Ancient ‘dead seas’ offer a stark warning for our own near future

Bristol during the pleiocene as envisaged by Lucas Antics.

The oceans are experiencing a devastating combination of stresses. Rising CO2 levels are raising temperatures while acidifying surface waters.  More intense rainfall events, deforestation and intensive farming are causing soils and nutrients to be flushed to coastal seas. And increasingly, the oceans are being stripped of oxygen, with larger than expected dead zones being identified in an ever broadening range of settings. These dead zones appear to be primarily caused by the runoff of nutrients from our farmlands to the sea, but it is a process that could be exacerbated by climate change – as has happened in the past.

Recently, our group published a paper about the environmental conditions of the Zechstein Sea, which reached from Britain to Poland 270 million years ago. Our paper revealed that for tens of thousands of years, some parts – but only parts – of the Zechstein Sea were anoxic (devoid of oxygen). As such, it contributes to a vast body of research, spanning the past 40 years and representing the efforts of hundreds of scientists, which has collectively transformed our understanding of ancient oceans – and by extension future ones.

The types of processes that bring about anoxia are relatively well understood. Oxygen is consumed by animals and bacteria as they digest organic matter and convert it into energy. In areas where a great deal of organic matter has been produced and/or where the water circulation is stagnant such that the consumed oxygen cannot be rapidly replenished, concentrations can become very low. In severe cases, all oxygen can be consumed rendering the waters anoxic and inhospitable to animal life.  This happens today in isolated fjords and basins, like the Black Sea.  And it has happened throughout Earth history, allowing vast amounts of organic matter to escape degradation, yielding the fossil fuel deposits on which our economy is based, and changing the Earth’s climate by sequestering what had once been carbon dioxide in the atmosphere into organic carbon buried in sediments.

Red circles show the location and size of many dead zones. Black dots show Ocean dead zones of unknown size. Image source: Wikimedia Commons/NASA Earth Observatory

In some cases, this anoxia appears to have been widespread; for example, during several transient Cretaceous events, anoxia spanned much of what is now the Atlantic Ocean or maybe even almost all of the ancient oceans. These specific intervals were first identified and named oceanic anoxic events in landmark work by Seymour Schlanger and Hugh Jenkyns.  In the 1970s, during the earliest days of the international Deep Sea Drilling Program (now the International Ocean Discovery Program, arguably the longest-running internationally coordinated scientific endeavor), they were the first to show that organic matter-rich deep sea deposits were the same age as similar deposits in the mountains of Italy. Given the importance of these deposits for our economy and our understanding of Earth and life history, scientists have studied them persistently over the past four decades, mapping them across the planet and interrogating them with all of our geochemical and palaeontological resources.

In my own work, I have used the by-products of certain bacterial pigments to interrogate the extent of that anoxia.  The organisms are green sulfur bacteria (GSB), which require both sunlight and the chemical energy of hydrogen sulfide in order to conduct a rather exotic form of bacterial photosynthesis; crucially, hydrogen sulfide is only formed in the ocean from sulfate after the depletion of oxygen (because the latter yields much more energy when used to consume organic matter). Therefore, GSB can only live in a unique niche, where oxygen poor conditions have extended into the photic zone, the realm of light penetration at the very top of the oceans, typically only the upper 100 m.  However, GSB still must compete for light with algae that live in even shallower and oxygen-rich waters, requiring the biosynthesis of light harvesting pigments distinct from those of plants, the carotenoids isorenieratene, chlorobactene and okenone. For the organism, this is an elegant modification of a molecular template to a specific ecological need. For the geochemist, this is an astonishingly fortuitous and useful synthesis of adaptation and environment – the pigments and their degradation products can be found in ancient rocks, serving as molecular fossil evidence for the presence of these exotic and diagnostic organisms.

And these compounds are common in the black shales that formed during oceanic anoxic events.  And in particular, during the OAE that occurred 90 million years ago, OAE2, they are among the most abundant marker compounds in sediments found throughout the Atlantic Ocean and the Tethyan Ocean, what is now the Mediterranean Sea.  It appears that during some of these events anoxia extended from the seafloor almost all the way to the ocean’s surface.


