Introduction to the principles of 18O.pdf

Introduction to the Principles of δ18
O, δ13
C,
and 87
Sr/86
Sr in the Palaeosciences.
Jack Thomas Rhodes Wilkin
Camborne School of Mines. College of Engineering, Mathematics and Physical Sciences.
University of Exeter. Penryn Campus. Penryn. Cornwall. TR10 9EZ
Keywords: δ18
O, δ13
C, 87
Sr/86
Sr, Palaeontology, Geochemistry.
This article has been submitted to the journal Frontiers in Earth Science
The stable isotopes of oxygen (O), carbon (C), strontium (Sr), hydrogen (H), and nitrogen (N)
have all been utilised for great effect in palaeoclimate, palaeoecological and palaeobiological
studies. Of these, O and C have been by far the most important and, in many types of study,
their use has become routine in universities and research institutions around the world. Stable
isotopes provide quantitative data about palaeotemperatures, metabolic rates, food webs,
palaeosalinity, palaeoprecipitation and evaporation rates as well as glacial ice volumes,
production and burial of organic carbon, and other processes related to
palaeoclimatic/biological/ecological change. Except for Sr, all the previously mentioned
isotopes (O, C, H, and N) directly record paleoclimatic, biological and palaeoecological
processes. Conversely, Sr reflects the composition of rocks at the Earth's surface, and its values
reflect on the climate indirectly as it is a proxy for global weathering rates and seafloor
spreading. This review will only be focusing on three isotopes commonly deployed by palaeo-
researchers: carbon, oxygen, and strontium.

Introduction
The first evidence for multiple isotopes for stable elements was found by Sir Joseph John
Thomson (Thomson, 1910, 1912). The term isotope was first coined by English radiochemist
Fredrick Soddy (1913) to distinguish chemical elements with different atomic masses, different
numbers of neutrons, but occupy the same position of the periodic table, same numbers of
protons and electrons. The term isotope is derived from iso meaning equal and the Greek topos
meaning place in reference to the same placement in the periodic table (Ellam, 2016).
Concentrations of isotopic species are represented by the relative abundance ratios of the heavy
to the light isotopes in each sample to the ratio of a given standard. The values of the 𝛿 can be
either positive, showing an enrichment of the heavier isotopes relative to the standard, or
negative, showing depletion of the heavier isotopes relative to the standard (Ellam, 2016).
There are two main types of isotope: radioactive isotopes and stable isotopes Radioactive
isotopes, also called radionuclides, are species of the same chemical element that contains
unstable nuclei and dissipate excess energy by spontaneity emitting radiation in the form of
alpha (α) beta (β) and gamma (γ) rays. Unlike stable isotopes, radioactive isotopes have
different isotopic mass by having differing numbers of electrons in their electron shells rather
than a different number of neutrons in the nucleus. Thus, this extra electron will give the isotope
a negative change, when the electron is released the isotope will drop down an energy level
becoming a different atom in a process called radioactive decay (Faure and Mensing, 2005;
Ellam, 2016). This phenomenon, whereby one radioactive substance transforms from one
element to another, was first observed by Rutherford and Soddy (1902).
In contrast stable isotopes as species of the same element that have differing numbers of
neutron and do not decay over time. Stable isotopes are of interest to palaeoclimatologists as
their preserve the original geochemical signatures unless acted upon by external forces such as
diagenetic alternation, from the past (Faure and Mensing, 2005; Ellam, 2016). There are several
stable isotopes that are commonly used by researchers to reconstruct past conditions such as
strontium which can be used in reconstructing sea-level change (Jiang et al., 2001; van der
Meer et al., 2017) and variations Mid-ocean ridge active (Jones et al., 1994) while boron which
is an ocean pH proxy in foraminifera (Foster and Rae, 2016).

