Using Heavy Isotopes in Marine Barite to Characterize Ocean Chemistry Changes

1. Using Heavy Isotopes in Marine Barite to Characterize Ocean Chemistry Changes Andrea M. Erhardt Stanford University University of California - Santa Cruz USAC Meeting Washington, DC July 14, 2009

2. 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

3. 5 um 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 Mearon et al., 2002

4. Formation of Marine Barite

5. Why is barite a useful proxy? Indicator of Primary Productivity Eagle et al., 2003

6. Examples of Productivity Records Glacial/ Interglacial Productivity Cycles Productivity Increase During OAE’s Paytan et al., 1996 Erhardt et al., in prep

7. 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 Ba Dehairs et al., 1990

8. Previous studies have utilized trace elements in marine barite Ca isotopes Srisotopes Mearon et al., 2003 Griffith et al., 2008

9. 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 Balance River Input 0‰ or 0.7 – 1.3 ‰ d98/95Mo = [(98Mo/95Mo)sample/(98Mo/95Mo)standard – 1] × 1000 Ocean 2.3 ‰ Oxic Sink -2.8 - -3.1‰ offset Average Crustal Rock: 0 ‰ (Standard) Euxinic Sink Approx. Complete Removal Hydrothermal Input 0.8‰ Anoxic Sink -0.7 ‰ offset Oxic Sediment: -0.8 ‰ Anoxic Sediment: 1.6 ‰ Euxinic Sediment: 2-2.3‰

11. Changes in d98/95Mo Correlates to Changes in the Relative Size of Sinks Modern Mid-Proterozoic Input Input 0‰ 0‰ Marine Barite would allow for a direct measurement of this ratio Oceans 1.1‰ Oceans 2.3‰ -3‰ -0.3‰ -3‰ -0.3‰ -0.7‰ -1.9‰ 2‰ 0.8‰ Arnold et al., 2004 Mn-Oxide Sediments Euxinic Sediments Euxinic Sediments Mn-Oxide Sediments Relative Flux: 75% 25% 25% 75%

12. Results from Precipitation Experiments

13. Results from Mo Isotopic Analysis Sample Concentration Average: 4.11 ng Average Blank Concentration: 0.82 ng • 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

14. 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

15. New Directions- Lead Isotopes Why Lead Isotopes? Different aged source rocks will have different Pb signatures • 232Th 208Pb t1/2 = 14.01 * 109 • 235U  207Pb t1/2 = 0.71 * 109 • 238U  206Pb t1/2 = 4.47 * 109 • 204Pb • Final ratios: • 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb • 208Pb/206Pb, 207Pb/206Pb

16. Why Lead Isotopes? • Used for provenance and weathering studies • Fe Mn nodules faithfully record Pb seawater signatures Correlation between Oxygen and Lead Records Foster and Vance, 2006

17. What are we measuring? Intermediate Water Changes • Description • Found between ~500 and 1500 m water depth • Distinctive salinity composition von Blanckenburg, 1999 • Significance • Important component of thermohaline circulation • Limited locations for characterization with current methods Pahnke and Zahn, 2005

18. 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

19. Results from Precipitation Experiment • Strong correlation between concentration Pb in Seawater and Pb in precipitated barite • Significantly higher concentrations than Mo

20. Core Top Analysis This Study Mid-Ocean Basalts Indian Ocean FeMn nodules Pacific Ocean FeMn nodules Pacific Bottom Seawater NHRL- Northern Hemisphere Reference Line Atlantic Bottom Seawater Organic pelagic sediments Ling et al., 1997; Frank, 2002 Core Top samples show a consistent source material

21. Downcore Record- Site 574 Upper Continental Crust 085-574 Multiple source fields- changing water masses?

22. Downcore Record – Site 574 Tasmania-Antarctic Passage Opens Oi-1 Glaciation Panama Seaway becomes shallow sill Tibetan Plateau uplift Mi-1 Glaciation Drake Passage Opens Panama Seaway Closes Correlation between marine barite Pb record and oxygen isotope record 085-574 Zachos et al., 2001

23. Similar trends between barite results, FeMn nodules from equatorial Pacific, and benthic oxygen record Christensen et al., 1997

24. 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

25. Future work • Mo methods will be applied to other systems, namely looking at changes in anoxia in coastal “dead zones” off the Oregon coast • 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 Additional Sample Locations for Cenozoic Record

26. Questions? Thanks again for the opportunity to pursue this research!

27. Additional Records Southern Ocean Nodules Vlastelic et al., 2005 Pacific Nodules, Ling et al., 1997