Tropical Pacific sea surface temperatures (SSTs) are a critical component of the global climate system with oceanic and atmospheric teleconnections through meridional and latitudinal heat transport. Understanding the climate drivers and dynamics of this region enables a better understanding of global climate. Orbital scale climate drivers for eastern and western Pacific SSTs have been studied; however, SSTs and thermocline structure have not been studied in the central equatorial Pacific (CEP). Studying temperature dynamics in the CEP upper water column can help determine which mechanisms control SST and thermocline structure and test previously proposed hypotheses. Here, I present CEP SST and subsurface temperature records from the Line Islands (ML1208-17PC) that span the last 380,000 years. Using two species of foraminifera, G. ruber and G. tumida, I respectively generated Mg/Ca based SST and subsurface temperature records and compared them to published records from the equatorial Pacific. This comparison indicates an expanded west pacific warm pool (WPWP) during interglacial periods but no expansion of the eastern Pacific cold tongue during glacial periods. Based on the thermocline depth proxy, the thermocline was deeper in glacial periods and shallower in interglacial periods. Cross-spectral analysis demonstrates which climate drivers are the likely forcings for CEP SST and thermocline behavior. The CEP SSTs are distinct from those to the east or the west as they are not directly driven by CO2 or insolation at orbital frequencies; instead, the CEP SST record is linked to subsurface temperature at eccentricity and obliquity bands. However, changes in thermocline conditions at the CEP are potentially driven by CO2 and Antarctic temperature changes. This study agrees and supports previous studies that indicate deeper thermocline depths in glacials and shallower depths in interglacials.

The tropical Pacific is a significant component of the ocean's climate system and a source of considerable global climate variability, including El Niño. Tightly coupled sea surface and thermocline dynamics influence tropical climate on a variety of spatial and temporal scales. Records of past climate surface and subsurface temperature variability, on long and short times scales, may aid in identifying the radiative and dynamic forcing important to tropical climate during different global climate states. During the Pliocene warm period (3.0-4.3 Ma) and Last Glacial Maximum (LGM, ~0.020 Ma) the global mean state was substantially different and may have influenced the behavior and strength of various processes and mechanisms that determine tropical climate. In this dissertation, I use geochemical techniques, including the novel application of the magnesium-calcite ratio of individual planktonic foraminifera, to investigate the different processes that determine tropical change on different time scales and background climate states.

In the first project, I generated stable isotope and minor element records using a subsurface dwelling planktonic foraminifera from a transect of sites across the Pacific cold tongue. Using subsurface temperatures as a proxy for thermocline depth, my records suggest there was a gradual shoaling of the eastern equatorial Pacific thermocline from the early Pliocene to present day. In the second and third projects, I use the minor element ratio of individual surface and subsurface dwelling foraminifera to monitor the SST and subsurface temperature background state, high-resolution variability and radiative and dynamic processes that determine tropical climate. The results suggest the tropical western Pacific has responded primarily to radiative forcing on glacial-interglacial time scales since the early Pliocene. In contrast, the eastern Pacific has responded to changes in dynamic and radiative forcing during the last glacial period, which influenced the background spatial pattern of the cold tongue region. Additionally, changes in individual foraminifera variability suggest a decrease in El Niño Southern Oscillation during the most recent glacial period in comparison to the Holocene. These records show that different scales of variability (i.e. long-term, glacial-interglacial, and seasonal/interannual) may illuminate the processes and mechanisms important to paleoclimate interpretations on various temporal and spatial scales.

