SHIFTING OCEAN PRODUCTIVITY PATTERNS DURING THE INTENSIFICATION OF NORTHERN HEMISPHERE GLACIATION

Previous studies have suggested that the onset of ocean stratification in the Subarctic Pacific and Southern Ocean gave rise to an abrupt decline in biological productivity in both these regions synchronous with the intensification of Northern Hemisphere Glaciation (NHG) (~2.75 Ma). Using the concentration of alkenones, organic compounds exclusively produced by a few species of haptophyte algae, derived from ocean sediments we document the continuous evolution of ocean productivity over the past 4 Myr at an array of sites in both hemispheres as well as in all ocean basins (ODP 982, DSDP 607, ODP 662, ODP 1014, ODP 1090, ODP 846, ODP 1123, ODP 722). We find that biological productivity in all high latitude regions declines precipitously in association with the intensification of NHG. In contrast, productivity in mid to low latitude regions of the Atlantic and Pacific Oceans, rises sharply during the same time interval eventually declining again between ~1.5 and 1 Ma. In mid latitude regions of the southern hemisphere, preliminary low-resolution data suggest a rise in productivity after ~1.5 Ma. These broad productivity patterns suggest major redistributions of ocean nutrients in association with Northern Hemisphere Glaciation as well as the mid-Pleistocene transition. High latitude regions in the North Pacific and Southern Ocean are instrumental in supplying nutrients to most mid and low latitude regions in the world's oceans. Thus, we hypothesize that nutrients that may have been inhibited from being utilized at high latitudes by the onset of water column stratification were instead entrained into the source waters for low and mid latitude regions causing the dramatic rise in productivity that occurred in these regions in association with NHG. The decline in productivity that occurred in northern hemisphere mid to low latitude regions between 1 and 1.5Ma and the corresponding rise in productivity in the mid latitude regions of the southern hemisphere may be related to the migration of the Southern Hemisphere polar front in association with the mid-Pleistocene transition.