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A
dv
a
ncing
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lanktonic foraminifer
a
Mg/Ca thermometry:
a microanalytical perspective
Aleksey Yu. Sadekov
A Thesis Submitted for The Degree of Doctor of Philosophy
of The Australian National University
Research School of Earth Sciences
This thesis represents original work conducted by the author,
except where specifically stated and acknowledged, during
full-time PhD candidature at the Research School of Earth Sciences,
The Australian National University. No part of the thesis has been
submitted to another University or institution.
Acknowledgments
I thank the following academics, technical specialists and colleagues who have
extended their help and support during the course of my PhD.
My supervisor, Dr. Stephen Eggins (ANU), for his guidance and patience and also
for introducing me to the world of geochemistry and in particular LA-ICPMS.
My adviser, Prof. Patrick De Deckker (ANU), for providing the opportunity for
me to come to Australia in the first place and for his continuous support and
insight throughout my PhD.
Other members of my advisory panel, Prof. Malcolm McCulloch (ANU), Dr.
George Chaproniere (ANU) and Dr. Michael Gagan (ANU), for their advice and
recommendations.
Ashley Norris (ANU) and Nick Ware (ANU) for their assistance with electron
microprobe analysis and Dr. Serguei Kalachev, wt.o prepared a program for
processing the microprobe data.
Roger Heady (ANU) for his help during my SEM sessions, and Judith Shelly
(ANU) for preparing samples. Bob Forrester from the Statistical Consulting Unit
at the ANU for his assistance with statistical analysis.
Prof.
J.
Erez (The Hebrew University, Israel), Prof. H. Spero (University of California, USA), and Dr P. Martin (The University of Chicago, USA), for their constructivecomments and suggestions which have helped improve the manuscripts in this
thesis.
Prof. D. Kroon (University of Edinburgh, UK) for providing the samples from the
ii
thesis. Prof. W. Kuhnt (University Kiel, Germany) for the opportunity to participate
in the RV "Sonne" cruise 185 in eastern Indian Ocean and for his great assistance
during this cruise.
Prof. G. Klinkhammer (Oregon State University, USA) for his collaboration and for
providing the Flow-through ICPMS data used in Chapter 3. Prof. U. S.
Ninnemann (University of Bergen, Norway) for the 8180 analyses he performed at
the stable isotope laboratory in BCCR/Department of Earth Sciences, University of
Bergen.
I thank Rose Berdin anc. Sophia Sadekova for their great support and
encouragement, which helped me complete this work. I also thank my friends and
colleagues at ANU and Canberra who created a fantastic scientific and living
environment during my stay in Australia.
This study was supported by provision of a PhD and research costs from the
Australian Research Council- via an ARC Discovery Grant DP0450358 to Prof. De
Deckker and Dr. Eggins, and by a tuition fee scholarship provided by The
Australian National University.
Abstract
Microanalytical techniques, specifically laser ablation - inductively coupled
plasma mass spectrometry (LA-ICPMS) and electron microprobe analysis
(EMPA), have been used to determine Mg/Ca compositional variation within and
between tests of planktonic foraminifera. Micro-scale characterization of this
increasingly widely-used seawater temperature proxy has allowed scrutiny of the
factors that limit its applicability as well as the development of new
microanalysis-based Mg/Ca proxy calibrations for sea surface temperature and
thermocline temperatures.
The variability of Mg/Ca compositions found within and between tests of
planktonic foraminiferal species greatly exceeds the possible range of test
calcification temperatures using established Mg/Ca-temperature sensitivities.
