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

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

Arctic Ocean North Pacific Ocean

Sigler et al. 2010

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

Sediment profiles of organic-matter oxidants 1 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 CH20 + 24 CO2 + 24 FE(OH)3→ 24 Fe2+ + 48 HCO3- + 18 48 H2O ΔG°=-0.79 (kJ mol-1) Denitrification 30 CH20 + 24 NO3- → 12N2 + 24 HCO3- + 6 CO2 + 18 H2O ΔG°=-2.66 (kJ mol-1) Manganese reduction 6 CH20 + 18 CO2 + 6 H2O + 12 MnO2→ 12Mn2+ + 24 HCO3- ΔG°=-2.38 (kJ mol-1) Aerobic respiration 6 CH20 + 6 O2 → 6 CO2 + 6 H2O ΔG°=-2.82 (kJ mol-1) Sulfate reduction 12 CH20 + 6 SO42- → 6 H2S + 12 HCO3- ΔG°=-0.309 (kJ mol-1) Free energies for glucose oxidation

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

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

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

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)

Rates of organic-carbon mineralization processes 30 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

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

Regional variation in aerobic versus anaerobic respiration

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

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

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

Nitrogen cycle in marine sediment NO 3 - 2 N O NH 4 + PON nitrification dentirification anammox Organic Rain Water column Sediment N2O Direct vs. coupled nitrification\denitrification and anammox

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

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

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

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

Rates and patterns of iron reduction 2 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 = 0.265 p = 0.003 0 5 10 15 20 Oxygen flux (mmol m-2 d-1)

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

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)

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?

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

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