The “Canfield Ocean” refers to a scientific theory proposing a distinct period in Earth’s ancient past when deep oceans were largely devoid of oxygen and instead contained high concentrations of hydrogen sulfide. This concept represents a significant shift in understanding Earth’s early environmental conditions, suggesting a more complex chemical landscape than previously thought. It has reshaped how scientists view the co-evolution of Earth’s atmosphere, oceans, and early life.
Earth’s Ancient Ocean Conditions
Before the emergence of the Canfield Ocean theory, the prevailing scientific understanding of Earth’s oceans after the Great Oxidation Event (GOE) was that they became largely oxygenated. The GOE, around 2.4 billion years ago, marked a dramatic increase in atmospheric oxygen, primarily due to photosynthetic microorganisms. It was widely assumed this led to widespread oxygenation of both shallow and deep ocean waters, suggesting a straightforward progression to an oxygen-rich planet.
This earlier model envisioned a global ocean where oxygen was readily available, supporting various forms of life. This view, however, did not fully account for certain geological observations that hinted at more nuanced ocean chemistry during the subsequent billion years. Re-examination of the geological record suggested the post-GOE ocean might not have been uniformly oxygenated.
Defining the Canfield Ocean
The Canfield Ocean theory describes a period during the middle to late Proterozoic Eon (1.8 to 0.8 billion years ago). During this time, deep ocean waters were anoxic (lacked oxygen) and sulfidic, containing high concentrations of dissolved hydrogen sulfide (H2S). This condition contrasts sharply with potentially oxygenated surface waters, creating a stratified ocean where a shallow oxygenated layer sat above a vast, sulfidic deep ocean.
Geochemical cycles involving sulfur, iron, and oxygen maintained this state. Sulfate, a common ion in seawater, could be reduced by microorganisms in the absence of oxygen, producing hydrogen sulfide. This process, known as dissimilatory sulfate reduction, played a significant role in accumulating sulfide in the deep ocean. The presence of abundant iron also influenced these cycles, forming iron sulfides.
Uncovering the Evidence
Scientists uncovered evidence for the Canfield Ocean by analyzing ancient sedimentary rocks, which preserve geochemical fingerprints of past ocean conditions. A primary line of evidence comes from stable isotope ratios of sulfur, particularly δ34S, found in minerals like pyrite (iron sulfide). These isotopic signatures indicate large-scale sulfate reduction in an anoxic environment, suggesting a significant presence of sulfide in the water column.
The distribution and types of iron formations also provide important clues. For example, the widespread disappearance of banded iron formations (BIFs) around 1.8 billion years ago aligns with the proposed rise of sulfidic deep waters. In an oxygen-poor, sulfidic ocean, dissolved iron reacts with hydrogen sulfide to form insoluble iron sulfides, which then precipitate out, preventing the widespread accumulation of iron oxides that characterize BIFs.
Impact on Early Life
The anoxic and sulfidic conditions of the Canfield Ocean had profound implications for the evolution and diversification of early life. Hydrogen sulfide in deep waters was toxic to organisms relying on oxygen for respiration. This environmental pressure restricted early oxygen-respiring life to shallow, oxygenated surface waters, potentially slowing the expansion and innovation of early eukaryotes.
Despite these challenges, these conditions also fostered specific microbial communities adapted to sulfidic environments. Sulfate-reducing bacteria thrived by utilizing sulfate and producing hydrogen sulfide. The Canfield Ocean thus represents an environmental filter that shaped microbial evolution and likely influenced the timing of the rise and diversification of more complex life forms, including early animals.