The research, published in Frontiers of Biogeography, focuses on two fossiliferous conglomerates: one at Tarrafal on Santiago Island in Cabo Verde, emplaced by a megatsunami triggered by the flank collapse of neighbouring Fogo volcano around 68,000 years ago during Marine Isotope Stage 4 (MIS 4), and another at Teno Bajo on Tenerife in the Canary Islands, deposited by a tsunami generated by the collapse of the Las Cañadas edifice around 170,000 years ago during MIS 6. Both events occurred when coeval sea level was about 80 metres lower than present, yet the waves stranded their sediments tens of metres above modern Holocene sea level, safely out of reach of the erosive action of subsequent interglacial highstands.

This preservation is the study’s methodological breakthrough. Glacial-age marine fossil deposits are rarely accessible worldwide because rising seas during the following interglacial erode them from the record. Previous work on long-distance dispersal of shallow-water molluscs in the Macaronesian islands tracked tropical species moving poleward during glacial terminations or early interglacials, but full glacial episodes remained a blind spot. Megatsunami deposits, by contrast, represent a geological process that lifts glacial seafloor sediments onshore and locks them in place, offering a unique window into the biogeography of cold periods.

The Tarrafal deposit yielded a remarkable haul. From seven one-kilogram bulk samples, the team recovered 2,260 specimens representing 209 specific taxa of molluscs: 39 bivalves, 169 gastropods and one polyplacophoran. Of the 150 taxa identifiable to species level, 134 are shallow-water species restricted to depths shallower than 50 metres, 14 inhabit deeper waters, and two are pelagic. The assemblage mixes hard-ground dwellers, coarse-sand and gravel species, epifaunal grazers, carnivorous gastropods and suspension-feeding bivalves, along with fish otoliths, barnacle plates, echinoderm spines, bryozoans, foraminifers, coral fragments and rhodoliths. Three endemic Cabo Verdean bivalves and 34 endemic gastropods are reported for the archipelago’s fossil record, most for the first time.

Crucially, 32 taxa from the MIS 4 tsunami sediments do not presently inhabit the warm waters of Cabo Verde, and for 14 of these the Canary Islands constitute their present-day southernmost location. The team detected successful equatorward latitudinal range expansion to Cabo Verde of 12 bivalves and 14 gastropods, plus longitudinal range expansion from West African shores of seven bivalves and nine gastropods. At Teno, of 101 taxa (28 bivalves and 73 gastropods), eight species are absent from the Canaries today. Equatorward expansion to the Canaries was detected for three bivalves, including the familiar Glycymeris glycymeris, Pecten maximus and Ostrea edulis, and three gastropods, including the common periwinkle Littorina littorea, with longitudinal expansion from Moroccan shores for four bivalves. None of these species was present in the archipelagos during the Last Interglacial, so their glacial presence conclusively supports long-distance dispersal and the establishment of viable populations during cold periods.

The dispersal mechanisms most probably involved planktotrophic larvae carried by sea-surface currents and trade winds, or rafting on natural floating devices such as wooden logs and algae for non-planktotrophic species, with attached eggs, juveniles or adults. The authors speculate that such equatorward dispersal was only possible if the Canary Current, today an oceanographic barrier between the Canaries and Cabo Verde, shifted its position southward or weakened during glacial episodes, allowing mid-latitude species to reach lower latitudes. Some cooling of sea-surface temperatures at Cabo Verde latitudes would have enabled settlement of temperate species. Longitudinal dispersal from Senegal and Mauritania towards Cabo Verde, meanwhile, is expected if the West African upwelling intensified, boosting marine productivity along the continental margin.

Mid-latitude North Atlantic records support this oceanographic picture. Sea-surface temperatures fell during both MIS 4 and MIS 6, driven by a weakening of the Atlantic Meridional Overturning Circulation. The AMOC reduction favoured a southward displacement of the Intertropical Convergence Zone, triggering very dry conditions over northwestern Africa and intensified northeast trade winds, which supplied huge amounts of dust to the adjacent ocean and intensified upwelling in the region. These coupled atmospheric and oceanic changes would have opened dispersal corridors for cold-water species moving equatorward, both between archipelagos and from continental shores to oceanic islands.

Beyond the biogeographic findings, the study reveals an unexpected sedimentological signature. When the team compared diversity indices from the Tarrafal samples with 24 one-kilogram samples from Last Interglacial (MIS 5e) raised beach deposits on Santa Maria Island in the Azores, the glacial megatsunami samples showed dramatically higher values across all Hill numbers used: species richness (q = 0), the exponential of Shannon’s entropy index (q = 1) and the inverse of Simpson’s concentration index (q = 2). The differences were statistically significant in an asymmetrical ANOVA, with p = 0.0000 for all three indices. The seven Tarrafal samples yielded 209 taxa but only about 2,257 specimens, whereas the 24 Azorean samples produced 136 taxa and 25,090 specimens, a pattern of high species richness with comparatively low individual counts that also characterised the Teno deposit.

