#################################################### # John F. McCarthy and Laura Liegerman # requesting 2 days, at minimum=2 days # beamline 33ID, Carbon Sequestration in Marine Sediments: Preservation by pores filled with organic matter # instrument 33ID-D USAXS #################################################### # top:/home/www/beamtime-requests/req00957.txt # UNICAT Member Beam Time Request #957 # created Wed Apr 19 13:57:06 CDT 2006 #################################################### apsrun: 2006-02 beamline: 33ID collaboration: Yes contact: jmccart1@utk.edu days: 2 description: Carbon flow through the organic matter (OM) of soil and seciments is essential to the functioning of terrestrial and marine ecosystems. Recently, this phenomenon has attracted great interest due to concerns about global warming and the potential for using soil as a sink for C released into the atmosphere by human activity. THE CURRENT REQUEST FOR BEAMTIME IS FOR A M.S. THESIS by graduate student, Laura Lieberman. She is focusing on OM preservation in marine sediments. Marine sediments are composed of aggregates much like terrestrial systems and OM could be contributing to the stability of the aggregate, and the protection of the organic matter. Her goal is to determining if the organic matter is indeed correlated to the stability of the marine aggregate and if the aggregate is providing physical protection for the organic matter. She has isolated silt-size fractions from multiple depths in a sediment core collected in the Gulf of Maine. She will evaluate the distribution of OM within the silt structure using USAXS to determine if the silt structure provides physical protection to OM in marine sediments. Her USAXS experiments will be similar to those employed by one of her mentors, John McCarthy, who focused the mechanisms of OM preservation by soil microaggregates because they have a greater capacity to protect OM against decomposition compared to other soil fractions, as demonstrated by the many-fold slower C turnover times. In those studies, USAXS was used to determine the distribution of OM in pores of soil microaggregates and found that OM protection and long-term accrual of OM in soil was associated with filling of porosity by OM. The novel mechanism of C-sequestration by OM-filled pores is postulated to act via spatial and kinetic limitations on microbial degradation. The OM pool available to microbes is restricted to the small area of the pore throat, while the large pool of OM residing in the pore body is inaccessible. Diffusional hindrances decrease to the efficiency of microbial exoenzyme-mediated degradation. The distribution of pores in microaggregates was measured in microaggregates before and after the OM was removed by combustion at 350oC. The total porosity was determined using data from the combusted, OM-free microaggregates. The distribution of the OM within the pores was determined by USAXS from differences in scattering curves for the combusted and intact samples. The scattering contrast, and thus the scattering intensity, increases substantially if an OM-filled pore is emptied due to removal of the OM by combustion. Therefore, for pores that are entirely filled with OM, the pore volume of the OM-filled pores can be estimated by difference curve resulting from subtracting scattering curve for the intact (OM-filled) microaggregates from that of the combusted (minerals-only) microaggregates: Äñ2OM-filled = (ñcombusted)2 – (ñnot-combusted)2 = (ñmineral)2 – [(ñmineral) - (ñOM)]2 The size frequency distribution of voids using evaluated via the Maximum Entropy method as implemented in Jan Ilavsky’s software package for SAS data analysis “Irena” (www.uni.aps.anl.gov/~ilavsky/irena.htm). A manuscript on the results of this study is undergoing final review by authors before being submitted to Geochimica Cosmochimia Acta. A second publication on water retention by OM-filled pores is in draft form. To better understand the processes underlying the formation of OM-filled pores, USAXS data were recently acquired on silt-size fractions within our soil microaggregates because they are thought to be critical in the formation of the microaggregate structure. Mayer et al. 2004 used N2 adsorption to measure the surface area and pore volume intact and combusted of marine sediments from a variety of sources to demonstrate that OM was not physically protected in small mesopores (2-50nm). We obtained USAXS data for selected samples intact and combusted of marine sediments to test the applicability of our approach to marine sediments. The USAXS analyses of those samples indicated that for some samples, a significant fraction of the total porosity was filled with OM, and that most of the OM-filled porosity was in size ranges much larger than mesopores (McCarthy, unpublished). We hesitated to use those data as a definitive analysis of the potential role of pore-filling as a PM-protection mechanisms in marine sediments because the samples were freeze-dried whole sediments; structured aggregates constituted an unknown fraction of the porosity. Laura Lieberman’s M.S. thesis study avoids that problem by focusing in stable silt-size aggregates. Further, her samples have a consistent mineralogy, and comparison of results from different depths in the core provides a systematic progression of the time the OM was buried, as well as the bulk density and water content of the sediment due to compression by overlying sediments. equipment_required: USAXS sample paddles experiment: Carbon Sequestration in Marine Sediments: Preservation by pores filled with organic matter foreign_nationals: none hazards: The experiments does not involve any hazardous materials, samples, or procedures. The sediments are from the relatively pristine Gulf of Maine and contain no hazardous contaminants; this area supports a large commercial fishery of bottom-dwelling organisms. The samples will be prepared at the University of Tennessee (UT), and an approximate monolayer of freeze-dried samples of the silt-size aggregates will be encased between two layers of kapton tape. These prepared samples will be brought with us to the APS and mounted on the USAXS sample paddle. At the end of the experiment, the samples will be brought back with us to UT. instrument: 33ID-D USAXS instrument_other: minimumdays: 2 name: John F. McCarthy and Laura Liegerman nonmembers: Layra Lieberman submit: Submit unacceptable_dates: z34ID_details: #REMOTE_HOST: john6966.roam.utk.edu #REMOTE_ADDR: 160.36.141.200 #CONTENT_LENGTH: 6171 #HTTP_REFERER: http://www.uni.aps.anl.gov/admin/unireq.html #HTTP_USER_AGENT: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)