Photo: MBARI
Organic carbon exchange between aging oceanic crust and the deep ocean through seamounts
Our lab has previously shown that microbial processes can be a dominant influence on the geochemistry of cool hydrothermal fluids circulating through aging oceanic crust, and that this vast volume of fluid-filled pore spaces in ocean crust represents a sink for both organic and inorganic carbon. (See North Pond project below.) But, to date, North Pond is the only location where fluids from the crust could be directly accessed through IODP CORK Observatories. In this project we will sample fluids naturally discharging from seamounts and use the same approach taken at North Pond to investigate subsurface microbial and geochemical processes. We will characterize the concentrations and isotopic composition of dissolved inorganic and organic carbon, the molecular composition of organic carbon, microbial cell counts and inorganic geochemistry. This will allow us to gain a better understanding of biogeochemical processes occurring globally in aging crust. This project involves an upcoming research expedition with the ROV Jason.
Funding: NSF Chemical Oceanography 2025-2030
MeThane Seep Dynamics and Biogeochemistry on the Continental Shelf & slope near the Delmarva Penninsula
On the shelf and slope of the North Atlantic margin, methane has been observed seeping from sediments at depths and locations where it was not predicted. In this project we employ lipid biomarkers and carbon isotopes to characterize modern methane seep biogeochemistry – depths and pathways of methane production and consumption. We will also investigate historical methane seep dynamics by using the same tracers on jumbo piston cores to be collected on an upcoming research expedition.
Funding: NSF Chemical Oceanography 2024-2027
Outreach:
Transformation and release of refractory organic carbon from hydrothermal vents, East Pacific Rise at 9°N
Although high-temperature hydrothermal vents on mid-ocean ridges are known to be a sink for oceanic dissolved organic matter, it has also been shown that these vents release aromatic and graphitic carbon that potentially persist for a very long time in the ocean. This component may help explain why deep-ocean dissolved organic matter is characterized by aged radiocarbon signatures. Through this project we will quantify how much of this aromatic carbon is released from vents and how far it propagates from the vent field in hydrothermal plumes and in sediments. Natural tracers (13C and 14C isotopes) will also resolve whether this aromatic carbon is produced by transformation of deep ocean organic carbon (e.g., diffuse dissolved organic carbon, chemosynthetic biomass) or is produced from mantle-derived carbon via abiotic synthesis pathways.
Funding: NSF Chemical Oceanography 2022-2026
Outreach:
Microbial and hydrothermal controls on organic carbon mobilization from Guaymas Basin sediment
Organic carbon stored in marine sediments and sedimentary rocks are a critical component of the global carbon cycle. This project uses drill core sediments from IODP Expedition 395 to Guaymas Basin in the Gulf of California to examine how hydrothermal heating, burial depth, organic carbon composition and microbial community composition impact the bioavailability of sedimentary organic carbon and mobilization of carbon out of the sedimentary reservoir. Results from this project will improve our understanding of how microbes reintegrate this subsurface organic carbon back into the “active” carbon cycle.
Funding: NSF Chemical Oceanography & Marine Geology & Geophysics 2020-2025
Publications:
McNichol, S. M., Quete, F. S., Loeb, S., Teske, A., Shah Walter, S. R., Mahmoudi, N. (2024) Dynamics of carbon substrate competition among heterotrophic microbes. ISME J, 18, wrae018. DOI: 10.1093/ismejo/wrae018
Outreach:
Archaeal Biogeochemistry and Methane Production Pathways in Delaware Coastal salt marshes and Rivers
The UD Marine Sciences campus in Lewes, DE is adjacent to the Great Marsh Preserve, a polyhaline salt marsh, and the Broadkill River which forms one of its borders. These environments have hosted multiple coastal projects of ours that investigate the distribution and controls on archaeal populations as well as their roles in coastal biogeochemistry with an emphasis on methane production pathways. We have identified blooms of free-living Marine Group II archaea that follow spring phytoplankton blooms in the Broadkill River and populations of methanogens in large suspended particles which are persistent if somewhat puzzling to find in this fully-oxygenated, tidally-flushed environment. In sediments from salt marsh tidal creeks, we find that methanogens are particular about their “food” and that conductive iron oxide minerals can enhance methane production.
Funding: University of Delaware Research Foundation 2020-2023 and UD startup funds
Publications:
Block, K. R., Arbetman, A., Slotznick, S., Hanson, T. E., Luther III, George W., Shah Walter, S. R. (2025) Influence of carbon source and iron oxide minerals on methane production and magnetic mineral formation in salt marsh sediments. Biogeosciences, in press. Discussion paper: DOI: 10.5194/egusphere-2025-822
Guider, J. T., Yoshimura, K. M., Block, K. R., Biddle, J. F., Shah Walter, S. R. (2024) Archaeal Blooms and Busts in an Estuarine Time Series. Environmental Microbiology, 26, e16584. DOI: 10.1111/1462-2920.16584
microbE-Mediated carbon cycling In Cool Oceanic Crust at North Pond
This project investigates biogeochemical processes and carbon loss in the largest actively flowing aquifer system on Earth – fluids circulating through cool, aged oceanic crust underlying the oceans and sediments. Fluids from the crust were recovered from seafloor observatories put in place by the International Ocean Discovery Program (IODP) at North Pond, a sediment pond on the western flank of the mid-Atlantic Ridge. This site is likely representative of the majority of global hydrothermal fluid circulation. Through integrated, multi-disciplinary techniques we described linkages between microbial activity and the production and loss of organic carbon in the cool, subseafloor biosphere. At North Pond there is a net loss of both organic and inorganic carbon and microbial populations find a way to “eat” dissolved organic carbon from the deep ocean that is generally untouched by open-ocean microbes while also producing and releasing chemically-distinct organic carbon of their own. At this relatively young, but cool study site, inorganic carbon is also lost suggesting crustal age is a more important control on carbonate mineral formation than fluid temperature. These results help establish the extent to which microbially-mediated processes in the subseafloor influence carbon cycling in the ocean.
Funding: NSF Biological Oceanography 2016-2020
Publications:
Shah Walter, S. R., Wood, L. J., Yoshimura, K. M., Gonski, S. F., Cai, W. J., Huber, J. A., Trembath-Reichert, E., Girguis, P. R. (2025) Microbial and abiotic drivers of carbon removal in aged oceanic crust. Geochimica et Cosmochimica Acta, 394, 1-14. DOI: 10.1016/j.gca.2025.02.012
Yoshimura, K. M., Shah Walter, S. R., Tully, B. J., Biddle, J. F. (2025) Highly dynamic archaeal and bacterial communities from the surface to the deep and shallow sediments in the Atlantic Ocean. Environmental Microbiology, 27, e70039. DOI: 10.1111/1462-2920.70039
Trembath-Reichert E., Shah Walter S. R., Fontánez Ortiz, M. A., Carter, P. D, Girguis, P. R., Huber, J. A. (2021) Multiple carbon incorporation strategies support microbial survival in cold subseafloor crustal fluids. Science Advances, 7, eabg0153. DOI: 10.1126/sciadv.abg0153
Shah Walter S. R., Jaekel U., Osterholz H., Fisher A. T., Huber J. A., Pearson A., Dittmar T., Girguis P. R. (2018) Microbial decomposition of marine dissolved organic matter in cool oceanic crust. Nature Geoscience, 11, 334-339. DOI: 10.1038/s41561 018 0109-5