ADINA_Ca - Mg cycle

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The Ca cycle - seawater Ca isotopes 
Elizabeth M. Griffith 
 
 
Road Map 
 
•  Essentials of calcium isotopes 
•  Ca isotopes and ocean acidification 
•  Examples from the geological record 
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Important Qualification! 
Ca in ocean ~10 mM (~400mg/kg) 
 
15x1018 moles of Ca in ocean (6x1017 kg) 
 
Residence time in ocean ~ 1 m.y. 
 
Analytical limits ±0.2‰ 
 
Difference between ocean and in/out put < 2‰ 
Change of >20% in Ca cycle must occur to 
be detected in δ44Ca of seawater 
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 δ44/40Ca = 1000 (44Ca/40Ca)sample - (44Ca/40Ca)standard
 (44Ca/40Ca)standard 
•  Standard = modern seawater or NIST SRM 915a 
•  Thermal Ionization Mass Spectrometry (TIMS) 
using double spike or MC-ICPMS 
Essentials of Calcium Isotopes: 
from Coplen et al., 2002; Russell et al., 1978 
40Ca 0.96941 44Ca 0.02086 
42Ca 0.00647 46Ca 0.00004 
43Ca 0.00135 48Ca 0.00187 
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Δ	
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Modern Ca Isotopes in the Ocean 
Sources & Sinks 
River Flux 
Source: River Water 
δ44Ca = -0.87 to -1.3‰ 
Seawater 
δ44Ca = 0.00‰ 
Carbonate Sedimentation 
Sink: Marine Carbonates 
δ44Ca = ~ -1.6‰ 
Δ	
Volcanic-Seawater 
Reactions 
Source: Hydrothermal Fluid 
δ44Ca = -0.96‰ 
Sink: Alteration of oceanic crust 
δ44Ca = -0.98 to -1.60‰ 
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 - dδ44Casw/dt 
 
+ dNCa /dt	
Fin < Fsed 
δ44Cain = Δ44Cased	
 + dδ44Casw/dt 
 
- dNCa /dt	
Fin > Fsed 
δ44Cain = Δ44Cased	
How does δ44Ca change in the ocean? 
( ) ( )
44/40
44/40 44/40 44/40Ca sw
in in sw sed sed
N d Ca F Ca Ca F Ca
dt
δ
δ δ= − − Δ Ca in sed
dN F F
dt
= −Ca
sed
N
F
τ =
Delivery > Burial : NEGATIVE EXCURSION – [Ca] increase 
Burial > Delivery : POSITIVE EXCURSION – [Ca] decrease 
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Global biogeochemical cycling of CaCO3 
Ridgwell and Zeebe, 2005 
Dominant processes, sources and sinks: 
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1 
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Calcium isotopes record the ratio of calcium fluxes into and out of 
seawater, linked to carbonate chemistry (Alk, DIC, pH) and pCO2 
Changes in δ44Ca indicate changes in [Ca] - may be related to [CO3] 
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Ca isotopes and ocean acidification 
•  High weathering flux (Fin > Fsed) = increase in ocean 
Alk., higher Ca concentrations and lower δ44Ca 
•  Increased CaCO3 burial (Fin < Fsed) = decrease in 
ocean Alk., lower Ca concentrations and higher δ44Ca 
•  High pCO2 – weathering 
•  High pCO2 - OA pH – dissolution 
7 Examples in the Geological Record 
 
Boron and calcium isotope composition in 
Neoproterozoic carbonate rocks from Namibia: 
evidence for extreme environmental change 
 
Kasemann et al., 2005, EPSL 
In the snowball Earth hypothesis, rapid melt back 
of the ice cover resulted in the transfer of 
atmospheric carbon dioxide to the oceans and 
hence deposition of postglacial cap carbonates. 
Such CO2 transfer to the oceans should have 
caused a rapid decrease in seawater pH. 
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A negative δ11B excursion – decrease in ocean pH (1-2 pH units) 
A negative δ44Ca excursion – increased weathering rates 
High pCO2 (7,000-90,000ppm) during the melt back of Neoproterozoic 
glaciations and precipitation of cap carbonates. Ca and C isotopes were 
coupled through silicate weathering with CO2 drawdown. 
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Examples in the Geological Record - 
P/T Extinction 
Upwelling from a highly alkaline deep ocean would cause 
carbonate precipitation and a drawdown in [Ca2+] and thus 
Ca Burial > Ca input - positive Ca isotope excursion 
Release of carbon dioxide from volcanic and sedimentary rocks 
could cause ocean acidification and carbonate dissolution 
(reduced ppt.) an increase in dissolved Ca concentration, 
Ca input > Ca output - negative Ca isotope excursion 
 