Isotopic palaeontology is a sub-discipline of palaeontology that uses the geochemical
composition of fossil remains of organisms to make inferences about the physical surroundings
of growth of the organisms (especially temperature), diets and palaeoclimates (Wefer and
Berger, 1991; Hudson and Marshall, 1997). Most research in this field revolves around marine
organisms -especially unicellular foraminifera from the K-Pg extinction onwards (e.g. Zachos
et al., 2001; Austin and James, 2008) and belemnites for most of the Mesozoic ( e.g. Sælen et
al., 1996; Prince and Sellwood, 1997; Wierzbowski and Joachimski, 2007, 2009; Wilmsen and
Niebuhr, 2017) – to work out past ocean chemistry and, therefore, ancient climates.
δ18
O
Oxygen (O), by mass, is Earth's most abundant element, forming the most common
components of the hydrosphere, biosphere and lithosphere (Faure and Mensing, 2005; Pederzani
and Britton, 2018). Oxygen naturally occurs, with decreasing abundance, as one of three stable
isotopes: 16
O (99.755%), 18
O (0.206%) and 17
O (0.039%) (Schoeller, 1999; Pederzani and
Britton, 2018)
Oxygen-isotopes has a long history in the palaeosciences since Urey (1947) first suggested that
𝛿18
O from fossil calcite could be used to estimate past temperatures. Temperature is the most
primary representation of the state of the climate, and the temperature of the oceans is critical
as the oceans are the single most crucial component of the Earth's climate system (Bigg, 2003).
Isotopes and trace elements can be used to estimate ancient temperatures. Isotopic ratios can
be taken from ice cores (e.g. Robin, 1977; Siegenthaler et al., 1988; Hendricks et al., 2000),
sediment samples (e.g. Jenkyns and Clayton, 1986; Wilmsen and Niebuhr, 2017) and fossil shells
and bones. Oxygen isotope analyses of marine carbonate fossils could substantiate the
modelled palaeoclimate evolution by providing information on oceanic palaeotemperatures.
Elements used for calculating palaeotemperatures are called palaeothermometers.
The following equation is used to calculate the 𝛿-value of oxygen:
δ18
O =
(
(
O
.
18
O
.
16 )
𝑆𝑎𝑚𝑝𝑙𝑒
(
O
.
18
O
.
16 )
𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑
− 1
)
∗ 1000
The 𝛿18
O and 𝛿13
C values have concentration units of per thousand, or 'per mil' (‰) relative to
the standard. For example, a 𝛿18
O value of 1.0‰ means that the sample has an 18
O/16
O ratio

that is 0.1% greater than the standard, or a 𝛿13
C value of -25% means that the sample has a
13
C/12
C ratio that is 2.5% lower than that of the standard (Ravelo and Hillaire-Marcel, 2007).
Many marine organisms are thought to have secreted their calcite shells in isotopic equilibrium
with seawater; consequently, their 𝛿18
O values are presumed to be a reliable
palaeothermometer (e.g. Sælen et al., 1996; Prince and Sellwood, 1997; Wierzbowski and
Joachimski, 2007, 2009). Temperature changes related to migrations and seasonal variations
can also be detected using oxygen isotopes (Rexfort and Mutterlose, 2006, 2009), hence the
importance of acquiring multiple samples from the same specimen. The first use of oxygen as
a paleothermometer was conducted by Urey et al. (1951) based on belemnites from the United
States, United Kingdom and Denmark.
Organisms with hard parts made of calcium carbonate (CaCO3) and phosphates combine
dissolved inorganic carbon in the form of bicarbonate (HCO-
3) from the water to make their
shells, skeletons, and tests in a process called marine biogenic calcification using the following
equation (Zondervan et al., 2001):
Ca2+
(aq)+2HCO-
3(aq) → CaCO3(s) + CO2+H2O.
To calculate palaeotemperatures from 𝛿18
O values, several different equations can be used:
• Anderson and Arthur (1983): T (°C) =16.0-4.14 (𝛿c- 𝛿w) +0.13 (𝛿c- 𝛿w)2
• Epstein and Mayeda (1953): T (°C) =16.5-4.3 (𝛿c- 𝛿w) +0.14 (𝛿c- 𝛿w)2
• Sharpe (2007): T (°C) =15.75-4.3 (𝛿c- 𝛿w) +0.14 (𝛿c- 𝛿w)2
Following advice from Dr Clemens Ullman of the Camborne School and Mines (personal
communication, February 25, 2019) the Epstein equation was not used in the final
palaeotemperature reconstructions for the Bathonian-Callovian Boundary as the formula was
incorrect due to lack of understanding regarding how water and carbonates fractionate during
mass spectrometry in the 1950s thus Epstein and Mayeda (1953) equation are seldom used, and
authors more commonly employ Anderson and Arthur (1983) or other more recent equations.
A δ18
O of seawater of −1‰ SMOW is generally used during greenhouse conditions, to account
for the absence of ice-sheets (Shackleton and Kennett, 1975). Therefore, when calculating
palaeotemperatures for much of the geological record, 1 needs to be added on for δ18
O values
to account for the negative SMOW.