This indicates that factors additional to temperature influence foraminiferal
Mg/Ca compositions and raise significant questions about their use as a seawater
temperature proxy. The latter are assessed by calibrating the relationship between
the Mg/Ca composition of Globigerinoides ruber determined by LA-ICPMS
microanalysis in core-top and plankton pump samples spanning a range of sea
surface temperatures (18.5 29.2 DC) from the eastern Indian Ocean. Results
demonstrate that despite very large inter- and intra-test compositional variability
the pooled mean Mg/Ca compositions for core-top and plankton pump samples
forms a strong exponential correlation with mean annual sea-surface
temperature. It is further shown that inter-test Mg/Ca. variability within core-top
samples is a significant source of uncertainty in Mg/Ca seawater temperature
estimates and that widely assumed uncertainties in Mg/Ca thermometry may be
underestimated. Reasons for the large intra and inter-test Mg/Ca variability are
attributed to seasonality ranges over centennial timescales and to a significant
iv
'vital effect' contribution. Statistical methods to minimize effect of these
uncertainties on Mg/Ca thermometry are described.
LA-ICPMS to foraminifera Mg/Ca analysis is employed to assess the nature and
effects of seafloor alteration on the distribution Mg/Ca within and composition of
foraminiferal tests, using several different approaches including a study of
core-top samples down a depth transect in the eastern Indian Ocean and, weak acid
dissolution under controlled laboratory conditions. Limited evidence is found for
selective dissolution of C. sacculifer high-Mg calcite and minimal if any
discernable dissolution effects are observed on the Mg/Ca compositions of G.
sacculifer tests above the calcite lysocline. These results are consistent with
previous studies of G. sacculifer, indicating that the effects of seafloor dissolution
on G. sacculifer bulk Mg/Ca compositions are small compared to the uncertainty in
bulk test compositions that derives from inter-test variability.
New methods for reconstructing paleoseawater temperatures are developed
based on LA-ICPMS Mg/Ca microanalysis of G. ruber, G. sacculifer and Pulleniatina
obliquiloculata. The Mg/Ca compositions of individual chambers and of specific
tests parts present within G. rubber and G. sacculifer and P. obliquiloculata, in
core-top samples are found to correlate best with seawater temperatures at different
and specific depths. This provides the basis for developing new Mg/Ca-based
methods for reconstructing surface mixed layer and upper thermocline
temperatures.
This study illustrates the potential for using novel microanalytical techniques to
advance Mg/Ca thermometry, by both providing a framework for better
understanding the nature of Mg/Ca dependence on seawater temperature and the
effects of complicating factors (e.g. seafloor dissolution, seasonality), as well as
providing insight into the underlying biomineralization mechanisms that control
Mg incorporation into and the formation of foraminiferal calcite.
Contents
Acknowledgments ... ...
lAbstract ... ... ...
iii
Rationale and background ... 1
Aims and outline of this study... 9
Chapter I
Characterisation of Mg/Ca distributions in planktonic foraminifera species by electron microprobe mapping. ...13
1. Introduction ... ...
13
2. Construction of planktonic foraminiferal tests and terminology used in this study ...
15
3. Methods ... '...
16
4. Results ...
17
4.1 Globigerinoides ruber, G. sacculifer, G. conglobatus ...
19
4.2 Globorotalia menardii and G. truncatulinoides ...
20
4.3 Neogloboquadrina dutertrei and N. incompta ...
22
4.4 Orbulina universa ... 23
5. Discussion ... '...
24
5.1 pH change in the foraminiferal microenvironment ... 25
5.2 Seawater temperature ... ...
26
5.3 Erez [2003] biomineralistion model ...
27
5,4 Other possible explanations for cyclic Mg/Ca banding ... 28
5.5 Implications of Mg/Ca heterogeneity for paleoseawater
vi
thermometry ... ... ... ... .... ...
