Paleoenvironmental Reconstructions of the Central Equatorial Pacific Ocean Using Uranium and Thorium Series Isotopes

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

Large deglacial shifts of the Pacific Intertropical Convergence Zone

The position of the Intertropical Convergence Zone (ITCZ) is sensitive to changes in the balance of heat between the hemispheres which has fundamental implications for tropical hydrology and atmospheric circulation. Although the ITCZ is thought to experience the largest shifts in position during deglacial stadial events, the magnitude of shifts has proven difficult to reconstruct, in part because of a paucity of high-resolution records, particularly those including spatial components. Here we track the position of the ITCZ from 150 to 110 ka at three sites in the central equatorial Pacific at sub-millennial time resolution. Our results provide evidence of large, abrupt changes in tropical climate during the penultimate deglaciation, coincident with North Atlantic Heinrich Stadial 11 (~136–129 ka). We identify this event both as a Northern Hemisphere increase in aeolian dust and as a shift in the mean position of the ITCZ a minimum of 4° southwards at 160° W

Spatial pattern and temporal evolution of glacial terminations of the last 800 ka

The second QUIGS workshop brought together 28 delegates to assess current knowledge and research needs on the spatio-temporal patterns of climate forcing, responses and feedbacks that characterize glacial terminations, i.e. transitions between glacial and interglacial periods

Productivity patterns in the Equatorial Pacific over the last 30,000 years

The equatorial Pacific traverses a number of productivity regimes, from the highly productive coastal upwelling along Peru to the near gyre-like productivity lows along the international dateline, making it an ideal target for investigating how biogeochemical systems respond to changing oceanographic conditions over time. However, conflicting reconstructions of productivity during periods of rapid climate change, like the last deglaciation, render the spatiotemporal response of equatorial Pacific productivity ambiguous. In this study, surface productivity since the last glacial period (30,000 years ago) is reconstructed from seven cores near the Line Islands, central equatorial Pacific, and integrated with productivity records from across the equatorial Pacific. Three coherent deglacial patterns in productivity are identified: (1) a monotonic glacial-Holocene increase in productivity, primarily along the Equator, associated with increasing nutrient concentrations over time; (2) a deglacial peak in productivity ~15,000 years ago due to transient entrainment of nutrient rich southern-sourced deep waters; and (3) possible precessional cycles in productivity in the eastern equatorial Pacific that may be related to Intertropical Convergence Zone migration and potential interactions with El Niño–Southern Oscillation dynamics. These findings suggest that productivity was generally lower during the glacial period, a trend observed zonally across the equatorial Pacific, while deglacial peaks in productivity may be prominent only in the east

The internal layering of Pine Island Glacier, West Antarctica, from airborne radar-sounding data

This paper presents an overview of internal layering across Pine Island Glacier, West Antarctica, as measured from airborne-radar data acquired during a survey conducted by the British Antarctic Survey and the University of Texas in the 2004/05 season. Internal layering is classified according to type (continuous/discontinuous/missing) and the results compared with InSAR velocities. Several areas exhibit disruption of internal layers that is most likely caused by large basal shear stresses. Signs of changes in flow were identified in a few inter-tributary areas, but overall the layering classification and distribution of layers indicate that only minor changes in ice-flow regime have taken place. This is supported by bed-topography data that show the main trunk of the glacier, as well as some of the tributaries, are topographically controlled and located in deep basins

Repeated storage of respired carbon in the equatorial Pacific Ocean over the last three glacial cycles

As the largest reservoir of carbon exchanging with the atmosphere on glacial–interglacial timescales, the deep ocean has been implicated as the likely location of carbon sequestration during Pleistocene glaciations. Despite strong theoretical underpinning for this expectation, radiocarbon data on watermass ventilation ages conflict, and proxy interpretations disagree about the depth, origin and even existence of the respired carbon pool. Because any change in the storage of respiratory carbon is accompanied by corresponding changes in dissolved oxygen concentrations, proxy data reflecting oxygenation are valuable in addressing these apparent inconsistencies. Here, we present a record of redox-sensitive uranium from the central equatorial Pacific Ocean to identify intervals associated with respiratory carbon storage over the past 350 kyr, providing evidence for repeated carbon storage over the last three glacial cycles. We also synthesise our data with previous work and propose an internally consistent picture of glacial carbon storage and equatorial Pacific Ocean watermass structure