The use of oxygen isotopes as a method to provide a geological thermometer has limitations
that must be considered. Firstly, the isotopic results, although calculated in degrees Celsius,
should be thought of as "isotopic temperatures" rather than "palaeotemperatures" (Stevens and
Clayton, 1971) providing relative temperatures rather than absolute results. Individual
temperatures should, therefore, not be interpreted too literally. Lowenstam and Epstein (1954,
p. 209) commented: "Our emphasis . . . will be on determining trends in temperature, both
geographical and chronological, rather than the determination of absolutely precise
temperatures."
A number of processes must be considered before interpreting oxygen-isotope data for
palaeoclimatic changes (Parrish, 1998):
• Fractionation within the atmosphere and during crystallisation
• Changes in the original isotopic composition of the water
• Biological fractionation
• Diagenesis
Fractionation of oxygen-isotopes occurs within the atmosphere when water is evaporated,
transported and finally precipitated. Fractionation happens on both local and global scales (see
Yurtsever, 1975).
Because light water (H2
16
O) preferentially evaporates, saline waters tend to have higher δ18
O
values. Elevated salinities have the same effect on isotope signatures as temperatures, that is
lower δ18
O values. Because saline waters can be warm, produced by evaporation at high
temperature, the effects of temperature and salinity on isotopic composition work against each
other. Thus, determining the relative effects of salinity and temperature can be difficult (see
Tourtelot and Rye, 1969). It should be noted, however, that many researchers ignore salinity
as a variable as its effects are relatively small (Parrish, 1998).
Isotopic measurements taken from ice cores measure the ƌD and ƌ18
O ratios. Isotopes
recovered from ice caps have lighter than ƌD and ƌ18
O ratios ocean water due to path
fractionation. Fractionation occurs when heavier isotopes are "rained out" as air pockets move
over land, condenses, and the water is lost during precipitation with heavier isotopes being
depleted at a faster rate than lighter ones so by the time precipitate over high latitudes there is
a disproportionately high ratio of O16
(Gat, 1996; Werner et al., 2018; Fig 1).

Figure 1: path fractionation of oxygen isotopes. Taken from Wilkin (2020)
In the high latitude, surface waters tend to have lower δ18
O values than deeper waters. These
low values are due to marine waters becoming contaminated with meltwater from ice fields,
thus returning the isotopically lighter 16
O to the oceans, while the marine waters of high salinity
were richest in 18
O. The observed relation between 18
O content and salinity of the oceanic
waters can be explained by process of multiple-stage distillation which produces a continuous
loss of freshwater to the ice regions from the surface waters of the warm oceans. The lower
salinities of cold ocean currents, such as the Alaskan and Californian currents, are due primarily
to mixing with meltwater from cold regions. The effect of glaciation upon the isotopic method
of measuring paleotemperatures is discussed by Epstein and Mayeda (1953) with a more recent
discussion provided by
Diagenesis is the term that encompasses all the chemical changes that occur to a fossil after
burial. Diagenetic processes include dissolution, recrystallisation and replacement by other
minerals: all these processes can affect the isotopic ratios in a given substance (e.g. Sælen,
1989; Blanchet et al., 2012; Ullmann and Korte, 2015). An understanding of diagenetic
processes is fundamentally crucial because meaningful data can only be collected from
unaltered material – that is containing the original atoms in the bone phosphate that was there
when the animal was still alive.
Diagenesis tends to homogenise seasonal signals, as it resets the original isotope values to those
of the surrounding sediment and pore fluids, and therefore alter and in extreme cases can

