using heavy isotopes in marine barite to characterize ocean chemistry changes andrea m. erhardt...
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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
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
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
Formation of Marine Barite
Why is barite a useful proxy?
Indicator of Primary Productivity
Eagle et al., 2003
Examples of Productivity Records
Erhardt et al., in prep
Paytan et al., 1996
Glacial/ Interglacial Productivity Cycles
Productivity Increase During OAE’s
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
Previous studies have utilized trace elements in marine barite
Sr isotopes Ca isotopes
Mearon et al., 2003Griffith et al., 2008
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
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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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
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
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
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
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
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
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
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
Results from Precipitation Experiment
• Strong correlation between concentration Pb in Seawater and Pb in precipitated barite
• Significantly higher concentrations than Mo
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
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
Christensen et al., 1997
Similar trends between barite results, FeMn
nodules from equatorial Pacific, and benthic
oxygen record
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
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
Questions?
Thanks again for the opportunity to pursue this research!
Pacific Nodules, Ling et al., 1997
Southern Ocean NodulesVlastelic et al., 2005
Additional Records