When Overturning Circulation Became Global: Insight into Timing of This Change

Abstract:

Crucial insight into the relationship between oceanic heat transport and deep-water circulation can be gained from paleoceanographic investigations of past warm climates. Sediment sequences recovered by four decades of international scientific ocean drilling provide abundant evidence that the climate during the Paleogene (40-65 Ma) was characterized by greenhouse warmth and atmospheric CO₂ in excess of ~1000ppm. Reconstructions of deep-water, or the overturning, circulation during this interval indicates a fundamentally different mode from the modern. During the Paleogene mode of circulation separate deep-water convection in each basin is indicated. This mode persisted at least into the early Neogene, the exact timing of when the mode of circulation transitioned from having very distinct and separate circulations, to having one global overturning circulation is unknown. In order to identify when this shift occurred, the connection between the transition in circulation and climate must be better understood.

Neodymium (Nd) isotopes are one of the most robust proxies of ancient water mass composition, acquiring their isotopic fingerprint in the region where they sink. The fossil fish teeth and bones record the Nd isotopic signature of the water mass at the sea floor and therefore create a stratigraphic record of fish teeth Nd at a given site, giving a temporal record of the water mass composition at that location. IODP Site U1438 is ideally positioned at a deep-water depth in the Northwest Pacific to capture the record of deep-water sourced from the Pacific sector of the Southern Ocean. With the expected loss of bipolar convection in the Pacific as the mode shift occurs there will be a manifestation in ε_Nd values as a divergence between the deep-water sites compared to the shallower sites. Preliminary data shows that there is indeed a shift to more unradiogenic values as time progressed closer to the modern (-4.9 ε_Nd at 13 Ma to -2.6 ε_Nd at 12 Ma). This indicates a divergence of the ε_Nd signature and the influence of a less radiogenic source being introduced, such as Southern Ocean bottom water.