The Earth’s carbon is distributed between terrestrial, oceanic and atmospheric reservoirs.
The transport of carbon between these stores operate on timescales from seconds to millennia. It includes a variety of familiar processes, such as respiration and photosynthesis, weathering and sedimentation, gas exchanges, and transportation by water.
Consequently, these “biologically active” zones - referred to as the biosphere - have a fundamental role in the climate system, providing both positive and negative feedback to climate change.
The advent of the industrial age has changed the way carbon is transported between the major carbon stores on the Earth. It does so by disturbing natural fluxes and creating new pathways and potential feedback mechanisms that influence Earth’s climate. For example, the impacts of global land use changes alone may have increased the amount of carbon stored as carbon dioxide (CO2) in the atmosphere by one third of pre-industrial estimates.
The increase of atmospheric CO2 is now of growing concern, as it is causing significant warming of the Earth by changing the heat and water balances between the surface and atmosphere.
The carbon cycle is inter-linked with the water cycle through various chemical, biological, and physical processes. Understanding the relationship between carbon and the water cycles is essential to gain a comprehensive understanding of the role that humans and ecosystems have in their overall function and in global climate.
Understanding the mechanisms that control the transport and fate of carbon and nutrients, collectively known as organic matter (OM) in Earth’s biogeochemical cycles is a global challenge underpinning the United Nations’ sustainable development goals (UN SDGs). It’s also the key focus of the Carbon-Water Dynamics group, led by Ryan Pereira, Ph.D., at the Lyell Centre, Heriot-Watt University, Edinburgh, UK.
While much progress has been made to quantify carbon stores and fluxes, our ability to model carbon and nutrient fluxes is not well constrained. That increases the uncertainty of future projections and our ability to respond and adapt to these changes. This is in part due to our limited knowledge of the source, composition and reactivity of OM during mobilization and transport between terrestrial, oceanic and atmospheric reservoirs, and how human activities modify natural cycles. These complex interactions impact short- and long-term food and water security, a central goal of the UN SDGs.
Unfortunately, carbon doesn’t just float along as an individual molecule. It's extremely heterogeneous. A variety of other elements are associated with it, including phosphorus and potassium, along with other macro elements like iron. That makes carbon an extremely complex molecule to track.
In water, carbon can be transported as a particle or dissolved substance called dissolved organic matter (DOM), and gas (such as greenhouse gases CO2 and methane (CH4).
DOM has three key characteristics:
Like every good scientist, Pereira’s group begins with a question and then develops some hypotheses to assess that question in an objective and robust way.
“I often liken my work to looking for questions rather than looking for answers,” he said. “Because once you identify better questions, you tend to get more reasonable answers.”
To do that, the team takes water, soil and sediment samples from diverse locations on Earth, including the Arctic, temperate, and tropical zones.
“To test our scientific questions, we have to travel to different parts of the world to be able to assess those questions,” he said. “We also need to incorporate a unique blend of fundamental and applied research that takes full advantage of novel technologies in collaboration between academic and industrial partners to drive this research forward.”
Pereira and his team use elemental quantification of carbon, fluorescence spectroscopy and next-generation liquid chromatography techniques to conduct measurements.
He uses a HORIBA Aqualog® spectrofluorometer to investigate the composition of DOM in water. The Aqualog is an ideal instrument for Pereira’s team, as it simultaneously measures both absorbance spectra and fluorescence Excitation-Emission Matrices, a proprietary technology known as A-TEEM™. His Aqualog is connected to an autosampler that allows seamless queueing of samples to provide high throughput and minimise technician time to acquire molecular fingerprints of DOM under study.
The research team makes numerous field measurements, although the higher-end instruments based in the university’s labs get its share of samples to examine. That’s where he uses the Aqualog.
“These carbon pools of DOM are extremely complex,” Pereira said. “We are interested in the coloured dissolved organic matter, but also the ‘invisible’ dissolved organic matter. These two different pools of carbon in the environment impact climate aspects of ecosystem function, greenhouse gas emission and uptake in aquatic systems, and the impact on drinking water supply.”
