Title: Paleoenvironmental Reconstructions of the Central Equatorial Pacific Ocean Using Uranium and Thorium Series Isotopes
Description: Uranium and thorium isotopes are powerful and sensitive tracers of a wide range of oceanographic and environmental processes. This thesis makes use of these isotopes in deep sea sediments to reconstruct dust fluxes and deep ocean respired carbon storage over the last 350 kyr in the central equatorial Pacific. The paleoenvironmental information obtained through the application of these isotopes as proxies reveals important information about the Earth’s ocean and atmosphere, and their connectivity on millennial and glacial-interglacial timescales. In Chapter 1 of this thesis I introduce the proxies and principals employed in our paleoenvironmental reconstructions. Subsequently, the first section of this thesis explores the use of 230Thxs,0-derived 232Th fluxes as a proxy for aeolian dust deposition at three sites beneath the shifting Pacific Intertropical Convergence Zone (ITCZ). The new records presented here improve upon existing records of tropical Pacific dust fluxes by increasing the temporal resolution ~5 fold and adding almost an order of magnitude more data.

Specifically, we reconstruct dust fluxes in two cores from 0-150 ka and from one core from 0-350 ka In addition to substantially improving constraints on tropical dust fluxes this work also utilizes the spatial transect of cores to infer past positions of the ITCZ on glacial-interglacial and millennial timescales. This proxy approach to reconstruct ITCZ position has only been applied and published once previously, in a relatively low-resolution study. Chapter 2, entitled “Large deglacial shifts of the Pacific Intertropical Convergence Zone,” presents evidence that the Pacific ITCZ experienced large shifts in latitudinal position, on millennial timescales during the penultimate deglaciation. The data resolve abrupt shifts in atmospheric circulation associated with deglaciation, in this case Termination II, at the boundary between the full glacial marine isotope stage (MIS) 6 and the peak interglacial MIS 5. These shifts are significant in that they appear to have occurred at the same time as changes in the North Atlantic driven by Heinrich Stadial 11 and may have played an important role in pushing the climate system over the threshold for deglaciation.

Indeed, this study is the first to show evidence of a millennial-scale ITCZ response at the time of the Heinrich Event 11 catastrophic iceberg discharge event. Additionally, the data point to the existence of a previously unidentified millennial peak in northern hemisphere dust abundance during the penultimate deglaciation. In Chapter 3, “Climate-related response of dust flux to the central equatorial Pacific over the past 150 kyr,” records of dust flux are used to provide strong evidence for an association between high latitude stadial events and tropical dust fluxes during the last 150 kyr. These high-resolution observations permit the drawing of conclusions about the meridional location of the Pacific ITCZ during six Greenland stadials. As with the shift of the ITCZ during Heinrich Stadial 11, these events were associated with perturbations of the interhemispheric thermal gradient and coincident movement of the ITCZ presents an important constraint on the sensitivity of the tropical atmosphere to high latitude perturbations.

The conclusions stemming from the interpretation of geochemical and paleoceanographic data presented in Chapters 2 and 3 are of broad relevance to a variety of geoscience disciplines that seek an understanding of the climate system. For example, these results confirm predictions made by modeling studies about the response of the ITCZ to high latitude climate forcing and provide an important new set of boundary conditions for modeling studies aimed at reconstructing changes in insolation forcing and tropical hydroclimate. The results show that paleo-reconstructions can constrain the magnitude of even abrupt ITCZ movement, demonstrating the potential to relate ITCZ changes to the magnitude of thermal forcing and to investigate thermal and hydrological components of other climate change events, past and future. Additionally, these results help improve understanding of the relationship between atmospheric dust abundance and climate, with implications for planetary albedo and micronutrient fertilization of the oceans.

The second portion of this thesis focuses on using authigenic uranium (aU) to reconstruct deep water chemistry with implications for paleocirculation. Chapter 4, “Repeated storage of respired carbon in the equatorial Pacific Ocean over the last three glacial cycles,” presents evidence that the Pacific was a significant reservoir for respired carbon during glacial periods over at least the last 350 kyr. This reconstruction is based on the precipitation of the redox sensitive metal uranium as a proxy for deep water oxygen concentrations. Because any change in oceanic storage of respiratory carbon must be accompanied by corresponding changes in dissolved oxygen concentrations, data reflecting bottom water oxygenation are of value in addressing questions of glacial carbon sequestration. The record reveals periods of deep ocean aU deposition during each of the last three glacial maxima. Export productivity data indicate these intervals are not associated with local productivity increases, indicating episodic precipitation of aU occurs in response to basin-wide decreases in deep water oxygen concentrations.

Not only does the aU record show the history of dissolved oxygen concentrations in the central equatorial Pacific, it also provides an opportunity for the reconciliation of records previously interpreted as incompatible with one another and with the storage of respired carbon. Synthesis of existing data suggests the existence of a ’floating’ pool of respired carbon between 2 and 3.5 km depth in the equatorial Pacific. This hypothesis permits the interpretation of existing proxy data reflecting abyssal LGM circulation and carbon storage without invoking a glacial watermass geometry significantly different from present. The new data and conclusions presented in Chapter 4 represent a significant advance in our understanding of where carbon was stored in the ocean during successive glacial periods. The perspective provided by the new aU time series is spatiotemporally unique and constitutes compelling evidence that hypotheses of marine carbon storage developed to explain the last glacial period are equally applicable to previous Pleistocene glacial periods.

The three studies presented in this thesis provide strong support for the utility of U and Th series isotopes in paleoenvironmental reconstructions. Not only does this work demonstrate the range of paleoclimatic proxy data that can be obtained from isotopes of U and Th, it also illustrates the value of interpretations derived from their analysis. The records presented here represent a substantial contribution to our knowledge of marine hydroclimate and ocean circulation over the last 350 kyr and motivate additional high resolution paleoclimate work using isotopes of U and Th.