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geochemistry

Stuck on you (for millions of years): Organic matter on oceanic minerals

Article: Emily Estes et al. Persistent organic matter in oxic subseafloor sediment. Nature Geoscience (2019) 12:126–131. DOI: 10.1038/s41561-018-0291-5 https://www.nature.com/articles/s41561-018-0291-5

You’re eating dinner. Did you ever hear your parent say ‘Clean the plate’? Or ‘You have to finish dinner before you get dessert!”? What happened if you had leftover food? Usually you would save it as leftovers, or scrape it off the plate and compost it, or maybe even sneakily feed it to your pet. But you usually didn’t leave it sitting around for millions years. But in deep sea sediments (think the mud at the bottom of the ocean), that’s what happens.

Organic matter in deep-sea mud

In these sediments, microscopic organisms like bacteria and fungi (microbes) live and die. While they are alive, many of them eat the food found in the sediment. Still, they leave a lot of it behind for some reason. This food is called organic matter, a term that describes all of the different types of carbon-y material, like proteins and carbohydrates. In other words, organic matter is anything from dead grass to what makes up your cells! Though organic matter long buried deep beneath the sea may not seem important, ocean sediments act as a carbon sink. This means they can lock away carbon over very long timescales, which can help regulate global climate by preventing more CO2 from entering the atmosphere.

In a lot of sediments, organic matter is scarce, so microbes getting to eat it isn’t as straightforward as walking to the freezer and grabbing a Hot Pocket. But scientists don’t totally understand why there is organic matter that remains uneaten by starving microbes over long timescales – think hundreds to millions of years. Some hypotheses for this organic matter persistence include that the type of organic matter is too hard to eat (like if we had to eat tree leaves all day), or that it is too dilute (like if we had to run across a football field just for a carrot). Another hypothesis it that the organic matter is stuck (sorbed) to minerals that occur in marine sediment, and that protects it from being eaten. It is really hard to test these hypotheses, especially since the breakdown of organic matter in deep sea sediment tends to go slowly, taking millions of years to occur.

The why and the how

Recently, Dr. Emily Estes of the Woods Hole Oceanographic Institute and her collaborators set out to find out more about why this organic matter remains uneaten for up to 24 million years using deep sea sediments from two regions – the North Atlantic and South Pacific Gyre (check out the map for where these are!).

Figure 1. Map of ocean currents with approximate locations of where the samples come from. Map credit to Dr. Michael Pidwirny.

 

Both the North Atlantic and South Pacific Gyre are exceedingly low productivity areas, meaning there is not a lot of photosynthesis because of a lack of nutrients. That causes the sediment under the ocean to take a lot time to build up, and very little organic matter is deposited with the minerals that are deposited. Estes and collaborators used very specialized tools to examine the organic matter stuck on the minerals to determine why that organic matter hadn’t been eaten by microbes. They used a technique called near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectroscopy requires using a huge piece of equipment called a synchrotron, which is a particle accelerator. The X-rays made by the synchrotron are used to zoom into tiny minerals, and can look at the types of organic matter stuck to these minerals. Different types of organic matter absorb different amounts of X-rays, which are measured as absorbance spectra, like seen in Figure 3.

Figure 2. Pictures of a synchrotron ring (top) and a mineralogy synchrotron station (bottom) at the ALBA synchrotron in Spain. (PC: Laura Zinke)

From the NEXAFS measurements, the scientists collected spectra, which they compared to spectra of known types of organic matter. These spectra revealed that most of the organic matter was made of proteins or products of protein degradation. This is true for samples tested from both locations (the North Atlantic and the South Pacific Gyre) and samples of different ages (11 million to 24 million years old). The authors of this study conclude that organics binding to minerals and being inaccessible to microbes is probably why this protein-derived organic matter sticks around for millions of years and isn’t eaten by microbes.

Figure 3. Examples of NEXAFS spectra of different types of organic molecules, demonstrating how different type of organic matter give off distinct spectra. PC: https://www-ssrl.slac.stanford.edu/stohr/nexafs.htm

The authors of the study also modeled how much organic matter is in marine sediments, and found that up to 1022 gram of carbon is stored as organic matter in sediments that are similar to the North Atlantic and South Pacific Gyre sediments. Not only is that a lot of organic matter, that’s up to twice as much as previous estimates of the organic matter in sediments worldwide.

 

This is important because there might be a lot more carbon locked up in sediments as organic matter than we previously thought. As we try to figure out how to sequester carbon – i.e. take it out of the atmosphere and have it stay in places where it doesn’t exacerbate climate change – it is important to know where carbon is stored, how much, and if it is going to be eaten by organisms and turned into more carbon dioxide. From what Dr. Estes and her colleagues found, it sounds like a lot of the organic matter in open ocean marine sediments is there to stay for a while!

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