Siberian Traps Volcanism 
Stratified Ocean / Ocean Overturn 
Payne et al., 2010, PNAS 
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Payne et al., 2012 
Overturn CO2 release 
δ44/40Ca δ44/40Ca 
Permian-Triassic δ44Ca Record 
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Marine Pelagic Barite 
BaSO4 
•  Precipitates authigenically in the 
 upper water column in association 
 with decaying organic matter 
•  Incorporates Ca2+ into crystal structure as a 
‘trace metal’, substituting for Ba2+ 
•  Advantages: 
•  Nonbiogenic phase 
•  Well preserved, high resolution potential 
•  Constant fractionation from seawater (-2.01‰) 
5 µm 
(Mearon et al., 2002) 
Griffith, Schauble, Paytan & Bullen, Geochim. Cosmochim. Acta 2008 
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n = 49 
Neogene seawater δ44Ca: marine barite 
1.What variations do we 
see from marine barite 
over this time in 
seawater δ44Ca? 
2. Are variations 
coincident with 
changes in the CCD? 
3. Can we quantify/model 
changes in seawater 
Ca2+? 
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n = 49 
Neogene seawater δ44Ca: marine barite 
Griffith, Paytan, Caldeira, Bullen & Thomas, Science 2008 
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Model: Ca concentrations (NCa) 
= fluid inclusion data (Horita et al., 2002) 
Model 
Inputs 
Model 
Results 
τ = 1.3 Myr 
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Results: Eocene-Oligocene Transition 
* 
n=25 
Griffith, et al., submitted 
Largest permanent change in 
CCD in Cenozoic 
(deepening) 
 
Associated with start of 
Antarctic glaciation and 
lowering of pCO2 and SST 
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Model: Determining dδ44Casw/dt, dNCa/dt 
1.  200kyr doubling of Fin (Rea and Lyle, 2005) with 
linear increase of Fsed to equal Fin at end of 200kyr 
2.  Initial δ44Casw = -0.2‰; NCa = 1.5 x modern 
 δ44Cain Δ44Cased δ44Casw NCa 
τ = 1.0 My -1.2‰ -1.2‰ -0.02‰ 110% 
τ = 0.5 My -1.2‰ -1.2‰ -0.16‰ 120% 
τ = 1.0 My -1.3‰ -0.7‰ -0.18‰ 110% 
τ = 0.5 My -1.3‰ -0.7‰ -0.28‰ 120% 
τ = 1.0 My -0.9‰ -1.6‰ +0.17‰ 110% 
τ = 0.5 My -0.9‰ -1.6‰ +0.29‰ 120% 
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Largest permanent deepening of CCD in Cenozoic: 
–  Scenario must include near balance of Ca2+ sources and 
sinks to the ocean (<20% change) 
–  Seawater Ca2+ concentration might have decreased at 
this time due to an increase in CaCO3 sedimentation 
(greater than any increase in weathering flux) 
–  On long time scales (millions of years), global CCD 
not coincident with extremely large changes in 
seawater Ca2+ concentration 
Implications from EOT seawater 
Ca-isotope record: 
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Paleocene-Eocene Thermal Maximum: 
Most dramatic shoaling of CCD during Cenozoic: 
–  CCD rise & CaCO3 dissolution spike in response to 
massive C input and following recovery of CCD 
could have affected seawater Ca-isotopes 
–  Potential constraint on short term imbalances of 
seawater Ca and C cycle 
High resolution data to clarify relationship 
between seawater δ44Ca, CCD, and C cycle 
–  Quantitative modeling to assess links & feedbacks 
 
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Paleocene-Eocene Thermal Maximum: 
*Nunes and Norris et al., 2005; 
 Murphy et al., 2006 
* 
* Leg 199 
Site 1221 
Griffith and 
Fantal in prep 
UNPUBLISHED 
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