Extreme Warmth in the Late Miocene North Atlantic

Climate in the late Miocene was unquestionably much warmer and less glaciated than the following Pliocene. This is implied by benthic oxygen isotope records, but as they necessarily combine the effects of temperature and ice volume, a precise climate history leading up to the onset of Northern Hemisphere glaciation is difficult to resolve. Unambiguous, continuous, and well-dated sea surface temperature (SST) records are required to characterize a pre-glacial world. Ideal sites 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 help to constrain the history of northern-sourced deep water, which plays a pivotal role in global climate. Located at the conflux of three major surface currents, 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. However, lack of abundant microfossils has prevented traditional isotope or faunal assemblage-based climate analysis from this Site. We present an alkenone-based SST and productivity (Total C37 alkenones/gram sediment = C37 total) record from this location, at moderate resolution (~50k between samples) covering the past ~14 ma. SSTs were as high as 23°C at ~14 ma, cooling gradually (~1.6°/my) to around 10°C by 6 ma. 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 crash is echoed by biogenic opal records, and presumably total productivity in the high-latitude North Atlantic was equally affected. The long-term trends (e.g. the Miocene cooling trend) of global benthic δ18O records correlate well with this alkenone-derived local record. The dramatic late Miocene cooling and the mid-Pliocene productivity crash, however, hint at precursor adjustments to the climate system that significantly preceded the establishment of permanent ice sheets in the northern hemisphere.