Deep biosphere oases help form biologically diverse ocean floor cold seeps ecosystems

What is it about?

Cold seeps are among the most biologically diverse ecosystems within Earth’s deep oceans. Scientific interest in these features is far reaching as these sites are largely fueled by chemosynthetic microorganisms that utilize reduced hydrocarbon-rich fluids for both energy and a source of carbon. These sites therefore represent a major hydrocarbon sink of greenhouse gas emissions because the fluids are largely believed to derive from the seepage of petroliferous hydrocarbons cracked from source rocks buried deep within the basin. While this is commonly the case, the model is incomplete as other mechanisms of cold seep formation are now known to occur. In this study, we use geochemical and geophysical surveys to resolve the location and mechanism of biogenic gas formation for fluids that are bubbling up in cold seeps along the Scotian Slope of Atlantic Canada. Many of these seeps are home to diverse communities of marine life that are vastly more complex than what is found on the surrounding spartan, muddy seafloor. Based on the collected data, we provide evidence that methanogenic archaea are living in the rocks buried more than a kilometre below these seeps. Here the methanogens form deep biosphere oases that are nested directly above even more deeply buried salt diapirs. The methane expelled by these organisms rises, moving up through the network of overlying faults and pore spaces of the overlying sedimentary rock units. Ultimately, the methane appears to be sustaining the biologically diverse cold seep ecosystems at places where this gas escapes to the seafloor. We further show that the occurrence of these deep biosphere oases is not random. They appear to be caused by a heat chimney effect whereby the higher heat flow produced from the deeply buried salt diapir acts to move formation waters and perhaps even the overlying seawater on to the salt edifice. The flow of this groundwater is hypothesized to enhance the supply of nutrient and dissolved inorganic carbon needed for methanogenesis. At these locations both the cold seeps and the underlying deep biosphere are therefore coupled ecosystems that collectively depend on the chemical energy provided by the salt diapir heat flux and faulting.

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Photo by Sarah Lee on Unsplash

Photo by Sarah Lee on Unsplash

Why is it important?

This study introduces a novel pathway for how cold seeps are formed and maintained over time. The cold seep – deep biosphere connected ecosystems may occur in other places as the geophysical conditions that gives rise to their formation on the Scotian Slope is a common feature of many passive margin, salt tectonic basins around the globe. Additionally, this study may also help to refine exploration strategies of future marine seep surveys as well as shed light on possible links on how the Earth’s deep biosphere adapts to its largely unknown habit.

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