Microbially mediated carbonate precipitation is globally significant and can provide a mechanism for understanding and potentially manipulating carbon sequestration. Arid-land caves, as semi-closed systems stripped of the influence of surface weathering, provide a particularly valuable window into the world of carbonate-precipitating microorganisms. Carbon moves from the surface to the subsurface, where it is preserved in carbonate precipitates. These precipitates, many of which are biogenic, record microbial influences, surface climate, and ecosystem changes. Our investigations will identify a suite of biosignatures useful in the search for these signals and will aid speleothem data interpretation in paleoclimate studies.
Prior work on speleothems by our team has shown a number of significant points. A variety of carbonate biomorphs exist in paleo- and active pools in Guadalupe Mountain caves in New Mexico (Melim et al. 2004; Queen and Melim, 2006). Fossil pool fingers from Hidden Cave revealed alternating microbial laminations with micritic textures and abiotic, clear spar layers (Melim et al. 2001). This distinct pattern has led us to hypothesize an episodic input of fixed carbon and other nutrients derived from the surface. Recent observations of forest fire-derived carbon visibly incorporated in nearby cave speleothems suggest that surface carbon may be incorporated into pool precipitates and provides a testable model of nutrient linkages between the surface and subsurface that fuel the geomicrobiology. We believe that the banding observed in these pool precipitates and episodic forest-fire-derived carbon mobilization are linked and more obvious in shallow caves more closely coupled to the surface than in deeper caves.
As an established multidisciplinary team, we will investigate these linkages and microbial and abiotic processes that produce these cave pool precipitates. To further the understanding of biogenic carbonate precipitation processes we propose to: 1) Quantify and characterize the movement of carbon and other possible energy sources that fuel geomicrobial metabolism from the surface to the subsurface as recorded in biogenic carbonate pool precipitates. 2) Characterize the pulsed input of carbon as recorded in layered cave pool precipitates as a reflection of surface ecosystem dynamics. 3) Characterize carbonate pool precipitates and their geologic setting to identify resulting biosignatures along a gradient of biogenic to abiologic, correlated with depth below the surface or hydrologic flow path. 4) Characterize the aqueous/gas geochemistry and microbial communities of living pools with and without pool carbonate precipitates and their corresponding drip waters. 5) Determine whether cultured microorganisms that form carbonate are reproducing textural, geochemical, and isotopic biosignatures found in natural cave settings.
Broader scientific impact of this work focuses on the determination of the degree of biogenicity of cave pool carbonate, which bears on interpretation of the paleoclimate, hydrology, and geochemistry of the vadose zone. Results will allow us to better understand carbon flow from the surface to subsurface and will have implications for global carbon storage.
Careful multidisciplinary work is essential to establish an active microbial role in cave carbonate formation and serves as an excellent platform for education efforts. Team members are involved in training a large number of students (graduate, undergraduate, and high school) including minority students through a number of UNM and NMT minority programs. This proposed work marks an important symbiosis with interdisciplinary education initiatives (EPSCOR and AGEP collaborations among UNM, NMT and NMSU). Importantly, WIU students have a summer research opportunity at UNM or NMT to encourage their pursuit of graduate education. Our research has a high broadcast and print media visibility helping to attract students to our work. We interact extensively with the general public via talks and other avenues.