29
5.6 Mg/Ca composition of final calcite crust versus gametogenic calcite. ...
30
6. Conclusions ... ... ... ...
31
Chapter II
Uncertainties in seawater thermometry deriving from intra-and inter-test Mg/Ca variability in Globigerinoides ruber .... ...33
1. Introduction ... ... ...
33
2. Materials and Methods ... ...
35
3. Results ... ...
38
3.1 Intra-test variability ...
38
3.2 Inter-test variability ...
41
3.3 Relationship between the sample mean Mg/Ca and annual SST.
42
3.4 Mg/Ca thermometer uncertainties- sample size ...43
3.5 Mg/Ca of G. ruber morphotypes (sub-species) ...
44
4. Discussion .... ... ... ... ...
45
4.1 Factors contributing to the uncertainty of Mg/Ca thermometry.
45
4.2 Comparison of Mg/Ca - SST relationship with previous studies 48 4.3 Error estimation for Mg/Ca thermometry ...50
5. Conclusions ... ... ... ...
53
Chapter III
Effects of seafloor and laboratory dissolution on the Mg/Ca composition of Globigerinoides sacculifer and Orbulina universa tests - a laser ablation ICPMS microanalysis perspective ...55
1. Introduction ... ... ... ... ... ... ... ...
55
2. Experimental methods and results... ... ...
58
2.1 Seafloor dissolution of G.sacculifer, eastern Indian Ocean ...
58
2.1.1 Method ...
58
2.2 Laboratory dissolution of O. universa and G. sacculifer ...
63
2.2.1 Method ...
63
2.2.2 Results ... ... ... ... ... ...
64
2.3 Flow-through ICPMS dissolution of Orbulina universa ...
67
3. Discussion ... ... ...
69
3.1 Evidence for selective dissolution of high Mg/Ca calcite ...
69
3.2 Laboratory dissolution ...
74
4. Summary ... ... ... ...
75
Chapter IV
Seawater temperature reconstruction usingMg/Ca
microanalysis of Globigerinoides ruber. G. sacculifer and Pulleniatina obliquiloculata. ... ... ...77
1. Introduction ... ... ... ... ...
77
2. Materials and Methods ... ... ... ... ...
80
3. Results and Discussion ... ... ... ... ...
82
3.1 Comparison of Mg/Ca variation within G. ruber, G. sacculifer and P. obliquiloculata ... ...
82
3.2 Low-Mg/Ca layer compositions in Globigerinoides ruber and G. sacculifer ... .... ... ... ... 84
3.3 Relationships between Mg/Ca compositions and sea surface temperature ... ... ... ... ... 87
3.4 Comparison of G. rube" G. sacculifer and P. obliquiloculata Mg/Ca-temperature calibrations ...
90
3.4.1 Seasonal species preferences ...
91
3.4.2 Differences in intra-test Mg/Ca distribution ...
91
3.4.3 Differences in species' habitat preference ...
94
3.5 Application to sea surface and upper thermocline temperature
reconstruction ... 100
4. Conclusions ... 102
Outlook and prospects for future research ...
103References ...
109Appendixes ... 121
Figures
Figure 1. Number of the publication in scientific... ...
1
Figure 1-1. Schematic illustration of the test and chamber waH construction in planktonic foraminifera ...
16
Figure 1-2. Maps of Mg/Ca values in chamber wall cross-sections of symbiont-bearing species ... ... ....
18
Figure 1-3. Frequency histograms of individual Mg/Ca values measured in cross sections ...
20
Figure 1-4. Maps of Mg/Ca values in chamber wall cross-sections of symbiont-barren species. ... ... ...
21
Figure 1-5. SEM images showing the characteristic surface morphology and internal structure of Orbulina universa ... ... 23
Figure 1-6. Maps of Mg/Ca values measured by electron microprobe in chamber wall cross-sections of selected Orbulina universa ... ...
24
Figure II-I. Map showing the location of core-top samples (stars) ...
35
Figure 11-2. Examples of laser ablation ICP-MS profiles of Mg/Ca (mmol/mol) through the final chamber walls of G. ruber ...
37
Figure 11-3. Mg/Ca profiles through the final four chambers of a single G. ruber test .. ... ... ...
40
Figure 11-4. Comparison of the mean Mg/Ca values obtained for different chambers ... ... ... ...
41
Figure 11-5. Plots of annual SST at each core-top site versus the measured Mg/Ca composition ...
