Source: Torres, M. A.; Moosdorf, N.; Hartmann, J.; Adkins, J. F.; West, A. J., Glacial weathering, sulfide oxidation, and global carbon cycle feedbacks, Proceedings of the National Academy of Sciences 2017, 114 (33), 8716-8721. DOI: doi:10.1073/pnas.1702953114
What causes glaciations?
What force drives the march and retreat of glaciers stumps scientists to this day. Every few thousand years, our planet shifts between glacial periods (full ice coverage) and interglacial periods (partial ice coverage) (Figure 1). This evolution of glaciers profoundly impacts the chemistry and biology of our planet, but we still don’t know what causes their onset, what controls how far the glaciers spread, and what allows for the fluctuations in ice cover.
The one clue we do have to their chronology is the close connection between atmospheric carbon dioxide (CO2) levels and the formation of glaciers. CO2 is a greenhouse gas: the more carbon dioxide is present, the warmer our climate. Carbon dioxide traps heat, and thus we conclude that CO2 in the atmosphere plays an important role in driving the formation and disappearance of glaciers. This leads us to the next question: what controls how much CO2 there is in the atmosphere?
When examining the composition of the atmosphere over the past million years through ice cores and other records, it is apparent that carbon dioxide levels have remained remarkably stable. This consistency hints at the presence of feedback loops; more specifically, some process that kicks in to increase CO2 when atmospheric CO2 levels start dropping (and the reverse when CO2 levels start rising). You can see the connection to glaciers here: if atmospheric CO2 starts declining, we’d expect global temperatures to drop and glaciers to form. But we know that glacial periods aren’t permanent. This means that next, some sort of feedback loop takes effect, adding carbon dioxide to the atmosphere to prevent and even reverse the formation of glaciers.
The research: weathering feedback loops
This study focuses on a group of chemical reactions thought to be central players in these feedback loops: weathering reactions. Chemical weathering is the name for reactions that occur when rock is newly exposed to the air. Depending on the kind of rock and reactions, chemical weathering can either absorb CO2 or add CO2 to the atmosphere. Torres and his colleagues examined two prevalent weathering reactions: silicate weathering, which takes CO2 out of the air, and sulfide oxidation, which supplies CO2 to the atmosphere. They sought to figure out which weathering reaction dominated during glacial periods. Depending on the balance between the two reactions, chemical weathering could either act as a negative feedback loop to limit and reverse the spread of glaciers, or as a positive feedback loop to accelerate the formation of glaciers (Figure 3).
To figure out which weathering reaction prevails during times of glaciation, the researchers collected samples of water trapped in glaciers. They first compared these samples to samples from non-glacial rivers and showed that the composition of the two were different. The glacial water contained higher amounts of positively charged chemicals (aka positive ions) than did the river water, affirming that the weathering chemistry happening during glacial periods is different from what’s happening when there’s less ice.
The researchers next looked only at the glacial samples and analyzed the compounds dissolved within. This time they focused on the ratio of positive ions to the amount of carbon in each of the samples. The production of positive ions decreases atmospheric carbon dioxide; if a sample has a high positive ion to carbon ratio, then the weathering reactions happening likely were a sink for atmospheric CO2. Conversely, if the samples had a low positive ion to carbon ratio, it means that the weathering reactions were adding CO2 to the atmosphere.
From their analysis, they found that sulfide oxidation (the reaction that adds CO2 to the atmosphere) dominated in glacial times. This result fits well with the prevailing picture that scientists have of glacial periods: when glaciers form, sulfide oxidation is enhanced, adding CO2 to the air, warming the atmosphere, and guiding our planet back to an interglacial state.
Why does more sulfide oxidation occur when there are glaciers? First, evidence suggests that more erosion occurs when glaciers are forming. This opens up more rock that can be weathered. While silicate weathering (CO2 sink) is always occurring, sulfide weathering (CO2 source) is “supply-limited” by the availability of the rock it occurs on. Therefore erosion during glaciation would enhance sulfide weathering without causing much of a change in the amount of silicate weathering happening.
It’s all connected…
When thinking about weathering (and any other earth process), it’s important to remember that the reaction isn’t occurring in isolation. Torres and his colleagues acknowledge that their study doesn’t consider the impacts of glacial erosion on other reactions that might affect atmospheric CO2 in other ways. The researchers also noted that the timing of when sulfide oxidation is enhanced due to glacier formation is still unclear. Despite these limitations, the feedback loop of enhanced sulfide oxidation during glacial erosion is likely to be a significant control on atmospheric CO2 from one glacial period to the next.
The consistency of the earth’s atmosphere and the ongoing oscillations of ice cover have mystified scientists for decades, but the mechanisms explaining these phenomena likely lie in the link between the two. Hundreds of scientists are studying different feedbacks in an effort to understand how each of these processes works in conjunction with one another to achieve the climate patterns we’ve observed. The possible feedback loop of glacial weathering holds promise as another way in which our planet regulates its own climate over thousands of years.
Julia is a PhD student at Scripps Institution of Oceanography in La Jolla, California. Her focus is on biogeochemistry, which, as the name suggests, centers on the combined effects of biological, geological and chemical processes on the earth system. She is advised by Dr. Ralph Keeling and is modeling the global carbon cycle to better understand how much carbon dioxide ends up in the atmosphere. When not at her computer writing code, Julia can usually be found reading and/or thinking about food.