The Eastern Equatorial Pacific (EEP) plays a key role in global climate and ocean biogeochemistry. Using a recently collected series of cores from the Peruvian margin, this collaborative research generates an integrated paleoceanographic data set from this region, including alkenones, trace elements, stable isotopes, and sediment composition. These data are used to document lateral and vertical gradients in key aspects of physical, biological, and geochemical properties of the cold tongue region of the EEP. The approach is structured around three time streams of increasingly high resolution: last glacial maximum (LGM) through Termination I, the Holocene, and the last 2 millennia. The research team tests the hypothesis that the paleoceanography of the EEP has been dictated by two distinct drivers: the remote influence of the high latitude southern ocean, which dominated surface and subsurface properties from the LGM to the middle Holocene, and tropical dynamics which drove the modern El Nino-Southern Oscillation (ENSO) system over the past ~5 ky. Broader impacts include graduate and post-doctoral training, internships for disadvantaged high school students, and collaborations with South American scientists.
The main objectives of this thesis were to examine the centennial and millennial temperature variability of different regions in the EP, to determine the cause of this variability and to understand the impact of this variability on the climate system. The primary temperature proxy used was Mg/Ca from Neogloboquadrina dutertrei, a planktonic foraminifera that lives between 75-150 m. Other proxies included alkenone Uk’37 ratios to reconstruct sea surface temperature (SST), Mg/Ca from benthic foraminifera to reconstruct bottom water temperature, d18O as a temperature and salinity proxy and d13C as a water mass tracer. Reconstructions are from the eastern equatorial Pacific (EEP). The primary results from both this thesis on all timescales suggest that drivers of millennial and multicentennial variability are distinct from interannual and decadal dynamics. Prior to 8 kyr B.P. EEP thermostad temperatures were warm which is arguably caused by the Southern Westerly Winds being located farther south. During the last 2 kyr, these subthermocline waters indicate the influence of external forcing and cool during the Little Ice Age and during periods of decreased radiative forcing. In contrast SST from the EEP did not respond to radiative forcing nor followed Northern Hemisphere temperatures suggesting large internal variability. Summaries of two chapters from Julie’s thesis are given below. These will be submitted as papers in the next few weeks. Southern Ocean links to the Eastern Equatorial Pacific Thermostad during the Holocene Abstract Temperature reconstructions from a shallow core (375 m) from the Peru Margin are used to test the influence of Subantarctic Mode Water (SAMW) on the eastern equatorial Pacific thermostad and the effect of southern high latitude climate on interior ocean heat content. The Mg/Ca temperature estimates, based on planktonic and benthic foraminifera (Neogloboquadrina dutertrei and Uvigerina spp., respectively) show higher temperatures in the early Holocene followed by a cooling of ~2° by 8 kyr. The temperature signal is similar in direction and timing to a rather robust Holocene climate signal from the southern high latitudes suggesting it originated there and was advected to the core site. Based on the N. dutertrei and Uvigerina d13C records and that the origin of the thermostad has been previously traced to SAMW, we conclude that SAMW acted as a conduit transporting the southern high latitude climate to the interior of the equatorial Pacific. We propose the early Holocene warmth is related to a southward migration of the Subtropical Front, which enabled greater influence of warm subtropical waters to the region of SAMW formation. The SAMW temperature signal was then transported to the thermostad in the equatorial Pacific. The subsurface temperature signal in the equatorial Pacific appears to be larger than the surface temperature expression in the cold tongue, indicating that the proposed mechanism may be important for sequestering heat in the ocean interior. External Forcing and Internal Variability influence on the Eastern Equatorial Pacific temperatures during the Common Era Abstract Using a high resolution sediment core from off the coast of northern Peru in the eastern equatorial Pacific (EEP), we generate an alkenone sea surface temperature (SST) record and Neogloboquadrina dutertrei sub-thermocline temperature (STT) record for the last 2,500 years. By reconstructing both SSTs and STTs, we examine the EEP upper water column response to external forcing including solar variability, large volcanic eruptions and Northern Hemisphere temperatures. The SST record increases non-monotonically by 0.8?C and does not parallel Northern Hemisphere temperatures or respond to solar variability or volcanic eruptions. In contrast, the STTs are ~0.3?C colder during the Little Ice Age and prior to 1800 CE and low STTs correspond to periods of low radiative forcing indicating that STTs are affected by external forcings. Because SSTs and STTs are decoupled, the signals from external forcings are likely transported to the base of the thermocline via the subtropics rather than surface water directly above. After 1800 CE STTs increase, which is prior to Northern Hemisphere temperatures, but when several equatorial Pacific records also change, suggesting a rather large transition in the equatorial Pacific caused by variability intrinsic to the equatorial Pacific. The cool STTs during the Little Ice Age implies that on centennial time scales the subsurface is in equilibrium with the ocean-atmosphere system. However, the relationship between radiative forcing and multi-decadal STT variability suggests a dynamic response to radiative changes that persist in the subsurface for several decades.
