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

The equatorial Pacific holds the potential to investigate the climate variability of the Earth as it connects both hemispheres via the atmospheric and oceanic circulation. 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. An increased primary production during glacials has often been attributed to an enhanced contribution of SOIW to equatorial sub-surface waters. However, there is growing debate over whether SOIW was capable of stimulating glacial equatorial productivity. This is in light of the fact that nutrients appear to have been trapped in glacial Southern Ocean waters. 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. Ultimately, the effect of these intermediate water mass changes on equatorial waters remains elusive. 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. However, it has been shown that the Apparent Calcification Depths (ACDs) of foraminiferal species are spatially non-uniform. Today, there are no ACD reconstructions from the equatorial Pacific based on multinet data. 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 (δ13C). In combination with published records of neodymium isotopes (eNd) and foraminiferal δ13C values from the subarctic Pacific, the eastern North Pacific, the eastern tropical North Pacific as well as the southeast and southwest Pacific, this thesis aims to improve our knowledge of end member contributions on EqPIW during the last two glacial-interglacial cycles, focusing in at higher resolution during Marine Isotope Stage (MIS) 2. The results of this thesis are presented in three manuscripts. The first manuscript examines foraminiferal calcification depths in the western equatorial Pacific using living planktonic foraminifera in combination with foraminiferal abundances. Despite the relatively deep thermocline in the Western Pacific Warm Pool (WPWP), the relative order of the five investigated species was IIIcomparable to other ocean basins. However, absolute ACDs differed due to the local hydrography in the WPWP. Surface mixed layer dwellers Globigerinoides ruber and Globigerinoides sacculifer were apparent at ~95 m and ~115 m water depth, and were found in low abundances during the sampling time. The comparatively deep thermocline between 130 – 230 m below sea level subsequently led to relatively deep calcification depths of Neogloboquadrina dutertrei and Pulleniatina obliquiloculata. Hence, both species occupy a depth habitat towards the top, and within, the thermocline. One of our major findings was that the planktonic species Globorotaloides hexagonus was found to occupy a deep habitat (~450 m water depth) within the Pacific. This subthermocline species seems to favour cool, nutrient-rich water masses and was shown to be a suitable archive for tracing nutrient-inflow of high latitude intermediate water masses on equatorial Pacific sub-thermocline. The second and third manuscripts deal with the ventilation of Glacial North Pacific Intermediate Water (GNPIW) and its influence on the EqPIW during the past 60 ka (Manuscript 2) and during the last two glacial-interglacial cycles (Manuscript 3). It was shown that δ13C records from the Bering Sea (as an indicator for GNPIW), the eastern tropical North Pacific and the EqPIW (measured on G. hexagonus) exhibit a similar temporal evolution during MIS 2. In addition, the absolute eNd signatures from the Bering Sea and the eastern North Pacific are similar during this time period. The δ13C difference between the equatorial record and northern and southern signatures, respectively, was calculated to infer the relative change of high latitude intermediate water contribution on equatorial sub-thermocline nutrient concentrations. Most interestingly, in times when the δ13C differences between the EqPIW record and two Southern Ocean cores are greatest (late MIS 2 and MIS 6), the difference in δ13C between the North Pacific and EEP is smallest. 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.