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The objectives of this project were to determine the paleoceanographic origins for the mid-Pliocene productivity shift from high- to low-latitudes. The preliminary data shown on the proposal suggested that near 2.8 Ma there was a marked shift in ocean productivity from the high arctic and subantarctic regions to the tropical upwelling zones.

The hypothesis to be tested by this ACS award was that this shift in productivity reflected a fundamental change in the source of water that ventilate the tropical thermocline. I proposed to test this idea using stable isotopic and Mg/Ca analyses of multiple surface and deeper dwelling planktonic foraminifera to reconstruct the upper ocean thermal gradients in an east-west transect of cores in the tropical Atlantic. The hypothesis would be accepted if both the east and western Atlantic sites showed antiphased SST trends over time (indicting a greater zonal SST gradient) and also antiphased zonal trends in the vertical thermal gradients (indicating shoaling of the thermocline to the east and deeping to the west).

The first year of the award was spent processing samples for picking, and isotopic and Mg/Ca analyses from ODP Site 925 (west Atlantic) and Site 662/3/4 (east Atlantic). This represented over 600 samples with three species picked for each sample, and two analyses (Mg/Ca and d18O) per species. The Site 925 data extend from 0-3.5 Ma, whereas the Site 662/3/4 data extend from 0-1 Ma, and then from 2.2-3.6 Ma (with a slumping hiatus in the middle section).

The Mg/Ca SST data for Sites 925 and Sites 662/3/4 show that the onset of the modern day zonal Atlantic tropical SST gradient commenced near 2.8 Ma. The multispecies oxygen isotope data document that the thermocline shoaled and cooled at this time, signaling stronger trade-wind driven upwelling. Our proxy for past changes in surface ocean productivity, biogenic opal percent at Site 662/3/4, shows a sharp increase in upwelling and opal burial at the same time as the thermocline and zonal SST changes near 2.8 Ma.

Collectively, these data support the hypothesis that the onset of upwelling was due to a shift toward stronger tropical wind-driven upwelling and productivity. I was also able to compare these tropical Atlantic data with other published multispecies isotope data from the eastern equatorial Pacific and remarkably data from this ocean parallels what we discovered in the Atlantic, suggesting a tropic-wide onset of greater upwelling and productivity after 2.8 Ma.

To investigate the cause of this shift to greater upwelling after 2.8 Ma, I compared these tropical data to a biogenic opal flux record from the SubAntarctic region (Site 1093) which was published in 2004. Impressively, these data show that opal productivity at 65S was dramatically reduced after 2.8 Ma and interpreted to reflect the onset of greater polar ocean stratification through cooling and lower salinities. I believe these data may be indicating that this high-latitude paleoceanographic shift affected how the tropical ocean thermocline was ventilated. Specifically, the high-latitude source waters which sink and flow equatorwards to ventilate the thermocline cooled significantly after 2.8 Ma.