In a groundbreaking new study published in Nature, researchers have unveiled a revised view of the history of marine dissolved organic carbon (DOC) — a key player in Earth’s carbon cycle that profoundly influences atmospheric and oceanic chemistry. Challenging prior models that predicted enormous buildups or minimal change in marine DOC across geological time, this research reconstructs a dynamic and nuanced evolution of DOC levels, shaped by shifts in ocean oxygenation, ecosystem complexity, and microbial activity throughout Earth’s history.

Marine dissolved organic carbon is a substantial reservoir of carbon in today’s oceans, consisting of a complex mixture of organic molecules that circulate through the water column, influencing everything from nutrient cycling to atmospheric greenhouse gas concentrations. However, understanding how this DOC pool has changed over Earth’s deep past has remained elusive, with past models often presenting conflicting views that either suggested vastly elevated DOC concentrations during the Neoproterozoic or little variation through time.

The new data-driven DOC record, developed by Galili and colleagues, diverges markedly from these previous estimates. Rather than an enormous, Neoproterozoic spike or a largely stable ancient DOC reservoir, their findings propose three distinct evolutionary phases that broadly correspond with planetary shifts in oxygen availability and marine ecological complexity spanning from the Palaeoproterozoic to the present day.

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During the first phase, extending from the late Palaeoproterozoic through the Mesoproterozoic, Earth’s oceans and continental shelves were severely hypoxic, meaning oxygen levels were extremely low, especially in deep waters. These conditions supported a predominantly microbial marine world dominated by small, single-celled organisms — both prokaryotes and simple eukaryotes. Within this environment, particles sank slowly, exacerbating a strong microbial loop: surface-produced organic matter was tightly recycled at shallow depths, leading to elevated surface DOC concentrations.

Importantly, however, the severe oxygen limitation in deeper waters restricted the respiration of organic carbon carried down by particle sinking. This combination meant that, while the surface held high DOC concentrations, the deep ocean did not accumulate the vast DOC reservoirs previously hypothesized. Instead, the total DOC pool remained roughly comparable in size to modern oceans, a notable refinement over former models estimating 100 to 1000 times current DOC levels in that era.

Transitioning into the Neoproterozoic, a second evolutionary shift occurred alongside the rise of larger cells, colonial prokaryotes, and early complex multicellular eukaryotes. Despite persistent severe hypoxia in continental shelves and deep basins, this phase witnessed faster sinking of organic particles, which effectively weakened the microbial loop by enhancing the export flux of organic carbon to sediments. This dynamic not only diminished DOC levels in surface waters but also altered sedimentary composition, as evidenced by the marked increase in particulate organic carbon (POC) accumulation rates seen in the geological record.

Intriguingly, stable carbon isotope data from crude oil, kerogen, and DOC across this time reveal that photosynthetic organisms may have undergone a shift toward larger isotopic fractionations—or that organic carbon pools contained higher proportions of compounds depleted in the heavier isotope carbon-13, such as lipids or methane-derived materials. These biochemical nuances reflect deeper changes in biological productivity, carbon cycling pathways, and possibly ecosystem structure during the Neoproterozoic.

The third phase, spanning the Phanerozoic, is characterized by fully oxygenated deep oceans and increasingly complex marine ecosystems. This oxygenation revolution coincided with the proliferation of larger animals, the advent of grazing, biomineralization, and the emergence of sponges, which reshaped marine food webs and altered organic matter cycling. While the microbial loop remained relatively weak compared to the earliest geologic eons, enhanced oxygen levels permitted greater particle solubilization and boosted microbial carbon pump activity, regenerating significant DOC reservoirs in the ocean interior.

This study also challenges hypotheses positing that terrestrial plant-derived DOC runoff primarily drove DOC increases during the Palaeozoic. The timing of DOC rise, particularly its sharp increase during the Ordovician period roughly 55 million years before the earliest known land plants, strongly suggests that oceanic processes and deep ocean oxygenation, rather than continental inputs, controlled marine DOC dynamics throughout this era.

Collectively, these findings imply that a large DOC reservoir cannot be invoked to explain prominent Neoproterozoic climate events such as Snowball Earth glaciations, nor can it fully account for carbon isotope excursions in this period. Instead, shifts in marine oxygenation and ecosystem complexity play fundamental roles in shaping global carbon reservoirs and, by extension, Earth’s climate system.

The study’s novel DOC reconstruction is grounded in analyses of iron oxide minerals as proxies, providing an independent geochemical line of evidence for historical organic carbon cycling. By integrating sedimentary, isotopic, and ecological data, the research offers a more mechanistic understanding of how DOC pools interacted with evolving marine biogeochemistry and Earth’s redox landscape.

Ultimately, this refined picture of DOC history underscores the intricate feedbacks between life and environment through deep time. It reveals that marine carbon reservoirs are not static but instead respond dynamically to biological innovation, oxygen availability, and sedimentation processes, thereby influencing global biogeochemical cycles and climate over hundreds of millions of years.

As Earth scientists continue to decode these long-term carbon cycle dynamics, this work sets a critical benchmark for understanding the co-evolution of ocean chemistry and life. It invites future research to explore how ancient microbial and eukaryotic communities engineered the chemistry of today’s oceans—and how these processes may respond amid contemporary environmental change.

Subject of Research: Geologic history and evolution of marine dissolved organic carbon (DOC) through Earth’s history, linked to ocean oxygenation and biological complexity.

Article Title: The geologic history of marine dissolved organic carbon from iron oxides.

Article References:
Galili, N., Bernasconi, S.M., Nissan, A. et al. The geologic history of marine dissolved organic carbon from iron oxides. Nature (2025). https://doi.org/10.1038/s41586-025-09383-3

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Tags: atmospheric greenhouse gas influencescarbon reservoirs in oceansdata-driven carbon researchecological phases of carbon dynamicsgeological evolution of marine DOChistory of carbon cyclingmarine dissolved organic carbonmarine ecosystem complexitymicrobial activity in oceansNeoproterozoic carbon modelsocean oxygenation effectsshifts in ocean chemistry