completely erase any geochemical data (Ullmann and Korte, 2015). Therefore, researchers
must assess the degree of alteration before continuing a geochemical analysis to ensure the data
they collect is reliable. The following list is adapted from Owocki et al. (2019).
• Comparison of the geochemical and isotopic composition of skeletal tissues
with various levels of preservation potential, e.g. enamel, dentine and bone.
• Analysis of the preservation of expected isotopic differences between related
taxa with known ecological or physiological differences and of seasonal cycles.
• Analysis of crystalline microstructure and composition as prerequisites to the
preservation of original isotope values as recrystallisation deforms crystals in
the fossils and alters their chemistry.
• Precipitation of secondary minerals through water-fossil interactions can result
in elemental enrichment of Fe and Mn, as these replace Ca. Therefore,
researchers apply "diagenetic cut-off points" to samples with specimens with a
Fe/Ca or Mn/Ca ratio over a given amount being omitted from further study (see
Ullmann and Korte, 2015 for a review of diagenetic screening techniques in
belemnites).
If the 𝛿18
O of the water and a bone is known, it is possible to calculate the body temperature of
extinct vertebrates using the following equation (Showers et al., 2002):
T°C=111.4-4.3(𝛿18
Ophosphate- 𝛿18
Owater)*1000.
Oxygen isotopes taken from dinosaur bones show little variation, implying a near-constant
body temperature. For example, Amiot et al. (2006) analysed Cretaceous dinosaurs from high
latitudes of different taxonomic groups. They found that dinosaurs maintained a constant body
temperature of 36-38°C, a range consistent with living endotherms, regardless of ambient
temperature. This conclusion implies that high metabolic rates were commonplace amongst
Cretaceous dinosaurs. The study tested the 𝛿18
Owater and 𝛿18
Ophosphate of sauropods, theropods,
ornithopods and ceratopsians. Such a similar thermophysiology and endothermy was a
common trait, synapomorphy, shared by all dinosaurs.

δ13
C
Carbon (C) is the sixth most abundant element in the solar system (Anders and Ebihara, 1982)
and plays an important role in the geochemical processes that occurs near the Earth's surface,
the biosphere, atmosphere, hydrosphere and in the lithosphere (in sedimentary, metamorphic
and igneous rocks) (Faure and Mensing, 2005).
The isotopic composition of C is expressed by the δ13
C values, defined as:
δ13
C =
(
(
C
.
13
C
.
12 )
𝑆𝑎𝑚𝑝𝑙𝑒
(
C
.
13
C
.
12 )
𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑
− 1
)
∗ 1000
where the standard is the PDB (Pee-Dee belemnite). When the δ13
C values are positive, the C
is enriched with 13
C relative to the standard, while negative δ13
C values imply depletion in 13
C
relative to the standard. The δ13
C values in marine carbonates are close to zero because the
P.B.D. standard itself is a marine carbonate (Catling and Kasting, 2017).
The isotopic geochemistry of C has been summarised by several authors (e.g. Farmer and
Baxer, 1976; Deines, 1980; Hazen, 2018) and by Faure and Mensing (2005, pp.753-802).
The fractionation factor for C isotopes is defined as:
𝛼
𝑎
𝑏
(𝐶13) =
𝑅𝑎
𝑅𝑏
Where a and b are C-bearing compounds in isotopic equilibrium with a certain temperature
and R is the isotopic 13
C/12
C ratio. The isotopic composition of C is biogenic carbonates and
organics, however, are an unreliable palaeotemperature proxy as these kinds of compounds do
not achieve isotopic equilibrium with major C reservoirs. In organic matter, it is because plants
discriminate against 13
C during photosynthesis in favour of the isotopically lighter 12
C and this
negative δ13
C bias is passed along to herbivorous and carnivorous animals (O'Leary, 1988). It
is possible, alongside δ18
O and δ15
N, to reconstruct food webs and diets on animals (e.g. DeNiro
and Epstein, 1978; Post, 2002; Hopkins and Ferguson, 2012; Parng et al., 2014) and humans
(e.g. Hakenbeck et al., 2010; Webb et al., 2013; Lamb et al., 2014) using stable isotope ratios.
One recently published example that shows the use of carbon-isotopes in establishing predator-
prey relationships is Owocki et al. (2019). The authors analysed oxygen and carbon isotopes

