The oceans are dying – and in some places, death is spreading faster than others. Take those so-called “dead zones”, for example. Bereft of oxygen, they often appear because fertilizer run-off causes a bloom in a specific type of algae; more algae means more dead algae, whose rain of corpses is a prime draw for bacteria.
When these bacteria break down these algae, they use up oxygen – and ultimately, the greater the algal numbers and subsequent die off, the greater the disappearance of dissolved oxygen in the water. Life in these anoxic zones is particularly sparse, and a new study suggests that they may one day be responsible for a new wave of extinctions.
Writing in the journal Science Advances, a team of researchers – led by Arizona State University – explain that they wanted to know how often these anoxic dead zones occurred in the past. Importantly, they wanted to conclusively discover whether or not the sudden drawdown of oxygen from the oceans took place because of these algal population explosions, or “blooms”.
The Cretaceous Period – the final chapter in the age of the dinosaurs – featured plenty of these dead zone-creating anoxic events. The researchers took a close look at sediment samples dating back to this time and found that around 94 million years ago, the deoxygenation of major segments of the world’s ancient oceans was in full sway.
This 50,000-year-long-event event is known as Ocean Anoxic Event-2, and it resulted in a major extinction event – although not a mass extinction, as some outlets have reported. The Spinosaurs, Pliosaurs, and possibly even the Ichthyosaurs – three major aquatic or semi-aquatic reptiles – all died out at the same time, along with 27 percent of all marine invertebrates.
Multiple causes have been suggested before, including volcanism – whose carbon dioxide and methane would have essentially forced the oxygen concentrations down – and algal blooms. However, according to this new analysis, it looks like both took place: the volcanism enhanced nutrient supply, which boosted algal populations.
Both factors triggered such a rapid deoxygenation event that, were it to happen today, roughly “16 percent of the modern seafloor area” would be starved of oxygen. This would easily be enough to cause a sizable extinction event – or contribute significantly to the mass extinction event humans are already driving.
The team go as far as to say that if we manage to drive oxygen from the sea at a rate similar to that seen during the Cretaceous Period, then we’ll be headed for a catastrophe. Within a few centuries, the massive dead zones appearing in the ocean today will double in size.
So if you want a glimpse of the near-future, you just need to take a look at the record-breaking man-made dead zone off the Gulf of Mexico.