Persistent tracers of historic ice flow in glacial stratigraphy near Kamb Ice Stream, West Antarctica

Variations in properties controlling ice flow (e.g., topography, accumulation rate, basal friction) are recorded by structures in glacial stratigraphy. When anomalies that disturb the stratigraphy are fixed in space, the structures they produce advect away from the source and can be used to trace flow pathways and reconstruct ice-flow patterns of the past. Here we provide an example of one of these persistent tracers: a prominent unconformity in the glacial layering that originates at Mt. Resnik, part of a subglacial volcanic complex near Kamb Ice Stream in central West Antarctica. The unconformity records a change in the regional thinning behavior seemingly coincident ( ∼ 3440±117&thinsp;a) with stabilization of grounding-line retreat in the Ross Sea Embayment. We argue that this feature records both the flow and thinning history far upstream of the Ross Sea grounding line, indicating a limited influence of observed ice-stream stagnation cycles on large-scale ice-sheet routing over the last  ∼ &thinsp;5700 years.</p

Coherent deglacial changes in western Atlantic Ocean circulation

Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. However, the exact timing and magnitude of past AMOC shifts remain elusive, which continues to limit our understanding of the driving mechanisms of such climate variability. Here we show a consistent signal of the 231Pa/230Th proxy that reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial-scale climate transitions. At the onset of deglaciation, we observe an early slowdown of circulation in the western Atlantic from around 19 to 16.5 thousand years ago (ka), consistent with the timing of accelerated Eurasian ice melting. The subsequent weakened AMOC state persists for over a millennium (~16.5–15 ka), during which time there is substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown, with a positive feedback sustaining continued iceberg calving and climate change during Heinrich Stadial 1NERC | Ref. NE/K008536/

230 Th normalization: new insights on an essential tool for quantifying sedimentary fluxes in the modern and quaternary ocean

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Costa, K. M., Hayes, C. T., Anderson, R. F., Pavia, F. J., Bausch, A., Deng, F., Dutay, J., Geibert, W., Heinze, C., Henderson, G., Hillaire-Marcel, C., Hoffmann, S., Jaccard, S. L., Jacobel, A. W., Kienast, S. S., Kipp, L., Lerner, P., Lippold, J., Lund, D., Marcantonio, F., McGee, D., McManus, J. F., Mekik, F., Middleton, J. L., Missiaen, L., Not, C., Pichat, S., Robinson, L. F., Rowland, G. H., Roy-Barman, M., Alessandro, Torfstein, A., Winckler, G., & Zhou, Y. 230 Th normalization: new insights on an essential tool for quantifying sedimentary fluxes in the modern and quaternary ocean. Paleoceanography and Paleoclimatology, 35(2), (2020): e2019PA003820, doi:10.1029/2019PA003820.230Th normalization is a valuable paleoceanographic tool for reconstructing high‐resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more diverse marine environments, the nuances of 230Th systematics, with regard to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of 230Th from across the global ocean at two time slices, the late Holocene (0–5,000 years ago, or 0–5 ka) and the Last Glacial Maximum (18.5–23.5 ka), and investigated the spatial structure of 230Th‐normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79–2.17 g/cm2kyr, 95% confidence) relative to the Holocene (1.48–1.68 g/cm2kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size‐dependent sediment fractionation, and carbonate dissolution on the efficacy of 230Th as a constant flux proxy. Anomalous 230Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that 230Th normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic marine settings (>1,000 m water depth).We thank Zanna Chase and one anonymous reviewer for valuable feedback. K. M. C. was supported by a Postdoctoral Scholarship at WHOI. L. M. acknowledges funding from the Australian Research Council grant DP180100048. The contribution of C. T. H., J. F. M., and R. F. A. were supported in part by the U.S. National Science Foundation (US‐NSF). G. H. R. was supported by the Natural Environment Research Council (grant NE/L002434/1). S. L. J. acknowledges support from the Swiss National Science Foundation (grants PP002P2_144811 and PP00P2_172915). This study was supported by the Past Global Changes (PAGES) project, which in turn received support from the Swiss Academy of Sciences and the US‐NSF. This work grew out of a 2018 workshop in Aix‐Marseille, France, funded by PAGES, GEOTRACES, SCOR, US‐NSF, Aix‐Marseille Université, and John Cantle Scientific. All data are publicly available as supporting information to this document and on the National Center for Environmental Information (NCEI) at https://www.ncdc.noaa.gov/paleo/study/28791