42
Figure 11-6. Plot showing the relationship between the standard error (SE) of the estimated SST ... ... ... ... ...
44
Figure 11-7. Comparison of the mean Mg/Ca composition of tests of the . two G. ruber morphotypes ...
46
x
Figure II-S. Comparison of published Mg/Ca seawater thermometers for G. ruber with the calibration obtained in this study... 49
Figure 11-9. Calculated sample size required to achieve a target temperature uncertainty using Mg/Ca thermometry...
52
Figure 111-1. Examples of Mg/Ca compositional profiles measured by LA-ICPMS across the final chambers of Pulleniatina obliquiloculata and Globigerinoides sacculifer ... ... ...
57
Figure 111-2. Core-top sample locations ...
59
Figure 111-3. Mean Mg/Ca compositions of the final chambers of individual Globigerinoides sa;;culifer test ... ... .... ...
61
Figure 111-4.1. Comparison of Mg/Ca chamber wall profiles measured through two Orbulina universa tests (specimens #1 and #4) ...
65
Figure 111-4.2 Comparison of Mg/Ca chamber wall profiles through two Orbulina universa tests (specimens #2 and #3) ...
66
Figure 111-4.3. Comparison of Mg/Ca chamber wall profiles measured through the final chambers of three Globigerinoides sacculi fer tests (specimens #2, #3, and #4) ...
66
Figure 111-4.4. Comparison of Mg/Ca chamber wall profiles through the final (F) and penultimate (F-l) chamber walls of three Globigerinoides sacculifer tests (specimens #5, #5 F-l, and #6 F-l) ... 67
Figure 111-5. Comparison of Orbulina universa tests in Flow Through I CPMS system... ... ... ... ... ... ... ... 68
Figure 111-6. Globigerinoides sacculifer Mg/Ca values from published studies of seafloor dissolution ... ...
71
Figure IV-1. Typical Mg/Ca profiles across the final chamber walls of Globigerinoides sacculifer and G. ruber tests ...
83
Figure IV-2. Relative proportion of tests with different numbers of Mg/Ca layers in core-top samples ...
83
Figure IV-3. Typical Mg/Ca profiles across the final chamber walls of Pulleniatina obliquiloculata b~sts . ... ... ... ... ... ... 84
Figure IV-4. Relative proportion of Pulleniatina obliquiloculata tests with
and without a low Mg/Ca cortex layer ...
85
Figure IV-5. Relationship between the average Mg/Ca composition and low Mg/Ca-Iayer compositions of the final chambers of Globigerinoides sacculifer and G. ruber ... ...
86
Figure IV-6. Relationships between sea-surface temperature and sample mean Mg/Ca values of different of test components .... ...
88
Figure IV-7. The percentage of low-Mg calcite comprising final chamber compositions of Globigerinoides sacculifer and G. ruber . ...
93
Figure IV-S. Reconstruction of the calcification depth of final test chambers of Globigerinoides sacculifer, G. ruber and Pulleniatina obliquiIoculata ... ... ...
94
Figure IV-9. Reconstructed calcification depths for the final chambers of Globigerinoides sacculifer tests ... ... ....
96
Figure IV-lO. Comparison of habitat depth-optimised Mg/Ca thermometers ... ... ...
97
Figure IV-H. Comparison of previously-published thermometers for G. ruber and MglCa thermometer calibrations derived from the average MglCa ...
98
Figure IV-l2. Reconstructed surface and subsurface temperatures based on our combined Mg/Ca thermometers ...
101
Tables
Table 11·1. Core-top and plankton pump sample locations ... 36
Table 11-2. Summary of the measured Mg/Ca values of individual G. ruber tests ...
39
Table 11-3. Summary of linear regression results ... 43
Table 111·1. Details of the core-top and plankton pump samples ...
60
Table 111-2. Summary of the station mean Mg/Ca compositions of Globigerinoides sacculifer ... ... ... ...
62
Table 111·3. Summary of the laboratory dissolution experiment ... 64
Table IV-I. The locations, sea-floor depths, 14C ages, and number of foraminiferal tests analysed ...
