Download 10-0129_Ca _Isotopes

Ca Isotopes
Cheryl Zurbrick
1/29/2010
Background
Background
40Ca
produced by β-decay of 40K
Most Ca from primordial earth
DePaolo
Reviews in Mineral Geology (2004)
Background
Methodology
Pros
TIMS
•Reproducible precision
(single collector) between runs
•Requires far less calibration
MC-ICP-MS
•Better precision of
individual measurements
Cons
•Can only accurately
measure 40Ca, 42Ca, 44Ca
•Long analysis times (hours
per sample)
•Larger instrument
fractionation
•Unaccountable drift;
worsens reproducibility
(requires bracketing)
Commonly measure 44Ca/40Ca or 42Ca/40Ca ratio
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)
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
Background
Fractionation as a result of:
– Igneous and metamorphic rocks, &
petrogenetic processes
– Weathering cycle
– Biology
The Biological Observation
δ44Ca decreases with increasing trophic levels
Mystery source???
Soft vs. Mineralized Tissue
• Soft tissues heavier than bones by ~1.3‰
• Mineralization responsible for fractionation
Skulan & DePaolo
PNAS (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 & DePaolo
PNAS (1999)
Bone Growth
During bone growth Vl << Vb:
Most Cadiet into bones:
•40Ca is taken up by bones
•δ44Casoft tissue > δ44Cadiet
Skulan & DePaolo
PNAS (1999)
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)
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)
Urine: a human biomarker of δ44Ca?
2 fractionations: bone/soft-tissue & blood-urine
Observed: δ44Ca urine > δ44Ca diet
Heuser & Eisenhauer
Bone (2010)
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)
Biomedical Application Pitfall
δ44Cadiet dependent (+ 0.2‰)
As bone loss
increases, urine
becomes lighter
Monitoring forested ecosystems in
Hawaii
δ44Ca indicates plant productivity and soil fertility
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 sourcedependent shift with time, but…
Wiegand Geophysical
Research Letters (2005)
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 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)
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