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. What is Marine Barite? Barium Sulfate- BaSO 4 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

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

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 Dehairs et al., 1990 Ba

8. Previous studies have utilized trace elements in marine barite Sr isotopes Ca isotopes 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. 10 Modern Mo Isotope Balance River Input 0‰ or 0.7 – 1.3 ‰ Hydrothermal Input 0.8‰ Oxic Sink -2.8 - -3.1‰ offset Anoxic Sink -0.7 ‰ offset Euxinic Sink Approx. Complete Removal Ocean 2.3 ‰ Oxic Sediment: -0.8 ‰ Anoxic Sediment: 1.6 ‰ Euxinic Sediment: 2-2.3‰  98/95 Mo = [( 98 Mo/ 95 Mo) sample /( 98 Mo/ 95 Mo) standard – 1] × 1000 Average Crustal Rock: 0 ‰ (Standard)

11. 11 Changes in  98/95 Mo Correlates to Changes in the Relative Size of Sinks Arnold et al., 2004 0‰0‰ 0‰0‰ Oceans 2.3‰ Input Oceans 1.1‰ -0.7‰ 2‰2‰ ModernMid-Proterozoic Mn-Oxide Sediments Euxinic Sediments 75%25% Relative Flux: -1.9‰ 0.8‰ Euxinic Sediments 25%75% -3‰ -0.3‰ Marine Barite would allow for a direct measurement of this ratio

12. Results from Precipitation Experiments

13. 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

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 232 Th  208 Pbt 1/2 = 14.01 * 10 9 235 U  207 Pbt 1/2 = 0.71 * 10 9 238 U  206 Pb t 1/2 = 4.47 * 10 9 204 Pb Final ratios: 208 Pb/ 204 Pb, 207 Pb/ 204 Pb, 206 Pb/ 204 Pb 208 Pb/ 206 Pb, 207 Pb/ 206 Pb

16. 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

17. 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

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 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

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

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

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

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 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

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

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