81
Table IV-2. Summary results for linear regressions between seawater temperatures and the natural logarithm of mean Mg/Ca values ...
89
Table IV-3. Summary of calcification depths (in metres ± standard error of the mean depth ... ... ... ...
95
Rationale and background
Mg/Ca thermometry is one of the most rapidly-developing and now widely-used
proxies for reconstructing past seawater temperatures. The number of scientific
publications related to and using Mg/Ca thermometry has experienced a dramatic
increase over the last decade (Fig. 1). This and the growing interest more generally
in reconstructing past climate parallels the rise in public awareness of global
climate change over recent decades, and the pressing need to better predict future
climate change and its associated impacts on societies and environments (Fig. 1).
The study of past climates and their evolution presents a powerful approach and
perspective to better understand modern climate and fu ture climate scenarios. A
crucial step in our understanding of past climates lies in our ability to accurately
reconstruct seawater temperatures because the oceans playa major role in driving
[image:16.559.214.484.499.725.2]and modulating global and regional climate change.
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2
MglCa thermometry is based on empirical observations that show the Mg/Ca
composition of calcite foraminifer tests (shells) increase systematically with
seawater temperature. Foraminifera are unicellular marine organisms that
precipitate microscopic calcium carbonate (calcite) tests comprising multiple
chambers and growth layeJ~s. Planktonic foraminifera inhabit the surface mixed
layer and upper thermocline of the oceans, spanning all latitudes from the Arctic
to the Southern Ocean. Fossil tests of planktonic foraminifera occur abundantly in
deep-ocean sediments and their stable isotopic «(5180, (513C) and elemental
compositions (Ba/Ca, Sr/Ca, Mg/Ca) are widely employed in paleoceanography to
constrain past seawater temperature and properties including salinity and
productivity.
In his pioneering work on the magnesium content of foraminifera tests, Chave
[1954] showed a positive correlation between magnesium concentration and
water temperature. This relationship was confirmed by subsequent studies
[Blackmon and Todd, 1959; Duckworth, 1977; Savin and Douglas, 1973], but the first
calibration and application of foraminiferal Mg/Ca as a proxy for seawater
temperature was not published until 1995 by Niirnberg [1995]. Since that study the
dependence of foraminiferal test Mg/Ca compositions on temperature has been
confirmed from core top samples [Dekens, et al., 2002; Elderfield and Ganssen, 2000;
Hastings, et al., 1998; Lea, et al., 2000; Rosenthal, et al., 1997; Rosenthal and Lohmann,
2002; Rosenthal, et al., 2000]" plankton net and sediments trap samples [Anand, et
al., 2003; McConnell and Thl<neli, 2005; McKenna and Prell, 2004], and in laboratory
cultures [Lea, et al., 1999; Mashiotta, et al., 1999; Niirnberg, et al., 1996]. An advantage
of MglCa thermometry over other paleo seawater palaeothermometers is that
Mg/Ca ratio of seawater is nearly constant throughout the ocean and, Mg and Ca
have long residence times (-1 Ma for Ca and 10 Ma for Mg) [Broecker and Peng,
1982; [Rosenthal, et al., 2000]. Moreover, the MglCa composition of planktonic
foraminifer tests is extraordinarily and consistently sensitive to seawater
Rationale and background
temperature, increasing by 9±1 % per
0c.
Calibration of test Mg/Ca composition changes with seawater temperature are
based on empirical relationships observed between Mg/Ca and seasurface
temperatures above core-top sediment sample sites, between Mg/Ca and
calculated 8180 calcification temperatures [Anand, et al., 2003; McKenna and Prell,
2004], or in laboratory culture experiments [Lea, et ai., 1999; Nurnberg, et al., 1996].
