U-series dating of diagenetically altered fossil reef corals and the application for sea level reconstructions [Elektronische Ressource] / presented by Denis Scholz

De
Dissertation submitted to the Combined Faculties for the Natural Sciences and for Mathematics of the Ruperto-Carola University of Heidelberg, Germany, for the degree of Doctor of Natural Sciences presented by Denis Scholz born in Ludwigshafen am Rhein, Germany Oral examination: 26.01.2005 U-series dating of diagenetically altered fossil reef corals and the application for sea level reconstructions Referees: Prof. Dr. Augusto Mangini Prof. Dr. Kurt Roth Zusammenfassung Mithilfe von fossilen Riffkorallen ist eine exakte Rekonstruktion der Meeresspiegelschwankungen der Vergangenheit möglich. Ein bekanntes Problem bei der 234 238Uranreihen-Datierung fossiler Riffkorallen sind initiale ( U/ U) Aktivitätsverhältnisse, die nicht durch den radioaktiven Zerfall in einem geschlossenen System erklärt werden können. Dieses Phänomen ist unter dem Begriff diagenetische Veränderungen bekannt. In der vorliegenden Arbeit werden fossile Riffkorallen aus Aqaba, Jordanien, und Barbados, Westindische Inseln, untersucht. Die Porites Korallen aus Aqaba zeigen einen hohen Grad diagenetischer Veränderungen und sind daher ideal geeignet, um die diagenetischen Prozesse zu untersuchen.
Publié le : samedi 1 janvier 2005
Lecture(s) : 29
Source : ARCHIV.UB.UNI-HEIDELBERG.DE/VOLLTEXTSERVER/VOLLTEXTE/2005/5287/PDF/DISSERTATION.PDF
Nombre de pages : 180
Voir plus Voir moins

Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany,
for the degree of
Doctor of Natural Sciences
























presented by
Denis Scholz
born in Ludwigshafen am Rhein, Germany
Oral examination: 26.01.2005
U-series dating of
diagenetically altered fossil reef corals
and the application for sea level reconstructions

















Referees: Prof. Dr. Augusto Mangini
Prof. Dr. Kurt Roth
Zusammenfassung
Mithilfe von fossilen Riffkorallen ist eine exakte Rekonstruktion der
Meeresspiegelschwankungen der Vergangenheit möglich. Ein bekanntes Problem bei der
234 238Uranreihen-Datierung fossiler Riffkorallen sind initiale ( U/ U) Aktivitätsverhältnisse, die
nicht durch den radioaktiven Zerfall in einem geschlossenen System erklärt werden können.
Dieses Phänomen ist unter dem Begriff diagenetische Veränderungen bekannt.
In der vorliegenden Arbeit werden fossile Riffkorallen aus Aqaba, Jordanien, und
Barbados, Westindische Inseln, untersucht. Die Porites Korallen aus Aqaba zeigen einen
hohen Grad diagenetischer Veränderungen und sind daher ideal geeignet, um die
diagenetischen Prozesse zu untersuchen. Mithilfe eines numerischen Modells wird gezeigt,
dass die Aktivitätsverhältnisse dieser Korallen durch nachträgliche Uranaufnahme gefolgt von
teilweisem Verlust des aufgenommenen Urans erklärt werden können. Auf dieser Basis wird
ein Korrekturverfahren (das Isochronenverfahren) entwickelt, das es ermöglicht,
glaubwürdige Uranreihen-Alter für Korallen zu berechnen, die nicht mit den herkömmlichen
Methoden datiert werden können.
Die Acropora palmata und Diploria strigosa Korallen aus Barbados zeigen einen
geringeren Grad diagenetischer Veränderungen. Es wird gezeigt, dass diese Korallen durch
einen anderen Prozess beeinflusst wurden als die Korallen aus Aqaba. Daher können sie nicht
mit der Isochronenmethode datiert werden. Dennoch ist es möglich, Höhe, Beginn und Dauer
des Meeresspiegel Hochstands während des Marinen Isotopen Stadiums (MIS) 6.5 (~175000
Jahre vor heute) zu bestimmen. MIS 6.5 ist eine außergewöhnliche Klimaphase innerhalb der
letzen 300000 Jahre. Die vorliegende Meeresspiegelkurve ist die erste, exakt datierte, direkte
Meeresspiegelrekonstruktion für das Marine Isotopen Stadium 6.5.

Abstract
The age/height relationship of fossil reef coral terraces provides an accurate measurement
of past sea level changes. An outstanding problem of U-series dating of fossil reef corals is
234 238that many corals show initial ( U/ U) activity ratios that cannot be explained by closed
system decay. This phenomenon is denoted by the term diagenetic alterations.
In this work fossil reef corals from Aqaba, Jordan, and Barbados, West Indies, are
investigated by U-series methods. The corals of the genus Porites from Aqaba show a very
high degree of diagenetic alteration. Therefore, they are ideally suited to study the diagenetic
processes. It is demonstrated by numerical modelling that the activity ratios of these corals
can be explained by post depositional U uptake followed by partial loss of the additional U.
On the basis of these results a correction technique (the isochron dating method) is developed
which enables to calculate reliable U-series ages for corals which could not be dated by the
conventional methods.
The corals of the genus Acropora palmata and Diploria strigosa from Barbados show less
degree of diagenetic alteration. It is shown that these corals have been affected by a different
process than the Aqaba corals. Therefore, they cannot be dated by the isochron dating
method. Nevertheless, the results allow an accurate reconstruction of the magnitude, timing,
and duration of the sea level peak during Marine Isotope Substage (MIS) 6.5 (~175,000
before present), a period of extraordinary climate signature within the last 300,000 years. The
presented record is the first precisely dated direct measurement of sea level during this period.
1

