Ocean circulation and climate variability in the western South Atlantic and eastern South America during the last deglaciation [Elektronische Ressource] / vorgelegt von Cristiano Mazur Chiessi

Marum

Description

Ocean circulation and climate variability in the western South Atlantic and eastern South America during the last deglaciation Dissertation zur Erlangung des Doktorgrades am Fachbereich Geowissenschaften der Universität Bremen vorgelegt von Cristiano Mazur Chiessi Bremen, April 2008 Tag des Kolloquiums: 25. Juli 2008 Gutachter: Prof. Dr. Gerold Wefer Prof. Dr. Dierk Hebbeln Prüfer:Prof. Dr. Katrin Huhn Dr. André Paul Table of contents Abstract iii Chapter 1 Introduction 1 1.1 Motivation 1 1.2 Objectives 4 1.3 Outline 5 Chapter 2 Environmental setting 7 2.1 Oceanic circulation 7 2.2 Atmospheric circulation 10 2.3 Geology 12 Chapter 3 Methods 15 3.1 Mg/Ca paleothermometry 15 3.2 Stable isotopes 16 3.3 X-ray fluorescence core scanner 20 3.4 Radiocarbon dating 21 3.5 The University of Victoria Earth System Climate Model 23 Chapter 4 Signature of the Brazil-Malvinas Confluence (Argentine Basin) in the isotopic composition of planktonic foraminifera from surface sediments 25 C. M. Chiessi, S. Ulrich, S. Mulitza, J. Pätzold, G. Wefer 4.1 Abstract 25 4.2 Introduction 26 4.3 Regional setting 27 4.4 Materials and methods 31 4.5 Results 35 4.6 Discussion 38 4.

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2008
• Nombre de lectures: 48
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Ocean circulation and climate variability

in the western South Atlantic and eastern South America

during the last deglaciation

Dissertation zur Erlangung

des Doktorgrades am

Fachbereich Geowissenschaften

en ität Bremder Univers

vorgelegt von

essi ur ChiCristiano Maz

en, April 2008 Brem

s: Tag des Kolloquium

25. Juli 2008

Gutachter:

Prof. Dr. Gerold Wefer

Prof. Dr. Dierk Hebbeln

Prüfer:

hn Prof. Dr. Katrin Hu

Dr. André Paul

Table of contents Abstract iii
1Chapter 1 Introduction Motivation 1 1.1 Objectives 4 1.2 Outline 5 1.3 7tal setting Chapter 2 Environmen2.1 Oceanic circulation 7
2.2 Atmospheric circulation 10
Geology 12 2.3 15Chapter 3 Methods 3.1 Mg/Ca paleothermometry 15
16Stable isotopes 3.23.3 X-ray fluorescence core scanner 20
21Radiocarbon dating 3.43.5 The University of Victoria Earth System Climate Model 23
alvinas Confluence (Argentine Basin) in the isotopic il-Mzthe BraChapter 4 Signature of composition of planktonic foraminifera from surface sediments 25
, S. Ulrich, S. Mulitza, J. Pätzold, G. Wefer C. M. Chiessi 25Abstract 4.1 Introduction 26 4.2 27Regional setting 4.34.4 Materials and methods 31
Results 35 4.5 38Discussion 4.64.7 Paleoceanographic implications and conclusions 42
Chapter 5 South Atlantic interocean exchange as the trigger for the Bølling warm event 45
A. Paul, J. Pätzold, J. Groeneveld, G. Wefer , S. Mulitza, C. M. Chiessi 45Abstract 5.1 Introduction 46 5.2 46Material and methods 5.3i

ts nnteoTable of c

48Results and discussion 5.4 Conclusions 53 5.55.6 Supplementary material 55
mmer American suChapter 6 Impact of the Atlantic Multidecadal Oscillation in the South 63monsoon Wefer Pätzold, G. ., S. Mulitza, JC. M. Chiessi 63Abstract 6.1 Introduction 64 6.26.3 Environmental setting and methods 64
Results 66 6.4 68Discussion 6.5 Conclusions 71 6.66.7 Supplementary material 73
77Chapter 7 Final remarks 7.1 Summary and conclusions 77
7.2 Future studies 79
ments 83Acknowledg 85References

