James R. and Shirley A. Kliegel Lecture in Geological and Planetary Sciences
The presence of atmospheric oxygen, which is so necessary for life as we know it, depends on globally distributed photosynthesis to support it. So dynamic is the oxygen budget, that were photosynthesis to stop, the atmosphere's oxygen inventory would be fully depleted in about 20 thousand years, a geologically short time-scale. Roughly half of this photosynthesis occurs within the ocean, within a few tens of meters of the sea surface. Below this surface layer, detritus from this biological activity gradually sinks, while bacterial colonies serve to consume this organic carbon, releasing inorganic (oxidized) carbon and nutrients and consuming dissolved oxygen. The net result is that to varying degrees, almost everywhere in the subsurface ocean dissolved oxygen is deficient (under-saturated) relative to what would be expected if the water were in chemical equilibrium with the atmosphere. The fact that dissolved oxygen is not completely gone in most of the ocean is due to the fact that the subsurface ocean is ventilated by physical processes that balance the continuing biologic demand for oxygen with the sinking, circulation, and mixing of waters that were at the sea surface and recharged with atmospheric oxygen. It's a good thing that this happens, at least from the viewpoint of the fish and marine mammals that depend on the presence of dissolved oxygen to survive. At the far end of this grand cycle, the old oceanographic adage holds true: "what goes down must come up", nutrient rich, oxygen depleted waters must return to the surface, fueling biological production (photosynthesis) anew. While the basics of the oceanic oxygen cycle have been known for more than a century, it has only been within the last few decades that we have gained insight into the rates and magnitudes of the moving parts in this grand machine. Moreover, the specter of global change and emerging evidence that oxygen deficient (and possibly biologically lethal) zones may be growing add further impetus to our need to understand the dynamics of this complex, global biogeochemical machine. But how do we measure these rates and processes on space- and time-scales that are relevant to developing a deeper and quantitative understanding? One approach is using transient tracers. In this lecture, I will tell you how we use these tools, what they can (and cannot) tell us about the global oxygen cycle, what we have learned from them so far, and a few residual puzzles that we have yet to solve.