[Enviro-lunch] Enviro-lunch Monday 9/27/2021 by Marisa Nicole Repasch on "The fate of organic carbon during source-to-sink river transit: the importance of mineral associations and sediment residence time"

[Manisha Dolui]

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Hello All,

Please join us this Monday 27th September for our Enviro-lunch seminar-series second guest speaker Marisa Nicole Repasch from 12 noon- 1 PM PST via ZOOM
https://ucmerced.zoom.us/j/175736103


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Title: The fate of organic carbon during source-to-sink river transit: the importance of mineral associations and sediment residence time


Abstract:

Rivers are primary conduits for organic carbon (OC) transfer from vegetation-rich uplands to long-term sinks, and thus are responsible for significant fluxes among different reservoirs of the carbon cycle. The balance between OC oxidation and preservation in rivers determines whether fluvial transport leads to net release or drawdown of atmospheric CO2. Unfortunately, there are large uncertainties associated with river OC flux estimates, because we do not fully understand the timescale of sediment and organic matter transit from source to sink or the mechanisms of organic matter preservation.

In this talk, I address the uncertainties in transit time and OC preservation mechanisms in a detailed study of a unique river system in Argentina - the Rio Bermejo. This river exports sediment and organic carbon from the central Andes and transports it nearly 1300 km downstream without tributaries. Using meteoric cosmogenic Beryllium-10 as a sediment tracer, I found that sediment resides in the river systems for ~8.5 kyr, accounting for multiple cycles of deposition and re-mobilization. Over the ~1300 km transit distance, radiocarbon measurements suggest that the fraction of modern OC (F14C) decreases from 0.92 to 0.86, corresponding to ~500 yr of aging. We also observed enrichment of δ13C values along the channel. Compound-specific F14C and δ13C measurements suggest there is an older organic matter pool that is preferentially preserved and ages during fluvial transit, while labile organic matter is turned over rapidly. Concentrations of oxyhydroxide metals correlate with F14C values, suggesting that organo-mineral complexation is an important carbon stabilization mechanism. These data are supported by nanoSIMS images showing organic carbon associations with clay minerals. By combining radiocarbon data with geomorphic rates, I developed a framework to estimate carbon turnover during fluvial transit, allowing us to compare carbon oxidation fluxes to river export fluxes for the first time. This framework will allow us to assess the carbon budgets of major rivers around the world.



Bio:

Marisa Repasch is a Postdoctoral Researcher at Lawrence Livermore National Lab, in the Center for Accelerator Mass Spectrometry. She recently completed her Ph.D. in Geochemistry at the University of Potsdam in Germany and performed most of her research at the German Research Center for Geosciences (GFZ Potsdam). She received her B.S. in Earth and Environmental Science from Lehigh University and her M.S. in Earth and Planetary Sciences from the University of New Mexico. In her doctoral research, she combined cosmogenic nuclide geochemistry, carbon isotopes, and geomorphology to understand the fate of organic matter transported through river systems. Her current research focuses on the impacts of landscape disturbance, like wildfires and permafrost thaw on the transfer of sediment and carbon across the landscape. Outside of her academic career, she enjoys mountain biking and volunteering with local conservation and trail stewardship groups.


We’ll see you there!



Regards,

Toshi, KJ , Manisha and Jennifer (Student and Post-doc coordinators), and Dr. Asmeret Asefaw Berhe (Faculty coordinator)



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