Respiration and calcium carbonate production by the invasive Asian clam, Potamocorbula amurensis, were calculated to assess their importance as CO₂ sources in northern San Francisco Bay. Production, calculated using monthly population density and size structure measured at three sites over 7 yr and a shell length/CaCO₃ conversion factor, averaged 221(±184)g CaCO₃ m⁻²yr⁻¹. Net calcium carbonate production by this exotic bivalve releases CO₂ at a mean rate of 18(±17)g C m⁻²yr⁻¹. Respiration by P. amurensis, estimated from secondary production, releases additional CO₂ at a mean rate of 37(±34)g C m⁻²yr⁻¹. Therefore, total net CO₂ production by P. amurensis averages 55(±51)g C m⁻²yr⁻¹ in an estuarine domain where net primary production consumes only 20g inorganic C m⁻²yr⁻¹. CO₂ production by P. amurensis in northern San Francisco Bay is an underestimate of the total CO₂ supply from the calcified zoobenthic communities of San Francisco Bay, and results from other studies have suggested that this rate is not unusual for temperate estuaries. Global extrapolation yields a gross CO₂ production rate in the world's estuaries of 1x10¹⁴g C yr⁻¹, which suggests that calcified benthic organisms in estuaries generate CO₂ equal in magnitude to the CO₂ emissions from the world's lakes or from planetary volcanism (the net source is determined by the highly variable rate of CO₂ consumption by carbonate dissolution). This biogenic CO₂ source is increasing because of the continuing global translocation of mollusks and their successful colonization of new habitats.