What triggered a dramatic decline in marine biodiversity 460 million years ago?

Produced by: Science Popularization China

Author: Zhang Junpeng Sheng Jie (Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences)

Producer: China Science Expo

Speaking of the "red tide" phenomenon, I believe everyone is familiar with it. This algal bloom phenomenon, which often occurs in lakes and semi-enclosed bays, is a very troublesome environmental problem.

"Red tide" is a common phenomenon of eutrophication of water bodies in modern times, mostly caused by large amounts of industrial wastewater rich in nutrients such as phosphorus and iron . When a red tide occurs, algae bloom on the surface of the water, the water's light transmittance decreases, the dissolved oxygen content decreases, chemical stratification occurs in the water, highly toxic components such as hydrogen sulfide appear at the bottom, and dead fish and other aquatic animals can be seen on the surface, and the dead bodies may even cause serious damage to the ecology.

Figure 1 Red tide

(Photo source: Pudong Environment)

Can you imagine that 460 million years ago, a large-scale "red tide" occurred in the low-latitude waters of the earth, which had a great impact on the marine ecological environment and killed countless marine creatures...

Recently, a Chinese and American research team led by researchers from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, conducted paleontological and sedimentary geochemical research on marine black shales preserved 460 million years ago and found that the expansion of anoxic water bodies in the ocean at that time had caused a significant reduction in marine biodiversity and pressed the "pause button" for the "Ordovician biological radiation" at that time. The relevant research results have been published in the comprehensive earth science journal Earth and Planetary Science Letters.

As for what the "Ordovician Bioradiation" is, why we need to start research on marine black shale, and what serious consequences the expansion of oxygen-deficient water bodies in the ocean will cause, if you are interested, let's read on together!

The flourishing of life: the Ordovician biological radiation

Over the long years, the process of biological evolution has been advancing at a relatively slow pace. However, during key geological periods such as the Great Biological Radiation, the ocean, the cradle of life on Earth, gave birth to a variety of organisms in a relatively short period of time. The more familiar "Cambrian Explosion" is a large-scale biological radiation event.

The "Ordovician biological radiation" that occurred 460 million years ago was once hailed as the largest biological radiation event in the history of biological evolution on Earth after the "Cambrian explosion".

This biological radiation event was not only manifested in the doubling of marine biodiversity at the genus and species level , but also in the gradual complexity and stability of the ecosystem , such as the establishment of a complex predator-prey network, the ecological expansion of planktonic and swimming animals, and the development of benthic ecological communities. The ocean at that time was full of vitality (Figure 2), and at the same time, early plants on land also completed their landing and began a long journey of expansion.

Figure 2 Ordovician marine ecosystem restoration map

(Image source: Fang Xiang et al., 2022)

The reason for the decline from prosperity: the expansion of hypoxic seawater

However, such a flourishing scene of life could not last long, and the same was true of the Ordovician biological radiation.

In recent years, with the application of supercomputers and in-depth research on the "Ordovician Biological Radiation", high-resolution biodiversity curves have shown that during this biological radiation event, biodiversity continued to decline by about 50% within about 10 million years after reaching its peak. This obvious turning point has attracted the attention of geologists.

According to previous studies, the Ordovician biological radiation benefited from environmental changes such as global climate cooling and increased atmospheric oxygen content that began in the Early Ordovician.

However, in the Middle and Upper Ordovician strata in Hunan, Jiangxi, Anhui, Zhejiang and other regions in South China, a large set of organic-rich mudstones (also known as "black shales") have developed, reflecting that the ocean floor at that time was probably oxygen-deficient .

Hypothetically, if the great biological radiation benefited from the increase in atmospheric oxygen content, then the large-scale hypoxia at the bottom of the ocean is likely the reason for the pause in the great biological radiation.

Researchers conducted further investigations. By analyzing the main and trace elements, iron composition and pyrite morphology, nitrogen isotopes and molybdenum isotopes of these sedimentary rocks, they found that eutrophication occurred on the surface of the waters far from the shore, and the anoxic waters at the bottom gradually expanded and even reached an anaerobic state, that is, a large amount of highly toxic components such as hydrogen sulfide appeared in the bottom waters.

