TEMPERATURE AND PRODUCTIVITY RECORDS FROM THE LATE MIOCENE NORTH ATLANTIC

The positive feedback associated with ice sheets is known to account for major portions of the high-amplitude temperature fluctuations observed during glacial periods in earth history. Temperature variability prior to bipolar glaciation is less well understood at orbital and longer timescales. The long-term cooling in the Miocene and Pliocene towards the initiation of Northern Hemisphere Glaciation (NHG), for instance, requires unambiguous, continuous, and well-dated sea surface temperature (SST) records to observe changes in climate feedbacks such as meridional heat transport, formation of deep water, and the extent of sea ice and continental ice sheets. Ideal sites for such a temperature record lie in the sensitive high latitudes, physically close to the growing ice sheets. ODP Site 907 (69°N, 12°E) is situated at a sensitively balanced location with respect to the world's ocean dynamics. A temperature record from this location may not only clarify local and global temperature changes, but also help to constrain the history of northern-sourced deep water. Site 907 experienced the effects of migrating wind fronts, sea-ice cover, and current migration. Magnetostratigraphy demonstrates continuous sedimentation at the level of polarity zonation, and provides an age model to ~14 Ma. Using the alkenone-based SST and productivity record from this location, at moderate resolution (~50 kyr between samples) a climate record covering the past ~14 Ma reveals both long-term, globally recognized trends, and also more local events. SSTs at Site 907 were as high as 23°C at ~14 ma, cooling gradually (~1.6°/my) to around 10°C by 6 Ma. The gradual cooling is punctuated by at least one large drop in temperature at ~9.5 Ma. This feature is coeval with the expansion of C4 plants in Asia as well as changing mineralogy at North Atlantic sites (ODP Sites 908, 909). These events hint at precursor adjustments to the climate system that significantly preceded the establishment of permanent ice sheets in the northern hemisphere. A small increase in C₃₇ total indicates a response to the 5 ma global biogenic bloom event observed in other records, but the most distinctive feature of the productivity record is a crash in haptophyte productivity at ~3.5 ma. This does not, however, coincide with any major change in temperature at Site 907. Indeed, a large cooling at this site occurred prior to 6 ma, well before NHG.