Today, the deep sea is a dark and empty world. It is a world of animals and Bacteria and Archaea – and relatively few of those. Unlike almost every other ecosystem on our planet, it is bereft of light and therefore bereft of plants.  The animals of the deep sea are still almost entirely dependent on photosynthetic energy, but it is energy generated kilometres above in the thin photic zone. Beneath this, both animals and bacteria largely live off the scraps of organic matter energy that somehow escape the vibrant recycling of the surface world and sink to the twilight realm below. In this energy-starved world, the animals live solitary lives in emptiness, darkness and mystery. Exploring the deep sea via submersible is a humbling and quiet experience.  The seafloor rolls on and on and on, with only the occasional shell or amphipod or small fish providing any evidence for life.

“Krill swarm” by Jamie Hall – NOAA. Licensed under Public Domain via Wikimedia Commons

And yet life is there.  Vast communities of krill thrive on the slowly sinking marine snow, can appear.  Sperm whales dive deep into the ocean to consume the krill and emerge with the scars of fierce battles with giant squid.  And when one of those great creatures dies and its carcass plummets to the seafloor, within hours it is set upon by sharks and fish, ravenous and emerging from the darkness for the unexpected feast. Within days the carcass is stripped to the bones but even then new colonizing animals arrive and thrive. Relying on bacteria that slowly tap the more recalcitrant organic matter that is locked away in the whale’s bones, massive colonies of tube worms spring to life, spawn and eventually die.

But all of these animals, the fish, whales, tube worms and amphipods, depend on oxygen. And the oceans have been like this for almost all of Earth history, since the advent of multicellular life nearly a billion years ago.

This oxygen-replete ocean is an incredible contrast to the north Atlantic Ocean during at least some of these anoxic events. Then, plesiosaurs, ichthyosaurs and mosasaurs, feeding on magnificent ammonites, would have been confined to the sunlit realm, their maximum depth of descent marked by a layer of surprisingly pink and then green water, pigmented by the sulfide consuming bacteria.  And below it, not a realm of animals but a realm only of Bacteria and Archaea, single-celled organisms that can live in the absence of oxygen, a transient revival of the primeval marine ecosystems that existed for billions of years before more complex life evolved.

We have found evidence for these types of conditions during numerous events in Earth history, often associated with major extinctions, including the largest mass extinction in Earth history – the Permo-Triassic Boundary 252 million years ago.  Stripping the ocean of oxygen and perhaps even pumping toxic hydrogen sulfide gas into the atmosphere is unsurprisingly associated with devastating biological change.   It is alarming to realise that under the right conditions our own oceans could experience this same dramatic change.  Aside from its impact on marine life, it would be devastating for us, so dependent are we on the oceans for our food.

The conventional wisdom has been that such extreme anoxia in the future is unlikely, that Cretaceous anoxia was a consequence of a markedly different geography.  North America was closer to Europe and South America only completely rifted from Africa about 150 million years ago; the ancient Atlantic Ocean was smaller and more restricted, lending itself to these extreme conditions.

And yet questions remain.  What was their trigger?  Was it really a happenstance of geography?  Or was it due to environmental perturbations? And how extensive were they? The geological record preserves only snapshots, limiting the geographical window into ancient oceans, and this is a window that narrows as we push further back in time. In one of our recent papers, we could not simulate such severe anoxia in the Atlantic Ocean without also simulating anoxia throughout the world’s oceans, a truly global oceanic anoxic event.  However, that model can only constrain some aspects of ocean circulation and there are likely alternative mechanisms that confine anoxia to certain areas.


Over the past twenty years, these questions have intersected one another and been examined again and again via new models, new geochemical tools and new ideas.  And an emerging idea is that the geography of the Mesozoic oceans was not as important as we have thought.

That classical model is that ancient oceans, through a combination of the aforementioned restricted geography and overall high temperatures, were inherently prone to anoxia.  In an isolated Atlantic Ocean, oxygen replenishment of the deep waters would have been much slower.  This would have been exaggerated by the higher temperatures of the Cretaceous, such that oxygen solubility was lower (i.e. for a given amount of oxygen in the atmosphere, less dissolves into seawater) and ocean circulation was more sluggish. Consequently, these OAEs could have been somewhat analogous to the modern Black Sea.  The Black Sea is a restricted basin with a stratified water column, formed by low density fresh water derived from the surrounding rivers sitting stably above salty and dense marine deep water. The freshwater lid prevents mixing and prevents oxygen from penetrating into deeper waters. Concurrently, nutrients from the surrounding rivers keep algal production high, ensuring a constant supply of sinking organic matter, delicious food for microbes to consume using the last vestiges of oxygen.  The ancient oceans of OAEs were not exactly the same but perhaps similar processes were operating. Crucially, the configuration of ancient continents in which major basins were isolated from one another, suggests a parallel between the Black Sea and the ancient North Atlantic Ocean.

