Ocean deoxygenation occurs when oxygen levels drop in the ocean, threatening marine animals and ecosystems that rely on small amounts of oxygen dissolved in water to survive. Known sources of deoxygenation include climate change, massive algae blooms, and nutrient pollution. In an attempt to better understand these events, researchers in a recent study looked to the past for clues about triggers and causes of deoxygenation events and found a surprising source to add to the list: ice sheet retreat.
Understanding deoxygenation matters for aquatic ecosystems because oxygen, necessary for the survival of nearly all animals including marine animals, is much harder to obtain in water than on land. Dissolved oxygen is the only form available to aquatic animals and microorganisms. Photosynthesizers like plants and marine algae can produce oxygen in the oceans but only near the surface where sunlight is abundant. As a result, mid-level and deep water can become low oxygen zones. However, too much of a good thing can quickly become a bad thing. If photosynthetic algae grow too fast, they can quickly shade out sunlight, decompose, and create severely hypoxic regions called “dead zones” where nothing that needs oxygen can survive.
Scientists have been attempting to understand the effects of climate change on ocean deoxygenation events by looking at past events. Figuring out what caused deoxygenation events in past eras of swift climate change, such as the last major deglaciation period, could allow researchers to determine potential future sources of deoxygenation.
In a paper published in the journal Nature in November, geologists examined the potential relationship between ocean deoxygenation events and the end of the last ice age 17,000 to 10,000 years ago. Jianghui Du, a postdoctoral scholar at the Swiss Federal Institute of Technology in Zürich and lead author of this study, recognized the urgency of understanding what processes can trigger deoxygenation. He chose to focus on the last deglaciation period when the Cordilleran ice sheet—a massive ice sheet bigger than Greenland that once covered the Pacific Northwest—shrank over the course of thousands of years.
To study ancient deoxygenation events like those that happened directly following the retreat of the Cordilleran ice sheet, Du and his colleagues used a variety of methods. To reconstruct ocean oxygenation, they assessed mineral assemblages—the abundance and composite of different elements including rhenium, cadmium, and uranium. Since different elements accumulate in sediments under differing oxygen conditions, the abundance of specific elements can allow scientists to estimate oxygen levels from thousands of years ago based on geological records. This work, combined with assessments of volcanic inputs, measurements of radiogenic isotopes, and a compilation of volcanic eruption records, allowed the researchers to evaluate the potential predictors and triggers of the large deoxygenation events following the retreat of the Cordilleran ice sheet.
Through this research, Du told GlacierHub that he found the retreat of the Cordilleran ice sheet “drove explosive volcanism” in the northern Pacific 17,000 to 10,000 years ago. This volcanism caused a large input of ash in the ocean, which according to Du, “fueled ocean productivity that triggered ocean deoxygenation.” While previous studies have hypothesized that ice sheet retreat might have induced volcanic eruptions, this study confirms that the deoxygenation events in the northern Pacific were a direct result of the volcanic eruptions caused by the Cordilleran ice sheet retreat.
As the Cordilleran ice sheet retreated, a phenomenon called isostatic rebound occurred. This process occurs when land rises after the massive weight of ice sheets is removed. Prior to retreat, the ice depresses the ground, putting pressure on the mantle underneath the rock. When the ice retreats, the rebound and upward movement of the land makes it unsteady and allows magma to rise through cracks toward the surface, triggering volcanic eruptions, earthquakes, and other tectonic events.
Volcanic eruptions in turn can fuel deoxygenation events in the same way that nutrient spills do. Volcanic ash contains high amounts of nutrients, including calcium, magnesium, and potassium, so large deposits of ash in the ocean quickly become areas where photosynthetic algae thrive. The resulting massive algae blooms can quickly cause water to become deoxygenated after the algae sink and die.
Since the ice coverage at present in the region is much smaller than the Cordilleran ice sheet was, current glacier and ice retreat is not likely to cause widespread volcanism to the same extent. However, understanding past volcanism and deoxygenation events can help scientists evaluate potential triggers of future events and prepare for them.
The study has key implications on the prospect of ocean iron fertilization, a climate engineering tactic popularized in the early 1990s. Since aquatic algae can remove carbon dioxide from the atmosphere via photosynthesis, proposals exist to disperse nutrients that are often limited, like iron, into the ocean to fuel more photosynthesis and reduce the amount of carbon in our atmosphere. Volcanic events like those studied by Du are examples of natural iron fertilization at a large scale. This study shows that while iron fertilization could indeed reduce the amount of carbon in the atmosphere, it comes at the cost of large deoxygenation events and potential for ecosystem damage.
Du emphasizes that collaboration between a wide variety of scientists is needed to understand past volcanic events. He says that even “slight decreases of oxygen in [low oxygen zones] may have a disproportionately large impact on the ocean ecosystem,” making it important to understand and prepare for deoxygenation events. Understanding the triggers of past volcanic eruptions and resulting low oxygen zones can help scientists and resource managers predict and mitigate the effects of such events in the future.