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Ca Isotopes
Cheryl Zurbrick1/29/2010
Background
Background
DePaolo Reviews in Mineral Geology (2004)
40Ca produced by β-decay of 40KMost Ca from primordial earth
BackgroundMethodology
Pros Cons
TIMS (single collector)
•Reproducible precision between runs•Requires far less calibration
•Can only accurately measure 40Ca, 42Ca, 44Ca•Long analysis times (hours per sample)
MC-ICP-MS •Better precision of individual measurements
•Larger instrument fractionation•Unaccountable drift; worsens reproducibility (requires bracketing)
Commonly measure 44Ca/40Ca or 42Ca/40Ca ratio
BackgroundDouble Spikes
Separate natural fractionation (+0.1% per mass unit) vs. instrumental fractionation (+0.5% per mass unit)
e.g. spike with 42Ca-48CaAnalyze 42/40, 44/40, and 48/40Solve equations iteratively for:
-spike/sample ratio-mass discrimination-sample 44/40 ratio
DePaolo Reviews in Mineral Geology (2004)
BackgroundStandards
• Established in 2008• Before 2008, labs used varying in-house standards
(seawater, terrestrial igneous rocks, and fluorite)• δ44/40Ca NIST SRM 915b and NIST SRM 1486
Background
Fractionation as a result of:– Igneous and metamorphic rocks, &
petrogenetic processes– Weathering cycle– Biology
The Biological Observation
Mystery source???
δ44Ca decreases with increasing trophic levels
Soft vs. Mineralized Tissue
• Soft tissues heavier than bones by ~1.3‰ • Mineralization responsible for fractionation
Skulan & DePaoloPNAS (1999)
Calcium Transport Model
Vd = flux from diet
Vex = flux excreted
Vb = flux into bones
Vl = flux leaving bones
Δb = fractionation between bone and soft tissue
Skulan & DePaoloPNAS (1999)
Bone Growth
During bone growth Vl << Vb:
Most Cadiet into bones: •40Ca is taken up by bones•δ44Casoft tissue > δ44Cadiet
Skulan & DePaoloPNAS (1999)
Bone RemodelingBones are gaining and losing Ca; net [Ca] is zero• δ44Casoft tissue reflects δ44Cadiet
• bones differ from diet by Δb:
Skulan & DePaoloPNAS (1999)
During bone loss Vl/Vd is important:
CUI = isotopic Ca use index
= 0 during bone remodeling
> 0 during bone growth < 0 during bone loss
• 40Ca is being lost from the bones • δ44Ca soft tissue < diet
Bone Loss
Skulan & DePaoloPNAS (1999)
Urine: a human biomarker of δ44Ca?
2 fractionations: bone/soft-tissue & blood-urine
Heuser & EisenhauerBone (2010)
Observed: δ44Ca urine > δ44Ca diet
Bone Growth vs. Loss Visibility
Young, healthy boy: bone growthElder woman, confirmed osteoporosis: bone loss
[Ca]urine twice as large for woman; δ44Ca urine, woman < δ44Ca urine, boy Heuser & Eisenhauer
Bone (2010)
Biomedical Application Pitfall
δ44Cadiet dependent (+ 0.2‰)
As bone loss increases, urine becomes lighter
δ44Ca indicates plant productivity and soil fertility
Monitoring forested ecosystems in Hawaii
Sr and Ca in soil, plants
Wiegand Geophysical Research Letters (2005)
Leaves & soil have isotopically similar Sr values; over time the source of plant available Sr is more marine aerosols than lava
Ca leaves & soil also see a source-dependent shift with time, but…
Ca in Soil, Leaves
Sr and Ca deviate from each other with time•Ca is leached from soil whereas Sr isn’t•Sr/Ca lower in leaves than in soil
δ44Ca in Plant Tissues
Sr vs. Ca uptakeFrom ocean (0.7092)
Wiegand Geophysical Research Letters (2005)
From basalt (0.704)
Marine aerosols δ44Ca = 0.00 +0.2 ‰Volcanic rocks δ44Ca = -1.1 +0.3 ‰
Conclusions
• δ44Ca fractionates as a result of bone formation (mineralization)
• δ44Ca fractionates as it is converted from blood to urine
• δ44Ca can be developed as a medical tool*• δ44Ca can be used to better understand the
biogeochemical cycle of terrestrial Ca which Sr alone cannot
Background painting Cows in field, courtesy of Peter Allsop