The Aqualog’s speed of processing samples played a key role in Pereira’s data collection.
“The Aqualog accounted for some of the high throughput of samples that were required,” he said. “For example, last year we were working in the tropics, and I think we brought something close to 3,000 samples back with us over a three-month period.”
That speed was needed because of this high time resolution that they were trying to capture.
By combining both fluorescence spectroscopy, elemental analysis and liquid chromatography, Pereira gets the top-down ability to quantify the amount of carbon in a particular system and an indication of its potential reactivity to a given process.
The Carbon-Water Dynamics group have two focus areas of research - rivers and surface water layers of the ocean.
Pereira’s research took his team to the tropical rain forests in South America. The tropics are a hotspot of untapped knowledge waiting to be uncovered.
“We spend a lot of time actually storm-chasing,” he said. “Unfortunately, this is not as glamorous as in the movies. We're looking for rain events, and when it starts raining, I send my poor students out to start taking samples every hour. And of course, in the tropics during the wet season, it rains a lot.”
In that particular project, Pereira and his team are trying to identify the sources of invisible, DOM in the system, and its rate of transformation into more labile, or easily altered organic substances or gases.
Oceans are a global reservoir of greenhouse gases, estimated to account for 20 to 40 percent of the post-industrial sink for anthropogenic CO2, according to Pereira.
However, quantifying the exchange of gases such as CO2, CH4, and N2O between the ocean and atmosphere is a major challenge. There are different layers within a body of water, each one with different physical and chemical properties. The surface microlayer, also called the skin layer, is around 400 μm thick, approximately twice the thickness of paper.
The ocean skin layer, known as the ocean surface microlayer, supports a unique bacterial community, and has an enrichment of carbon compounds - dissolved organic matter. This layer is more concentrated than the underlying water layers.
Because of the concentration, and that the surface is directly interacting with the atmosphere, it has a direct control on any process between the atmosphere and a body of water, and in particular, the uptake of greenhouse gases like CO2. Understanding how the ocean’s organic skin layer modulates this exchange is critical to estimating the intrinsic oceanic sinks and sources of these key greenhouse gases both now and in the future.
Some of Pereira’s work has been looking at DOM’s influence on gas exchange.
“These processes that are ongoing can be impacted by DOM,” he said. “Organic substances in the skin layer, known as surfactants, span across traditional operational definitions and are derived from multiple sources undergoing biotic and abiotic transformations along the land-ocean continuum.”
Work by Pereira and other investigators in the UK have shown that reduced gas exchange by surfactants, known as the “surfactant suppression effect,” (SSE), has been shown to reduce the amount of CO2 annually stored by about 9 percent in the Atlantic Ocean (Pereira et al., 2018), and to control the spatio-temporal flux of N2O (Holding et al., 2019).
Importantly, there appears to be a strong relationship to ocean temperature, suggesting that warmer surface oceans may absorb less atmospheric greenhouse gases due to the increased presence of DOM in the ocean skin layer.
Pereira and his team have demonstrated that organics like DOM have a very large role to play in the process.
“If we are overestimating, it means that the amount of gas being taken up by the ocean is not what we're expecting,” Pereira said. “That means that the rate of ocean acidification may not be as high as we previously thought. It may also mean that there is more of a buffer to the ocean in its ability to absorb greenhouse gases.”
Pereira considers himself an environmental scientist in the midst of a long journey.
“I do feel that in a lot of what I do, whether or not this is the microlayer work or the invisible carbon story, we are very much in the early stages of our scientific understanding in where these are contributing to climate change matters,” he said. “And I think that's because of our limited understanding.”
It's a difficult area to get into because there are so many unknowns, and you need to convince a lot of people, he said.
“But I find that it’s the most exciting stuff - my science, which a colleague and friend described, is disruptive rather than iterative. And well, that was quite a compliment.”
“I'm trying to add new knowledge in a way that we didn't know anything about beforehand, which then obviously challenges our perceptions.”
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