As ancient oceans warmed, carbon export and fish productivity surged

October 29, 2026 feature As ancient oceans warmed, carbon export and fish productivity surged David Appell Author Sadie Harley Scientific Editor Robert Egan Senior Editor Ocean fish stocks have been increasingly exploited over time, and remote sensing shows they have declined in the open ocean over the last three decades, particularly in the tropics. But it's uncertain how fish populations will fare in the future, as climate models show both increases and decreases in global trends by the end of this century. Climate change, warmer oceans, ocean acidification, weakened upwelling and increasing ocean stratification all play a part in determining the trend. A new paper looks at changes in fish biomass during a past episode of global warming and finds that sustained warming can stimulate open-ocean biomass growth as well as carry carbon taken up by marine plants and algae into the deeper ocean. But it takes a long while to happen. The study is published in the journal Nature Communications. A warming episode 55.9 million years ago The prior episode of global warming considered is the Paleocene-Eocene Thermal Maximum (PETM) that occurred about 55.9 million years ago. Large amounts of greenhouse gases such as carbon dioxide and methane were released into the atmosphere, most likely from large volcanic traps—very large regions of stacked volcanic flows that spread over a major region. In the case of the PETM, the most likely culprit was the North Atlantic Igneous Province (NAIP), a large, solidified magma region in the North Atlantic, centered on Iceland. (Research on the exact cause(s) of the PETM has been going on for decades and continues today.) In any case, the beginning of the PETM saw rapid emissions occurring over only a few thousand years. About 3,000 to 10,000 billion metric tons of carbon were released into the atmosphere. Open-ocean temperatures rose by about 5°C, though varying with latitude and depth, and the global-average near-surface air temperature rose 5–6°C. The warming lasted for about 100,000 years. The PETM is considered perhaps the best analog to today's global warming. The PETM's overall carbon emission rate is about an order of magnitude lower than ours in the fossil fuel era; today's rate of warming is about 100 times faster, so far, than the PETM's average rate over its estimated 100-millennium lifetime. Fish teeth track ancient productivity To investigate past stocks of pelagic (open ocean) fish, the researchers—four from China, including lead author Xiuwen Zhou of the Southern University of Science and Technology in Shenzhen, and one in Canada—examined proxy indicators that are a function of fish stocks. They combined measurements of the accumulations of fossil fish teeth and dermal denticles, together called ichthyolites—the second in the pair being tiny, flat V-shaped scales that are more like teeth than fish scales, found only in cartilaginous fish like sharks and rays—from six core sites in ocean sediment. Ichthyolites are deposited on the ocean floor through natural shedding or excretion of indigestible parts by predators. The ichthyolite accumulation rate (IAR) served as the scientists' proxy of pelagic fish productivity for the PETM. The group also examined marine barite records from the same or nearby locations. The mineral barite (BaSO₄) in an ocean water column is produced, directly and indirectly, in association with decaying marine organic matter. Its distribution in both the water column and, for these purposes, ocean floor sediments corresponds closely to the organic carbon falling anywhere in the water column from the surface to the deep ocean. Productivity surged unevenly across ocean basins Across sites, the group's IARs varied from nearly zero to as high as 150 ichthyolites per square centimeter per thousand years. Several sites, especially in the northern Pacific and northern Atlantic oceans, showed spikes in the IARs at the beginning of the unusual carbon accumulation and another one about 50,000 years later. At these sites, and one in the central Pacific Ocean, peak IAR values were 5 to 11 times higher than the background values. Increases in the South Atlantic and southeast Indian Ocean were more modest. "The increase in the ichthyolite flux to marine sediments," the researchers write, "can be attributed to increased trophic transfer efficiency [the fraction of biological production at one feeding level that becomes new biological production at the next feeding level] and/or elevated fish productivity." Here, fish productivity means the rate at which a fish population produces new biomass through growth and reproduction. (Caution: in nonscientific usage, "productivity" means the fish catch taken by the fishing industry.) Carbon export fueled the increase The group incorporated the cGENIE Earth-system biogeochemical model to determine the interaction of carbon, nutrients and more across the ocean, atmosphere, sediments and terrestrial systems. Doing so revealed that the widespread increase in pelagic fish productivity closely tracked the rate at which organic carbon was transported to the deep ocean, in the trends and in the spatial patterns across the seafloor. This suggests, they conclude, "the increase was fueled by enhanced carbon export from the surface ocean." Thus, under elevated carbon levels in the ocean, the effects of an enhanced nutrient supply seemed to outweigh the effects of ocean stratification—the separation of ocean waters into horizontal layers determined by density—which suppresses productivity because it obstructs mixing of heat, carbon, oxygen and other nutrients. Long-term gains are no short-term forecast Importantly, the researchers concluded that changes ultimately seen during the PETM might be different from the changes seen in the modern world (which, granted, are due to a variety of human factors beyond climate change). "Our findings highlight that the long-term response of marine productivity to sustained greenhouse warming may differ fundamentally from the short-term declines observed in the modern ocean." Written for you by our author David Appell, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details Xiuwen Zhou et al., Coupled increases in pelagic fish productivity and export productivity during the Paleocene-Eocene Thermal Maximum, Nature Communications (2026). DOI: 10.1038/s41467-026-77863-9 Journal information: Nature Communications © 2026 Science X Network