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Strontium isotopes: systematics and ecological applications

Overview

• What’s so special about strontium? Where does it come from and how is it different from other isotopes we’ve been covering? If there are four species why are we only looking at one?

• Brief systematics: How is it cycled in the earth. What standard is it compared to. How has this standard changed over geologic time. What factors affect concentrations and enrichment (wind and weathering). What are some baseline values for a range of ecosystems. How is it incorporated into the cute fuzzy things we care about? Mixing models.

• Application 1: Looking at the Ca flow through a temperate forest. By looking at the relative ratios of 87Sr/86Sr Capo et al. were able to see where most of the calcium being supplied to the forest is coming from and that acid rain probably won’t effect its supply.

• Application 2: On the trail of mammoths. Looking at teeth to see just how far these big guys ranged.

• Application 3: On to the fish. Here we look at the use of 87Sr/86Sr ratios in the otolith of Pacific salmon. Because of underlying geological differences stream waters where the fish are raised they have slightly different ratios allowing us to reconstruct freshwater migrations and assign them to a river if we catch them at sea.

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Strontium’s stable isotopes

Capo RC, Stewart BW, Chadwick OA (1998) Strontium isotopes as tracers of ecosystem processes: theory and methods. Geoderma 82: 197-225

87Rb half life is (87Ga) so the 87Sr that exists on Earth is what was there when the earth was created plus the daughter material from decay

Natural concentrations are 88Sr 82.53%, 87Sr 7.04%, 86Sr 9.87% and 84Sr 0.56%

90Sr is also present in the modern environment (a product of fission reactions) but has a relatively short half-life (≈ 30 years)

Brief systematics:

• When a system is created the amount of 88Sr, 86Sr and 84Sr is set, but the amount of 87Sr increases due to the decay of 87Rb

• Ratio of 87Sr/86Sr is used because of their similar relative abundances

• Ratio a result of geochemical origin and age

• Because of the long residence time of Sr in the ocean and mixing the value of sea water is quite stable 87Sr/86Sr of ≈ 0.70917

• This is the value used for comparison however it does change over geologic time

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Global seawater value over time

• Data from marine shells

(coccolithophore, foraminifera and

molluscs)

• Shift is thought to be from the increase

in weathering

• Associated with major tectonic events

(e.g. the uplifting of the Himalaya)

• Fluctuates between average values of

terrestrial weathering and hydrothermal

exchange with mid-ocean ridge basalts

For example

Bedrock

weathering

Precipitation

Dry deposition

δ87Sr

Geologic

Atmospheric

Ca/Sr

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Mixing

• Substrate contribution is

not necessarily a linear

relationship

• Many processes affect the

fractions of a sample that

come from different

sources

Examples of δ87Sr in soils and mixing

Varies by depth

based on parent

material

...and by other factors

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Incorporation by vegetation

• They take up what is available to them

• Sr/Ca can come from water, soil, and air

• No real biological fractionation (because of its size)

• Atmosphere can be dominant source of Sr and Ca (65-75%)

• Upper soil and plants tend to reflect atmospheric Sr/Ca

• Conifers seem to trap Sr-bearing aerosol whereas deciduous trees do not

In animals

• No well documented fractionation

• Incorporated into the teeth, bone and other “hard parts” of animals that include Ca atoms

• In terrestrial animals this mainly comes from diet whereas in aquatic animals it comes from their environment

• Accreting tissues are used to track feeding habits and migration

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Application 1: Ca budgets

• Blum JD et al (2002) Mycorrhizal weathering of apatite as an important calcium source for base-poor forest ecosystems. Nature 417: 729-731

• Calcium weathering of plagioclase rocks previous thought not to be at the same pace the Ca is being exported (due to acid rain)

• (i.e. the Ca coming out of the system is greater than the flux in through known sources--based on Na measurements)

• Authors used an analysis of Sr/Ca ratios to show that apatite represents a pool of Ca comparable to other know inputs

What is a soil horizon?

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Digests

• Main Ca bearing minerals

present in the soil are

plagioclase, K-feldspar,

hornblende, pyroxene and

apatite

• Different digests used to

determine their composition in

soil

• 12% apatite, 12% hornblende

and pyroxene, 76% feldspar

Mixing of sources

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Application 1: contributions

• Soil exchange pool dominated by a mix of Ca and Sr from silicate and atmospheric deposition

• Ca/Sr ratios indicate the amount of Ca coming from apatite

• 87Sr/86Sr nearly identical for all species

• However, based on this you can tell where most of the calcium in a species is coming from (e.g. the Spruce)

• An alternate hypothesis could be that the plants are selectively uptaking Ca or Sr, but there is little evidence to support this

