Abstract

Mantle degassing continually releases gases onto the earth's surface. Over geologically long time intervals, a general equilibrium probably exists between mantle CO ₂ release and uptake by surficial sinks. However, during periods of rapid plate movement, or continental flood basalt volcanism, the increased rate of mantle CO ₂ release may exceed that of uptake, leading to CO ₂ accumulation in the atmosphere and the marine mixed layer (top 50-100 m). This in turn triggers chemical changes in the mixed layer, climatic warming, and bioevolutionary turnover. The Cretaceous/Tertiary ( K / T ) transition at 65 Ma seems to have been a time of major mantle degassing which induced a perturbation of the carbon cycle. During the K / T transition, Deccan Traps volcanism, perhaps the greatest episode of continental flood basalt volcanism in the Phanerozoic, flooded an estimated 2.6 × 10 ⁶ km ² of India with basaltic lavas, releasing 5 × 10 ¹⁷ moles of CO ₂ into the earth's atmosphere over a duration 0.53-1.36 Ma at the rate of 3.9 × 10 ¹¹ to 9.6 × 10 ¹¹ moles CO ₂ per year. The modern mean annual rate of mantle CO ₂ release from all sources is 4.1 × 10 ¹² moles CO ₂ per year; assuming a comparable rate of release prior to the Deccan Traps volcanism, the Deccan Traps addition would have elevated the rate of mantle CO ₂ release by 10-25%. Sluggish marine circulation and warm, deep, oceans (14-15°C) would have exacerbated CO ₂ buildup in the atmosphere, accounting for the Cretaceous to Tertiary drop in oxygen-18 via climatic warming, and, in the marine mixed layer (top 50-100 m), explaining the selective nature of the terminal Cretaceous marine extinctions via a pH change. The extinctions were most severe amongst the calcareous microplankton of the mixed layer; calcareous microplankton (planktonic foraminifera and coccolithophorids) begin to have pH problems at 7.8 and 7.5, respectively. Failure of the coccolithophorids would have disrupted the Williams-Riley pump (algal productivity-gravity pump of CO ₂ from the atmosphere and mixed layer into the deep oceans) producing dead ocean conditions (severely reduced photosynthesis and CaCO ₃ production). Failure of the Williams-Riley pump is reflected in the extinctions themselves, and in the loss of biogenic CaCO ₃ to the sea floor, causing the K / T boundary hiatus and (or) the K / T boundary clay. Failure of the pump today would elevate atmospheric pCO ₂ severalfold; the K / T failure would have responded comparably. Dead ocean conditions would, in themselves, have produced a major CO ₂ buildup. Early Tertiary "Strangelove" conditions in the mixed layer, characterized by a dominance of the thoracosphaerids, braarudosphaerids and small planktonic foraminifera, were coeval with the main pulse of Deccan Traps volcanism. Overall, the record is one of gradual K / T bioevolutionary turnover during a period of disequilibrium between the rate of mantle CO ₂ degassing and uptake by sinks. Mantle degassing during the Deccan Traps volcanism unifies the K / T biological and physicochemical records.