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