The best fit relationship typically has the exponential form
Mg/Ca =A exp (B*T) (1)
where A is pre-exponential constant, B the exponential temperature sensitivity
constant, and T is seawater temperature. The exponential increase is consistent
with thermodynamic control of Mg incorporation in:o calcite [Nu rn berg, et al.,
1996]. Experiments on inorganic calcite precipitation from seawater also indicate
that the distribution coefficient for Mg/Ca (K in equation 3) correlates positively
with temperature. [ Mucci and Morse, 1983; Mucci, 1987; Gomori et ai, 1987;
Nurnberg, et al., 1996]
CaC03 + Mg2+ <=> MgC03 + Ca2+ (2)
(3)
where X is the activity of the component phase in calcite solid solution or ion in
seawater.
Despite the growing evidence in support of the exponential temperature
dependence of the Mg/Ca composition of foraminiferal calcite, the accuracy and
reliability of Mg/Ca thermometry in reconstructing past ocean temperatures
remains debated [Barker, et al., 2005; Barker, et ai., 2003; Bentov and Erez, 2006;
Eggins, et al., 2004; Rosenthal, et al., 2004]. Questions surrounding the accuracy and
reliability of Mg/Ca thermometry can be grouped into three observation-based
4
themes:
Theme I: What is the "true" nature MglCa-temperature relationship?
a) The MglCa composition of planktonic foraminifer tests is almost
two orders of magnitude lower than in inorganic calcite precipitation
from seawater [Katz, 1973]. Furthermore, foraminiferal calcite and
inorganic calcite have significantly different absolute distribution
coefficient (K) values (see equation 3) [Mucci, 1987; Mucci and Morse,
1983; Nilrnberg, et al., 1996]. These differences indicate that Mg
incorporation into foraminifer test calcite is subject to additional
biological controlling factors (so called "vital effects"), which bias
foraminiferal calcite compositions away from inorganic compositions.
The cause of this bias remain unknown.
b) "Vital effects" have been shown to be species-specific. For example,
Dekens et al [2002.] showed that for the same temperature range in the
tropics, Globigerinoides ruber Mg/Ca values are on average 5-15% higher
than the Mg/Ca values of G. sacculifer, and 49-55% higher than
Neogloboquadrina dutertrei. Similarly, Anand et al [2003] have determined
different MglCa··thermometer calibration fits for a wide variety of
individual species of planktonic foraminifera. The origin of these
different species-specific calibrations has been attributed to differences
in intra-test MgJCa distribution but is highly speculative given the
limited knowledge of the Mg/Ca distribution within tests of different
species.
c) Different studies have reported Mg/Ca-seawater temperature
calibrations that vary greatly for the same species. For example,
calibrations for Globigerinoides ruber yielded pre-exponential factors
between 0.3 and 0.48, and exponential factors between 0.058 and 0.102
Rationale and background
(see Anand et ai., 2003) producing temperature estimates that differ by
up to 4-5
0c.
The reasons for these calibration discrepancies remainunresolved.
Theme II: What is the nature and origin of intra and inter-test variability in
MglCa composition within planktonic foraminifer species? And more
specifically, how does this variability affect the reconstruction of seawater
temperatures?
a) Planktonic foraminifera from core-top sediment samples show
increasing Mg/Ca compositions with increasing test size for many
species [Eider field, et aI., 2002]. This size effect corresponds to between
a 12-36% increase in Mg/Ca values or 1.5-4.5°C over the full range of
test sizes. It has been suggested that this size-dependence may reflect
an increase in calcification rate [Elder field, et al., 2002] or symbiont
activity with increasing test size [Hanisch and Hemming, 2004]. The
origin of the size effect remains unknown.
