ALBERT

Description: Highlights • Re-organization of the West Pacific Warm Pool at ~ 1.7 – 1.35 Ma. • West Pacific Warm Pool and South Pacific Convergence Zone located further to the NE prior to ~ 1.5 Ma. • High amplitude variations at thermocline and deep thermocline depths after ~ 1.5 Ma. • West Pacific Warm Pool thermocline dynamics linked to southern-sourced mode waters. Abstract The internal development of the tropical West Pacific Warm Pool and its interaction with high latitude ocean regions on geological timescales is only poorly constrained. Based on two newly recovered sediment cores from the southeastern margin of the West Pacific Warm Pool (northern and southern Manihiki Plateau), we provide new aspects on the dynamically interacting ocean circulation at surface, subsurface, thermocline, and deep thermocline levels during the Pleistocene (~ 2.5–0.5 Ma). Notably, the variability of thermocline and deep thermocline (~ 150–400 m water depth) foraminiferal Mg/Ca-based temperatures with up to ~ 6 °C amplitude variations exceeds those at shallower depths (down to ~ 120 m) with only ~ 2–3 °C temperature variations. A major gradual reorganization of the West Pacific Warm Pool oceanography occurred during the transitional time period of ~ 1.7–1.35 Ma. Prior to ~ 1.7 Ma, pronounced meridional and latitudinal gradients in sea-surface to subsurface ocean properties point to the eastward displacement of the West Pacific Warm Pool boundaries, with the South Pacific Convergence Zone being shifted further northeastward across Manihiki Plateau. Simultaneously, the low amplitude variations of thermocline and deep thermocline temperatures refer to an overall deep and stable thermocline. The meridional and zonal gradients in sea-surface and subsurface ocean properties within the West Pacific Warm Pool reveal a pronounced change after 1.5 Ma, leading to a more southward position of the warm South Pacific Convergence Zone between ~ 1.35–0.9 Ma and ~ 0.75–0.5 Ma. Synchronous to the changes in the upper ocean, the deeper water masses experienced high amplitude variations in temperature, most prominently since ~ 1.5 Ma. This and the dynamically changing thermocline were most likely associated to the impact of southern-sourced mode waters, which might have developed coincidently with the emergence of the East Pacific Cold Tongue and high latitude sea-surface cooling.

Description: Highlights • First apparent calcification depth assessment of living foraminifera in the SE WPWP • Deep surface mixed layer causes deep apparent calcification depths. • Deep-dwelling G. hexagonus traces nutrient conditions in equatorial water masses. Abstract Insight into past changes of upper ocean stratification, circulation, and nutrient signatures rely on our knowledge of the apparent calcification depth (ACD) and ecology of planktonic foraminifera, which serve as archives for paleoceanographic relevant geochemical signals. The ACD of different species varies strongly between ocean basins, but also regionally. We constrained foraminiferal ACDs in the Western Pacific Warm Pool (Manihiki Plateau) by comparing stable oxygen and carbon isotopes (δ18Ocalite, δ13Ccalcite) as well as Mg/Ca ratios from living planktonic foraminifera to in-situ physical and chemical water mass properties (temperature, salinity, δ18Oseawater, δ13CDIC). Our analyses point to Globigerinoides ruber as the shallowest dweller, followed by Globigerinoides sacculifer, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotaloides hexagonus inhabiting increasing greater depths. These findings are consistent with other ocean basins; however, absolute ACDs differ from other studies. The uppermost mixed-layer species G. ruber and G. sacculifer denote mean calcification depths of ~ 95 m and ~ 120 m, respectively. These Western Pacific ACDs are much deeper than in most other studies and most likely relate to the thick surface mixed layer and the deep chlorophyll maximum in this region. Our results indicate that N. dutertrei appears to be influenced by mixing waters from the Pacific equatorial divergence, while P. obliquiloculata with an ACD of ~ 160 m is more suitable for thermocline reconstructions. ACDs of G. hexagonus reveal a deep calcification depth of ~ 450 m in oxygen-depleted, but nutrient-rich water masses, consistent to other studies. As the δ13C of G. hexagonus is in near-equilibrium with ambient seawater, we suggest this species is suitable for tracing nutrient conditions in equatorial water masses originating in extra-topical regions.

Description: Insight into past changes of upper ocean stratification, circulation, and nutrient signatures rely on our knowledge of the apparent calcification depth (ACD) and ecology of planktonic foraminifera, which serve as archives for paleoceanographic relevant geochemical signals. The ACD of different species varies strongly between ocean basins, but also regionally. We constrained foraminiferal ACDs in the Western Pacific Warm Pool (Manihiki Plateau) by comparing stable oxygen and carbon isotopes (δ18Ocalite, δ13Ccalcite) as well as Mg/Ca ratios from living planktonic foraminifera to in-situ physical and chemical water mass properties (temperature, salinity, δ18Oseawater, δ13CDIC). Our analyses point to Globigerinoides ruber as the shallowest dweller, followed by Globigerinoides sacculifer, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotaloides hexagonus inhabiting increasing greater depths. These findings are consistent with other ocean basins; however, absolute ACDs differ from other studies. The uppermost mixed-layer species G. ruber and G. sacculifer denote mean calcification depths of ~95mand ~120 m, respectively. These Western Pacific ACDs are much deeper than in most other studies and most likely relate to the thick surface mixed layer and the deep chlorophyll maximum in this region. Our results indicate that N. dutertrei appears to be influenced by mixing waters from the Pacific equatorial divergence, while P. obliquiloculata with an ACD of ~160 m is more suitable for thermocline reconstructions. ACDs of G. hexagonus reveal a deep calcification depth of ~450 m in oxygen-depleted, but nutrient-rich water masses, consistent to other studies. As the δ13C of G. hexagonus is in near-equilibrium with ambient seawater, we suggest this species is suitable for tracing nutrient conditions in equatorial water masses originating in extra-topical regions.

Description: 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 (εNd) 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 εNd 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.

Description: The eastern equatorial Pacific (EEP) is a key area to understand past oceanic processes that control atmospheric CO2 concentrations. Many studies argue for higher nutrient concentrations by enhanced nutrient transfer via Southern Ocean Intermediate Water (SOIW) to the low-latitude Pacific during glacials. Recent studies, however, argue against SOIW as the primary nutrient source, at least during early Marine Isotope Stage 2 (MIS 2), as proxy data indicate that nutrients are better utilized in the Southern Ocean under glacial conditions. New results from the subarctic Pacific suggest that enhanced convection of nutrient-rich Glacial North Pacific Intermediate Water (GNPIW) contributes to changes in nutrient concentrations in equatorial subthermocline water masses during MIS 2. However, the interplay between SOIW versus GNPIW and its influence on the nutrient distribution in the EEP spanning more than one glacial cycle are still not understood. We present a carbon isotope (δ13C) record of subthermocline waters derived from deep-dwelling planktonic foraminifera Globorotaloides hexagonus in the EEP, which is compared with published δ13C records around the Pacific. Results indicate enhanced influence of GNPIW during MIS 6 and MIS 2 compared to today with largest contributions of northern-sourced intermediate waters during glacial maxima. These observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past EEP nutrient concentrations. A switch from increased GNPIW (decreased SOIW) to diminished GNPIW (enhanced SOIW) influence on equatorial subthermocline waters is recognized during glacial terminations and marks changes to modern-like conditions in nutrient concentrations and biological productivity in the EEP.