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«Dissertation for the degree philosophiae doctor (PhD) at the University of Bergen Dissertation date: Calibration, validation and application of the ...»

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Dissertation for the degree philosophiae doctor (PhD)

at the University of Bergen

Dissertation date:

Calibration, validation and application of the foraminiferal Mg/Ca

Scientific environment

This thesis was submitted for the degree doctor philosophiae (dr. philos.) at the

Department of Earth Sciences (University of Bergen, Norway). The research

has been funded by European Science Foundation (ESF) EuroMARC with

support from the Research Council of Norway (RCN) and NRF with a working place at Bjerknes Centre for Climate Research and Uni Research Climate AS.

I Calibration, validation and application of the foraminiferal Mg/Ca Acknowledgements This is the time to thank all of you that contributed to make this thesis possible and undeniably enjoyable… First of all, I would like to thank my supervisor Trond Dokken for your constant support during this PhD. Thank you, Trond, for giving me the opportunity to work with an exciting and challenging research field. Your positive attitude, your availability at all times for discussions, your interest and great enthusiasm about my work have been more than expected and made always working pleasant. I am grateful also, Jeanne Gherardi-Scao, which it was always a pleasure to share the office but also new results, discussions and doubts. Thank you for your undeniable support. Frank Peeters, many thanks for your contribution and especially for your careful and rich comments. Thank you, Claire Waelbroeck, for taking time to assess my papers. Øyvind Lie, thank you very much for the great help and your contribution during the last phase of my thesis.

The most important part of the thesis is the result of many hours, days, and weeks at the Bergen Geo Analytical facility at Department of Earth Science, UiB. I would like to acknowledge Siv Hjorth Dundas for technical support and particularly Vincent Scao, for your expertise on Mg/Ca and discussions in the cleaning room. I learned a lot about techniques. In the same occasion, I am grateful to Ulysses Ninnemann and Rune Søraas for technical support with stable isotopes analysis.

I am grateful to my colleagues and friends at Bjerknes Centre for Climate Research and at Uni Research Climate, for the friendly atmosphere. In particular, Jørund, Kerim, Camille, Bjørg, Carin, Nil, Elin, Lea, Emil, Charla… I have been lucky to take part by twice of oceanographic cruises. First, on board of G.O. SARS and on board of R/V Marion Dufresne in an irresistible tropical area. I would like to thank the cruise team for good time and nice company.

I would like to acknowledge European Science Foundation (ESF), the Research Council of Norway (RCN) and the RETRO project to supporting this work.

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Finally, last but not least I would like to thank my family. I think in particular about my parents for always believing in me and always supported me. Thank you, Ronan, for giving your support, for believing in me and for sharing my life. And thank you, Lena and Nils, to join us during this PhD. Your smiles, your big hugs and your distraction make relief when I was oversaturated…

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IV Calibration, validation and application of the foraminiferal Mg/Ca

Abstract

of dissertation The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the northward distribution of heat. It is a key mechanism in the climatic system that needs to be further understood. Most studies focused on surface or deep water conditions. Relatively little is known about the role of subsurface, thermocline and intermediate waters, where heat uptake and/or transport occur. Our aim is to give a better representation of the hydrography during the past by reconstructing the upper water column from surface to intermediate waters. Understanding of how changes in the upper water masses properties is crucial for determining the role of oceanic heat distribution involved in the AMOC and is the main objective of this PhD project.

The Western Tropical Atlantic is a sensitive oceanic region connected with surface and intermediate waters that flow from the Southern Hemisphere into the North Atlantic and a deep water mass from the Northern Hemisphere. This region is unique in identifying the upper limb pathway of the AMOC and associated water masses.

In this context, using sediment cores from depth transects is therefore crucial for estimating not only the magnitude of change but, particularly the orientation of water mass distribution change. Seven coring sites have been retrieved that fall along a depth transect extending from 600 to 1000 m water depth.

