Because of underlying geological differences stream waters where the fish are ....
strontium isotopes. Geology. •Using 87Sr/86Sr ratios to look at migrations of.
and ecological Strontium isotopes: systematics 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 Atmospheric Geologic
Precipitation
Bedrock weathering Dry deposition
δ87Sr
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 Srbearing 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 basepoor 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
Application 1: bottom line •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
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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.
Mastodon Mammoth
• Some hypothesize that they both migrated great distances and this may have contributed to their extinction
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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
0.7143
Rodent sites 0.7117
0.7092
0.7087
0.7090
0.7095
0.7085
• 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
Fossil sites • Enamel analyzed from 58 individuals from the 5 different locations • Bulk samples from proboscideans compared to sympatric species
Mastodons have higher range, evidence for migration Mastodon 0.7143
Rodent sites 0.7117
0.7090
Mammoth Modern Deer/Tapir 0.7092
0.7087
0.7095
0.7085
Fossil sites
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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
2 yrs.
• Mammoths on the other hand appear to have only ranged locally
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
• 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
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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.
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