A pioneering study from Stanford University has, for the first time, reconstructed how marine biomass—the total mass of living organisms in the ocean—has changed over the past 540 million years. Published in Current Biology, the research reveals a long-term upward trend in marine biomass, punctuated by sharp declines during major mass extinction events. This work, led by postdoctoral researcher Pulkit Singh, fills a critical gap in understanding how life in the oceans has evolved in terms of abundance, not just diversity.
While previous studies have focused on biodiversity—the number and variety of species—this research highlights the importance of quantifying marine biomass, which reflects the energy available in ecosystems and the number of organisms they can support. Biomass indicates productivity, health, and resilience of ecosystems, and understanding its historical trends can offer insights into long-term planetary health.
To uncover these trends, Singh and his team analyzed over 7,700 marine limestone samples spanning the past half-billion years. The researchers compiled data from more than 100 studies and conducted new analyses using a technique called petrographic point-counting. This method involves slicing rock samples thinly enough to be examined under a microscope, allowing scientists to calculate the proportion of shell material—remains of marine organisms such as mollusks, corals, algae, and single-celled protists.
The data revealed a consistent pattern: during the Cambrian period (starting 540 million years ago), shell content was low, averaging under 10%. As the Ordovician period began around 485 million years ago, this percentage increased, reflecting the so-called “Cambrian Explosion”—a rapid diversification of life. Over the following hundreds of millions of years, shell material often exceeded 20% of rock content, suggesting a major rise in marine biomass.
However, this increase was not steady. Mass extinction events, like the Late Devonian (about 375 million years ago) and the catastrophic Permian-Triassic extinction 250 million years ago—also known as the “Great Dying”—caused dramatic drops in shell content, indicating major declines in biomass. During the Permian-Triassic extinction, shell content plummeted to around 3%.
After each extinction, ecosystems recovered and marine biomass rebounded. The Cenozoic era, beginning 66 million years ago, saw the highest biomass levels to date, with shell content reaching over 40% thanks to abundant mollusks and reef-building corals. This surge is likely due to increased ecological specialization, improved nutrient recycling, and greater energy efficiency in marine food webs.
To ensure their conclusions were robust, the researchers tested whether geographic or environmental biases—such as differences in water depth or latitude—could explain the pattern. Even after subdividing the data across various conditions, the upward trend in biomass remained strong and consistent.
The study’s findings have important implications for today’s world. Human-driven threats like overfishing, pollution, and ocean acidification may now be reversing millions of years of biological growth. With the sixth mass extinction underway, biodiversity loss could reduce biomass and long-term ecosystem productivity.
Ultimately, the study shows that biodiversity and marine biomass are deeply connected, and both are essential for sustaining the ocean’s health. Protecting biodiversity may be key not just to preserving species, but to maintaining the energy and resilience of life in the sea.
https://phys.org/news/2025-06-marine-biomass-million-years.html