There was no industrial wastewater discharge into the Ordovician oceans, so why did phenomena similar to "red tides" occur?

As for the mechanism behind these phenomena, researchers explained: "In the middle of the Ordovician period, the temperature dropped further and significantly , causing the cold currents in the deep seas of high latitudes to intensify . Large amounts of nutrients were carried to the coastal basins of mid- and low-latitudes in the form of upwelling currents, causing eutrophication of surface waters, which is similar to the red tide phenomenon in modern lakes."

Based on this inference, researchers studied the contemporary strata in other parts of the world and found that this phenomenon was not limited to the South China Plate at that time. The North China Plate, Tarim Plate, Laurent Plate in America and Baltic Plate in Europe all had contemporary black shale deposits . This shows that the ocean at that time was not as oxidized as previous studies believed. At the same time, the molybdenum isotope evidence and simulation results of the Middle Ordovician black shale in South China reported for the first time in this study can also confirm this.

Figure 3 A comprehensive diagram of climate-ocean environment changes and biogeological events in the middle Ordovician, from top to bottom: carbon isotope evidence, oxygen isotope evidence and its reflection of sea temperature change trends, sea level changes and distribution of anoxic water bodies, molybdenum isotope evidence, and biodiversity data (species level), etc.

(Image source: Zhang Junpeng et al. 2022)

The ocean's "deadly breath" - the huge impact of lack of oxygen in the ocean

So far, we know that in the middle and late stages of the Ordovician radiation, the oxygen-deficient water bodies in the ocean expanded significantly. So, what impact did this phenomenon have on the marine ecological environment?

Climate cooling will affect the ecology of surface water bodies. At the same time, eutrophication of water bodies will cause a large amount of organic carbon to sink to the seabed. The oxidation and decomposition of organic matter consumes the oxygen dissolved in the water, which in turn causes a large-scale expansion of oxygen-deficient seawater. Water bodies lacking dissolved oxygen will also be enriched with hydrogen sulfide, causing fatal damage to marine ecosystems (especially benthic communities).

Therefore, this series of climate and environmental changes is considered to be the main cause of the turning point in the great biological radiation in the Middle to Late Ordovician .

Similar phenomena have occurred many times in geological history. The most famous one is the "End-Permian Extinction" 250 million years ago, which is the largest known biological extinction event and is also related to global ocean hypoxia.

The idea of ​​"learning from the past to guide the present" - gaining environmental protection inspiration from exploring geological history

In addition to the biological extinction events in geological history, the disasters that may be caused by the Earth's climate breaking the "steady state" are also full of inspiration for us today.

Global warming is a consensus among earth scientists in various countries and regions. However, when we pay attention to climate change, we should not simply understand it as an increase or decrease in average temperature, but should consider the series of extreme weather and environmental disasters that occur when the earth's climate breaks the "steady state" as in geological history , such as the "Carnian flood event" that lasted for a million years in the late Triassic period 230 million years ago.

At the same time, the "dual carbon" goal proposed by our country in recent years is a major strategy to actively respond to the global issue of climate change . Academician Ding Zhongli of the Chinese Academy of Sciences mentioned in his latest important book "Carbon Neutrality: Logical System and Technical Requirements" that "carbon neutrality should be understood from the two aspects of carbon emissions (carbon sources) and carbon fixation (carbon sinks)". In these two processes, nature occupies an important proportion .

During the geological history, volcanic eruptions were an important form of carbon emissions, marine carbonate factories and organic carbon burial were important processes of carbon fixation, and "black shale" is an important product and evidence of marine carbon burial during the geological history.

Therefore, exploring the "carbon" of geological history and conducting in-depth research on the climate-environmental changes related to it can provide important reference and inspiration for us to implement the "dual carbon" strategy today. Only by using the past to discuss the present and learning from the past can we better practice it in the present and benefit the future.

Editor: Guo Yaxin