[Eoas-seminar] Friday Seminar

[eoas-seminar at lists.fsu.edu]

Please join us Friday (11/20) at 3:30 on zoom for this week's EOAS Colloquium speaker:

Dr. Paul Schroeder of the University of Georgia

If you would like to meet with Paul please email Jeremy Owens (jdowens at fsu.edu<mailto:jdowens at fsu.edu>)

Join Zoom meeting:
https://fsu.zoom.us/j/92748669753
Meeting ID: 927 4866 9753

Understanding rates of Critical Zone change on geologic and human time scales: Vantages of potassium and clay minerals

              The Calhoun Critical Zone observatory (CCZO) in SC provides valuable insight into the interactions between all biotic and abiotic components at the Earth's surface where rock meets life. Biotite [K(Mg,Fe)3AlSi3O10(OH)2] and phengite [K(Mg,Al,Fe)2(Al,Si)4O10(OH)2] from a deep core (0 to 20 meters) in the CCZO are thought to influence the cycling of potassium, a factor of plant nutrition at depth. As these micas weather and structural ferrous iron oxidizes, potassium is released in order to satisfy a layer charge balance. More intensely weathered biotite and phengites contain lower relative abundances of potassium due to higher levels of oxidation, where the 2:1 structure is maintained. As potassium content of weathered micas near the surface and 2:1 layers become less abundant, the 2:1 layers are restructured to 1:1 layers kaolinite [(Al2Si2)O5(OH)4]. Using an electron microprobe, sand-sized grains were analyzed for their elemental composition using energy dispersive spectroscopy (EDS). Whole grain mounts were also examined in optically. Notably, weathered grains displayed "frayed" ends when viewed perpendicular to the principle c-axis. Analysis of these weathered grain ends in comparison to less weathered centers of grains yielded less relative percentage of potassium on the ends. Near the surface, many more kaolinite grains were observed yet a continuous range of compositions between end member mica and kaolinite were found at all depths. Evidence for fixed potassium in biotite and phengite supports the notion that reservoirs are available to supply rooted zones as uplift and chemical erosion proceeds. Although the loss of potassium occurs, persistence of lower amounts of potassium in the near-surface clays suggest that the degraded micas can still serve as a stock for nutrient cycling. As subsurface conditions become reducing due to low oxygen supply during wet conditions, then the ferric iron in the 2:1 structure can be reduced, necessitating uptake of ions such as potassium and ammonium to compensate for layer charge changes. This implies that oscillating seasonal reducing and oxidizing cycles (fall/winter and spring/summer, respectively) create the potential for degraded biotite and phengite to act as refugia for nutrients in the subsurface CZ.
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