Table of contents


1 Introduction ..................................................................................5
2 Basics .............................................................................................9
2.1 U-series dating methods .......................................................................... 9
2.1.1 U, Th, and Pa geochemistry ............................................................................. 10
2.1.2 The calculation of Th/U- and Pa/U-ages.......................................................... 11
2.1.3 Correction for detrital contamination............................................................... 13
2.2 U-series dating of fossil reef corals....................................................... 15
2.2.1 The behaviour of U-series isotopes in seawater............................................... 15
2.2.2 Stony or scleractinian corals and reef development......................................... 16
2.2.3 The incorporation of U-series isotopes into coral skeletons ............................ 20
2.2.4 Testing the dating assumptions for fossil reef corals ....................................... 21
2.2.5 Modelling diagenetic effects in fossil reef corals............................................. 26
3 The application of fossil reef corals as paleoclimate archives 31
3.1 Reconstruction of past sea level fluctuations ........................................ 31
3.1.1 The utilization of stable isotopes measured in deep sea sediments.................. 31
3.1.2 The utilization of the height/age relationship of fossil reef corals................... 34
3.1.3 The utilization of submerged speleothems....................................................... 37
3.1.4 Comparison of the different reconstruction methods....................................... 38
3.1.5 The Late Quaternary sea level curve from coral dating................................... 39
3.2 Corals as high resolution paleoclimate archives 40
4 Results and discussion................................................................43
4.1 The study of fossil reef corals from Aqaba, Jordan .............................. 43 2 Table of contents
4.1.1 Description of the study region and coral sampling......................................... 43
4.1.2 Pre-treatment of the coral samples................................................................... 47
4.1.3 Results of the U-series measurements.............................................................. 48
2324.1.4 Discussion of Th content.............................................................................. 50
238 234 230 2384.1.5 U content, δ U, and ( Th/ U) activity ratio ................... 52
4.1.6 Numerical simulation of the Aqaba coral data................................................. 60
4.1.7 The calculation of isochron ages...................................................................... 66
4.1.8 The paleo sea level reconstruction at Aqaba.................................................... 70
4.1.9 Improving the calculation of isochron ages and errors .................................... 72
4.1.9.1 The recalculation of the isochron ages......................................................... 73
4.1.9.2 The recalculation of the isochron age errors ................................................ 74
4.1.9.3 Results and discussion of the recalculation of the Aqaba isochron age
errors............................................................................................................. 80
4.1.9.4 Using additional information to improve the isochron age accuracy........... 82
4.1.10 Conclusions I.................................................................................................... 84
4.2 The study of fossil reef corals from Barbados, West Indies ................. 87
4.2.1 Description of the study region ........................................................................ 87
4.2.2 The studied coral reef terraces ......................................................................... 88
4.2.3 Coral pre-treatment and U-series measurements ............................................. 93
4.2.4 The corals from the middle unit (Termination II)............................................ 94
4.2.4.1 Results and discussion of the U-series measurements ................................. 94
4.2.4.2 The application of isochron dating............................................................... 95
4.2.4.3 Discussion of the U-series ages.................................................................... 96
4.2.5 The corals from the lower unit (MIS 6.5) ........................................................ 96
4.2.5.1 Climate during Marine Isotope Substage 6.5............................................... 96
4.2.5.2 Results of the U-series measurements.......................................................... 99
4.2.5.3 The application of isochron dating............................................................. 100
4.2.5.4 Investigation of the diagenetic processes................................................... 104
4.2.5.5 Identification of reliable U-series ages....................................................... 114
4.2.5.6 Magnitude, timing and duration of the MIS 6.5 sea level peak ................. 116
4.2.6 Conclusions II ................................................................................................ 120
5 Summary and outlook..............................................................123
TUTTTUUUTUUTUTTTPUUTTUUUPUTTTTTTUTTUTUUUUTUUUTUUTTUUUTUPTUUTTUTUTUUTUPTTTUUUTUUTTUUTUUTTUUTTTUUPTUTTTUUTUUUTUUUTUUTUUTUUUTUUTTTUUTUUUPUUTTUUUUUTUUUPTUUTTUTTUUUPUUTUTTTTTUTUUTUTTUUTTUTUPUUUTTUTUUUTUTUUUTTUTTUUTTTUTUUTTUTTTUTTTUTTUUUTUUTTUTTUTTTTUUTTUUUUUUUTUUUTTUTTPUTTTUUTUTUUTTTable of contents 3
Appendix .........................................................................................127
A Thermal ionisation mass spectrometry (TIMS)......................................127
A1 Sample preparation............................................................................................. 127
A2 Thermal ionisation mass spectrometry (TIMS) ................................................. 127
A3 Blanks................................................................................................................. 129
A4 U and Th standards and spikes........................................................................... 130
B Visual Basic Scripts ................................................................................135
B1 Calculation of U-series ages, initial δ234U and the corresponding errors......... 135
B2 U uptake/loss models ......................................................................................... 140
B3 Calculation of coral isochron age errors ............................................................ 145
C TIMS data ...............................................................................................152
D List of samples collected during the Barbados excursion ......................159
References .......................................................................................161
UUUUUTTUTTTUTTTTTUTUUTTUTTUUTTTTTTTUUUUUTTUUUTTTTUTTUUUUUUUTUTTTUUUTUUTUUTUUTTUTTUTTTUUTTUUTUUUT

Soyez le premier à déposer un commentaire !

17/1000 caractères maximum.