using heavy isotopes in marine barite to characterize ocean chemistry changes andrea m. erhardt...
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![Page 1: Using Heavy Isotopes in Marine Barite to Characterize Ocean Chemistry Changes Andrea M. Erhardt Stanford University University of California - Santa Cruz](https://reader035.vdocument.in/reader035/viewer/2022062717/56649e495503460f94b3c9ef/html5/thumbnails/1.jpg)
Using Heavy Isotopes in Marine Barite to Characterize Ocean
Chemistry Changes
Andrea M. Erhardt
Stanford UniversityUniversity of California - Santa Cruz
USAC MeetingWashington, DC
July 14, 2009
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Overview
• What is Marine Barite? Why is it a useful proxy?
• New directions- Mo and Pb isotopes– Mo isotopes- a proxy for anoxia– Pb isotopes- a proxy for water provenance
• Future Directions
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What is Marine Barite?
• Barium Sulfate- BaSO4
• An inorganic precipitate that forms in association with organic matter
• Highly resist to degradation• Distinguishable from other
forms of barite (hydrothermal, diagenetic) by size, shape, and Sr isotopic composition
5 um
Mearon et al., 2002
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Formation of Marine Barite
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Why is barite a useful proxy?
Indicator of Primary Productivity
Eagle et al., 2003
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Examples of Productivity Records
Erhardt et al., in prep
Paytan et al., 1996
Glacial/ Interglacial Productivity Cycles
Productivity Increase During OAE’s
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Why is it a useful proxy? Archive of paleo ocean chemistry
• Forms within the water column– Much of the barite forms
at the depth of maximum organic matter regeneration
– Captures water column conditions, not surface or bottom water
• Multiple Elements (Ca, Sr, Pb, Mo, Ra, Nd) have been shown to substitute into matrix
Particulate barite and dissolved oxygen in the Southern Ocean
Dehairs et al., 1990
Ba
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Previous studies have utilized trace elements in marine barite
Sr isotopes Ca isotopes
Mearon et al., 2003Griffith et al., 2008
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New Directions
• Looking at Heavier trace elements, namely Mo and Pb– Mo an established proxy for anoxic conditions– Pb is used as a indicator of weathering and
seawater provenance
• These new proxies will allow for the characterization of the water column chemistry through time
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10
Modern Mo Isotope BalanceRiver Input 0‰
or0.7 – 1.3 ‰
Hydrothermal Input0.8‰
Oxic Sink-2.8 -
-3.1‰ offset
Anoxic Sink
-0.7 ‰offset
Euxinic Sink
Approx.CompleteRemoval
Ocean2.3 ‰
Oxic Sediment:
-0.8 ‰ Anoxic
Sediment: 1.6 ‰
Euxinic Sediment: 2-2.3‰
98/95Mo = [(98Mo/95Mo)sample/(98Mo/95Mo)standard – 1] × 1000
Average Crustal
Rock: 0 ‰ (Standard)
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11
Changes in 98/95Mo Correlates to Changes in the Relative Size of Sinks
Arnold et al., 2004
0‰ 0‰
Oceans2.3‰
Input Input
Oceans1.1‰
-0.7‰ 2‰
Modern Mid-Proterozoic
Mn-Oxide Sediments
Mn-Oxide Sediments
EuxinicSediments
75% 25%Relative Flux:
-1.9‰ 0.8‰
EuxinicSediments
25% 75%
-3‰ -3‰-0.3‰ -0.3‰
Marine Barite would allow
for a direct measurement
of this ratio
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Seawater and Precipitated Barite Mo Concentrations
y = 1.587x + 1E-07
R2 = 0.9315
y = 2.0004x
R2 = 0.8312
0.E+00
1.E-07
2.E-07
3.E-07
4.E-07
5.E-07
6.E-07
7.E-07
8.E-07
9.E-07
0.0E+00 5.0E-08 1.0E-07 1.5E-07 2.0E-07 2.5E-07 3.0E-07 3.5E-07 4.0E-07
Mo/S Molar Ratio- Seawater
Mo/S Molar Ratio- Barite
Mo precip samplesNIST barite standardAverage Coretop SamplesLinear (Mo precip samples)Linear (Mo precip samples)
Lower Limit will be further defined in future
Modern Conditions
Results from Precipitation Experiments
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Results from Mo Isotopic Analysis