from tooth enamel of Tarbosaurus bataar, a close relative of Tyrannosaurus from Mongolia,
and a sauropod (Nemegtosaurus) and hadrosaurs (Barsboldia and Saurolophus) from the
Nemegt Formation (Maastrichtian, Upper Cretaceous). The 𝛿13
C values of herbivorous
dinosaur tooth enamel suggested a closed canopy forest of Araucaria, which is supported by
petrified wood found in multiple localities throughout the Nemegt Formation. By comparing
the 𝛿13
C signals of the different dinosaur fauna, Owocki et al. (2019) concluded that
Tarbosaurus was the apex predator in its organism. The isotopes show that the tarbosaurs were
feeding on giant sauropods though it is possible, however, that were scavenging sauropod
carcasses (see Holtz, 2008 for an in-depth review of the tyrannosaur scavenger hypothesis).
The method employed by Owocki et al. (2019) works because different plant groups have
different δ13
C values due to the way they take in carbon during photosynthesis – a process
called fractionation. As carbon moves through food webs, it slowly becomes enriched
(therefore the δ13
C values increases), so the values need to be adjusted due to a process called
trophic fractionation. A study by Wings et al. (2014) compared 𝛿13
C from theropod and
sauropod enamel from the Jurassic Qigu and Shishugou formations of western China and found
that theropod δ13
C was ~1.7‰ lower than the contemporaneous herbivores. Using Wings et al.
(2014) results, Owocki et al. (2019) then adjusted their tarbosaur results accordingly. The
researchers found that the carbon-isotope values overlapped both hadrosaur and sauropod δ13
C
values providing clear evidence of a predator-prey relationship and carbon transfer (Fig 2). The
results in both studies are consistent with what has been found in modern mammalian predators
who have about 1.3-2.6‰ lower δ13
C values in their enamel than their prey (Bocherens et al.,
1995).
Isotopic measurements can also be taken from the teeth of grazing mammals (e.g. Cerling et
al., 1997; Clementz, 2012). Those eating modern grasses have a C4 signal and those eating
more primitive grasses have a C3 signal. C4 grasses are adapted to arid environments whilst C3
grasses indicate cold conditions. There was a major climate shift around 5 to 7 million years
ago when the main grass type changed from predominantly C3 to C4. This shift is linked to the
cooling and drying of the climate in the Late Miocene and the start of the Ice Age. The reasons
for this change remain unclear but is thought to be related to changes in atmospheric CO2
concentrations (Wallace, 2004).

Figure 2: δ13C values from tooth enamel of various dinosaurs analysed by Owocki et al.
(2019). To account for trophic fractionation, the Tarbosaurus data was adjusted by ~1.7‰
following Wings et al. (2014). Redrawn from Owocki et al. (2019).
When an increase in nutrients stimulates biomass production in the surface of the water column,
the difference in δ13
C values between pelagic and benthic species increases. The loss of C from
the surface of the water is compensated by the addition of CO2 from the atmosphere
(Shackleton et al., 1983) and thus decreasing atmospheric CO2 concentrations which can result
is the cooling of the global average temperature. Such a scenario leads to a positive feedback
loop in the global climate system as a decrease in water surface temperature increases the
solubility of CO2 in the oceans which causes additional loss of CO2 in the atmosphere further
decreasing global temperatures.
Throughout geological time, there have been numerous negative carbon isotope excursions
(nCIEs) which are interpreted as episodes of massive carbon release. The study of such events
is crucial as it helps connect past carbon cycling and climate variability. The study of nCIEs is
important as it parallels current global warming trends. As of 2019, anthropogenic fossil fuel
emissions have already caused the isotopic composition of atmospheric CO2 (δ13
CCO2) has
already declined by ~1.5 ‰ (Vervoort et al., 2019) in the Suess Effect first described by Suess
(1955). According to Norris et al. (2013), current anthropogenic warming will be recorded as
a nCIE in future sedimentological records. Multiple nCIEs in the geological record may be
associated with coeval increases in average global temperatures, ocean acidification and even

