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

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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