GANDERIA, NEW CALEDONIA, THE CARBONATOR, AND LARGE POINT SINKS

Ganderia, during its transfer from Gondwana to Laurentia, must have perturbed the global carbon cycle at least twice, first when it underthrust seafloor in the earliest Ordovician (Penobscot orogeny) and second as a side effect of late Ordovician arc-arc collision along the Hurricane Mountain suture (Red Indian Line). Ganderia's modern cousin, the Norfolk Ridge ribbon continent east of Australia, jammed the subduction zone of the Loyalty arc during the latest Eocene; the obducted New Caledonian seafloor is a viable candidate for precipitating growth, via pCO₂ drawdown, of the Antarctic ice sheet. The “carbonator” refers to a scenario such as would result if a large mass of seafloor was translated through present-day Mt. Cook: surprisingly large factors, including volume of eroded material (for New Caledonia, <600,000 km³), exhumation rate, high acid-neutralizing capacity of peridotites (<9 mol CO₂ kg^-1), hydrothermally-enhanced carbonation rate (>1000-fold increase at 185 vs. 25ºC), and fraction dissolved, combine to produce surprisingly large flux (>0.1 Tmol yr^-1). The last factor is most uncertain, and ferruginous, dolomitic carbonates in foreland basin sequences that occur beneath chromite-bearing sandstones should be examined as candidates for the actual carbon sequestered by the dissolution of the adjacent seafloor. The emplacement of single large igneous provinces has been linked to CO₂-forced global warming. Conversely, seafloor exhumation, such as took place in New Caledonia and along the GRUB line, may create “point CO₂ sinks” large enough to cool the globe.