MODELED ESTIMATES OF GLOBAL REEF HABITAT AND CARBONATE PRODUCTION SINCE THE LAST GLACIAL MAXIMUM

Estimated changes in reef area and CaCO3 production since the last gla cial maximum (LGM) are presented for the first time, based on a model (ReefHab) which uses measured environmental data to predict global dis tribution of reef habitat. Suitable reef habitat is defined by tempera ture, salinity, nutrients, and the depth-attenuated level of photosynt hetically available radiation (PAR). CaCO3 production is calculated as a function, of PAR. When minimum PAR levels were chosen to restrict r eef growth to 30 m depth and less, modern reef area totaled 584-746 x 10(3) km(2). Global carbonate production, which takes into account; to pographic relief as a control on carbonate accumulation, was 1.00 Gt y r(-1). These values are close to the most widely accepted estimates of reef area and carbonate production and demonstrate that basic environ mental data can be used to define reef habitat and calcification. To s imulate reef habitat changes since the LGM, the model was run at 1-kyr intervals, using appropriate sea level and temperature values. These runs show that at the LGM, reef area was restricted to 20% of that tod ay and carbonate production to 27%, due primarily to a reduction in av ailable space at the lower sea level and secondarily to lower sea surf ace temperatures. Nonetheless, these values suggest that reef growth p rior to shelf flooding was more extensive than previously thought. A c rude estimate of reef-released CO2 to the atmosphere since the LGM is of the same order of magnitude as the atmospheric CO2 change recorded in the Vostok ice core, which emphasizes the role of neritic carbonate s within the global carbon cycle. This model currently addresses only the major physical and chemical controls on reef carbonate production, but it provides a template for estimating shallow tropical carbonate production both in the present and in the past. As such, the model hig hlights several long-standing issues regarding reef carbonates, partic ularly in terms of better defining the roles of light, temperature, ar agonite saturation state, and topography on reef calcification.