extinction events (e.g. Kaiho et al.,2009; Erba et al., 2010; Stanley, 2010; Bartlett et al., 2018;
Elling et al., 2019) and these effects are being observed in the early twenty-first century (e.g.
Thomas et al., 2004; Hansen et al., 2006; Pandolfi et al., 2011).
One widely studied example of such an event is the Toarcian Ocean Anoxic Event, a major
ocean redox perturbation (Jenkyns, 1988) that occurred during the Early Jurassic epoch, c. 183
million years ago (McArthur et al., 2008). This event is characterised by wide-spread
deposition of organic-rich marine shales as well as a sudden perturbation in the global carbon
cycle (Hesselbo et al., 2007). This event is associated with a negative carbon excursion which
implies large scale inputs of carbon into the oceans which were triggered by the release of
methane gas hydrates (Hesselbo et al., 2000; Beerling et al., 2002), thermally metamorphised
sedimentary organic matter, as well as global warming caused by an increase in atmospheric
CO2, increased continental chemical weathering which may have been up to 530% higher than
the pre-event baseline (Them II et al., 2017).
The increased which then increased sediment supply due to higher weathering rates (Cohen et
al., 2004; Hesselbo et al., 2007) increased productivity resulting in shallow-water anoxia
creating a negative feedback loop on atmospheric pCO2 levels (Them II et al., 2017). The
upsurge in marine primary productivity (Them II et al., 2017; Xu et al., 2018) led to the
widespread deposition of organic-rich sediments reaching up to 20% in some localities
(Jenkyns, 1988; Schouten et al., 2000; Bailey et al., 2003) and left a pronounced negative
carbon isotope shift in the geological record (e.g. Jenkyns and Clayton, 1986; Schouten et al.,
2000; Hesselbo et al., 2000; Hesselbo et al., 2007; McArthur et al., 2008). The increase in
atmospheric CO2 led to a rise to seawater temperatures (Bailey et al., 2003). These changes in
the climate resulted in a mass extinction event (Harries and Little, 1999; Wignall et al., 2005;
Caruthers et al., 2013). The Toarcian turnover was a seventh largest extinction event is Earth's
history (Owens and Them II, 2018).
The isotopic composition of C in carbonates depends on several factors as outlined by Faure
and Mensing (2005, pp.764-765):
1) isotope fractionation between CO2 gas in the atmosphere and biogenic/abiogenic
CaCO3.
2) Isotope fractionation among the aqueous carbonate species (CO2(aq), H2CO3, HCO3,
and CO3
2-
) and pH-dependent species.

3) Temperature of the water at the time of secretion.
4) The mineral composition of the CaCO3 – calcite or aragonite.
5) Introduction of non-atmospheric CO2 formed by oxidisation of organic matter.
6) Vital effects related to metabolic processes of organisms.
7) Alteration of CaCo3 during diagenesis (and in extreme example metamorphism) and
the deposition of secondary CaCO3 in fractures/cavities from groundwater/subsurface
brines.
The potential of diagenesis to alter primary δ13
C values must be evaluated on a case by case
basis (Marshall, 1992). For examples, in samples connected from outcrops (as all the material
in this study were), the isotopically lighter 12C may have been introduced by meteoric waters
thus producing more negative δ13
C values. Meteoric water diagenesis generally also results in
lowered δ18
O values, therefore a negative correlation between δ13
C and δ18
O values is expected
(Banner and Hanson, 1990; Knauth and Kennedy, 2009). One example of meteoric diagenesis
in the geological record is reported by Fujioka et al (2019) who analysed δ13
C and δ18
O values
from Pliocene brachiopod shells and assessed the degree of diagenetic alteration using minor
element (Mn, Fe and Sr) concentrations, cathodoluminescence and shell microstructure.
The depth within the water column would also have affected the δ13
C values of biogenic calcite.
Shackleton and Kennet (1975) demonstrated that pelagic foraminifera are enriched in δ13
C
relative to benthic foraminifera due to the preferential removal of 12
C from the surface layer of
the oceans by photosynthetic organisms. The subsequent decay of organics at the ocean floor
enriches deeper waters in 12
C and thus causes benthic species to be depleted in 13C compared
to pelagic forms. Such a phenomenon is observed between belemnites, which are often
interpreted to have been pelagic (e.g. Jeletzky, 1966; Donovan, 1977; Arkhipkin et al., 2012),
and benthic bivalves. Several studies (e.g. Podlaha et al., 1998) have shown δ13
C depletion in
bivalves and enrichment in belemnites, a result consistent with Shackleton and Kennet's (1975)
foraminifera.
Differences in δ13
C can also be observed between juveniles and mature specimens of
foraminifera. Such discrepancies depends on (i) distinct "vital effects", (ii) changes of habitat
during the life cycle and seasonal shifts in environmental conditions, (iii) the deposition of
secondary calcite in large sinking shells, and (iv) differential dissolution of shells depending
on the thickness of their walls during early diagenetic processes (e.g. Lohmann, 2006).

