the presence of specific calcareous nanoplankton place this interval in the late Valanginian subzone NK3B, the very window classically associated with the Weissert Event elsewhere in the world, and linking our local curve to the global record.

The dark beds do more than carry isotopes. They are loaded with mineral and microscopic evidence that oxygen in bottom waters dropped sharply during certain pulses. Under SEM we see framboidal pyrite and small cubic aggregates, often growing inside foraminiferal tests, mingled with illite-smectite clays and microbial fabrics. Redox-sensitive trace elements peak in the same horizons: vanadium, nickel, chromium, molybdenum, uranium, thallium, and sulfur all show marked enrichments where total organic carbon is highest. At the same levels, detrital indicators such as Al, Si, Ti, and Li also increase, suggesting stronger terrigenous flux -probably the fingerprint of enhanced runoff and weathering on land.

Bioturbation tells a complementary story. In the thick gray limestones, burrows are common and the bioturbation index often reaches 3–4: the seafloor was oxygenated enough for infauna to churn the sediment. In the organic-rich marls, the index drops to 1–2, and X-ray images reveal only faint, incomplete structures. These are intervals where oxygen availability was restricted enough to suppress most benthic activity.

Put together, the picture that emerges is not a single, monolithic “black shale” horizon but a series of deoxygenation pulses, closely tracking the negative carbon isotope excursion. During those pulses, Los Órganos lay beneath waters that were more stagnant or less ventilated, but more superficially nutrient-rich. Organic matter arriving at the seafloor was less efficiently destroyed, sediment input from land was boosted, and trace metals were scavenged from the water column into the accumulating muds.

Why does this matter beyond the satisfaction of matching a Cuban cliff to a named global event? First, it demonstrates that the Proto-Caribbean Seaway did not sit on the sidelines of Valanginian climate change. It participated fully in the Weissert disturbance. The same isotope signal, the same style of organic-rich sedimentation, and comparable redox signatures appear here as in better-known Tethyan sections. Second, the section at La Lata shows how a marginal to hemipelagic basin can register global forcing factors like higher CO₂, weathering, in its own local language of facies, microfossils, and chemical inventories.

Finally, there is an uncomfortable echo with the present. The rocks at La Lata, Sierra de los Órganos, formed under long-term CO₂ buildup, intensified weathering, and nutrient loading. The ocean responded by reorganizing circulation and oxygen distribution, carving “dead-zone” intervals into the sedimentary archive. Today we are driving the carbon cycle far faster, but the physics and chemistry of seawater have not changed. Ancient barcodes like those of the Pons Formation are not predictions, but warnings. When the carbon cycle is pushed hard, oceans everywhere, from open basins to narrow seaways, can slide toward deoxygenation affecting not just the carbon cycle, but all life in earth.

In that sense, each black band in the quarry is both a record of a vanished ocean and a quiet reminder that Earth’s climate system has thresholds. Our goal at La Lata was to read that record as clearly as possible, and to place the Proto-Caribbean firmly on the map of ocean anoxic event research. The story those limestones tell is simple and stark = when the planet breathes carbon too quickly, the oceans are often the first to lose their breath.

Recommended paper

Our pen-access article: Orihuela J., Melinte Dobrinescu M.C. and Maurrasse F.J-M.R. (2025) “Characterization of the Weissert oceanic anoxic event in lower Cretaceous limestones of the Guaniguanico terrain, Sierra de los Órganos, Western Cuba,” published in Frontiers in Earth Science (Vol. 13, 1549274) on 20 April 2025 (doi: 10.3389/feart.2025.1549274).