Marine organic matter is one of Earth‘s largest actively cycling reservoirs of organic carbon and nitrogen. The processes controlling organic matter production and removal are important for carbon and nitrogen biogeochemical cycles, which regulate climate. However, many possible cycling mechanisms have hindered the ability to quantify marine organic matter transformation, degradation and turnover rates.
Lawrence Livermore National Laboratory scientist Tom Guilderson and colleagues from UC Irvine(link is external), Stony Brook University(link is external) and UC Santa Cruz(link is external)analyzed existing and new measurements of the carbon-to-nitrogen ratio and radiocarbon age of organic matter, spanning sizes from large particulate organic matter to small dissolved organic molecules. The research appears in the Nov. 14 edition of the journal, Nature Geoscience(link is external).
“Detrital (non-living) marine organic matter can be converted to CO2 via many mechanisms,” said Brett Walker of UC Irvine and lead author of the study. “In our study, microbial degradation (or particles and molecules being eaten by bacteria) is the most important. Larger marine particles and molecules tend to have more nitrogen and cycle on faster timescales — impacting climate on shorter (modern to centennial) timescales. In contrast, small molecules tend to be carbon-rich, older and persist on longer (milllenial) timescales — affecting long-term climate trends.”
Using radiocarbon dating at LLNL‘s Center for Accelerator Mass Spectrometry, Guilderson and Walker determined production rates of small dissolved organic matter molecules and found that they stay in the deep ocean waters longer than larger molecules and particles.
It is important to measure how much organic matter is in the ocean because this is a vast reservoir of carbon, approximate in size to that of all atmospheric CO2.
“By radiocarbon dating organic matter size fractions, we can come up with estimates of the turnover time of these carbon reservoirs,” Walker said. “Essentially, we’d like to know how carbon is stored and recycled in the ocean before being converted back to CO2 and re-entering Earth‘s atmosphere. This is important because the ocean is where most of the extra carbon dioxide that humans are putting into the air will be stored.”
The findings suggest that organic matter is increasingly chemically degraded as it decreases in size, and small particles and molecules are stored in the ocean longer than their larger counterparts. The nearly ubiquitous relationships between size, age and composition quantify the rate of carbon sequestration in the deep ocean and indicate that the production of small molecules is a major pathway.