b) The occurrence of Mg/Ca compositional variation within and
between individual foraminiferal tests has been documented since the
earliest studies of foraminifer test chemistry [Emiliani, 1955; Krinsley,
1960]. Studies of the planktonic species Globorotalia truncatulinoides by
Lipps and Ribbe [1967] and Bender et aI. [1975] documented variations in
bulk test Mg contents up to 40% and 120%, respectively. In a detailed
investigation of individual G. truncatulinoides tests using the
electron-microprobe, Duckworth [1977] showed that different calcite layers
contained Mg/Ca values between 5 and 8 mmol/mol. More recently,
direct evidence of Mg/Ca variation within individual foraminifera tests
came from studies using electron microprobe and laser ablation
ICPMS, which document Mg/Ca variation within and between tests of
6
up to 120%, which corresponds to an apparent change in calcification
temperature of -15°C [Brown and Elderfield, 1996; Eggins, et al., 2003;
Eggins, et al., 2004,; Elderfield and Ganssen, 2000; Hathorne, et al., 2003; Jha
and Elderfield, 2000; Lohmann and Rosenthal, 1993; McKenna and Prell,
2004; Nurnberg, et al., 1996; Puechmaille, 1994]. The cause of this
intra-and inter-test variability in Mg/Ca composition, intra-and in particular the
role of termperature in its origin, presents a major uncertainty for the
application of Mg/Ca thermometry to seawater temperature
reconstructions.
Theme III: How do postdepositional processes, specifically seafloor
dissolution, affect test MglCa compositions and, consequently, the accuracy of
MglCa thermometry?
a) Numerous studies have documented the presence of detrital
minerals, organic: matter, and diagenetic-mineral overgrowths on fossil
tests. A variety of cleaning methods have been proposed to remove
these contaminants from foraminiferal calcite in efforts to analyse the
composition of the original foraminiferal calcite. However, these
cleaning methods involve the use of chemical reagents that can also
dissolve test calcite and modify its Mg/Ca composition. For example, a
number of studies have documented a significant and systematic
decrease (15 % or - 1.5°C) in bulk test Mg/Ca compositions after
applying a reductive cleaning process [Barker, et al., 2003; Rosenthal, et
al., 2004]. Currently. there is no agreement on what cleaning methods
should be used for test cleaning prior to Mg/Ca analysis.
b) A decrease in bulk test Mg/Ca composition with progressive test
dissolution has ':Jeen shown to occur in studies of fossil foraminifer
tests from deep-sea sediment samples [Brown and Elderfield, 1996;
Rntionale and background
Dekens, et al., 2002; Lohmann, 1995; Regenberg, et al., 2006; Rosenthal and
Lohmann, 2002; Savin and Douglas, 1973]. Partial dissolution of tests can
occur in seafloor sediments well above the calcite lysocline and has
been shown to produce an approximately 12% decrease in test Mg/Ca
composition per kilometre of water depth. This translates into a
temperature bias of about -1.3°C per kilometre depth [Benway, et al.,
2003; de Villiers, 2003; Dekens, et al., 2002; Rosenthal and Lohmann, 2002].
Despite evidence for the effect of seafloor dissolution on foraminiferal
test Mg/Ca values, its nature and extent remain controversial.
Aims and outline of tllis study
The aims of this study are to exploit recent advances in microanalysis techniques,
in particular laser ablation-inductively coupled plasma mass spectrometry
(LA-ICPMS), aided by electron microprobe analysis (EPMA), to:
o improve the understanding of the nature and cause of Mg/Ca variation
within and between individual tests of key species of planktonic
foraminifera used in palaeoceanography.
o assess the effects of postdepositional dissolution on the distribution of
Mg/Ca within foraminiferal test and specific test parts.
o evaluate the effect of intra and inter-test Mg/Ca variability on the
accuracy and precision of Mg/Ca thermometry.
o develop new microanalysis-based approaches to reconstructing
calcification temperatures of planktonic foraminifera.
These four aims are addressed in each of the four chapters of this thesis, brief
descriptions of which are outlined below.
