ATMOSPHERIC CARBON DIOXIDE AND OCEAN CARBONATE ION CONCENTRATION DURING THE LAST GLACIAL CYCLE

Continuous production and sinking of fixed carbon out of the thin sunlit layer of the ocean maintains a steep vertical gradient in ocean CO2, keeping most of the carbon in the deep ocean and out of contact with the atmosphere. Favored explanations for the 80 ppm decrease in atmospheric CO2 that accompanied the earth’s glacial cycles involve some change in this biological pump at low or high latitudes. One way to study the pump is to reconstruct the vertical profile in the carbonate ion concentration, useful because the carbonate ion concentration is inversely proportional to CO2. Today the carbonate ion concentration is greatest at the tropical sea surface, low between 1000-5000 m, with a minimum between 500-1500 m. We extend our previous glacial reconstruction to make a complete profile of the glacial ocean from the surface to 5000 m, and reconstruct the entire glacial cycle (150,000 years BP to present) at the critical surface and upper ocean (1500 m) depths. We find that the vertical gradient outside the North Atlantic steepened in the upper 1500 m during glacial times. Reconstructions over the last glacial cycle reveal an 80 micro-mol/kg increase in carbonat ion at the sea surface during glacial times relative to present, a small increase (10-20 micro-mol/kg) at 1500 m, and little change below. The reconstructions have large (+10 micro-mol/kg) uncertainty and possible biases in absolute value, and are deficient in cores representing 100-1500 m, yet the major conclusion, that over much of the ocean, the carbonate ion concentration (and) CO2 gradient between 0-1500 m steepens during glacial times, appears robust. The change is large, well above measurement uncertainty, supported by different proxies, and is geographically widespread outside the North Atlantic. Mechanisms capable of explaining the increased vertical gradient in carbonate ion concentration include reduced vertical diffusion and thermocline ventilation (the primary mechanisms acting against the biological pump), and/ or a stronger/ more effective biological pump. Recognizing that over thousands of years the deep ocean carbonate ion concentration is essentially invariant due to the buffering capacity of calcium carbonate that lines the seafloor, the simplest explanation is that changes driven by the steeper glacial thermocline keep more carbon out of the surface ocean during cold glacial times.