87
Sr/86
Sr
Strontium (element number 38 in the periodic table) is a trace element commonly found in
many minerals usually with average concentrations in the Earth's crust averaging 300 µg/g. Sr
is the 15th most abundant element on Earth, making up 0.034% of the crust (Höllriegl &
München, 2011) and is much rarer in the Earth's mantle having a concentration of
approximately 30 µg/g (Jagoutz et al., 1979). Because it is chemical behaviour is very similar
to that of calcite, having doubly changed positive ions in most natural substances, it readily
substitutes for calcium in minerals (Höllriegl & München, 2011). Sr is usually present as an
impurity, replacing Ca, with elevated concentration is mineral such as plagioclase and –
importantly for this work – calcite. Though it is worth noting that strontium can form its own
minerals for example strontianite (SrCO3) or celestine (SrSO4).
Strontium has four stable isotopes are found in nature –
𝑆𝑟;
38
46
𝑆𝑟;
38
48
𝑆𝑟 and 𝑆𝑟
38
50
first discoved by Nier (1938) –
38
49
and 16 radioactive isotopes have
been produced artificially. Most of the radioactive isotopes of strontium are short-lived and of
little interest. The exception is strontium-90 (half-life 29.1 years), a beta-emitter that is
produced as a fission fragment.
In addition to being a stable isotope, 87
Sr is also the radiogenic daughter isotope produced by
the β-decay from the radioactive alkali metal 87
Rb. The half-life of 87
Rb is 4.967(32) x 10(10)y.
(i.e. more than three times longer than the current age of the universe; Kossert, 2003). Thus,
there are two sources of 87
Sr in any material: primordial, formed during nucleosynthesis in stars
(Watson et al., 2019) - along with 84
Sr, 86
Sr and 88
Sr; and that formed by radioactive decay of
87
Rb (Davis et al., 1977; O'Nions, 1977; Kossert, 2003).
Strontium is released from the Earth's crust into the atmosphere as a result of natural processes
such as entrainment of dust particles, resuspension of soil by wind, and sea spray. In addition,
human activities, including milling and processing of strontium compounds, burning of coal,
land application of phosphate fertilisers, and the usage of pyrotechnic devices, may cause
strontium to be released into the atmosphere (Höllriegl & München, 2011).
Strontium-isotopes are not directly indicative of climate but are rather reveal changes in process
that could either cause or be the result of climate change. Strontium is commonly regarded as
a proxy for the rate of chemical weathering of the continents and rate of seafloor spreading
(Blum and Erel, 1995; Clow et al, 1997). Sr is well mixed in the oceans, so can provide a global
signature of seawater chemistry (e.g. Stille, 1992; Chakrabarti et al., 1996). The Sr isotope ratio