• Similar variability in results from downcore and precipitation study samples• Ocean is a well mixed reservoir of 2.3‰- no consistent fractionation from this value shown•Blank represents ~20% of signal on average- blank isotopic composition varied so blank corrections could not be applied
Sample Concentration Average: 4.11 ng
Average Blank Concentration: 0.82 ng
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Conclusions from Mo Work
• Sample concentrations, with current methods, are too similar to blank to generate a reliable signal
• Blank has already been lowered to below work from other labs
• We will apply these methods to other environments where small sample sizes could provide for high resolution analysis
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New Directions- Lead Isotopes
Why Lead Isotopes?Different aged source rocks will have different Pb signatures
• 232Th 208Pb t1/2 = 14.01 * 109
• 235U 207Pbt1/2 = 0.71 * 109
• 238U 206Pb t1/2 = 4.47 * 109
• 204Pb
Final ratios: 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb
208Pb/206Pb, 207Pb/206Pb
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Why Lead Isotopes?• Used for provenance and weathering studies
• Fe Mn nodules faithfully record Pb seawater signatures
Foster and Vance, 2006
Correlation between Oxygen and Lead Records
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What are we measuring? Intermediate Water Changes
von Blanckenburg, 1999
Pahnke and Zahn, 2005
Description• Found between ~500 and
1500 m water depth• Distinctive salinity
composition
Significance• Important component of
thermohaline circulation• Limited locations for
characterization with current methods
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Results for Pb Isotopes
• Three Experiments Conducted:– Precipitation study to confirm that Pb is
incorporated into the barite crystal– Core top calibration to check for reliability of
signal– Downcore record for last ~40 Ma to determine if
Pb ratios are sensitive to changes through time
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Results from Precipitation Experiment
• Strong correlation between concentration Pb in Seawater and Pb in precipitated barite
• Significantly higher concentrations than Mo
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Core Top Analysis
NHRL- Northern Hemisphere Reference Line
Mid-Ocean Basalts
Indian Ocean FeMn nodules
Pacific Bottom Seawater
Organic pelagic sediments
Atlantic Bottom Seawater
This Study
Core Top samples show a consistent source material
Pacific Ocean FeMn nodules
Ling et al., 1997; Frank, 2002
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Downcore Record- Site 574
085-574
Upper Continental Crust
Multiple source fields- changing water masses?
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Correlation between marine barite Pb
record and oxygen isotope record
Downcore Record – Site 574
085-574
Zachos et al., 2001
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Christensen et al., 1997
Similar trends between barite results, FeMn
nodules from equatorial Pacific, and benthic
oxygen record
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Conclusions
• Marine Barite appears to be recording the Pb isotopic signature of intermediate water
• Trends generally follow the Cenozoic oxygen curve
• Trends generally follow the FeMn nodule record, though the magnitude of the changes are greater
• Proxy holds promise for reconstruction of intermediate water changes
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Future work– Mo methods will be applied to other systems, namely
looking at changes in anoxia in coastal “dead zones” off the Oregon coast
Additional Sample Locations for Cenozoic Record
– Pb work• Generate results from
additional cores for Cenozoic record
• Samples from Expedition 320 will provide a great continuous record
• These samples will also be processed for the barite paleoproductivity proxy
• Construct a glacial/interglacial record
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Questions?
Thanks again for the opportunity to pursue this research!
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Pacific Nodules, Ling et al., 1997
Southern Ocean NodulesVlastelic et al., 2005
Additional Records