Euxinia

Lore & Background

Euxinia most frequently occurred in Earth's ancient oceans, but its distribution and frequency are still under debate. The original model suggested it was quite constant for approximately a billion years, though some meta-analyses have questioned this based on small black shale deposits. Before the Great Oxygenation Event, there was little free oxygen in the atmosphere or ocean. It was originally thought that the ocean accumulated oxygen soon after the atmosphere, but this was challenged in 1998 when it was proposed that instead of becoming oxidizing, the deep ocean became sulfidic. This hypothesis is partially based on the disappearance of banded iron formations from the geological record 1.8 billion years ago. The argument was that enough oxygen entered the atmosphere to erode sulfides in continental rocks, but not enough to mix into the deep ocean, resulting in an anoxic deep ocean with increased sulfur flux from continents. The sulfur would strip iron ions from seawater, forming iron sulfide (pyrite), and when sulfide became the major oceanic reductant instead of iron, the deep water became euxinic—a model known as the Canfield ocean, backed by increases in δ³⁴S in sedimentary pyrite and evidence of the first sulfate evaporites.

Reader's Guide

Euxinia is significant in understanding both ancient and modern aquatic chemistry. The basic requirements for its formation are the absence of oxygen, the presence of sulfate ions, organic matter, and sulfate-reducing bacteria. These bacteria use sulfate as an oxidant and organic matter as a reductant, producing hydrogen sulfide. Although sulfate reduction occurs worldwide, most modern waters are oxygenated, limiting such reduction to seabed sediments. The Great Oxygenation Event increased atmospheric oxygen, leading to oxidative weathering of sulfides and a major source of sulfate to the ocean. However, sulfate reduction is not energetically preferred by most bacteria over oxygen or nitrate reduction, so euxinia requires nearly zero concentrations of these other oxidants. Persistent euxinia also requires anoxic waters, high nutrient levels, and a stratified water column. Nutrient traps—where high phosphate input from rivers, recycling from sediments, and slow vertical mixing create a positive feedback loop—help sustain euxinia. Geographic factors such as silled basins with estuarine circulation also promote these conditions. A warming climate can exacerbate euxinia by decreasing oxygen solubility, increasing respiration, and enhancing the hydrologic cycle, which increases weathering and nutrient concentrations. The legacy of the Canfield ocean model lies in its challenge to earlier assumptions about ocean oxygenation and its explanation of geological evidence such as the disappearance of banded iron formations.

Did You Know?

• The word 'euxinia' is derived from the Greek name for the Black Sea, Εὔξεινος Πόντος, meaning 'hospitable sea'.
• The Canfield ocean model was proposed by American geologist Donald Canfield in 1998.
• Sulfate-reducing bacteria produce hydrogen sulfide by using sulfate as an oxidant and organic matter as a reductant.