Main strategy is to use different planktonic species having different depth habitat to reconstruct the upper surface to about 200-400 m water depth, and furthermore benthic species from depth of the coring sites to reconstruct water mass changes further below to 1000 m.

The PhD project has two major objectives. The first is to calibrate and validate relationship between Mg/Ca ratio and temperature in benthic foraminiferal shells.

Mg/Ca ratio in several benthic foraminifer species from core-tops were analysed to develop Mg/Ca calibrations over the 0-6 degrees Celsius temperature range in the Atlantic Ocean (Paper I). Apart from temperature, other factors may exert additional influences on foraminiferal Mg/Ca. For that purpose, the effect of the seawater

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carbonate chemistry on the magnesium incorporation into the foraminiferal calcite have been investigated for the benthic foraminifer species Cibicidoides wuellerstorfi (Paper II).

In the second part of this project, we have applied the improved C. wuellerstorfi Mg/Ca-temperature calibration to examined surface, subsurface, thermocline and intermediate water conditions during the last deglaciation. Combining with water densities estimates and ventilation ages, we are trying to bring important insights on discerning sources of subsurface and intermediate waters (Paper III).

This PhD work introduces a relatively new concept of investigating the hydrology along a core section in the upper water column. Looking to the future, we recommend a parallel strategy of applying proxies with refinements in methods and understanding of mechanisms that control the proxy.

VI Calibration, validation and application of the foraminiferal Mg/Ca Contents Scientific environment

Acknowledgements

Abstract of dissertation

Contents

1 Introduction

2 Scientific background

3 Response of tropical Atlantic surface and intermediate waters to AMOC changes.. 4 4 Results overview

5 Perspectives

References

Paper I

Paper II

Paper III

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This PhD work has been accomplished within the Research Council of Norway (RCN) and the ESF-Euromarc project “Response of tropical Atlantic surface and intermediate waters to changes in the Atlantic meridional overturning circulation” (RETRO). The research presented in this thesis advances our understanding of the behaviour and the role of surface and intermediate oceanic circulation in the tropical Atlantic Ocean during the last deglaciation. This study is based on analysis and interpretation of geochemical multi-proxy records from marine sediments. The PhD thesis consists of an introduction, three papers and a conclusion/perspectives section.

The introduction provides the scientific background and the main purposes of this study and summarizes the articles presented in the thesis. The last section concludes with some discussions and further investigations.

The three manuscripts included in this thesis are listed below:

Paper 1: Refining benthic foraminiferal Mg/Ca-temperature calibration using coretop from the western tropical Atlantic: Implication for paleotemperature estimation.

Manuscript published in G3 (May 2013).

Paper 2: Revisiting the carbonate ion saturation effect on Mg/Ca in the benthic foraminifer C. wuellerstorfi in the Atlantic Ocean. Manuscript in preparation.

Paper 3: Deglacial evolution of surface to intermediate water mass characteristics in the western tropical Atlantic. Manuscript in preparation.

The first paper presents new benthic foraminifera Mg/Ca calibrations. The second paper is focused on the carbonate system and how the benthic foraminiferal C.

wuellerstorfi Mg/Ca ratio can be affected or not by the carbonate ion saturation. The third paper is a compilation of benthic and planktonic Mg/Ca-temperature and δ18O signal over the last deglaciation. These results are used to reconstruct temperature and hydrological changes along a western tropical Atlantic section covering a depth range from surface to 1000 m water depth.

Calibration, validation and application of the foraminiferal Mg/Ca

–  –  –

Changes in ocean circulation are known to have played a critical role in the Earth’s climate evolution during the last deglaciation (Figure 1), between the Last Glacial Maximum (LGM 26-19 kyr; e.g. Clark et al., 1999) and the present warm interglacial period, the Holocene (~10-0 kyr) (Clark et al., 2002; McManus et al., 2004;

Rahmstorf, 2002, 2006). This deglacial evolution in the North Atlantic included cold episodes, the Younger Dryas and Heinrich 1 events, affecting ocean properties and consequently global climate (Broecker, 1994; Hu et al., 2004; Marcott et al., 2011).

