Abstract
The Guaymas Basin in the Gulf of California is a young marginal rift basin characterized by active seafloor spreading and rapid deposition of organic-rich sediments from highly productive overlying waters. The high sedimentation rates in combination with an active spreading system produce distinct oceanic crust where the shallowest magmatic emplacement occurs as igneous intrusion into overlying sediments. The intrusion of magma into organic-rich sediments creates a dynamic environment where tightly linked physical, chemical, and biological processes regulate the cycling of sedimentary carbon and other elements, not only in a narrow hydrothermal zone at the spreading center but also in widely distributed off-axis venting. Heat from magmatic sills thermally alters organic-rich sediments, releasing CO2, CH4, petroleum, and other alteration products. This heat also drives advective flow, which distributes these alteration products in the subsurface and may also release them to the water column. Within the sediment column, the thermal and chemical gradients created by this process represent environments rich in chemical energy that support microbial communities at and below the seafloor. These communities may play a critical role in chemical transformations that influence the stability and transport of carbon in crustal biospheres. Collectively, these processes have profound implications for the exchange of heat and mass between the lithosphere and overlying water column and may determine the long-term fate of carbon accumulation in organic-rich sediments. The fate of carbon deposited in Guaymas Basin, throughout the Gulf of California, and more broadly within similar marginal seas throughout the world, depends on the relative efficiencies of interacting physical, chemical, and microbial processes, some working to sequester carbon and others working to release carbon back to the ocean and the atmosphere. Drill core samples from Expedition 385 to Guaymas Basin will enable us to study these processes, their interactions, and their ultimate effects on carbon cycling. Samples obtained from scientific drilling are crucial to these goals, which include Quantifying the sedimentary and elemental inputs to the system through time and their variation with oceanographic and climatic conditions; Sampling igneous sills and the surrounding sediments to determine the products and efficiency of alteration and key hydrologic factors such as sediment type, faulting, and permeability evolution; and Studying subsurface microbial communities hosted by alteration products to determine their efficiency at capturing carbon-bearing alteration products and to further our understanding of the conditions that limit life in the deep biosphere.
Highlights
Guaymas Basin in the Gulf of California (Figure F1) is a young marginal rift basin characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments from highly productive overlying waters and by terrigenous sediments from nearby continental margins (Van Andel, 1964)
The juxtaposition of active seafloor spreading and thick sedimentary sequences has resulted in a dynamic environment where tightly linked physical, chemical, and biological processes regulate the cycling of sedimentary carbon
Thermal and chemical gradients that form in response to heating of sediments and fluid flow create environments rich in chemical energy that support microbial communities at and below the seafloor that may play a critical role in chemical transformations that influence the stability and transport of carbon in crustal biospheres
Summary
Guaymas Basin in the Gulf of California (Figure F1) is a young marginal rift basin characterized by active seafloor spreading and rapid sediment deposition, including organic-rich sediments from highly productive overlying waters and by terrigenous sediments from nearby continental margins (Van Andel, 1964). Faunal observations, mineralogical analyses, thermal gradient measurements in surficial sediments, and pore water chemistry demonstrated that this site retained hydrothermal activity These results were used to flesh out the characterizations of proposed Ringvent drill sites and to adjust their positions in the Proposal Addendum 833-Add. Guaymas Basin provides an exemplary opportunity to observe the processes that create our planet’s passive margins both today and throughout Earth’s history and to investigate how those processes mobilize and reinject sedimentary organic carbon into the ocean and the atmosphere (Lizarralde et al, 2011). Hypothesis: the overarching hypothesis motivating Expedition 385 is that chemical, mass, and thermal budgets in Guaymas Basin and other similar settings are controlled by the interplay of tectonic processes that create conditions for sediment deposition; magmatic processes that provide energy to the sedimentary system and drive fluid circulation; oceanographic processes that control biogenic sediment accumulation; and microbial processes that represent a source and sink for carbon compounds within the sediment pile, water column, and potentially the atmosphere. We will target all aspects of these exchanges, including the thermal drivers of hydrologic flow; the hydrologic properties of sediments, sediment diagenesis, altered sediments, and sill/sediment complexes; the alteration processes that dissolve minerals and evolve hydrocarbons; and the microbial processes that consume and transform the products of this alteration
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