In the quest to unravel Earth’s deep past, researchers often grapple with a scarcity of direct evidence, as ancient events leave behind elusive traces. But now, a groundbreaking discovery by ETH Zurich’s Professor Jordon Hemingway and his team has introduced a novel, tangible record of Earth’s early marine carbon reservoir—tiny, egg-shaped iron oxide structures known as ooids. These minuscule formations, previously mistaken for simple grains of sand, function as natural archives, locking within their layers critical information about the organic carbon content of primordial oceans for up to 1.65 billion years.
Unlike inert sand grains, ooids are dynamic assemblages formed by the accretion of mineral layers as they are continuously tumbled along ancient seafloors by wave action. During this rolling process, organic carbon molecules adhere and become embedded within the growing crystalline structure. These organically laden iron oxides thus capture, layer by layer, a biochemical record of the marine environment’s carbon content, preserving data about organic carbon flux that were hitherto inaccessible through traditional sedimentary proxies.
By meticulously analyzing the organic carbon impurities within these iron oxide ooids, Hemingway’s team has succeeded in developing a new analytical methodology capable of directly gauging the dissolved organic carbon (DOC) reservoirs of ancient oceans. Their findings, published in the prestigious journal Nature, challenge long-held assumptions about Earth’s biogeochemical history. Contrary to prior beliefs that dissolved organic carbon levels spiked dramatically between 1,000 and 541 million years ago, the team has demonstrated that these reserves were in fact 90 to 99 percent lower than contemporary levels during this critical era.
This revelation bears significant implications for our understanding of Earth’s climatic and biological evolution, particularly during the Neoproterozoic era when complex life and global glaciations emerged in tandem with fluctuating oxygen levels. Previous models linked high dissolved organic carbon concentrations to the rise of atmospheric oxygen and the attendant “oxygen catastrophes,” which shaped the trajectory of life on Earth. However, the new evidence compels scientists to reconsider these paradigms, prompting fresh inquiry into the interplay between ocean chemistry, oxygenation, and biological innovation.
Organic carbon enters the oceans through a dual mechanism: firstly, through the dissolution of atmospheric carbon dioxide into seawater, and secondly, via the biological production of organic compounds by photosynthetic microorganisms such as phytoplankton and certain bacteria. These minute life forms utilize sunlight to convert CO2 into complex organic molecules, which upon death, descend as marine snow towards the seafloor. If undisturbed by scavengers, this organic detritus becomes sequestered in marine sediments, forming a substantial long-term carbon sink facilitating Earth’s biogeochemical cycles.
Additionally, microbial degradation processes recycle organic matter by breaking down dead organisms and waste products, releasing dissolved organic carbon that permeates ocean waters. This dissolved organic carbon reservoir surpasses the carbon embodied in living marine organisms by a factor of approximately 200, underscoring its critical role in marine ecosystems and the global carbon cycle. The isotopic and molecular signatures entrapped within iron oxide ooids now offer a direct window into how this reservoir fluctuated through deep time.
Historical geochemical anomalies in sedimentary records had suggested that dissolved organic carbon levels were exceptionally high during the late Proterozoic, a period coinciding with two major oxygenation events—the so-called “oxygen catastrophes.” These transitions saw atmospheric oxygen rise from near-absent to modern levels, fundamentally reshaping Earth’s surface environment and enabling the evolution of energetically demanding complex life. The correlation of ice ages with oxygen surges lent credence to models positing an abundance of organic carbon fueling these dramatic changes.
However, the ooid-derived data reveal a contrasting narrative. The oceanic reservoir of dissolved organic carbon was substantially depleted relative to today’s oceans during this interval. It was only following the second oxygenation event, around 541 million years ago, that DOC concentrations rebounded to their current magnitude, approximately 660 billion tonnes of carbon. This finding disrupts the long-standing assumption that DOC accumulation drove oxygen increases and advocates for alternative causal mechanisms connecting marine biogeochemistry to Earth’s evolutionary milestones.
The leading explanation offered by the research team links the dramatic decline in dissolved organic carbon to ecological shifts involving the rise of larger multicellular organisms. These organisms’ increased biomass and altered trophic dynamics accelerated the sinking rates of organic matter, intensifying marine snowfall processes. Consequently, carbon-rich particles settled more rapidly to the seafloor, limiting their recycling in oxygen-minimal deep waters and precipitating a marked contraction of the dissolved organic carbon reservoir.
This revised model also emphasizes the role of oxygen distribution in the ocean interior. The deep ocean remained largely anoxic for much of this era, restricting microbial degradation of sinking organic matter and enhancing carbon burial at the seafloor. Only with increased oxygenation of the deep ocean did complete recycling resume, allowing the dissolved organic carbon pool to grow to present-day levels. Hence, the interplay of marine oxygenation, biological complexity, and physical carbon cycling presents a nuanced framework for interpreting Earth’s deep-time environmental changes.
Beyond reconstructing Earth’s ancient past, these novel insights bear provocative implications for planetary science and contemporary environmental challenges. Enhanced understanding of the marine carbon reservoir’s evolution informs models of exoplanet habitability, where ocean chemistry may similarly mediate atmospheric composition and biological potential. Moreover, the study underscores how anthropogenic impacts—particularly ocean warming and deoxygenation—might echo geological precedents, foreshadowing changes in oceanic carbon storage with profound consequences for Earth’s biosphere.
In sum, the pioneering utilization of iron oxide ooids as biochemical time capsules casts new light on the intricate history of marine dissolved organic carbon, challenging entrenched geochemical dogmas and inviting a reevaluation of the relationships between carbon cycling, oxygen dynamics, and life’s evolution on Earth. As more refined data emerge, the geological narrative of our planet’s ancient oceans will continue to evolve, offering deeper insights into the forces that shaped life and environment over billions of years.
This research not only illuminates an obscure chapter of Earth’s early ocean chemistry but also serves as a powerful reminder of the complex feedback mechanisms that regulate planetary systems over vast temporal scales. The implications resonate well beyond academic curiosity, touching on the urgent need to preserve oceanic health as humanity navigates rapid environmental change.
Subject of Research:
The geologic history and quantification of marine dissolved organic carbon reservoirs through deep time, analyzed via iron oxide ooids.
Article Title:
The geologic history of marine dissolved organic carbon from iron oxides
News Publication Date:
13 August 2025
Web References:
https://doi.org/10.1038/s41586-025-09383-3
References:
Galili N, Bernasconi SM, Nissan A et al.: The geologic history of marine dissolved organic carbon from iron oxides. Nature, 13 August 2025, doi:10.1038/s41586-025-09383-3
Image Credits:
Credit: Nir Galili / ETH Zurich
Keywords:
Marine dissolved organic carbon, iron oxide ooids, primordial ocean, carbon reservoir, carbon cycling, ocean oxygenation, Neoproterozoic, oxygen catastrophes, marine snow, biogeochemical cycles, deep ocean anoxia, Earth history
