For decades, the scientific community held a fundamental belief about the ocean's role in combating climate change: microscopic plants called phytoplankton were the heroes. Through photosynthesis, they absorbed carbon dioxide at the surface, and upon death, their remains sank, locking carbon away in the deep seabed via the "biological carbon pump." A groundbreaking new study, however, has dramatically shifted this understanding, revealing that physical forces like monsoon behaviour and river flow are far more critical in determining the ocean's long-term carbon burial capacity.
Sediment Delivery Trumps Plankton Productivity
The research, presented at the 15th International Conference on Paleoceanography in Bengaluru, is based on an extensive analysis of a 50,000-year record of carbon accumulation in the northeast Indian Ocean. Scientists from the National Institute of Oceanography (NIO) in Goa examined chemical data from 718 modern surface sediments and 19 sediment cores across the Bay of Bengal and the Andaman Sea.
Their findings, published in Nature in October 2024, were startling. "We see essentially no correlation between surface chlorophyll — a proxy for surface ocean productivity — and sedimentary carbon content," said the study's lead author, Dr. Rajeev Saraswat, a geological oceanographer at NIO, Goa. This means that more plankton growth does not automatically translate to more carbon being permanently stored on the ocean floor.
Instead, the study identified the real determinants: how sediments are delivered and preserved. The strength of monsoon-driven currents, the volume of mud and sand carried by rivers, and the texture of the sediment itself were found to be the primary factors controlling whether carbon survives long enough to be buried. This applies to both organic carbon from marine and land sources and calcium carbonate from plankton shells.
A Tale of Two Climates: Ice Age vs. Holocene
The research uncovered a stark contrast in carbon burial patterns between different climate epochs, powerfully illustrating the new theory.
During the Last Glacial Maximum (26,000 to 20,000 years ago), monsoons were weaker and plankton productivity declined. Paradoxically, carbon burial increased. With less river-borne sediment entering the ocean, organic matter was less diluted. Furthermore, slow-moving deep waters created oxygen-poor conditions that slowed decay, leading to better preservation.
The pattern completely reversed during the Holocene period (11,600–4,200 years ago). Stronger monsoons delivered a massive influx of sediments—up to 1.4 billion tonnes annually—into the Bay of Bengal. This deluge diluted organic carbon. Heavy rainfall also created a stable freshwater surface layer that limited the mixing of nutrients, reducing both plankton productivity and the ultimate burial of carbon.
Preservation Belts and a Climate Warning
The study successfully mapped specific zones where carbon is most likely to persist. Organic carbon burial peaked at depths of 200 to 1,000 meters within oxygen minimum zones, where low oxygen levels slow decay. Fine-grained mud in these regions acts as a seal, creating a preservation belt along India's continental margins. Overall, total carbon burial was highest at depths beyond 2,900 meters.
Dr. Saraswat emphasized the dynamic nature of the ocean's role, stating, "The same ocean can be either a source or sink depending on boundary conditions. We need process-based understanding, not just empirical relationships."
This insight carries a significant warning for climate strategies that consider the ocean as a carbon buffer. The carbon stored in sediments is not guaranteed to remain locked away. Future warming, ocean acidification, and shifts in circulation could disturb these long-term stores, potentially re-releasing carbon back into the system.
Other scientists, while acknowledging the importance of the study, urge caution. Kaustubh Thirumalai of the University of Arizona noted that interactions between sediments and chemical processes can mask evidence of carbon originally from marine life. Oscar Branson of the University of Cambridge added that while broad pathways are understood, "predicting this system is full of devilish details."
The researchers concluded that improving climate predictions will require closer monitoring of ocean chemistry and more robust climate models, particularly in monsoon-driven regions like the Indian Ocean, where the dance of wind, water, and earth ultimately writes the fate of atmospheric carbon.