But over the past twenty years, that model has proved less and less satisfactory.  First, it does not provide a mechanism for the limited temporal occurrence of the OAEs.  If driven solely by the shape of our oceans and the location of our continents, why were the oceans not anoxic as the norm rather than only during these events? Second, putative OAEs, such as that at the Permo-Triassic Boundary occur at times when the oceans do not appear to have been restricted.  Third, coupled ocean-atmosphere models indicate that although ocean circulation was slower under these warmer conditions, it did not stop.

But also, as we have looked more and more closely at those small windows into the past, we have learned that during some of these events anoxia was more restricted to coastal settings.  And that brings us back to the Zechstein Sea. We mapped the extent of anoxia at an unprecedented scale in cores drilled by the Polish Geological Survey, and we discovered an increasing abundance of GSB molecular fossils in rocks extending from the carbonate platform and down the continental slope, suggesting that anoxia had extended out into the wider sea.  But when we reached the deep central part of the basin, the fossils were absent.  In fact, the sediments contained the fossils of benthic foraminifera, oxygen dependent organisms living at the seafloor, and the sediments had been bioturbated, churned by ancient animals. The green sulfur bacteria and the anoxia were confined to the edge of the basin, completely unlike the Black Sea.  This is not the first such observation and this is consistent with new arguments mandating not only a different schematic but also a different trigger.  And perhaps that trigger was from outside of the oceans.

If the trigger was not solely a restriction of oxygen supply then the alternative is that it was an excess of organic matter, the degradation of which consumed the limited oxygen. A likely source of that organic matter and one that is consistent with restriction of anoxia to ocean margins is a dramatic increase in nutrients that stimulated algal blooms – much like what is occurring today.  And that increase in nutrients, as elegantly summarized by Hugh Jenkyns, could have been caused by an increase in erosion and chemical weathering, driven by higher carbon dioxide concentrations, global warming and/or changes in the hydrological cycle, all of which we now know occurred prior to several OAEs. And again, similar to what is occurring today.

It is likely that today’s coastal dead zones are due not to climate change but to how we use our land and especially to our excess and indiscriminate use of fertilisers, most of which does not help crops grow or enhance our soil quality but is instead washed away to pollute our rivers and coastal seas. And yet that only underscores the lessons of the past.  They suggest that global warming might exacerbate the impacts of our poor land management, adding yet another pressure to an already stressed ecosystem.

Runoff of soil and fertiliser  during a rain storm. Image source: Wikimedia Commons

The Zechstein Sea study is not the key to this new paradigm (and that ‘paradigm’ is far from settled).  There is probably no single study that marked our change in understanding.  Instead, this new model has been gradually emerging over nearly 20 years, as long as I have been studying these events. New geochemical data, such as the distribution of nutrient elements, suggest that many of these anoxic episodes, whether local or global, were associated with algal blooms.  And other geochemical tools, such as the isotopic composition of trace metals, provide direct evidence for changes in the chemical weathering that liberated the bloom-fueling nutrients.

Science can move in monumental leaps forward but more typically it evolves in small steps. Sometimes, after years of small steps, your understanding has fundamentally changed. And sometimes that change means that your perception of the world, the world you love and on which you depend, has also changed.  You realize that it is more dynamic than you thought – as is its vulnerability to human behaviour.

This blog is by Prof Rich Pancost, Director of the Cabot Institute at the University of Bristol
A shortened version of this blog can be found on The Conversation.

Prof Rich Pancost

This blog has also appeared in IFL Science and The Ecologist.

Withdrawn: Reflections on the past and future of our seas

On the 23rd of August, and as part of Bristol 2015 European Green Capital, I have the privilege of participating in a conversation about the future of our coastal seas that has been inspired by Luke Jerram’s ethereal and evocative Withdrawn  Project in Leigh Woods.  The conversation will include Luke, but also the esteemed chef, Josh Eggleton  who has championed sustainable food provision and is providing a sustainable fish supper for the event, and my University of Bristol Cabot Institute colleague, Dani Schmidt, who is an expert on the past and current impacts of ocean acidification on marine ecosystems.

