New directions in limnology and oceangraphy using cosmogenic radionuclides

1. New directions in limnology and oceangraphy using cosmogenic radionuclides Erik Brown Large Lakes Observatory Department of Geological Sciences University of Minnesota Duluth

2. Cosmic ray interaction on Earth Cosmic radiation •High energy subatomic particles •Nuclear interactions with matter •Produces “cosmogenic nuclides” 10Be, 14C, 36Cl, 3He, 32P, 33P Cosmogenic nuclide production •Atmosphere (mostly) •Earth’s surface •Decreases exponentially •10x higher at 4500m than at sealevel •2x lower 40 cm into rock than at surface

4. Radiocarbon Accelerator Mass Spectrometry (AMS) has become the method of choice. Sample size ~1 mg. High precision until uncertainty in instrument background becomes significant, typically for materials older than ~40,000 years. b-counting. Requires larger samples (~4 g), but can provide good precision in older samples.

6. Where is this useful?? 40 ka > age > 80 ka. Large samples available (~ grams of carbon) Ancient coral. Calibration of radiocarbon timescale.

7. Typically U-series dates

8. Biological productivity in Lake Superior Limited by phosphate availability Knowledge of P cycling is key to understanding ecosystem Cosmogenic P isotopes have been used in marine systsms. C. Benitez-Nelson (U. South Carolina)

9. 32P, 33P formed by spallation reactions in the atmosphere • 32P t½ =14.3 d • 33P t½ = 25 d • 32P/ 33P t½ = 33.4 d • Advantages: • P is a nutrient used by all living organisms. • Radioisotope half-lives relevant to biological timescales. • In-situ tracers avoid issues associated with ‘bottle effects.’ • Ratio of isotopes minimizes changes due to dilution. • Disadvantages: • Large sample volumes and extensive purification: • 5 tons per sample!!! Several hour shiptime.

10. Background: 32P t½ = 14.3 days Emax = 1.71 MeV Strong Beta Emitter. Gas proportional counter with background count rates of 0.18-0.20 cpm 33P t½ = 25. 3 days Emax = 0.249 MeV Weak Beta Emitter. Suffers from high self absorption and can’t separate from stable P. Requires measurement with LSS counter with typical backgrounds of 0.85 – 1.25 cpm depending on quench levels 32P and 33P activities in RAIN water: 0.5 to 4 dpm/L Fluxes (dependent on rain): Range from 800 to 2000 dpm/m2/y 32P and 33P activities in Seawater: 0.5 – 4.0 dpm/1000 L 32P and 33P activities in particles: 0.05 – 0.4 dpm/1000 L

11. Atmospheric Deposition 33P/32P ratio avoids complications with changes in flux. 32P, 33P Phytoplankton HPO42- CO2 Bacteria DOP Zooplankton Heterotrophic Protozoa Sinking Particles Upwelling of Inorganic Nutrients

12. Increasing Age All errors are 2s

13. 33P/32P = 1.4 33P/32P = 1.15 33P/32P = 0.82 33P/32P = 1.05 33P/32P = 0.95 H2PO4 H2PO4 DOP H2PO4 DOP Part. P H2PO4 DOP Part. P H2PO4 DOP Part. P 33P/32P ratios in dissolved and particulate P result from the source ratio (i.e. you are what you eat) and the P residence time.

14. 60 days Increasing Age 30 days All errors are 2s

15. HPO4= HPO4= HPO4= HPO4= HPO4= HPO4= HPO4= HPO4= Phosphate Dissolved Organic P Total Diss. P Low 33P/32P Ratio + = Rapid Turnover Low 33P/32P Ratio “Small” molecules Rapid Turnover Low 33P/32P Ratio “Large molecules” Slow Turnover High 33P/32P Ratio Total Diss. P High 33P/32P Ratio + = Rapid Turnover Low 33P/32P Ratio “Large molecules” Slow Turnover High 33P/32P Ratio

16. Conclusions A range of questions in ocean/lake science can be addressed using cosmogenic nuclides. Radiocarbon dating for large samples older than 40 kyr can provide important complement to AMS, if 14C:12C background is <10-16. Many major questions regarding P-cycling (and hence overall marine or lake productivity) remain unresolved. Some of these can be addressed using cosmogenic P isotopes, but present approaches limit sampling. Decreasing background can reduce sampling requirements 5-fold, permitting more innovation in field strategies.

17. Note that the residence time of P INCREASES with increasing primary production. This is not intuitive. One might expect that as more organisms grow and consume nutrients that the residence time of P within the dissolved phase would DECREASE. The Increase appears to be due to the different forms of P in the water. The more bioaviable, low 33P/32P ratio compounds are being consumed first, leaving the older, higher 33P/32P compounds in solution. Combined, this causes an INCREASE in the 33P/32P ratio in the dissolved phase. The particle 33P/32P ratios are always low, supporting this theory.

19. Lake Superior Extremely low soluble reactive P Low productivity, mostly limited by P (but also by Fe) Annual input of P supplies less than 10% of that required for biological activity Bacteria play a major role

20. 30 days Increasing Age All errors are 2s

21. PO4 33P/32P = 0.55 DOP1 33P/32P = 0.55 DOP2 33P/32P = 0.75 DOPn 33P/32P > 0.75 Rain at Sta. ALOHA 33P/32P = 0.55  0.19 Short Residence Time Long Residence Time Low Bioavailability High Bioavailability “Normal” Growth “P-Stressed” “P-Starved” Bacteria and Algae Bjorkman and Karl (2003) measured SRP turnover rates: 10 - 14 days BAP turnover rates: 4 - 21 days PP turnover rates: < 2 - 10 days

22. This work suggests two important points: • Open ocean organisms are responding rapidly to their environment, and in times of stress will consume both inorganic and organic P REGARDLESS of the inorganic P present in the system. • Particulate P has a surprisingly short residence time regardless of the P source in the upper ocean Wouldn’t it be nice to do this on volumes less than 5000 L and collect samples over both depth and region? Reducing background by 10x will reduce sample volume requirements to ~1000L.

23. Increasing Age All errors are 2s

24. If there is abundant sample material, b-counting can provide better precision than AMS for samples older than ~40ka • There is a diminishing return for reducing background