The Long Term Carbon Cycle Model (LTCM) is a schematic ocean circulation and carbon cycle model constructed for the simulation of long-term carbon cycle. LTCM is a modified and extended model of the High-Latitude Exchange/Interior Diffusion-Advection Model (HILDA) of Shaffer and Sarmiento (1995). The low latitude ocean has a vertical resolution of 37 levels and the high latitude ocean is represented by a surface box and a well-mixed deep ocean box. The ocean is coupled with an energy-balance atmposhere model. Transport processes considered are: diffusive exchange between low latitude boxes; advective flow within both low and high latitude ocean; exchange between low and high latitude ocean; and convective transport between surface and deep high latitude ocean. Physical transport parameters are tuned against data-based natural 14C profile. A component of marine biology is included to represent biological carbon uptake. Export production of organic matter is limited by the availability of surface phosphate via the Michaelis-Menten scheme, and the production of calcium carbonate is linked to that of organic matter through a fixed rain ratio. The remineralization of both organic matter and calcium carbonate in the ocean interior is parameterized by a form of exponential decay. A 1D sediment column lies at the bottom of each ocean layer following ocean hypsometry and each column has a total depth of 10cm and 10 vertical levels. The solid component of sediment includes CaCO3 and refractory materials, and dissolved inorganic carbon and alkalinity in the pore water exchanges with those of ocean water through diffusion. A parameterization of changes in carbonate and silicate weathering as function of temperature and CO2 concentrations are included based on the GEOCARB model of Berner and Kothavala (2001). In addition, the flux of calcium carbonate deposited to shallow-water sediment is linked to aragonite saturation state at the surface ocean. Here is a schematic illustration of the model