{"id":40076967,"date":"2016-02-05T00:00:00","date_gmt":"2016-02-04T18:30:00","guid":{"rendered":"http:\/\/china-dialogue-ocean-staging.darkbluehq.com\/uncategorized\/757-ancient-dead-seas\/"},"modified":"2022-02-10T23:00:08","modified_gmt":"2022-02-10T17:30:08","slug":"757-ancient-dead-seas","status":"publish","type":"post","link":"https:\/\/dialogue.earth\/en\/ocean\/757-ancient-dead-seas\/","title":{"rendered":"Ancient \u2018dead seas\u2019 offer a stark warning for our own near future"},"content":{"rendered":"

For billions of years, life on Earth remained relatively simple. Only single-celled organisms that could live with little or no oxygen were able to survive in the seas.<\/p>\n

Eventually, the rise of oxygen led to a proliferation of diverse, multicellular life. However the oceans have not remained unchanged since that chemical and biological revolution. At several times in geological history, they have partially reverted back to their original bacterially-dominated, oxygen-free state \u2013 and they could do so again.<\/p>\n

Today rising CO2\u00a0levels are making the oceans warmer and more acidic. Deforestation and intensive farming are causing soils and nutrients to be flushed into the sea. And increasingly, the oceans are being\u00a0stripped of oxygen<\/a>, leaving large \u201cdead zones\u201d in the Gulf of Mexico<\/a>, the Baltic Sea and\u00a0the Atlantic off West Africa<\/a>.<\/p>\n

These dead zones<\/a>, smaller-scale revivals of the primeval oceans that existed before complex life, appear to be caused by poor land management, such as\u00a0fertilisers draining from farms<\/a>\u00a0into the sea. It is a process that could be exacerbated by climate change \u2013 as has happened in the past.<\/p>\n

How oceans become \u2018dead\u2019<\/h2>\n

Oceans lose their oxygen when animals and bacteria consume it faster than it can be replenished. This usually comes about in stagnant or algae-rich waters.<\/p>\n

In severe cases, all oxygen can be consumed, rendering the waters \u201canoxic\u201d and inhospitable to animal life. This happens today in isolated\u00a0fjords and basins<\/a>. And it has happened on a larger scale throughout Earth\u2019s history, especially during the Cretaceous period, towards the end of the dinosaur era 145-66\u00a0million years ago. Then, large parts of the ancient oceans became anoxic, allowing vast amounts of organic matter to escape degradation, and in many cases forming\u00a0deposits of oil and gas<\/a>.<\/p>\n

\"Hood
Hood Canal (actually a fjord) in Washington state, US, often suffers from dead zones. Walter Siegmund<\/a>, CC BY-SA<\/a><\/figcaption><\/figure>\n

We can examine the extent of anoxia by looking for a certain type of \u201cgreen sulphur bacteria<\/a>\u201d which require both sunlight and oxygen-depleted waters in order to conduct their rather\u00a0exotic form of photosynthesis<\/a>. Evidence of their presence can be found in ancient rocks \u2013 molecular proof that anoxia once extended from the seafloor almost all the way to the ocean\u2019s surface.<\/p>\n

These oceans thrived with microbial life. But animals need oxygen, and vast portions of these ancient oceans would have become \u201cdead\u201d to them.<\/p>\n

Life in the deep sea<\/h2>\n

Unlike almost every other ecosystem on our planet, the deep sea is bereft of light and plants. Animals down there largely live off\u00a0marine snow<\/a>, the scraps of organic matter that somehow escape from the surface world and sink to the twilight realm below. In this energy-starved world, creatures live solitary lives in emptiness, darkness\u00a0and mystery<\/a>.<\/p>\n

And yet life is there. Krill thrive on the slowly-sinking snow. Sperm whales dive deep to consume the krill and emerge with scars from giant squid. And when a whale dies and\u00a0its carcass plummets to the seafloor<\/a>, it is set upon by sharks and fish who emerge from the darkness for the unexpected feast. Within days the carcass is stripped to the bones \u2013 but even then, massive colonies of tube worms spring to life.<\/p>\n

All of these animals, the fish, whales and worms, depend on oxygen. Our oxygen-rich seas are an incredible contrast to the North Atlantic during some ancient anoxic events. Then, plesiosaurs (pictured at the top) and ichthyosaurs, feeding on magnificent ammonites, would have been confined to the sunlit, oxygen-rich realm near the surface, their maximum depth of descent marked by a layer of pink and then green water, pigmented by bacteria. And below it, where the deeper waters were anoxic, only single-celled organisms.<\/p>\n

Could this happen again?<\/h2>\n

Conventional wisdom has been that such extreme anoxia in the future is unlikely, that Cretaceous \u201cdead zones\u201d were a consequence of a markedly different geography. The ancient Atlantic Ocean was smaller and more restricted, lending itself to these extreme conditions. This is a bit like\u00a0the modern Black Sea<\/a>, a restricted basin where fresh river water sits stably above salty and dense marine deep water.<\/p>\n

\"Cretaceous
During the Cretaceous the Atlantic Ocean was much smaller. Ron Blakey, NAU<\/a><\/figcaption><\/figure>\n

But the Black Sea doesn\u2019t quite match up with what we know about ancient anoxic oceans. For a start, if driven solely by geographical shape, why were the oceans not anoxic as the norm rather than only at certain times? Sometimes much larger oceans became dead zones, or the\u00a0anoxia was restricted to coastal areas<\/a>. And although ocean circulation was slower during warm climates, it did not stop \u2013 unlike in the Black Sea.<\/p>\n

This suggests geography was important but not exclusively so. Algal blooms are a more likely trigger. These algae would have flourished after dramatic\u00a0increases in nutrients<\/a>\u00a0caused by erosion and chemical weathering, driven by higher carbon dioxide concentrations, global warming and\/or changes in the hydrological cycle \u2013 all of which we now know occurred\u00a0prior to several anoxic events<\/a>.<\/p>\n

It is likely that today\u2019s coastal dead zones are due not to climate change but to our\u00a0excessive use of fertilisers<\/a>. And it is unlikely that our future will revisit the widespread ocean anoxia of the past. But the lessons of the past do suggest global warming could exacerbate the impacts of our poor land management, adding yet another pressure to already stressed ecosystems.<\/p>\n

A earlier version of Richard Pancost\u2019s article appears here<\/a> on The Conversation.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

Ocean acidification related to climate change, and the use of fertilisers, are threatening a mass extinction in the world’s oceans<\/p>\n","protected":false},"author":0,"featured_media":40066483,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[50039901],"tags":[519,40027778],"hashtags":[],"country":[],"class_list":["post-40076967","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-ocean","tag-climate-impacts","tag-marine-protection"],"acf":[],"yoast_head":"\nAncient \u2018dead seas\u2019 offer a stark warning for our own near future | Dialogue Earth<\/title>\n<meta name=\"description\" content=\"Ocean acidification related to climate change, and the use of fertilisers, are threatening a mass extinction in the world's oceans\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" 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