However, many of the mechanisms involved in the Atlantic Meridional Overturning Circulation (AMOC) changes during the deglaciation remain unresolved.

Figure 1: (left panel) Simplified sketch of the NADW formation and water distribution accross the GreenlandScotland Ridge (source: www.whoi.edu; by E.P. Oberlander) and (right panel) schematic representation of the three modes of ocean circulation during different times of the last glacial period (Rahmstorf, 2002).

Freshwater forcing from melting ice in the North Atlantic has been suggested as a key mechanism (Broecker, 1994; Clark et al., 2012; Hu et al., 2009; Zhang, 2007), but it is still debated how and to what degree do the deglacial meltwater pulses impact the ocean circulation. Similarly, a slowing of the overturning circulation in the North Atlantic Ocean is expected to reduce northward heat transport in the ocean and to induce warming in the Souhern Ocean and particularly in the thermocline tropical Atlantic, as described as the bipolar see saw mechanism (Figure 2) (Arz et al., 1998, 1999; Chang et al., 2008; Cheng et al., 2007; Chiang et al., 2008; Dahl et al., 2005;

Manabe and Stouffer, 1997; Ruhlemann et al., 2001; Ruhlemann et al., 2004; Stouffer Calibration, validation and application of the foraminiferal Mg/Ca et al., 2006; Vellinga and Wood, 2002; Weldeab et al., 2006; Zhang, 2007).

Furthermore, this tropical heat retention might have played an important role in abrupt climate changes during the last deglaciation.

Figure 2: (A) Schematic Atlantic Ocean section during abrupt cold events in the North Atlantic. The thermocline, which is presented by a black line indicates the highest temperature gradient. Zonal mean temperature anomaly in the Atlantic Ocean averaged over the last 401st to 500th year, which is presented by gray lines in the profile (A and B), shows heat anomaly centered to the intermediate tropical water depth (adapted from Manabe and Stouffer, 1997). (B) Chart of the Brazilian transect shows the core-top and downcore used to reconstruct the climate variability (see paper III for further details). TSW: Tropical surface water,

GNAIW: Glacial North Atlantic intermediate water, GAABW: Glacial Antarctic bottom water, GAAIW:

Glacial Antarctic intermediate water.

Unfortunately, crucial evidence of changes in the ocean’s most active water: the intermediate ocean is lacking. How did intermediate ocean circulation vary across the last deglaciation? One of the objectives of this PhD project is to remedy to this unresolved issue.

Calibration, validation and application of the foraminiferal Mg/Ca

–  –  –

The aim of the research project RETRO is to reconstruct circulation changes within the surface, thermocline, intermediate and deep waters in the tropical Atlantic Ocean during periods of reduced AMOC, in order to understand the controlling mechanisms linking the high and low latitudes. One of RETRO goals is to investigate large changes in tropical ocean parameters during the transition from the LGM to the present interglacial. This thesis is part of this context.

RETRO focuses on three different depth layers in order to assess the response of the tropical Atlantic climate under different AMOC states: the surface and the subsurface, the intermediate depth including the permanent thermocline, and the deep waters.

- What mechanisms affect the upper limb/branch of the AMOC?

- How these changes in intermediate waters can affect the climate generally or locally?

- What is the role of the thermocline and intermediate waters on the abrupt climate changes during the last termination?

We aim to give insight into these research questions.

3.2 Strategy for reconstructing the tropical thermocline structure Constraining the different thermocline/intermediate water mass regimes at present and during the past is challenging. One of the objectives of this PhD work is to examine and describe surface, subsurface and intermediate temperature and density changes associated with an AMOC change.

- Can we show thermocline/intermediate layers warming as predicted by models?

Calibration, validation and application of the foraminiferal Mg/Ca

- What was the spatial extent of the warming along the water column?

- What was the state of the intermediate circulation during extreme reduction in the deeper branch of the AMOC?

This work will expose some hypothesis.



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