Late Pleistocene History Of Ocean Atmosphere Interaction In The Eastern Equatorial Pacific

The vast area of the North Pacific, spanning ~55˚ longitude, represents a challenge for documenting and understanding the geologic history of ocean, atmosphere, and terrestrial environmental change. Nevertheless, its importance for many issues, including our fundamental understanding of ocean and atmospheric circulation patterns and teleconnections with natural modes of climate variability through time, has led to a steady rise in the numbers of study sites and proxy types. By bringing together a wide range of proxies and timescales that examine the impacts of paleoclimate on ecosystems, water, carbon, and humans, and interactions between marine and terrestrial processes, this Research Topic contributes to an improved understanding of the region’s significance at global, hemispheric, and regional scales.

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.

This volume is one outcome of the 6th International Conference on Paleoceano graphy (ICP VI). The conference was held August 23-28, 1998 in Lisbon, Portugal. The meeting followed the traditional format of a small number of invited oral presentations complemented by a large number ofcontributed posters. Over 550 participants attended, representing thirty countries and nearly 450 posters were presented. The invited speakers addressed the main themes of the 5oral sessions. The session topics were: Polar-Tropical and Interhemisphere Linkages; Does the Ocean Cause, or Respond to, Abrupt Climatic Changes?; Biotic Responses to Major Paleoceanographic Changes; Past Warm Climates; and Innovations In Monitoring Ocean History. This is the first time in ICP history that the Conference Proceedings are published. The aim of the organisers with the publication of this book is two-fold: to provide a useful review of the field and to document the ideas/controversies raised during the con ference that may stimulate future work. The book reflects the initial intentions of the conference, but it is not a conven tional conference proceedings, given that the papers have been reviewed by formal exter nal referees. Each of the conference topics is introduced by a review article designed to summarize the state of the art in each theme followed by articles prepared by the invited speakers. As with most conference proceedings, each theme is covered heterogenously. Some topics have all the expected contributions, others are less well covered.

Preface This book is the culmination of a workshop jointly organized by NATO and CEC on Climate-Ocean Interaction which was held at Lady Margaret Hall, Oxford University during 26-30 September 1988. The objective of the ARW was to assess the current status of research on climate-ocean interaction, with a major focus on the development of coupled atmosphere-ocean-ice models and their application in the study of past, present and possible future climates. This book contains 16 chapters divided into four parts: Introduction; Observations of the Climate of the Ocean; Modelling the Atmospheric, Oceanic and Sea Ice Components of the Climatic System; and Simulating the Variability of Climate on Short, Medium and Long Time Scales. A fifth part contains the reports of the five Working Groups on: Climate Observations, Modelling, ENSO Modelling and Prediction, Climate-Ocean Interaction on TIme Scales of Decades to Centuries, and Impact of Paleoclimatic Proxy Data on Climate Modelling. Preface ix Acknowledgements I thank Howard Cattle and Neil Wells for their guidance and assistance as members of the Workshop Organizing Committee. I particularly thank Michael Davey for all his efforts as Local Organizer to make the ARW a success. I also thank the staff of Lady Margaret Hall, Oxford University, for their help with the arrangements for the ARW.

The tropical Pacific is a region where nutrient delivery, upper ocean dynamics, and global climate variability are tightly coupled. For example, the depth of the eastern equatorial Pacific (EEP) thermocline, for the most part, determines the delivery of the essential nutrient nitrate, but it is also a key aspect of an ocean-atmosphere feedback system responsible for global climate variability--the El Nino Southern Oscillation (ENSO). To provide a unique view of the upper ocean processes directly responsible for nutrient and climate dynamics in this region, I apply a single tracer--the ratio of 15N to 14N (hereafter "[delta]15N")--in modern seawater and sediment throughout the tropical Pacific. I find that the [delta]15N of nitrate is homogeneous throughout the tropical Pacific and is elevated by 2 / relative to the Southern and Atlantic Oceans by mixing with the denitrified waters of the eastern tropical Pacific. The results of this modern ocean survey are used to devise a new application of [delta]15N in eastern and western equatorial Pacific (WEP) sediments that provides a record of EEP nitrate consumption over the past 1,200,000 years. This record strongly suggests that the depth of the EEP thermocline--and therefore the upper ocean conditions driving the tropical climate system--has little to no response to high-latitude processes such as icesheet dynamics. Instead, as has been suggested by coupled ocean-atmosphere models, the east/west thermocline tilt responded primarily to changes in local seasonal insolation over thousands of years (a product of planetary axial precession). Some of the long-term changes in these deep-sea sediment [delta]15N records would seem to suggest an alteration of the original surface ocean signal, but additional [delta]15N measurements of sedimentary size fractions and components support the fidelity of bulk sedimentary [delta]15N as an archive of surface ocean nutrient cycling.

The Early-Middle Pleistocene transition (around 1.2 to 0.5 Ma) marks a profound shift in Earth's climate state. Low-amplitude 41 ka climate cycles, dominating the earlier part of the Pleistocene, gave way progressively to a 100 ka rhythm of increased amplitude that characterizes our present glacial-interglacial world. This volume assesses the biotic and physical response to this transition both on land and in the oceans: indeed it examines the very nature of Quaternary climate change. Milankovitch theory, palaeoceanography using isotopes and microfossils, marine organic geochemistry, tephrochronology, the record of loess and soil deposition, terrestrial vegetational change, and the migration and evolution of hominins as well as other large and small mammals, are all considered. These themes combine to explore the very origins of our present biota.