In a groundbreaking study recently published in Nature Communications, researchers have unveiled critical new insights into how carbon was transported across the Pacific Ocean during the deglacial period—a time of profound climatic transformation that saw the planet emerge from the last Ice Age. The team, led by Karas, Nürnberg, and Lambert, explored the dynamics of southern-sourced intermediate and mode waters and their enhanced role in sequestering carbon at depth, a process that likely had a significant impact on atmospheric carbon dioxide levels and global climate regulation.
Understanding the mechanisms behind past carbon fluxes in the oceans is fundamental to deciphering Earth’s climate history. The deglacial interval, roughly spanning 20,000 to 10,000 years ago, represents a pivotal era when vast amounts of carbon were released from oceanic reservoirs, contributing to the rise in atmospheric CO2 that helped warm the planet. What remained unclear, until now, was the specific role played by Pacific waters originating from the Southern Hemisphere—waters characterized by intermediate depths and unique physical properties known as mode waters.
Karas and colleagues employed an innovative combination of geochemical proxies, sediment core analyses, and sophisticated climate modeling to reconstruct past ocean circulation patterns and quantify carbon transport. Their research pinpointed that these southern-sourced intermediate and mode waters enhanced carbon export in a manner previously underestimated. This revelation redefines our understanding of ocean-atmosphere carbon exchange during the deglacial centuries and underscores the Pacific’s vital role as a carbon sink.
Unlike surface waters that directly interact with the atmosphere, intermediate and mode waters are formed at depths between the surface mixed layer and deep ocean basins, typically spanning 200 to 1000 meters. These water masses serve as conduits, carrying dissolved inorganic carbon from their formation zones toward the ocean interior, effectively locking it away from atmospheric exchange for extended durations. The study illustrates that during the deglaciation, shifts in the formation and flow of these waters amplified carbon sequestration by increasing offshore transport and storage, thus modulating atmospheric CO2.
Notably, the southern Pacific, influenced by Antarctic processes and prevailing wind patterns, emerged as a hotspot for enhanced formation of these intermediate and mode waters. This enhancement is believed to have been driven by shifts in Southern Hemisphere wind systems and changes in sea surface temperature gradients. These physical changes dynamically altered ocean stratification and mixing, allowing greater subduction of carbon-rich waters into the ocean’s mid-depth strata.
One particularly compelling aspect of the research is the use of carbonate system proxies—such as boron isotopes and radiocarbon ages—embedded within deep-sea sediment cores to trace historic carbon inventories. These proxies provide a timeline of how carbon concentrations changed with depth and time, revealing patterns that coincide with climatic events. The patterns traced suggest that carbon-rich waters from the south were injected into the Pacific interior in pulses, correlating with phases of rapid climate warming.
The implications of this discovery extend beyond reconstructing past climate states; it offers valuable analogues for the future. As the modern climate system undergoes rapid change, understanding how oceans naturally control carbon storage is crucial for predicting their capacity to buffer anthropogenic CO2 emissions. The enhanced deglacial carbon transport mechanisms described by Karas et al. indicate that similar processes could be modulated by ongoing shifts in wind patterns and ocean circulation under global warming scenarios.
Additionally, this study sheds light on the intricate interplay between oceanic physical processes and biogeochemical cycles. By elucidating the pathways that intermediate and mode waters used to ferry carbon, it bridges gaps in how we conceptualize the ocean’s role as a dynamic climate regulator. This further emphasizes the necessity of incorporating detailed ocean circulation structures into Earth system models to improve climate projections.
Previous research predominantly focused on the Atlantic Ocean’s deep-water formation as the main driver of glacial-interglacial carbon changes, often sidelining the Pacific Ocean’s contributions. This new evidence elevates the Pacific’s southern intermediate and mode waters to primary players in deglacial carbon cycling, thereby reshaping conventional narratives and highlighting the complexity of global ocean carbon dynamics.
Critically, the research acknowledges that while the Pacific Ocean stores vast carbon quantities, its mechanisms for exporting and trapping carbon at intermediate depths were substantially more active during deglacial periods than previously thought. This finding suggests a more interactive ocean carbon system where regional processes can have profound global signals. Timing and magnitude of these processes, as depicted in the study, also align with the broader atmospheric CO2 concentrations reconstructed from ice cores.
The methodology underpinning this study relied on interdisciplinary collaboration, combining oceanography, geochemistry, paleoceanography, and computational modeling. This allowed for high temporal resolution insights into ocean carbon dynamics that surpass the limitations of singular disciplinary approaches. The precision and depth of data afford unprecedented clarity on how natural processes modulate carbon storage and release over millennia.
From a technical perspective, the study utilized advanced coupled ocean-atmosphere models calibrated against proxy records derived from sediment cores collected across strategic sites in the Pacific. By integrating dynamic physical ocean parameters with chemical tracers, the research team teased apart the complex feedback loops influencing deglacial carbon transport. This synergy of observation and modeling underscores the future direction for climate science, emphasizing holistic, data-integrative approaches.
Moreover, the findings offer vital context for interpreting modern changes in mode and intermediate water masses, which recent studies suggest are shifting in response to global warming. Variations in these water masses’ properties could alter the contemporary ocean’s capacity to sequester carbon, influencing future climate feedbacks. Therefore, the insights gained from past analogues furnish important baselines for assessing the resilience and vulnerability of ocean carbon sinks.
In sum, the work by Karas et al. heralds a paradigm shift in our perception of the Pacific Ocean’s role in the Earth’s deglacial carbon cycle. By unmasking the enhanced carbon transport capacity of southern-sourced intermediate and mode waters, the study contributes a critical piece to the puzzle of how past climatic events unfolded and offers vital perspectives for forecasting the oceans’ role in our changing climate.
As the climate science community grapples with refining models and assessing carbon budgets amid increasing anthropogenic pressures, such detailed reconstructions from the geological past provide invaluable benchmarks. The deglacial period serves as a natural experiment showcasing how ocean circulation reorganization can dramatically influence atmospheric CO2, and this latest research forms a cornerstone for future explorations into oceanic carbon sequestration.
Carrying forward these revelations, further research may focus on unraveling regional specifics of intermediate and mode water production, their sensitivity to atmospheric forcing, and feedbacks with ecosystems. Such knowledge will be instrumental in adapting climate mitigation strategies that incorporate the ocean’s complex carbon cycling pathways, making this study not just a retrospective insight, but a roadmap for future planetary stewardship.
Subject of Research: Enhanced transport of carbon during the deglacial period by Pacific southern-sourced intermediate and mode waters, and their impact on global carbon cycling and climate dynamics.
Article Title: Enhanced deglacial carbon transport by Pacific southern-sourced intermediate and mode water
Article References: Karas, C., Nürnberg, D., Lambert, F. et al. Enhanced deglacial carbon transport by Pacific southern-sourced intermediate and mode water. Nat Commun 16, 5245 (2025). https://doi.org/10.1038/s41467-025-60551-5
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