Madrid 2 (European Press)
Sediment cores reveal a frequent relationship between hot climates and sudden outbursts of low-oxygen “dead zones” in the sub-Arctic North Pacific over the past 1.2 million years.
The results of the new study, led by researchers at the University of California Santa Cruz (UCSC) and published in the journal Science Advances, provide important information for understanding the causes of oxygen starvation or hypoxia in the North Pacific and for predicting the occurrence of future hypoxic conditions.
“It is imperative that we understand whether climate change is pushing oceans toward a ‘tipping point’ of sudden and severe hypoxia that would devastate ecosystems, food sources and economies,” first author Carla Knudson said in a statement. Graduate student in Earth sciences at the University of California.
The researchers based their findings on the analysis of deep sediment cores from a site in the Bering Sea. Over long periods of time, sediment settles and accumulates on the sea floor.
The activity and mix of organisms living in seafloor sediments often changes as they accumulate, but if hypoxia kills those organisms, an orderly pattern of stratification is maintained. Thus, scientists can find a record of past hypoxia events in the form of these stratified or “lamellar” sediments in the perforated cores of the seafloor.
Scientists have long known that a major episode of generalized hypoxia occurred in the North Pacific at the end of the last Ice Age, when melting ice led to a massive influx of fresh water into the ocean. Now, the new study provides the first records of previous low oxygen events, and shows that the last one was not representative of most of these events in terms of mechanisms or time.
“It doesn’t take as much perturbation, as the melting of the ice sheets,” explains corresponding author Anna Christina Ravello, professor of oceanography at UCLA. “These episodes of sudden hypoxia are common in the geological record and are not usually associated with decomposition. They almost always occur during warm icy periods, like the ones we live in now.”
Hypoxia occurs after intense growth of phytoplankton (seaweed) in surface waters. When phytoplankton die, they sink to the depths of the ocean and decompose, depleting oxygen and releasing carbon dioxide into the water below the surface. However, it is not yet clear what led to these phenomena. Ocean warming, rising sea levels, and iron availability (a limiting factor for phytoplankton growth) appear to play a role.
Our study indicates that elevated sea levels, which occur during warm icy climates, contributed to these hypoxia phenomena, Knudson notes. “During elevated sea levels, dissolved iron can be transported from flooded continental shelves to the open ocean and promote intensive growth. From phytoplankton in surface waters.”
Although sea level rise is an important background condition, it is not sufficient to trigger a hypoxia event on its own. He adds that changes in ocean circulation, including the intensification of surface water waves to bring more nutrients to surface waters and stronger currents that can carry iron from the continental shelf to the open ocean, could play a critical role.
Regional dead zones are currently occurring in coastal regions around the world due to the temperature effects of global warming, as well as the enrichment of coastal waters with nutrients from agricultural fertilizers. But even the massive dead zone at the mouth of the Mississippi River pales in comparison to the widespread hypoxia that occurred across the North Pacific at the end of the last Ice Age.
Because the new study is based on sediment cores from a single site, the researchers don’t know the extent of the dead zones it records: whether they are confined to the Bering Sea or extend across the entire North Pacific rim, as happened in the most recent case.
“We don’t know how widespread it was, but we do know that it was very severe and lasted longer than the well-studied dissolution event,” says Ravello, who was a scientist involved in Expedition 323 from the Integrated Ocean Drilling Program, which recovered the core of the Bering Sea in 2009 .