Global warming caused widespread ocean anoxia 93 million years ago, study of deep-sea sediments finds

Marine anoxia is characterized by a severe lack of dissolved oxygen in the oceans, which makes them toxic and thus has devastating effects on the organisms that inhabit them. One of these events, known as Oceanic Anoxic Event 2 (OAE2), occurred approximately 93.5 million years ago across the Cenomanian-Turonian boundary of the Late Cretaceous and lasted until ‘at 700,000 years.

In such scenarios, organic matter is buried at a high rate, producing distinct layers of black shale in the geologic record, which are depleted of the isotopically heavier carbon-13, thereby generating a positive carbon isotope excursion of approximately 6‰ for this study. period.

The specific factors triggering OAE2 are still debated, but the most widely supported is the volcanism of the Caribbean Large Igneous Province and the High Arctic Large Igneous Province, which increases atmospheric carbon dioxide and therefore warms the planet.

Among the many impacts of a warmer planet are increased land weathering, with fluvial processes transporting these materials to the oceans, providing essential nutrients to ocean surface primary producers. Increased primary productivity produces more oxygen, but trophic food chains ultimately consume more of this oxygen in their metabolic processes.

This phenomenon, combined with a decrease in oxygen solubility in warmer oceans, is leading to widespread deoxygenation of Earth’s marine realm, the focus of new research published in Climates of the past.

Dr Mohd Al Farid Abraham, of Universiti Malaysia Sabah, Malaysia, and colleagues turned to deep-sea sediments drilled during an exploratory expedition from the Demerara Rise, in the equatorial North Atlantic Ocean, which in Cenomanian, was located at a latitude of approximately 5°N. .

Revealing the importance of this work, Dr Abraham said: “Our research explores the secrets of the ancient oceans, particularly a period 93.5 million years ago when much of the ocean was devoid of ‘oxygen. By studying natural chemical fingerprints preserved in marine sediments, we are discovering how past volcanic activities and global warming have led to drastic deoxygenation of the oceans. Understanding this in depth is crucial, as they reflect the challenges we face today with the current climate crisis, helping us predict and mitigate future consequences.

By taking samples of organic matter from the drilled cores, the research team isolated compounds of biological origin stable over geological periods of several million years, called biomarkers. Dr Abraham explains that biomarkers are known as “molecular fossils”, adding: “Biomarkers are chemical compounds found in sedimentary rocks from living organisms millions of years ago. Think of them as molecular fossils that, unlike bones or shells, are not easily visible to the naked eye. These compounds, once part of living organisms, have remained chemically stable over vast geological time scales.

“We carefully extract them using a series of chemical procedures and a technique known as gas chromatography-mass spectrometry in the laboratory to isolate these compounds from the drilled sediments and avoid contamination.

“Analyzing these biomarkers helps us reconstruct past environmental conditions, such as temperature and oxygen levels in the oceans, but linking their presence to specific historical environmental conditions requires meticulous laboratory work and in-depth understanding geochemical processes.”

The scientists found that the percentage of total organic carbon content in the samples increased over the study period (3.8 million years), peaking at around 28% by weight at OAE2, compared at initial levels of 1 to 17% by weight. This occurred alongside an increase of around 5-8°C in sea surface temperature, to around 43°C.

The key biomarkers 28,30-dinorhopane and lycopane are indicative of this warming and decrease in oxygen, forming a Cenomanian oxygen minimum zone, similar to those observed today in the Black Sea. These data are associated with a notable reduction in the abundance of benthic foraminifera (single-celled microorganisms living on the ocean floor) at the end of the Cenomanian, as they were not able to survive in a low-oxygen environment.

These persistent oxygen-poor layers increase in number and size with increased ocean warming, forming a thick zone at depth beneath a thin, highly productive, oxygen-rich surface layer. C biomarkers₃₅ Hopanoid thiophene and isorenieratane reveal this expanded water-column (both anoxic and sulphide) Ilsinia eventually reaching the surface photic zone across the Cenomanian-Turonian boundary at OAE2.

The movement of water masses, such as that of the Tethys Sea moving warm, salty waters from the bottom of Demerara, likely played a role in the distribution of nutrient-rich but oxygen-poor conditions across the ocean basin. Previous research has suggested that around 93 million years ago, up to 50% of Earth’s oceans were anoxic during OAE2, with this process potentially beginning around 2 million years earlier during the Middle Cenomanian event.

Eventually, this oceanic anoxic event ended, a halt that Dr. Abraham and colleagues attribute to the depletion of nutrient reserves in surface waters, which led to a collapse in primary productivity. Additionally, the termination may have been influenced by changes in the paleogeography of the equatorial gateway to the Atlantic. This gateway, which appeared between what is today northeastern South America and West Africa, modified ocean circulation in the North Atlantic, preventing it from becoming a nutrient trap likely to support primary productivity.

Envisioning the future of Earth’s oceans with expanding oxygen minimum zones, Dr. Abraham says: “In today’s world, ocean conditions are generally hypoxic but have not yet reached extremes. anoxic levels in open oceans. However, closed basins or seas are more dangerous. likely to become anoxic.

“With ongoing global warming, it is predicted that oxygen minimum zones will expand both horizontally and vertically. Warmer waters contain less oxygen and increasing surface temperatures may lead to more strong stratification of ocean layers, thereby reducing the mixing that normally replenishes oxygen in deeper waters.

“Additionally, global warming may increase biological activity in surface waters, which would result in more organic matter sinking to depths, where it would consume oxygen as it decomposes, an obvious process during OAE2.

“Today, oxygen minimum zones are found primarily in the Pacific and Indian Oceans, with conditions that make life difficult for many marine species. With the current trend of global warming, these zones are expected to expand , reducing habitable marine space and negatively affecting marine biodiversity and fisheries.

“By the end of this century, if the current trajectory of warming and nutrient runoff continues, we could see a significant increase in anoxic and Ilsinic conditions in our oceans, threatening marine ecosystems and the services they provide. provide to humanity.”

Understanding the role that warmer oceans may play in the cycling of oxygen and nutrients in the water column is crucial, especially as Earth’s oceans enter an uncertain future due to climate change in course. Past episodes of marine anoxia (like OAE2) teach us about Earth’s history and remind us how vital it is to take care of our oceans. As we face new climate-related challenges, looking back on these past events can help us make better decisions for the future of our planet.

“It is both fascinating and alarming to see how deeply the story resonates in our current environmental crisis,” says Dr. Abraham.

“Ancient oceans tell a story of resilience and rebirth, but also offer a warning. The OAE2 event, although taking place over millions of years, shows us the profound impact that changes in the atmosphere can have on marine life. “

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