Modelling the Deep Biosphere over Deep Geological time at the Nankai Trough, Japan

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Recent work has demonstrated that microbes living within deep ocean sedimentary rocks can be descended from the organisms present within the rock's depositional environment. This implies that the microbes experienced a long geological history and in many cases the microbes experienced significant changes in their environment as the sediment was buried and converted to rock.Computer models of the conversion of sediments into sedimentary rock have been adapted to model the life and death of individual biomes within their host geological formations. However, to date, this has only been done for examples of sub-seafloor cases where there is little movement of groundwater or other fluids between geological formations. However, there are many instances where the transfer of fluids between formations occurs. Examples include oil and gas migration, as well as the catastrophic movement of fluids associated with earthquakes and tectonic activity. The Nankai trough is one such place where both process may occur and subsurface microbes have been detected at depth. At the moment current models of microbes living within sediments can not accurately model the transfer of microbes between geological horizons, and this would prohibit the modelling deep subsurface biomes in these important environments.An IODP expedition to the Nankai trough will explore the subsurface to investigate the temperature limits of life. This expedition will provide an opportunity to acquire data that will be used to develop better models of the deep biosphere that incorporate the subsurface movement of cells. To achieve this it is necessary to have access to important data characterizing the microbial populations found in the Nankai trough. It is also essential that the boundaries between geological formations, and migration pathways for microbes within the subsurface are recorded and described. To accurately model past temperatures experienced by microbes as their host sediment is converted into rock, it is necessary to make geochemical measurements that record past episodes of thermal alteration. The type of measurement needed depends on the temperatures concerned, and in this case measurements must span a considerable temperature range including mild temperatures experienced in the near surface, as well as the higher temperatures that generate oil and gas. This later group is common practice for scientists the former is not.Observations of the boundaries between geological formations must be parameterised; e.g. converted into numerical inputs that can be put into a model. Two things need to be parameterised: 1) Basic information describing how the boundaries permit the passage of fluids. These are routine measurements that are easily made. 2) How different rock textures interact with cells as they move through rocks. This is not common information and will be determined by experiments that take account of specific circumstances appropriate to the Nankai trough sediments; e.g. the presence of gases and fluids, and specific rock textures.Models built using this data can then be used for addressing fundamental science questions concerning life in the deep subsurface, life on other planetary bodies and the evolution of life over geological time. They will also have practical applications for predicting petroleum occurrences and forward modelling microbial enhanced oil recovery and contaminated ground remediation.