Chapter 1
Characterization of Mg/Ca distributions in planktonic foraminiferal
species by electron microprobe mapping
This chapter reports the distribution of Mg/Ca ratio values within the tests of a
range of planktonic foraminifer species, including symbiotic and asymbiotic
species. A Cameca SX100 electron microprobe has been employed to map the
distribution of Mg and Ca within the test cross sections. Each of the investigated
species is found to display large variations in Mg/Ca composition within
individual tests. Symbiotic species display cyclic Mg/Ca compositional banding
that is characterized by narrow «1-3 jJ.m), high-Mg/Ca (typically 8-11 mmol/mol)
bands, intercalated between broader low Mg/Ca (typically 1-5 mmol/mol) bands.
Symbiont-free species tend to have fewer and broader compositional bands. These
may more closely reflect changes in calcification temperature and develop as
these species have migrated within the water column.
10
Chapter 2
Uncertainties in seawater thermometry deriving from intra- and inter-test
Mg/Ca variability in Glob~gerinoides ruber
Laser ablation ICPMS microanalysis is used to characterise variations of Mg/Ca
composition both within and between individual tests from fossil and
live-collected Globigerinoides ruber from the eastern Indian Ocean. The extent of
inter-test and intra-inter-test compositional variability is shown to exceed that attributable to
seasonal and interannual calcification temperature, at any particular sample site.
The pooled mean Mg/Ca composition obtained for core top samples between the
equator and -300
S correlate exponentially with mean annual sea surface
temperature (Mg/Ca mmol/mol 0.52 exp(0.076*SSTOC), r2 = 0.99). The inter-test
Mg/Ca variability within individual core-top samples is shown to be a source of
significant uncertainty in derived Mg/Ca seawater temperature estimates and is
notably site specific. It is demonstrated that widely-assumed uncertainties in
Mg/Ca thermometry are likely underestimated, and that statistical power analysis
in combination with individual test analyses can be used to evaluate the number
of tests needed to achieve a target uncertainty for any sample.
Aims and outline •••
Chapter 3
Effects of seafloor and laboratory dissolution on the Mg/Ca composition of
Globigerinoides sacculifer and Orbulina universa tests - a laser ablation
ICPMS microanalysis perspective
Laser Ablation ICPMS has been used to investigate the effect of seafloor and
artificial test dissolution on test Mg/Ca compositions. Three different approaches
are used: 1) the analysis of core-top samples collected at different water depths
along a depth transect in the NE Indian Ocean; 2) the analysis of test subjected to
laboratory dissolution experiments using weak acid; and 3) the analysis of tests
prior to their dissolution and subsequent analysis using a "Flow-through" ICPMS
(see more for method in Haley and Klinkhammer [2002]). No evidence is found for
significant selective dissolution of tests parts that have different Mg/Ca
compositions (specifically higher Mg/Ca values) in either G. sacculifer and O.
universa during natural seafloor dissolution above the lysocline or during
laboratory dissolution or Flow-Through ICPMS analysis. The integration of these
with previously published results for G. sacculifer, reveal little if any significant
seafloor dissolution affect on the Mg/Ca composition of this species above the
calcite lysocline.
Chapter 4
Seawater temperature reconstruction using l\1g/Ca microanalysis of
Globigerinoides ruber, G. sacculifer and Pulleniatilla obliquloculata.
This study is aimed at understanding the extent and possible nature of differences
in Mg/Ca composition between three foraminifer species (G. ruber G. sacculifer and
p. obliquiloculata), and to assess Mg/Ca-temperature calibrations based on
LA-ICPMS microanalysis. It is found that differences in intra-test Mg/Ca distributions
between species cannot account for the observed differences in species Mg/Ca
composition in subtropical to tropical regions. Based on best regression model fits
12
to depth-temperature profiles at each site, and also the calculated 8180
calcification temperature of C.
sacculifer
tests, differences in Mg/Ca thermometercalibrations for the different species are attributed to their different preferred
habitat depths. A comparison of Mg/Ca calibrations based on preferred depth
habitat temperatures indicate that all three species may share a similar or
common temperature sensitivity and pre-exponential constant that determines
their test Mg/Ca compositions. The potential for reconstructing upper water
column temperature profilE's using these species and for using the Mg/Ca content
