Modeling Chemical Reaction and Energetics: development of simple input/output programs to allow use of existing codes (EQ3/6 and Geochemist's Workbench) over wider ranges of temperature and pressure


DOEI Project Funded: 2006

Two of the three broad research themes of the Deep Ocean Exploration Institute are concerned in part with chemical reactions and how they modify the earth, and how they can provide energy for life deep in the ocean, and below the seafloor. Fluid flow and water-rock reaction within the ocean crust occur over broad ranges of temperature and pressure, and result in significant modification of both the composition of the crust and of the oceans. As a result of these reactions, there are places both within the seafloor and at the interface of the crust and ocean where microorganisms, if present, can exploit chemical disequilibria, gaining energy by catalyzing geochemical reactions. The study of fluid flow and resultant chemical reaction is thus intimately linked to studies of the unusual fauna present at the seafloor at sites of hydrothermal venting, and of the subsurface biosphere.

Quantification of the chemical reactions that occur as fluids interact with various rock types within the crust (e.g., basalt, peridotite, sediment), and as different fluids mix either beneath or at the seafloor (e.g., hydrothermal fluid, ambient seawater), is needed 1) to understand the magnitude and significance of fluxes of heat and mass from the earth to the oceans, and 2) to estimate the potential metabolic energy available to organisms in different environments. Over the past 30 years a number of software tools have been developed for these purposes.  However, the thermodynamic databases needed for use by these programs are not adequately suited for calculations of fluid flow and water-rock reaction in the ocean crust.  The most appropriate databases contain data from 0 to 300ºC and at one atm pressure below 100ºC, and along the vapor pressure of water at higher temperature (Bethke, 1998). Unfortunately, many of the problems of interest in the ocean crust require data at higher pressures (e.g., at 150 to 500 bars, and with pressure not varying with temperature), and up to higher temperatures (e.g., to 350º to 400ºC for many active seafloor vents).  A program exists to calculate data for these conditions (SUPCRT92; Johnson et al., 1992), and it is also possible to modify the existing databases; however, this is a painstaking process given the large numbers of chemical reactions usually being considered. Over the past 15 years, those using the EQ3/6 software package have compiled databases for use at constant pressures (e.g., 250 bars for vents at 2500 m depth, or 500 bars for consideration of water-rock reactions at depth beneath the seafloor) and for temperatures up to 400ºC.  However, these databases are not compatible with GWB, i.e., they cannot be used as GWB input. 

Accurate quantification of chemical reactions within the ocean crust and at the crustseawater interface requires access to thermodynamic databases at appropriate temperature and pressures.  This proposal, if funded, will result in public-access software that will allow creation of thermodynamic databases compatible with either the EQ3/6 or GWB software packages. Specifically it will provide: 1) a program to convert thermodynamic databases compatible with EQ3/6 (EQ3/6 format databases) into databases compatible with GWB (GWB format databases) = Therodynamic Database Conversion (TDC) code; 2) a program to take data from SUPCRT92 output files and create new EQ3/6 and GWB format databases = Thermodynamic Database Generation (TDG) code. 

These programs will facilitate investigations of fluid flow within the deep ocean crust, and how resultant chemical reactions modify the earth, and provide energy for life deep in the ocean, and below the seafloor.