in seawater changes gradually over time, often showing no detectable variation over large time
scales (Depaolo and Ingram, 1985; Clemens et al., 1993).
One of the key variables determining which processes (at which temporal resolution) can be
traced by an elemental proxy like Sr in marine sediments is "residence time". Residence time
is defined as the average time that a particle resides in a reservoir (Bolin and Henning, 1973).
Sr in seawater is controlled by riverine discharge (Palmer and Edmond, 1992) or Sr flux from
the upper mantle at Mid Ocean Ridges/hydrothermal vents (), and diagenesis and dissolution
of deep-sea carbonates (Parrish 1998, pp. 25):
N dRSW/dt = Jr (Rr – RSW) + Jh (Rh – RSW) + Jc(Rc – RSW).
In the above equation, N is the number of moles of Sr in the ocean; Jr, Jh, Jc are influxes of
riverine hydrothermal and carbonate (either diagenesis or dissolution) Sr respectively; and Rr,
Rh, Rc, R.S.W. (Parrish 1998)
Assuming – and this may be a big if – that an element cycle is in a stable state and therefore
the amount being introduced into the reservoir equals the amount leaving the reservoir is equal
at any time, this residence time can be calculated easily via the following equations:
𝑟𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 𝑡𝑖𝑚𝑒 (𝑡𝑖𝑚𝑒) =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑟𝑒𝑠𝑒𝑟𝑣𝑜𝑖𝑟 (𝑚𝑎𝑠𝑠)
𝑖𝑛𝑓𝑙𝑢𝑥 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 (𝑚𝑎𝑠𝑠 𝑝𝑒𝑟 𝑡𝑖𝑚𝑒)
𝑟𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 𝑡𝑖𝑚𝑒 (𝑡𝑖𝑚𝑒) =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 𝑖𝑛 𝑟𝑒𝑠𝑒𝑟𝑣𝑜𝑖𝑟 (𝑚𝑎𝑠𝑠)
𝑜𝑢𝑡𝑓𝑙𝑢𝑥 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 (𝑚𝑎𝑠𝑠 𝑝𝑒𝑟 𝑡𝑖𝑚𝑒)
For strontium, one can calculate the residence time using equation 1 and employing an influx
estimate of 4.04 * 1012
g per year (Pearce et al., 2015) and an oceanic reservoir size of 1.02 *
1019
g (de Viliers, 1999):
𝑟𝑒𝑠𝑖𝑑𝑒𝑛𝑐𝑒 𝑡𝑖𝑚𝑒𝑆𝑟 =
1.01 ∗ 1019
𝑔 𝑆𝑟
4.04 ∗ 1012 𝑔
𝑎
𝑆𝑟
= 2.52 ∗ 106
𝑎
It follows that the average strontium ion remains dissolved in seawater for c. 2.5 million years
before being precipitated in a carbonate mineral.

The observation that modern carbonate shells have the homologous 87
Sr/86
Sr ratios across the
globe (see Brand et al., 2003 for a modern example involving brachiopod data), makes
strontium incredibly useful for global stratigraphic correlations as one can use the 87
Sr/86
Sr of
marine carbonate, so long as the ratios have not been severally altered by diagenetic processes,
as global stratigraphic marker.
Furthermore, the 87
Sr/86
Sr ratio of seawater changes slowly over geologic time spans as a
consequence of global tectonic and climatic processes that caused significant changes in the
balance of Sr fluxes from the continents and Mid Ocean Ridges. These changes are typically
slow, but highly precise measurements allow for detecting small differences in the 87
Sr/86
Sr
ratios of marine fossils through time represented by sedimentary sequences. Typically, one can
hope for a time resolution of a few hundred thousand to a few million years. 87
Sr/86
Sr
stratigraphy is therefore not highly precise but can be of enormous help when other – higher
precision – tools for stratigraphic correlation do not work (Depaolo and Ingram, 1985).
It is worth stating, however, that 87
Sr/86
Sr is not an absolute dating technique but instead relies
on a detailed knowledge about the strontium-isotope ratio of seawater at any given point in
time (Peucker-Ehrenbrink and Fiske, 2019). To obtain such look-up chart that lists 87
Sr/86
Sr
ratios throughout geological time, strontium-isotope measures have to be conducted for a large
sample base of well-preserved fossils of known age. One such data set is the LOWESS 5 Fit,
a collection of strontium isotope measurements dated every 0.05 million years (McArthur et
al., 2012).
Conclusion
The purpose of this review was to provide a brief introduction to the application of stable
isotopes in geochemistry. The stable isotopes of carbon, oxygen, and strontium have played an
especially important role in furthering our understanding of past climates, biology, diets and
stratigraphy.
• Some organisms make their bones, teeth, tests and shells in isotopic equilibrium with
their environment.
• Oxygen is an important palaeothermometer used to reconstruct past ocean
temperatures.
• Oxygen isotopes have been used, for example, to reconstruct dinosaur body
temperatures and some that they were endothermic.

• Carbon isotopes are primarily used as a proxy for changes in the global carbon cycles
and have been instrumental in identifying Ocean Anoxic Events (e.g. the T-OAE)
• Carbon isotopes can be used to establish food webs.
• Strontium is a proxy for mid-ocean ridge/hydrothermal vent activity and can be used to
date strata.
• Strontium is homologous in the oceans and 87
Sr/86
Sr ratios vary slowly over time,
making them ideal for dating and correlating strata.
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