My engagement with Withdrawn has been inspired on multiple levels, primarily the enthusiasm of Luke but also arising from my role as Cabot Director and my own research on the oceans. Withdrawn inspires reflection on our dependence on the sea and how we have polluted and depleted it, but also on how we obtain our food and the people at the heart of that industry.

All of these issues are particularly acute for our island nation, ringed by nearly 20,000 kilometres of coastline and culturally and economically dependent on the sea. Beyond our own nation, over 2.6 billion people  need the oceans for their dietary protein, a point driven home when I interviewed Sir David Attenborough on behalf of Cabot (see video below). He passionately referred to the oceans as one of our most vital natural resources. And of course, as Withdrawn reminds us, the oceans have vast cultural and spiritual value. It also reminds us that those oceans and those resources are at profound risk.

I’ve spent over 25 years studying our planet and its oceans. However, my first ocean research expedition did not occur until 1999, and it was a profoundly eye-opening experience. We were exploring the deep sea communities fuelled by methane extruded from the Mediterranean seafloor.  Isolated from light, the ocean floor is a largely barren world, but in parts of the Mediterranean it is interrupted by explosions of colourful life, including tubeworms, bacterial colonies, fields of molluscs and strange and lonely fish, all thriving in exotic mountains of carbonate crusts cut by saline rivers. These are vibrant ecosystems but so far removed from the surface world and light that they instead depend on chemical energy sourced from deep below the bottom of the ocean. And even here we found human detritus, plastic and cans and bottles.

Those were powerful observations, in large part because of their symbolism: our influence on the oceans is pervasive and quite often in ways that are challenging to fully comprehend and often invisible to the eye. These include, for example:

  • The potentially devastating impact of plastic on marine ecosystems, including plastic nanoparticles that are now, for all intents and purposes, ubiquitous.  Of course, pollutants are not limited to plastic – our lab now identifies petroleum-derived hydrocarbons in nearly every ocean sediment we analyse.
  • The decreasing pH of the oceans, due to rising CO2 levels, an acid when dissolved in water. We acidifying the oceans, apparently at a rate faster than at any other time in Earth history, a deeply alarming observation. We are already seeing some consequences of ocean acidification on organisms that make calcium carbonate shells. However, what concerns most scientists is how little we know about the impacts of rapid ocean acidification on marine ecosystems.
  • Ocean warming. A vast amount of the energy that has been trapped in the Earth system by higher greenhouse gas concentrations has been absorbed by the oceans.  Its impact on marine life is only beginning to be documented, but it has been invoked, for example, as an explanation for declines in North Sea fisheries.

And these represent only a few of the chemical and environmental changes we are making to the marine realm. They do not even begin to address the numerous issues associated with our over-exploitation and poor management of our marine resources.

Compounded, these factors pose great risk to the oceans but also to all of us dependent on them. As Cabot Institute Director, I engage with an inspiringly diverse range of environmental scientists, social scientist, engineers, doctors and vets.   And in those conversations, of all the human needs at threat due to environmental change, it is water and food that concern me the most.  And of these, our food provision seems the most wildly unpredictable. The synergistic impact of warmer temperatures, more acidic waters, and more silt-choked coastal waters on a single shellfish species, let alone complex ecosystems such as coral reefs or North Sea food webs, is very difficult to predict. This uncertainty becomes even more pronounced if we factor in nutrient runoff from poorly managed land, eutrophication and ocean anoxia leading to more widespread ‘dead zones’. Or the impact of plastic, hydrocarbon, and anti-biofouling pollutants. The ghost ships of Withdrawn quietly tell the story of how our increased demand and poor management have led to overexploitation of fish stocks, causing an industry to face increasing uncertainty. But they also invoke deeper anxieties about how environmental change and pollution of our seas could devastate our food supply.

But Withdrawn, like other Bristol Green Capital Arts projects and like all inspiring art, does not telegraph a simple message.  It does not shout to ‘bring back local fisherman’ or ‘save our oceans’.  These messages are present but subtly so, and for that both Luke and the National Trust should be celebrated. The boats themselves are captivating and draw you into the fisherman’s efforts; they acknowledge our dependence on the ocean and that we must continue to exploit it. To others they are suggestive of some past catastrophe, a tsunami that has somehow deposited fishing boats in a wildly unanticipated place. And yet to others, they suggest the changing character of seas, seas that once stood 100 m higher than they do today and which almost certainly will do so again if all of our coal and oil is burned into carbon dioxide.