•Trees appear to be accessing Ca and Sr from

apatite in the Bs2 horizon

•Studies in northern European coniferous forests

have suggested that ectomycorrhiza mycelia are

able to take up Ca and P directly from apatite

•Depth at which trees acquire minerals also

determines which pools they are accessing

•Root systems directly weathering apatite to get Ca

which means there is no Na reslease

•≈ 30% of Ca comes from the atmosphere,

35%from the silicate minerals, and 35% from

apatite

Application 1: bottom line

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Application 2: Tracking megafauna

•Hoppe KA, Koch PL, Carlson RW, Webb SD (1999) Tracking mammoths and mastodons: reconstruction of migratory behavior using strontium isotopes. Geology.

•Using 87Sr/86Sr ratios to look at migrations of two large extinct proboscideans

•Compared values from teeth to the values for different regions of the southeast US

•Results have interesting implications for theories about the life histories and cause of extinction for the species

Mammoth vs. Mastodon

• The American mastodon roamed North America for 3.75 million years, until its extinction 10,000 year ago. Reached a size of 2.5 to 3 m and weighed an estimated 4-6 tons. Mostly a browser. Very different teeth than a mammoth.

• Three mammoth species lived on North America until about the same time. Were taller and heavier than mastodons. Thought to be a grazer.

• Some hypothesize that they both migrated great distances and this may have contributed to their extinction

MammothMastodon

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87Sr/86Sr and bedrock geology

• In an herbivore, the 87Sr/86Sr values are an average of the plant material that is ingested

• The vegetation in turn reflects the 87Sr/86Sr of the bulk soil (again from bedrock, weathering and atmospheric deposition)

• Values were mapped using modern plant and water samples from relevant areas

• Enamel analyzed from 58 individuals from the 5 different locations

• Bulk samples from proboscideanscompared to sympatric species

Rodent sites

0.7143

0.7092

0.70870.7117

0.7090

0.7095

0.7085

Fossil sites

Mastodons have higher range, evidence for migration

Rodent sites

0.7143

0.7092

0.70870.7117

0.7090

0.7095

0.7085

Fossil sites

Mastodon

Mammoth

Modern Deer/Tapir

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Kind of like an mastodon otolith

• Variety of values indicates that the

animal was quite mobile

• However the random pattern in

samples points towards a nomadic

ranging rather than a seasonal

migration similar to modern

elephants

• At most they ranged ≈ 500 km if

they went to the Appalachians and

at least 120~300 km if they

followed river valleys

• Mammoths on the other hand

appear to have only ranged locally 2 yrs.

Application 3: Tracking Atlantic salmon

• Kennedy BP, Klaue A, Blum JD, Folt

CL, Nislow KH (2002) Reconstructing

the lives of fish using Sr isotopes in otoliths. Canadian Journal of

Fisheries and Aquatic Sciences 59:

925- 929

• Exploration of 87Sr/86Sr values for

Atlantic salmon

• Comparing these to values from fish

that have lived their whole lives in one

location

• Attempting to use isotopes instead of something like radio telemetry

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At least they got to spawn!

• Four adult salmon randomly chosen from a run of 154

• Otoliths removed implanted in resin and sanded down to a thin

wafer

• Rings indicate age

• Sampling 87Sr/86Sr from a given

ring tells you the isotopic

composition of the water that the

fish was living in

• 87Sr/86Sr in river water is fairly

stable because it is mostly from bedrock

Where’d did they go!?

• Three of the four salmon

appear to have similar

locations before smolting

• 87Sr/86Sr values indicate location

• All four fish adopted different strategies (in terms of

geography)

• All show a similar value for

their time at sea

87S

r/86S

r

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Other fishy studies of interest

• Barnett-Johnson, R., Ramos, F.C., Grimes, C.B., and R.B. MacFarlane. 2005. Validation of Sr isotopes in

otoliths by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS): opening avenues in fisheries science applications. Canadian Journal of

Fisheries and Aquatic Sciences 62:2425-2430.

• Walther and Thorrold (2006) Water, not

food, contributes the majority of strontium and barium deposited in the otoliths of a marine fish. MEPS

• People are trying to do this with marine

species, but seem to not be having success

Summary:

• Sr serves as a proxy for Ca in most organisms and

systems

• They provide a natural tag for a number of migratory

species as well as a way to differentiate influxes of Ca to a system

• Sr isotopes have a great potential for use in ecology

• However great care must be taken to know the values of

inputs into your “box” and proper mixing models must be used

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West, J. Bowen, G., Dawson, T., and K. Tu (eds.) Isoscapes: Understanding movement patterns

and processes on earth through isotope mapping. Springer Press, Berlin, Germany.