Reports: AC2
47626-AC2 Origin of the Mid-Pliocene Pole-to-Equator Ocean Productivity Shift
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 results so far are extremely promising n terms of testing the hypothesis as proposed – the measured signal thus far is quite large and robust and supports the initial hypothesis as proposed.
The Mg/Ca SST data for Sites 925 and Sites 662/3/4 show a remarkable divergent trend after 2.8 Ma, confirming that coincident with the onset of greater opal productivity after 2.8 Ma, the east-west SST gradient also increased indicating greater wind-driven equatorial upwelling. Additionally, our preliminary subsurface dwelling species isotopic data from eastern tropical Atlantic Sites 662/3/4 show that this increase in zonal SST gradient was accompanied by a shallower thermocline. Together, these data support the hypothesis that the onset of upwelling was due to a shift toward stronger tropical wind-driven upwelling and productivity.
Additional data are needed to investigate the remaining, important question concerning changes in the source of the waters upwelled after 2.8 Ma. To address this question we will need to complete the d18O and Mg/Ca analyses of subsurface dwelling species at western Site 925. These samples have been picked and are in line for analysis. We had a mass spectrometer fail in early 2009 and this significantly delayed these analyses, but this instrument was repaired in Dec. 2009 and is now ready for samples again. The subsurface species’ Mg/Ca analyses at Site 925 are in the process of being run.
I anticipate that we will have these results completed by May 2010 and we can commence writing the results for publication at that point.
