Intramolecular Isotopic Variation in Acetate from Soils and Sediments

Open Access
Thomas IV, Randal Burton
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
September 10, 2008
Committee Members:
  • Michael Allan Arthur, Dissertation Advisor
  • Michael Allan Arthur, Committee Chair
  • Christopher Howard House, Committee Chair
  • Katherine Haines Freeman, Committee Member
  • Jennifer Macalady, Committee Member
  • Robert David Shannon, Committee Member
  • intramolecular isotopic analysis
  • wetland biogeochemistry
The stable isotopes of carbon are heterogeneously distributed within natural organic compounds. This heterogeneity presents an opportunity to probe natural processes because intramolecular isotopic distributions may contain evidence related to the natural formation and fate of organic matter. This thesis concerns the measurement and interpretation of intramolecular isotopic variation in acetate, a key intermediate in anaerobic metabolism, from soils and sediments. The first chapter in this thesis introduces the processes that convert organic matter into gaseous products, methane and carbon dioxide. It also introduces the use of stable isotopic analyses to follow carbon flow in anaerobic sediments and soils. This chapter outlines the differences between organic matter burial in soils and sediments to frame the consequential nature of carbon cycling in both terrestrial and marine systems. Since two of the largest reservoirs of potentially labile carbon today are found in the deep organic rich soils of northern peatlands and the vast marine reservoir of dissolved organic carbon. The isotopic tools we will apply in subsequent chapters relate specifically to understanding and interpreting the isotopic content of dissolved compounds in these two important reservoirs. The second chapter presents an amended method for the intramolecular analysis of acetate. Following on the heels of work by Dias et al., (2002) at Penn State, we have adapted their pyrolytic conditions for use with direct aqueous injection of acidified samples. This work also presents the use of a lyophilization preconcentration technique for dilute porewater samples. It represents a novel advance because it eliminates the need for extraction and significantly reduces sample volume requirements of prior techniques. The third chapter presents the results from an incubation experiment with an acetoclastic methanogen in which the exchange of the acetate carboxyl carbon with DIC is evident. The exchange is rapid and may also be associated with an equilibrium isotopic effect at stationary phase growth of the methanogen. Furthermore, we believe this exchange is analogous to the exchange observed by other authors for the phenol carboxylic acid carboxyl group and DIC. The existence of carboxyl-DIC exchange in nature would have important consequences and one of those might be the alteration of DOC isotopic content via exchange processes. We hypothesize about the role of this process in nature and outline a case for the exchange process as one piece that may help explain the current paradoxically old age of deep marine DOC. The fourth chapter presents the application of a reverse tracer technique for the estimation of the time to peeper equilibrium in sediments and soils. We have designed and constructed a peeper apparatus that allows for the repeated deployment and sampling in reproducible locations. Furthermore, we present the use of a conservative tracer in peeper cell deployments as a means to both estimate the rate of diffusion and to correct all measured values for the extent of equilibrium. The fifth chapter is devoted to the use of the peepers described in chapter four along with isotopic analyses of gases and acetate. This chapter presents a time-series of pH and isotopic measurements from Bear Meadows, and acidic wetland in central Pennsylvania. The intramolecular acetate isotopic analyses support the role of autotrophic acetogens in producing as much as 30% of the acetate in porewaters. These analyses also present the possibility that isotopic estimates of the pathway of methane formation may be unable to distinguish between acetoclastic and hydrogenotrophic methanogenesis. The observation that as much as 1/3 of the acetate could be derived from carbon dioxide means that nearly every carbon atom product of fermentation must go through the acetate pool. We hypothesize about the fate of the acetate production in acidic wetlands as an electron donor for the reduction of fulvic and humic acids in the peat catotelm. The potential ecological consequence of acetate production in acidic wetlands may be to reduce DOC to an amorphous particulate peat matrix that is relatively hydrophobic and helps produce a uniquely hydrophobic peat associated with sphagnum moss.