1. 222Rn, oxygen, nutrients (nitrate, ammonia, phosphate)
Research interests:
Effects of benthic organisms on particles and porewater
Porewater constituents of interest and “bioirrigation”:
222Rn, oxygen, nutrients (nitrate, ammonia, phosphate)
Current research topics: mercury methylation, denitrification
Particles and “bioturbation”:
Radionuclides, beads, contaminants, organic matter, pigments
dinoflagellate cysts

2. Bioturbation, organic matter mineralization and the cycling of nitrogen and iron in Bering Sea sediments
Geographic domains of the Bering Sea
Patterns and pathways of sedimentary organic matter mineralization
Denitrification
Iron reduction
Implications and future directions

3. Arctic Ocean
North Pacific Ocean

4. Sigler et al. 2010

5. Principal domains of the Bering Sea
Saint Lawrence Island
Alaska
Northern shelf
Middle
shelf
Coastal
domain
Outer shelf
Off shelf
50 m
100 m
200 m

6. Sediment profiles of organic-matter oxidants1 2 3 4 5 6 7 8 9 10 20 40 60
Dissolved Oxygen (µM)
Nitrate (µM)
Station KNR 53, 2009
2800 m depth
Fe (µmole ml -
sediment)
Pore water Fe
oxide
Mn (µmole ml Mn
Iron reduction
6 CH CO FE(OH)3→
24 Fe HCO H2O
ΔG°=-0.79 (kJ mol-1)
Denitrification
30 CH NO3- → 12N HCO3-
+ 6 CO H2O
ΔG°=-2.66 (kJ mol-1)
Manganese reduction
6 CH CO2 + 6 H2O + 12 MnO2→
12Mn HCO3-
ΔG°=-2.38 (kJ mol-1)
Aerobic respiration
6 CH O2 → 6 CO2 + 6 H2O
ΔG°=-2.82 (kJ mol-1)
Sulfate reduction
12 CH SO42- → 6 H2S + 12 HCO3-
ΔG°= (kJ mol-1)
Free energies for glucose oxidation

7. Decreased benthic OC flux
Saint Lawrence Island
Alaska
Decreased benthic OC flux
Decreased benthic OC flux
50 m
100 m
200 m
Expected geographic
patterns organic carbon
flux to the benthos

8. Hypotheses regarding Organic Carbon Mineralization
Organic matter oxidation pathways vary with latitude, water depth, and among Bering Shelf “domains”
Rate of organic matter mineralization decreases from: Northern shelf →Middle shelf → Outer shelf → Off shelf
Ratio of anaerobic to aerobic respiration decreases from: Northern shelf → Middle shelf → Outer shelf → Off shelf

9. Principal domains of The Bering Shelf Saint Lawrence Island
Northern shelf
Middle
shelf
Coastal
domain
Outer shelf
Off shelf
50 m
100 m
~ 125 stations
200 m
Principal domains
of The Bering Shelf

10. Relevant measurements
234Th →
O2 consumption in flux core incubations analyzed by Optode and MIMS (O2/Ar)
N2 production in flux cores by MIMS and IRMS (NO3- reduction + ANAMMOX)
Mn- and Fe-oxide reduction from concentration profiles + bioturbation rates
SO4- reduction by 35SO4- incubation
Quantitative samples of benthic infauna
234Th (24 d)

11. Rates of organic-carbon mineralization processes30
2.5 25 2 20
Oxygen consumption (mmol m-2 d-1)
N2 production (mmol m-2 d-1)
1.5 15 1 10 5
0.5
Northern
Shelf
Middle
Shelf
Outer
Shelf
Off
Shelf
Northern
Shelf
Middle
Shelf
Outer
Shelf
Off
Shelf 10 10 8 8
Fe reduction (mmol m-2 d-1) 6
Sulfate reduction (mmol m-2 d-1) 6 4 4 2 2
Northern
Shelf
Middle
Shelf
Outer
Shelf
Off
Shelf
Northern
Shelf
Middle
Shelf
Outer
Shelf
Off
Shelf