Withdrawn is about all of those things. And consequently, at its deepest level, I think Withdrawn is about change.

Ammonite by Alex Lucas as part of Cabot Institute’s Uncertain World art project.

Geologists have a rather philosophical engagement with the concept of change – on long enough timescales, change is not the exception but the defining character of our planet and life. I should clarify that the aforementioned Mediterranean expedition was my first proper research excursion to the modern seas, but it came long after numerous visits to ancient ones.  In 1993, my PhD co-supervisor Mike Arthur took a group of us to Colorado where we collected samples from sedimentary rocks that had been deposited in the Cretaceous Western Interior Seaway 90 million year ago, a Seaway from a hotter, ice-free world, in which higher oceans had invaded a downflexed central North American basin. That might not seem like a proper marine experience but to a geologist you can reconstruct an ocean in startling clarity from the bold clues preserved in the rock: current flows that tell you the shape of the coastline; fossils that reveal the ecosystem, from cyanobacterial mats on the seafloor to inoceramids  and ammonites  to great marine reptiles in the waters above; and the rocks themselves that reveal a shallow sea in which limestone was deposited across a great platform.

But it was only like this at some times.  The fascinating aspect of these rocks is the complex pattern of sedimentation – from limestones to shales and back again – limestones that were much like the lime cliffs of Lyme Regis, switching in a geological blink of the eye to oil shales similar to those in Kimmeridge Bay, from which, further North and at greater depths and pressures, North Sea oils derive. Limestone. Shale. Limestone. Shale. A pattern repeated hundreds of times.  In the Western Interior Seaway.  Along the Jurassic Coast. Across the globe, from the Tarfaya, Vocontian and Maracaibo basins to the Hatteras Abyss, from Cape Verde to the Levant Platform. Cycles and cycles of astonishingly different rock types – all bundled up in patterns suggesting they were modulated by the ever changing character of Earth’s orbit.  These cycles are change, from a sea with clear waters, little algal growth and ringed with reefs to one fed with nutrients and gorged with algal blooms and stripped of oxygen.

Change is a necessary and inevitable feature of our planet.  And of the human condition.

But we seem incapable of resisting the urge to impose a value judgment for or against change. It is either viewed as a technocratic marvel to be celebrated or a violation against the natural state of the world and to be resisted.  But often, change is conflated with loss.  And there is something of loss in Withdrawn. These are the ‘Ghost Ships’ of Leigh Woods.  Ghosts of a way of life that no longer exists. Ghosts of the animals these boats once hunted.  Ghosts of some past and inexplicable event.

Of course, change will always be about progress vs loss, its value neither solely good nor bad but nonetheless inevitable.  But just because a geologist recognises the inevitability of change does not mean he thinks we should be passive to it. Change will come but should be managed, a significant challenge given its rapid pace over the past 150 years. In fact, one of the main observations of Dani Schmidt’s research is that our current rate of environmental change appears to be essentially unprecedented in Earth history, let alone human experience.

My hope is that Withdrawn has caused people to engage with the concept of change. How do we manage change in the 21st century?  How do we recognise those things that can and should be let go. As one visitor said, ‘We want to resist romanticising the past.’  Conversely, how do we decide what change must be moderated, because its cost is too high?  We can reduce our plastic consumption and waste, and we can enforce more rigorous regulations to stop the pollution of our planet – and we should.  More complicated questions arise from how we manage our dependencies on these precious marine resources, but it is clear that we can eat fish more sustainably, and chefs like Josh Eggleton are showing the way. We can create marine reserves that will not only conserve species but serve as biodiversity hotspots benefitting all of the oceans.

Perhaps most importantly, how do we recognise those things that must be preserved?  When I see the ghost ships of Withdrawn, I feel the poignant loss of our connection with nature and our connection with what it provides. Our food is now produced far away, delivered to sterile supermarkets via ships, trains and lorries; maybe that is necessary on a planet of over 7 billion people but if so, we must strive to preserve our connection to the sea – to our whole planet – understanding what it provides and understanding its limits.


This blog is by Prof Rich Pancost, Director of the Cabot Institute at the University of Bristol.

Prof Rich Pancost
The final Withdrawn talk at Leigh Woods will be taking place on 23 August 2015 and will feature Cabot Institute scientists, Luke Jerram and chef Josh Eggeleton who will be cooking up a sustainable fish and chip supper for attendees.  This event is sold out.