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

Cosmic radiation High energy subatomic particles Nuclear interactions with matter Produces “cosmogenic nuclides” 10 Be, 14 C, 36 Cl, 3 He, 32 P, 33 P 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 Cosmic ray interaction on Earth

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.  -counting. Requires larger samples (~4 g), but can provide good precision in older samples.

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

Typically U-series dates

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)

32 P t ½ =14.3 d 33 P t ½ = 25 d 32 P/ 33 P t ½ = 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. 32 P, 33 P formed by spallation reactions in the atmosphere

Background: 32 P t ½ = 14.3 days Emax = 1.71 MeV Strong Beta Emitter. Gas proportional counter with background count rates of cpm 33 P t ½ = days Emax = 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 32 P and 33 P activities in RAIN water: 0.5 to 4 dpm/L Fluxes (dependent on rain): Range from 800 to 2000 dpm/m 2 /y 32 P and 33 P activities in Seawater: 0.5 – 4.0 dpm/1000 L 32 P and 33 P activities in particles: 0.05 – 0.4 dpm/1000 L

DOP HPO 4 2- CO 2 Upwelling of Inorganic Nutrients Heterotrophic Protozoa Bacteria Phytoplankton Zooplankton Sinking Particles Atmospheric Deposition 32 P, 33 P 33 P/ 32 P ratio avoids complications with changes in flux.

Increasing Age All errors are 2 

H 2 PO 4 DOP H 2 PO 4 DOP Part. P H 2 PO 4 DOP Part. P H 2 PO 4 DOP Part. P 33 P/ 32 P = P/ 32 P = P/ 32 P = P/ 32 P = P/ 32 P = P/ 32 P ratios in dissolved and particulate P result from the source ratio (i.e. you are what you eat) and the P residence time.

Increasing Age 60 days 30 days All errors are 2 

HPO 4 = + Total Diss. P Low 33 P/ 32 P Ratio Rapid Turnover Low 33 P/ 32 P Ratio “Large molecules” Slow Turnover High 33 P/ 32 P Ratio “Small” molecules Rapid Turnover Low 33 P/ 32 P Ratio HPO 4 = + Rapid Turnover Low 33 P/ 32 P Ratio “Large molecules” Slow Turnover High 33 P/ 32 P Ratio PhosphateDissolved Organic P = Total Diss. P High 33 P/ 32 P Ratio =

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 14 C: 12 C background is < 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.

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.

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

Increasing Age All errors are 2  30 days

Rain at Sta. ALOHA 33 P/ 32 P = 0.55  0.19 PO 4  33 P/ 32 P = 0.55 DOP 1 33 P/ 32 P = 0.55 DOP 2 33 P/ 32 P = 0.75 DOP n 33 P/ 32 P > 0.75 “Normal” Growth“P-Stressed”“P-Starved” Bacteria and Algae High Bioavailability Low Bioavailability Short Residence Time Long Residence Time Bjorkman and Karl (2003) measured SRP turnover rates: days BAP turnover rates: days PP turnover rates: < days

This work suggests two important points: 1)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. 2)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.

Increasing Age All errors are 2 

1.If there is abundant sample material, b-counting can provide better precision than AMS for samples older than ~40ka 2.There is a diminishing return for reducing background