Figure

Worldwide changes in sea level due to geotectonic mechanisms during the Phanerozoic have had pronounced effects upon the partitioning of carbon and sulfur among the exogenic reservoirs.

Analyses of mineralogically homogeneous brachiopod shell material (for ^dgr¹³C_ox) and associated micrite organics (for ^dgr¹³C_red) support the secular variability of whole-rock isotopic data compiled from the literature. Together, the data indicate the ratio of reduced carbon to total carbon in sediments is low at times of global high sea level and high at times of global low sea level. Conversely, the reduced to oxidized sulfur ratio as calculated from ^dgr³⁴S values of evaporites follow an opposite trend.

A global tectonic model of carbon and sulfur cycling consistent with these observations is proposed with the following implications.

End_Page 971------------------------------

High spreading rates result in large mid-ocean ridge volumes giving rise to high sea levels, lowered erosion and terrigenous sedimentation rates, extensive shelf carbonate (C_ox) deposition, and concomitant mass transfer of calcium from evaporites to carbonates and of sulfur from evaporites to sedimentary sulfides. The observed evidence of elevated global temperatures during such times may result from higher atmospheric CO₂ levels due to an increased rate of production of CO₂ from the decarbonation of limestones and the formation of calc-silicates at subduction zones.

During global low sea levels, higher erosion and terrigenous sedimentation rates restrict carbonates leading to the mass transfer of calcium from carbonates and of sulfur from sulfides to extensive evaporites. Total organic carbon (C_red) storage in sediments is greater (although preservation and concentration of organic carbon may also be high during anoxic high CO₂, high sea level times). Lower atmospheric CO₂ levels during these episodes may have been conducive to the observed evidence of epochs of glaciations and lowered world temperatures.

End_of_Article - Last_Page 972------------