Tiny Algae Rewrote the End of a Six-Million-Year Ocean Bloom

Between roughly 9 and 3.5 million years ago, the world’s oceans were in overdrive. Sediments accumulating on the seafloor during this stretch of the late Miocene and early Pliocene contain an extraordinary pulse of biogenic material, chiefly calcium carbonate and opal, the mineral fingerprints of two groups of phytoplankton: coccolithophores, which build tiny calcite plates, and diatoms, which construct glassy silica shells. Scientists call this extended episode the late Miocene to early Pliocene biogenic bloom, and it records a period when export production, the flux of organic and mineral matter sinking from the sunlit surface ocean, was elevated across most ocean basins. But pinning down when and why this planetary productivity spurt finally fizzled out has proven stubbornly difficult, with different ocean basins apparently telling different stories.

A new study by Boris-Theofanis Karatsolis, Joseph D. Asanbe, and Jorijntje Henderiks, published in the Journal of Micropalaeontology, tackles that puzzle by reading the fine print written by the algae themselves. Rather than relying solely on bulk carbonate accumulation rates, the team compared the species composition of calcareous nannofossil assemblages at two astronomically tuned deep-sea archives: ODP Site 1264 on Walvis Ridge in the South Atlantic and IODP Site U1463 on the northwest Australian shelf in the Indian Ocean. Their conclusion is striking. The timing of the biogenic bloom’s demise was not dictated by a single global trigger acting uniformly everywhere, but by which fast-growing coccolithophore species happened to dominate the ecosystem in each region.

The backdrop to this work is a genuine scientific tension. A 2022 meta-analysis of multiple deep-sea records, led by Karatsolis, pointed to a synchronous termination of the biogenic bloom in low latitudes between 4.6 and 4.4 million years ago, an abrupt step toward reduced paleoproductivity that coincided with a phase of decreasing eccentricity and low-amplitude obliquity in Earth’s orbital cycles. That orbital configuration, the authors proposed, may have weakened monsoon systems and cut the nutrient supply to the surface ocean, throttling biogenic sedimentation across the tropics. Yet an independently tuned carbonate accumulation record from South Atlantic Site 1264 told a different tale: productivity there declined around 4 million years ago, but elevated carbonate burial persisted in the southern mid-latitudes until roughly 3.3 million years ago. Something regional, not global, seemed to be controlling the schedule.

To investigate, the team analyzed 21 sediment samples from Site 1264 spanning 5 to 3 million years ago, taken at depths aligned with the astronomical tuning ties of the site’s high-resolution age model. Using a dilution-based drop technique to prepare microscopy slides, they counted at least 300 specimens per sample at 1000-times magnification, classifying the dominant Noelaerhabdaceae coccolithophores, the genera Reticulofenestra and Gephyrocapsa, into small, medium, and large size categories using a 3 and 5 micrometer cutoff. From these counts they calculated relative and absolute abundances, nannofossil accumulation rates, and, crucially, the carbonate mass contributed by each taxon, converting coccolith size and shape into estimates of the actual calcite each group delivered to the seafloor.

The results reveal a carbonate factory in transition. At Site 1264, small Reticulofenestra, opportunistic, bloom-forming algae with coccoliths under 3 micrometers, climbed steadily to around 90 percent of the assemblage by about 4.4 million years ago, then began a long decline toward roughly 20 percent by the late Pliocene. Small Gephyrocapsa, the species that stole the show in the Indian Ocean, was consistently present at Site 1264 between 4.2 and 3.3 million years ago but never exceeded about 8 percent of the assemblage. Meanwhile, medium-sized Reticulofenestra faded early, then rebounded after 3.8 million years ago to reach about 60 percent by 3.2 million years ago, likely reflecting the appearance of the species Reticulofenestra minutula. Absolute abundances peaked spectacularly, with concentrations exceeding 30 billion coccoliths per gram of sediment around 4.4 and 3.5 million years ago.

When the team compared these ecological shifts against the carbonate mass accumulation rates, a two-step pattern emerged. Median carbonate fluxes at Site 1264 dropped significantly after 4.1 million years ago, falling to levels resembling those before the bloom even began, and then declined again at 3.3 million years ago. The first step coincided with reductions in the abundance and carbonate mass contribution of small and large Reticulofenestra, while the second matched a broader decline in sedimentation rates that pushed small Reticulofenestra nearly to disappearance. Throughout, nannofossils, not foraminifera, drove the long-term carbonate trends, confirming that these microscopic algae were the engine of carbonate burial at the site.

The contrast with the northwest Australian shelf could hardly be sharper. At IODP Site U1463, the end of the biogenic bloom was marked by an abrupt collapse of small Reticulofenestra between 4.6 and 4.4 million years ago, followed by the rise of small Gephyrocapsa, which by 4.2 million years ago had become the dominant species and, by mass, the most effective carbonate producer, sometimes contributing more than 45 percent of the nannofossil-derived calcite. At Site 1264, no such takeover ever occurred. Instead, medium-sized Reticulofenestra filled the ecological space left vacant by their shrinking relatives. The same species turnover that looked like a universal biostratigraphic marker in the Indian Ocean simply never materialized in this corner of the South Atlantic.

The explanation, the authors argue, lies in regional oceanography. Small Gephyrocapsa acmes, short-lived intervals of dramatic abundance, have now been documented across the Indian Ocean, in the Tasman Sea gateway, and at Atlantic sites bathed by Indian Ocean water, including ODP Site 1088 on the Agulhas Ridge, where the species exceeded 60 percent of the small coccolith assemblage. Sites within the South Atlantic Gyre, like Site 1264, never saw the acme. Modern descendants of Gephyrocapsa prefer warm, stratified, tropical-to-subtropical waters, and the early Pliocene distribution of their ancestors appears to have followed the same rules, tracking the reach of Indian Ocean-influenced surface currents. The ecological dominance of small Gephyrocapsa, in other words, was an adaptation to the warmer, more stratified post-bloom ocean, but only where those conditions actually prevailed.

Why does this matter beyond the Miocene-Pliocene boundary? Because coccolithophores remain one of the planet’s most important carbonate pumps, and this study demonstrates that the species-level identity of the dominant bloom-formers, not just total productivity, controls how much calcite reaches the seafloor and when. A global climate perturbation that lowers nutrient supply will produce different sedimentary signatures depending on whether the local ecosystem is built around Reticulofenestra or Gephyrocapsa, species that likely differed in growth rates, calcification rates, and ecological tolerances. The delayed termination of the biogenic bloom at mid-latitude Site 1264, the authors conclude, is best explained by the regional ecological success of the coccolithophores that controlled carbonate export there. Bridging the gap between short-term phytoplankton dynamics and long-term biogeochemical change, they suggest, will require exactly this kind of high-resolution, basin-to-basin comparison, along with morphometric and geochemical work to pin down what made these ancient opportunists tick.

Subject of Research: Coccolithophore assemblage changes across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans

Article Title: Coccolithophore signatures across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans

Article References: Karatsolis, B.-T., Asanbe, J. D., & Henderiks, J. (2026). Coccolithophore signatures across the termination of the late Miocene to early Pliocene biogenic bloom in the Atlantic and Indian oceans. Journal of Micropalaeontology, 45(1), 475-486. https://doi.org/10.5194/jm-45-475-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-475-2026

Keywords: coccolithophores, biogenic bloom, Pliocene, Miocene, calcareous nannofossils, paleoproductivity, carbonate burial, Reticulofenestra, Gephyrocapsa, ODP Site 1264, IODP Site U1463, paleoceanography