12. Rates and approximate pathways of organic-carbon mineralization
Carbon Oxidation
(mmol m-2 d-1)
Carbon Oxidation
(mmol m-2 d-1)

13. Regional variation in aerobic versus anaerobic respiration

14. What causes this pattern?
Variation in organic carbon supply
Variation in benthic communities and bioturbation
Mean abundance of deposit feeders Mean bioturbation rate from 234Thxs

15. Significance of organic carbon mineralization pathways
Total carbon mineralization estimates export of carbon to seafloor
Rates of anaerobic carbon respiration vary slowly, and potentially respond to longer time-scale changes in Bering Sea
Some of these pathways significantly effect ecosystem function

16. Organic carbon mineralization pathways
Carbon Oxidation
(mmol m-2 d-1)

17. Nitrogen cycle in marine sedimentNO 3 - 2 N O NH 4 +
PON
nitrification
dentirification
anammox
Organic
Rain
Water column
Sediment
N2O
Direct vs. coupled nitrification\denitrification and anammox

18. Sedimentary denitrification rates (N2/Ar)
Spring Summer
Denitrification: (mmol N m-2 d-1)
Membrane-inlet Mass spectrometry Isotope-ratio mass spectrometry

19. Nitrogen in the water column
N** = DIN−(PO4−3 *15:5) + 5.9
2009
Bottom waters depleted in N
Surface waters in summer depleted in phosphorus
2010
Water column depleted in N spring and summer

20. Significance of denitrification in Bering Sea sediments
Little nitrogen regeneration in Bering Sea sediments – sediments are a strong sink for nitrogen
Denitrification is primarily coupled nitrification-denitrification
Large water column deficits in nitrogen result in winter
Denitrification removes about 7 TgN y-1 or about 16% of N uptake by phytoplankton
Denitrification exacerbates N limitation of primary productivity on the Bering shelf
Avg DiN flux across the shelf was zero

21. Calculating rates of iron reduction
Bioturbation Sedimentation Reactions
Bioturbation rate DB
Iron reduction rate

22. Rates and patterns of iron reduction2 4 6 8 10
Northern
Shelf
Middle
Outer
Off
Fe reduction (mmol m-2 d-1) 10 5 2 1
0.5
0.2
0.1
.05
Fe reduction (mmol m-2 d-1)
R2 = p = 0.003
Oxygen flux (mmol m-2 d-1)

23. Variation in water-column N and Fe
Aguilar-Islas et al 2007

24. Significance of Fe reduction in Bering Sea sediments
Fe reduction accounts for 10% of organic carbon oxidation in the Northern Bering Sea
Just 3% of reduced Fe flushed from the sediments via bioirrigation would satisfy the iron requirements of phytoplankton in the Bering Sea
Benthic processes could account in part for the shift from nitrogen limitation to iron limitation of primary productivity in the Bering Sea
(0.01 mol Fe: 106 mol C)

25. New questions What is the rate of Fe flux in Bering Sea sediments?
How does this rate vary with rates of bioirrigation?
What mixing processes might transport Fe in bottom water to the euphotic zone?

26. Bioturbation, organic matter mineralization and the cycling of nitrogen and iron in Bering Sea sediments
Regional variation in organic carbon mineralization pathways reflects
Organic carbon export
Bioturbation
Benthic mineralization processes have important consequences for Bering Sea ecosystem function
Denitrification exacerbates N limitation on the Bering Shelf and lowers nitrogen concentrations
Rates of iron reduction on the shelf are rapid and possibly contribute to the switch from N to Fe limitation moving offshore
Lots of unanswered questions

27. 222Rn, oxygen, nutrients (nitrate, ammonia, phosphate)
Research interests:
Effects of benthic organisms on particles and porewater
Porewater constituents of interest and “bioirrigation”:
222Rn, oxygen, nutrients (nitrate, ammonia, phosphate)
Current research topics: mercury methylation, denitrification
Particles and “bioturbation”:
Radionuclides, beads, contaminants, organic matter, pigments
dinoflagellate cysts