Euxinia

Euxinia, or euxinic conditions, describes water that lacks oxygen and contains high levels of hydrogen sulfide. This special kind of water is found in places where the layers of water don't mix well. The top layer has oxygen and supports many living things, but the deeper water has no oxygen and lots of hydrogen sulfide, which is poisonous to most life.

The term "euxinia" comes from the ancient Greek name for the Black Sea, which means "hospitable sea." Long ago, during a time called the Proterozoic eon, many oceans may have been like this. Scientists, such as Donald Canfield, study these ancient oceans to learn more about Earth's history.

Today, euxinia is rare but still exists in some places, like the Black Sea and certain fjords. These areas help scientists understand how ancient oceans may have worked and what life was like in those conditions.

Background

Euxinia, which means water that has no oxygen and contains hydrogen sulfide, mostly happened in Earth's ancient oceans. Scientists are still learning how often it occurred. Before the Great Oxygenation Event about 2.3 billion years ago, there was very little oxygen in the air or the ocean. A scientist named Canfield suggested that instead of the deep ocean getting oxygen, it actually got filled with hydrogen sulfide. This idea is called the Canfield ocean.

Today, euxinia is rare and mostly found in special kinds of lakes and places like the Black Sea. Less than half a percent of the ocean floor today has these conditions.

Causes

Euxinic conditions happen when water has no oxygen and contains hydrogen sulfide. This needs special conditions like the absence of oxygen, sulfate ions, organic matter, and special bacteria that turn sulfate into hydrogen sulfide. In most oceans today, there is too much oxygen because of photosynthesis and gas exchange with the air. But in some places, like deep sediments, sulfate reduction can still happen.

For euxinia to last, the water must be without oxygen, have lots of nutrients, and stay layered. This can happen in places where nutrients get trapped, like in certain ocean basins or during warm climates. When the climate gets warmer, water holds less oxygen, and more nutrients come from rivers, helping create conditions where hydrogen sulfide can form.

Evidence for euxinic events

Black shales are dark, rich-in-organic-matter rocks that often show there was no oxygen in the water below. This lack of oxygen helps preserve the organic material. Scientists also look at special chemicals in ancient rocks to learn about past conditions. For example, they study sulfur and molybdenum to find signs of water that had no oxygen and lots of hydrogen sulfide.

They also look at tiny remains of special bacteria that live where there is light and no oxygen. These remains can tell scientists about past water conditions. Another clue comes from pyrite, a mineral that forms when hydrogen sulfide meets iron. Finding lots of this mineral can suggest past conditions with no oxygen and lots of hydrogen sulfide.

Euxinic events in Earth's history

The Proterozoic era shows a shift from oceans without oxygen to ones with oxygen. Some scientists believe that anoxia lasted longer, and the end of banded iron formations happened because of sulfide, not oxygen. Evidence from Canada and Australia supports the idea of widespread euxinia, where water had no oxygen and lots of hydrogen sulfide. This euxinia might have lasted until about 0.8 billion years ago.

During the Phanerozoic eon, there were likely many periods of euxinia, especially linked to mass extinctions like the Late Devonian and Permian–Triassic. In the Paleozoic, evidence from China and Canada shows periods of euxinia. In the Mesozoic, Ocean Anoxic Events sometimes included euxinia, but it wasn’t everywhere. The Cenozoic has few signs of euxinia in the record.

Modern euxinia

Euxinic conditions happen when water has no oxygen and contains hydrogen sulfide, a gas with a strong smell like rotten eggs. Today, such conditions are rare in the open ocean but still exist in some small places. These areas often have layers of water that don't mix well, which helps keep the oxygen-poor, sulfur-rich water at the bottom.

One well-known example is the Black Sea, which helps scientists understand ancient ocean conditions. It has layers of water, with oxygen at the top and sulfur at the bottom, separated by a sharp boundary called the chemocline. Other places, like some fjords and lakes, can also have euxinic conditions, especially when their layers don't mix well. This can sometimes lead to the release of dangerous gases, but these events are rare.