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64 73 77 77 79 85

Abstract

The growing impact of human activities on the climate system adds a new dimension of
complexity and urgency to climate change research. Human activities may have the potential to
push key components of the climate system past critical states into qualitatively different modes of
many the This possibility requires additional efforts fromoperation, i.e. to exceed a tipping point. branches of climate change research in order to improve the accuracy of climate change
useful area of climate mate records has proven to be a veryion of past cliinatprojections. The examchange research, allowing, for instance, the verification of outputs from climate models, as well as
an evaluation of the range of responses from the climate system to different forcings, the timing of
s involved. these responses and the feedback mechanism This thesis tackles two elements of the climate system whose tipping points are currently
rculation (AMOC) and South onal overturning ci critical: the Atlantic meridiconsidered veryAmerican precipitation. The major goal of this work is to better understand and work out a detailed
reconstruction of ocean circulation and climate variability in the western South Atlantic and eastern
South America during the last deglaciation, with special emphasis on abrupt climate change.
Marine sediment samples from the western South Atlantic were used as archives of
oceanic and climatic signals. The methods applied included radiocarbon dating of planktic
foraminifera, stable isotopes and Mg/Ca ratios of planktic and benthic foraminifera, and Ti
intensities in bulk sediment. Exceptionally high sedimentation rates in the study area during the last
, outputs Additionallyses. poral resolution on the analydeglaciation, allowed sub-decadal-scale temfrom an Earth system climate model of intermediate-complexity have been used to validate the
s for paleoceanographic changes. echanismical coherence of the suggested msphy This investigation first determined how different species of planktic foraminifera record
the present-day properties of the upper water column of the western South Atlantic (focused on the
Brazil-Malvinas Confluence (BMC)) in their isotopic compositions. For this purpose, the oxygen
and carbon isotopic compositions of Globigerinoides ruber (pink and white varieties analyzed
separately), Globigerinoides trilobus, Globigerina bulloides, Globorotalia inflata and Globorotalia
truncatulinoides (left- and right-coiling forms analyzed separately) were measured. A latitudinal

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transect of 56 surface sediment samples from the continental slope off Brazil, Uruguay and
Argentina between 20 and 48oS were used. Lowest oxygen isotopes values were found in G. ruber
(pink), followed by G. ruber (white) and G. trilobus reflecting the highly stratified near surface
water conditions north of the BMC. Globigerina bulloides was present mainly south of the BMC
and Globorotalia inflatastudies. and records subsurface conditions supporting earlier plankton tow G. truncatulinoides (left and right) were both available over the whole transect and calcify in the
depth level with the steepest temperature change across the BMC. Accordingly, the oxygen
isotopic compositions of these species depict a sharp gradient of 2 ‰ at the confluence with
remarkably stable values north and south of the BMC. The data show that the oxygen isotopic
composition of G. inflata and G. truncatulinoides (left and right) are the most reliable indicators for
BMC. the the present position of As a second step, changes in the upper water column of the western South Atlantic
during the last deglaciation were addressed, and the implications for abrupt climate change were
fodiscussed. The high latitudes of the North Atlantic experienced an abrupt temperature increase 9oC within a couple of decades during the transition from Heinrich event 1 (H1) to the Bølling
warm event (at ~14.7 cal kyr BP). Nevertheless, the mechanism responsible for this warming
H1 to the ins uncertain. Records presented in this thesis show that during the transition fromaremBølling, the western South Atlantic experienced a warming of ~6.5oC and an increase in the oxygen
isotopic composition of seawater (18Osw) of 1.2 ‰ at the permanent thermocline. Simultaneously,
a warming of ~3.5oC with no significant change in 18Osw was determined for intermediate depths.
Most of the warming can be explained by tilting the South Atlantic east-west isopycnals from a
flattened towards a steepened position, associated with a collapsed (H1) and strong (Bølling) AMOC, respectively. However, this zonal seesaw explains an increase of just 0.3 ‰ in permanent
thermocline 18Osw. Considering that 18Osw of the South Atlantic permanent thermocline is
strongly influenced by the inflow of salty Indian Ocean upper waters, the data suggest that a
H1 to the Bølling, and that ook place at the transition fromstrengthening in the Agulhas Leakage tthis is responsible for the change in 18Osw recorded in the western South Atlantic. The temperature
anomalies between the “Heinrich-like” and the “Bølling-like” climate states simulated with the
University of Victoria Earth System Climate Model were consistent with the proxy-based
reconstructions. Taken together, these results highlight the important role played by Indian-Atlantic
interocean exchange as the trigger for the resumption of the AMOC and the Bølling warm event.
Finally, the multidecadal variability in precipitation over eastern South America was
on the the impact of the Atlantic Multidecadal Oscillation (AMO) , reconstructed. More specifically

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South American Summer Monsoon (SASM) was investigated, using marine records of the La Plata

River drainage basin (PRDB) discharge. The records are based on stable oxygen isotopic

composition of shallow-dwelling planktic foraminifera (controlled by the PRDB plume) and Ti

intensity in bulk sediment (controlled by the source of the terrigenous sediments), and cover a

period of approximately 4500 years of the last deglaciation. Spectral and wavelet analyses of the

me pluears in both the extension of the PRDB yrecords indicate a periodic oscillation of about 60

and the source of the terrigenous sediments. The observed oscillation most probably reflects

variability in the SASM activity associated to the AMO. During negative (positive) AMO phase,

the anomalously warm (cold) South Atlantic would increase (decrease) SACZ activity and displace

the main belt of SASM precipitation to the south (north). Amplified (reduced) SACZ activity

rainfall over the PRDB and the basin’s isotopically low discharge intowould increase (decrease)

the western South Atlantic, affecting the composition of the upper water column above the core

site. The southward (northward) displacemSACZ would increase (decrease) rainfall and nt of the e

entually increasing (decreasing) the Ti content erosion on the southern Ti-rich half of the PRDB, ev

of the terrigenous fraction of the sediments delivered to the core site. The results point out

of the AMO on the SASM. pact mclear i