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Understanding Calcium Isotope Fractionation in Biological Systems

Explore the background, methodology, and applications of calcium isotope ratios in bones, tissues, urine, and plant ecosystems. Learn how δ44Ca can indicate bone growth, loss, and more. Discover the potential of δ44Ca as a medical diagnostic tool.

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Understanding Calcium Isotope Fractionation in Biological Systems

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  1. Ca Isotopes Cheryl Zurbrick 1/29/2010

  2. Background

  3. Background 40Ca produced by β-decay of 40K Most Ca from primordial earth DePaolo Reviews in Mineral Geology (2004)

  4. Background Methodology Commonly measure 44Ca/40Ca or 42Ca/40Ca ratio

  5. Background Double Spikes Separate natural fractionation (+0.1% per mass unit) vs. instrumental fractionation (+0.5% per mass unit) e.g. spike with 42Ca-48Ca Analyze 42/40, 44/40, and 48/40 Solve equations iteratively for: -spike/sample ratio -mass discrimination -sample 44/40 ratio DePaolo Reviews in Mineral Geology (2004)

  6. Background Standards • 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

  7. Background Fractionation as a result of: • Igneous and metamorphic rocks, & petrogenetic processes • Weathering cycle • Biology

  8. The Biological Observation δ44Ca decreases with increasing trophic levels Mystery source???

  9. Soft vs. Mineralized Tissue • Soft tissues heavier than bones by ~1.3‰ • Mineralization responsible for fractionation Skulan & DePaolo PNAS (1999)

  10. 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 & DePaolo PNAS (1999)

  11. Bone Growth • During bone growth Vl << Vb: • Most Cadiet into bones: • 40Ca is taken up by bones • δ44Casoft tissue > δ44Cadiet Skulan & DePaolo PNAS (1999)

  12. Bone Remodeling Bones are gaining and losing Ca; net [Ca] is zero • δ44Casoft tissue reflects δ44Cadiet • bones differ from diet by Δb: Skulan & DePaolo PNAS (1999)

  13. Bone Loss 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 Skulan & DePaolo PNAS (1999)

  14. Urine: a human biomarker of δ44Ca? 2 fractionations: bone/soft-tissue & blood-urine Observed: δ44Ca urine > δ44Ca diet Heuser & Eisenhauer Bone (2010)

  15. Bone Growth vs. Loss Visibility Young, healthy boy: bone growth Elder woman, confirmed osteoporosis: bone loss [Ca]urine twice as large for woman; δ44Ca urine, woman < δ44Ca urine, boy Heuser & Eisenhauer Bone (2010)

  16. Biomedical Application Pitfall δ44Cadiet dependent (+ 0.2‰) As bone loss increases, urine becomes lighter

  17. Monitoring forested ecosystems in Hawaii δ44Ca indicates plant productivity and soil fertility

  18. Sr and Ca in soil, plants 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… Wiegand Geophysical Research Letters (2005)

  19. 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

  20. δ44Ca in Plant Tissues

  21. Sr vs. Ca uptake From ocean (0.7092) From basalt (0.704) Marine aerosols δ44Ca = 0.00 +0.2 ‰ Volcanic rocks δ44Ca = -1.1 +0.3 ‰ Wiegand Geophysical Research Letters (2005)

  22. 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

  23. Background painting Cows in field, courtesy of Peter Allsop

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