The explanation lies in the genesis of the deposits. Raised beach sediments accumulate gradually over hundreds or thousands of years, producing time-averaged death assemblages biased towards the most abundant and taphonomically durable species. Megatsunami deposits, by contrast, are instantaneous events at geological timescales: a few mega-waves en-masse transport and deposit sediments with minimal sorting or destruction, sampling a varied biological community from different depths and habitats in a single event. The result is a highly biodiverse, mixed-zonation assemblage characterised by the absence of dominant species, with well-preserved micromolluscs and even pelagic species included. The authors propose that this extremely high palaeobiodiversity and evenness constitutes a new criterion for distinguishing megatsunami deposits from normal highstand deposits, adding to established traits such as chaotic textures, extremely poor sorting, distribution at variable elevations and fossils never found in life position.

The study also carries a broader message for island biogeography theory. A collaborative horizon-scanning exercise identified the 50 most fundamental questions for the field, but only one palaeoecology question survived the final cut, reflecting the usual lack of geological and palaeontological expertise among biologists. This work demonstrates that integrating biology, palaeontology and geology through multidisciplinary teams is a superior approach. In times of accelerated global change, knowing how species distributions shifted during Pleistocene glacial and interglacial periods is crucial for understanding biogeography, ecology and evolution over the past million years in fragile marine ecosystems such as archipelagic settings. The cold-water travellers of the ice ages, preserved by catastrophe, now write a new chapter in the story of how life moves across the ocean.

Subject of Research: Megatsunami deposits reveal glacial equatorward range expansion of temperate marine molluscs in Macaronesia

Article Title: Megatsunami deposits and range expansion of cold-temperate marine species towards the tropics in glacial times

Article References: Ávila, S. P., Paris, R., Ramalho, R. S., Melo, C. S., Martín-González, E., Rolán, E., Madeira, P., Ávila, G. C., Porteiro, J. M., Medeiros, A. M., Naughton, F., Abrantes, F., Martins, G. M., Johnson, M. E., & Madeira, J. (2025). Megatsunami deposits and range expansion of cold-temperate marine species towards the tropics in glacial times. Frontiers of Biogeography, 18, Article e138319. https://doi.org/10.21425/fob.18.138319

Image Credits: AI Generated

DOI: 10.21425/fob.18.138319

Keywords: biogeography, megatsunami, Macaronesia, Cabo Verde, Canary Islands, molluscs, glacial periods, MIS 4, MIS 6, paleontology, deposits, range

Silicate weathering is one of the fundamental thermostats of the Earth system. When rainwater, charged with atmospheric carbon dioxide in the form of carbonic acid, attacks silicate minerals in rocks, the carbon is converted into dissolved bicarbonate and carried by rivers to the ocean, where much of it is ultimately buried in carbonate sediments. Over geological timescales, this process acts as a long-term sink for atmospheric CO2, and its intensity is conventionally assumed to track temperature and runoff: hot, humid climates accelerate chemical denudation, while cold, dry glacial climates slow it down. That assumption underpins much of how scientists model the carbon cycle over glacial-interglacial cycles, which is precisely why the new findings are so consequential. The study suggests that during the very intervals when the conventional model predicts weakened weathering, an enormous and previously overlooked weathering engine was running at full tilt on the exposed mid-latitude shelves of East Asia.

The evidence comes from Site CSDP-1, a core recovered by the Chinese Continental Shelf Drilling Program in the South Yellow Sea. The team conducted a comprehensive analysis of the mineralogical and geochemical properties of the sediments, layer by layer, building a continuous archive that spans six hundred millennia of alternating glacial and interglacial conditions. Among the key indicators they examined was the Chemical Index of Alteration, or CIA, a widely used proxy that measures the degree to which feldspars and other primary silicate minerals have been chemically transformed into clay minerals through hydrolysis. They also tracked the ratio of potassium to aluminum, K/Al, which decreases as labile potassium-bearing minerals are leached away during weathering, and the ratio of smectite to illite plus chlorite, which captures the balance between newly formed alteration products and relatively unweathered detrital minerals transported from the river basin.

All of these proxies told the same story. The weathering intensity recorded in glacial shelf sediments was consistently and markedly higher than that recorded in interglacial sediments. In quantitative terms, the team calculated that the rate of carbon consumption through silicate weathering during glacial periods was approximately 1.5 times that of interglacial periods. This inversion of the expected pattern, in which colder climates should yield weaker chemical denudation, is the heart of the discovery. The researchers interpret it as evidence that glacial weathering on the shelf was decoupled from climate in the conventional sense. It was not temperature or precipitation over the source basin that controlled the signal, but rather the physical state of the sediment itself and the amount of time it spent exposed at the surface.