The Guaymas Basin, Gulf of California, is an important site for paleoclimate study due to ideal preservation conditions and high sedimentation rates, which allow for high-resolution analysis of paleoclimate records and investigation of climatic and oceanographic processes that operate over timescales not resolved by modern instrumental records. Furthermore, the Guaymas Basin receives some water advected at depth from the Eastern Tropical North Pacific (ETNP), a region where oceanographic processes are closely coupled to the global climate system. The past climate and oceanography of the Gulf of California and ETNP have been well studied at low resolution over glacial-interglacial timescales: the goal of this study is to elucidate Mid- to Late Holocene oceanographic and climate changes at a higher resolution than previous studies in order to resolve decadal to centennial variance. Three main conclusions can be drawn from the new high-resolution records presented here. First, a careful comparison of radiocarbon dating and layer counting data calls into question the completeness of varved marine sequences in the Guaymas Basin. Second, stable isotopes, elemental composition data, XRF core scan trace metal counts, and smear slide analysis are used to identify a significant change in mean conditions and ecosystem structure at ~2800 yr BP, which is probably driven by southward displacement of the Inter-tropical Convergence Zone (ITCZ). Notably, mean bulk δ15N is ~0.6‰ lower after 2800 yr BP, suggesting decreased denitrification and improved ventilation. Third, important frequencies of variance at centennial and decadal periodicities are identified. Comparison of these new high-resolution proxy records to previous work in other regions suggests that Mid- to Late Holocene changes in the Guaymas Basin may be linked to climate changes across a larger spatial scale.

Paleoceanographic studies typically employ stable nitrogen isotopic measurements of total combustible N (δ15NTN) from marine sediments to infer temporal changes in surface ocean marine N dynamics. However, δ15NTN is a highly non-specific measurement, susceptible to confounding issues including non-marine N sources, and offers little, if any, information regarding degradation and/or alteration of primary δ15N signals. Recent development of novel biomarker and microfossil based proxies provide new analytical capabilities to circumvent issues with δ15NTN.

Here, I explore the utility of compound specific nitrogen isotopic analysis of individual amino acids measurements (δ15NAA) of proteinaceous material isolated from marine sediments and deep-sea coral. With these data I infer past changes in nitrate δ15N fueling primary production at the base of the food web, ecosystem structure, and the extent of heterotrophic resynthesis influencing sedimentary and coral-skeletal δ15NTN records. Within sediments and corals, I demonstrate that δ15NAA based parameters reflect that of sinking particles and surface collected planktonic biomass and thus serve as a robust, albeit complex proxy to infer ecosystem and nutrient dynamic changes for at least the last few hundred years on the California Margin. In sediments I find an unexpected δ15N offset in major operational fractions and molecular compound classes. This result (1) challenges a major assumption organic nitrogen in sediments is dominated by amide functionality (i.e., proteinaceous N) and (2) highlights possible issues with standard protein hydrolysis techniques.

Lastly, I analyze the δ15N composition of foraminifera-bound N (FB-δ15N) to assess the Plio-Pleistocene history of whole ocean nitrogen dynamics from the oligotrophic North Pacific for the last 5 million years. This region contains calcareous nannofossil rich sediments, ideal for preserved FB-δ15N records. These new FB-δ15N records support an expansion of suboxic regions, consistent with previous evidence, but also indicate that the whole ocean δ15N budget was stable throughout the Plio-Pleistocene. This latter result implies that kinetic isotopic fractionations of marine N transformations and the ratio of pelagic to benthic denitrification, must not have changed during the Plio-Pleistocene transition, despite drastic global scale changes in sea level, shelf area, and volume of the major oxygen deficient zones at this time.

Compound-specific nitrogen (N) isotopic analysis (δ15N) of amino acids (AA), or CSI-AA, is a novel approach to understand N cycling. We expand upon initial observations at a productive, hypoxic margin to provide insight into source and transformation of sedimentary organic nitrogen from varied depositional conditions in a complex N cycling zone, the eastern tropical Pacific. δ15NAA patterns are generally well preserved, matching original mixed plankton inputs with some evidence for microbial degradation. There is appreciable δ15N offset between total N (bulk) and total AA N, where AA N is enriched. Source AA, previously understood to record changing baseline δ15N, are fundamentally different in marine sediments. We use modeling to rigorously test whether the total AA δ15N instead tracks changing baseline δ15N. Lastly we employ this new understanding of δ15NAA in sediments to compare with published whole sediment (δ15Nbulk) records, demonstrating the utility and wealth of information provided by CSI-AA.