Changing dominance of high-latitude intermediate waters and its impact on the equatorial nutrient-budget : Implications from foraminiferal geochemistry

Abstract

This dissertation assesses the changing dominance of high-latitude intermediate waters and its impact on the equatorial nutrient-budget. The modern Equatorial Pacific Intermediate Water (EqPIW) is fed by three end-member components: Southern Ocean Intermediate Water (SOIW), Pacific Deep Water (PDW) and, by a smaller proportion, North Pacific Intermediate Water (NPIW). This modern configuration of end-members in the EqPIW results in low productivity of siliceous phytoplankton in the Eastern Equatorial Pacific (EEP) today as SOIW is depleted in silicic acid compared to other nutrients. However, there is growing debate over whether SOIW was capable of stimulating glacial equatorial productivity. Furthermore, recent studies point towards a change in the lateral and vertical extent of both SOIW and NPIW during glacials, impacting the supply of nutrients to the EEP. Most upper ocean water mass reconstructions are based on planktonic foraminifera tests. Different foraminiferal species preferentially dwell in distinct water depths and thus, the calcitic tests of these species can be used to infer past climate conditions. This thesis assesses equatorial foraminiferal ACDs to identify a species suitable to trace nutrient-inflow of extra-tropical intermediate water masses. Using this determined species, this thesis then reconstructs the effect of variable nutrient injections from extra-tropical water masses on the equatorial Pacific upwelling waters using benthic and planktonic foraminiferal carbon isotopes (d13C). It was shown that d13C records from the Bering Sea (as an indicator for glacial NPIW (GNPIW)), the eastern tropical North Pacific and the EqPIW exhibit a similar temporal evolution during MIS 2. These results indicate increased GNPIW ventilation during glacials that spreads southward towards the eastern tropical North Pacific. During peak glacials the southward expansion of GNPIW was at a maximum and extended into the equatorial Pacific. Together with newly published evidence for a shallower penetration of relatively nutrient-depleted SOIW during glacials, these results point towards repeated episodes of reduced southern-sourced nutrient-injections into EqPIW during peak glacials. In contrast, the enhanced ventilation of nutrient-elevated GNPIW resulted in a comparatively increased nutrient contribution to the EqPIW. This intensified GNPIW nutrient-inflow possibly relaxed the nutrient limitation in the EEP, stimulating primary productivity in the EEP during peak MIS 2. As a consequence, the invigorated glacial biological pump would have sequestered more carbon dioxide (CO2) from the atmosphere into the ocean. And thus, in summary, this thesis has contributed important new insights into the role of the dynamics of the EEP in driving the glacial reduction in atmospheric CO2 concentrations