The mechanism hinges on sea level. During glacial periods, global sea level fell by well over a hundred meters as water was locked into continental ice sheets. This regression exposed the loosely consolidated, unconsolidated sediments that had accumulated on the continental shelf during earlier highstands. Rather than being rapidly buried beneath the ocean, these fine-grained deposits were left stranded on the emergent plain, where they remained for thousands of years, subjected to repeated wetting and drying, oxidation, and infiltration by CO2-charged meteoric water. Because the sediments were fine-grained, freshly deposited, and largely unweathered when first delivered by the Yellow River, they presented an enormous reactive surface area to chemical attack. The long residence time of these materials on the exposed shelf gave weathering reactions the opportunity to proceed far beyond what would have been possible had the sediment been quickly sequestered on the seafloor.

Interglacial conditions reversed this arrangement. As sea level rose, the river mouth retreated landward and the previously exposed shelf was flooded once more. Fine-grained material eroded from the basin was then rapidly transported through the estuary and dispersed across the shelf and into the deep sea, sharply reducing both the time and the space available for reweathering of the sediment. In other words, the same sediment that lingered and weathered during glacial lowstands was efficiently exported and buried during highstands. The sediment routing system, rather than the climate system, emerged as the dominant control on chemical weathering intensity in this basin-shelf configuration, a conclusion that challenges how weathering signals in marine sediments are commonly interpreted.

To appreciate the scale of the overlooked sink, the researchers extended their quantitative estimates beyond the Yellow Sea. Taking into account the exposed shelves of the Yellow Sea and the East China Sea, along with comparable low-latitude marine areas that experience similar sea-level-driven exposure, they calculated that the increased silicate weathering of shelf sediments during glacial periods may account for approximately 9.3 percent of the total reduction in atmospheric CO2 during glacial times. For a mechanism that had previously gone essentially unrecognized in glacial carbon budgets, nearly a tenth of the glacial drawdown is a substantial contribution. It implies that the mid-latitude shelf region functioned as an active and underestimated silicate weathering carbon sink precisely during the intervals when the global climate was coldest and atmospheric CO2 was at its lowest.

This finding also reframes comparisons with earlier work in low-latitude seas, where enhanced glacial weathering had been documented but attributed to specific tropical conditions. The new study demonstrates that the phenomenon is not confined to the tropics. Mid-latitude shelves fed by major sediment-discharging rivers, of which the Yellow River is one of the largest on Earth, can host equally vigorous weathering during lowstands. Given the abundance of such river-shelf systems around the world, from the Sunda Shelf of Southeast Asia to the shelves of other major river depocenters, the implication is that shelf weathering may be a globally significant and systematically underestimated term in glacial carbon cycle accounting.

Methodologically, the study is notable as the first quantitative reconstruction of the Late Quaternary history of silicate weathering carbon consumption in the Yellow River basin-continental shelf system. By combining multiple independent weathering proxies with calculated carbon consumption fluxes, the team was able to move beyond qualitative statements about alteration intensity and place firm numbers on the carbon sink, both through time at the study site and extrapolated across the broader shelf systems of the western Pacific margin. The 600,000-year record captures numerous glacial-interglacial cycles, giving the researchers the statistical confidence to distinguish a systematic glacial enhancement of weathering from episodic or local effects.

The broader significance of the work lies in the perspective it opens for understanding the mechanisms of glacial-interglacial climate evolution. If exposed continental shelves acted as a meaningful carbon sink during ice ages, then models of the glacial carbon cycle must incorporate sea-level-driven shelf exposure as a first-order process, alongside the ocean’s biological pump, changes in ocean circulation, and carbonate compensation dynamics that have traditionally dominated the discussion. The study, authored by Zhang, Wan, and colleagues in SCIENCE CHINA Earth Sciences, thus adds a continental margin process to the roster of mechanisms that helped the planet cool into glacial states and, by implication, helped release that carbon as shelves were reflooded during deglaciation. For a field that has long focused its attention on the open ocean and the terrestrial biosphere, the message from the muddy floor of the South Yellow Sea is that some of the most important carbon chemistry of the ice ages may have happened in plain sight, on land that is now underwater.

Subject of Research: Enhanced silicate weathering on the South Yellow Sea continental shelf during glacial periods and its role in the glacial carbon cycle

Article Title: Enhanced silicate weathering on the South Yellow Sea shelf during glacial periods and its influence on the glacial carbon cycle

Article References: Enhanced silicate weathering on the South Yellow Sea shelf during glacial periods and its influence on the glacial carbon cycle. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: silicate weathering, South Yellow Sea, continental shelf, glacial periods, carbon cycle, atmospheric CO2, sea level, Chemical Index of Alteration, Yellow River, CSDP-1, Science China Earth Sciences, Quaternary climate