Research interests Effects of benthic organisms on particles

Research interests: Effects of benthic organisms on particles and porewater Porewater constituents of interest and “bioirrigation”: 222 Rn, 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 Outer shelf Off shelf Middle shelf Coastal domain 50 m 100 m 200 m

Sediment profiles of organic-matter oxidants 0 0 1 2 3 4 5 6 7 8 9 10 Dissolved Oxygen (µM) 20 40 60 Mn (µmole ml-1 sediment) 2 4 6 Pore water Mn-oxide 0 0 1 2 3 4 5 6 7 8 9 10 Nitrate (µM) 20 40 60 Station KNR 53, 2009 2800 m depth Fe (µmole ml-1 sediment) 0 20 40 60 Pore water Fe-oxide Iron reduction Denitrification Manganese reduction Aerobic respiration Sulfate reduction -612 CH FE(OH) 30 CH 24 →H 612 N HCO 624 O +S 2 12 Mn. O 624 → 242 -+36 -→ CO +26 H O→ 6 OCO SO ++212 HCO 324 220++18 20 2→ 2 NO 2 H 33 --1 - -1 24 HCO +ΔG°=-2. 82 6 Fe CO 2+2+2 ++ 48 18 H 12 Mn 24 HCO (k. J mol ΔG°=-0. 309 (k. J mol 33 )+ )18 48 H 2 O 2 O ΔG°=-0. 79 (k. J mol-1) ΔG°=-2. 66 ΔG°=-2. 38

Saint Lawrence Island Decre ased Alaska ic OC benth flux ed s a re thic n e b Dec Expected geographic patterns organic carbon flux to the benthos flux C O 50 m 100 m 200 m

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

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

Relevant measurements Depth (cm) • O 2 consumption in flux core incubations analyzed by Optode and MIMS (O 2/Ar) • N 2 production in flux cores by MIMS and IRMS (NO 3 reduction + ANAMMOX) • Mn- and Fe-oxide reduction from concentration profiles + bioturbation rates • SO 4 - reduction by 35 SO 4 - incubation • Quantitative samples of benthic infauna 238 U → 234 Th Excess 234 Th (dpm g-1) 0 50 0 0, 5 234 Th 1 1, 5 2 2, 5 (24 d)

25 20 15 10 5 0 Northern Middle Shelf Outer Shelf Off Shelf 10 8 6 4 2 0 Northern Middle Shelf Outer Shelf Off Shelf N 2 production (mmol m-2 d-1) 30 Sulfate reduction (mmol m-2 d-1) Oxygen consumption (mmol m-2 d-1) Fe reduction (mmol m-2 d-1) Rates of organic-carbon mineralization processes 2. 5 2 1. 5 1 0. 5 0 Northern Middle Shelf Outer Shelf Off Shelf 10 8 6 4 2 0

te lfa Su n io n ct io du re ct du n n tio ct io du re -re Fe n- ca Fe t -re ion Su lfa duc ti te re on du ct io n n tio du c nre M bi ro c n 8 M ifi itr De n Ae at io xid lo ta To 10 ca c 3, 5 3 2, 5 2 1, 5 1 0, 5 0 ifi itr bi ro n Outer Shelf Ae io at xid n ct io du re tio c -re n d uc lfa ti te re on du ct io n Su Fe n- M ca bi n Northern Shelf De n lo ta To n io n ct du re ct du n tio ct io du re -re te lfa Su Fe n- M ca 9 8 7 6 5 4 3 2 1 0 ifi ro Ae at io xid itr De n lo ta To Carbon Oxidation (mmol m-2 d-1) 12 10 8 6 4 2 0 ifi c n bi ro Ae io at xid itr De n lo ta To Carbon Oxidation (mmol m-2 d-1) Rates and approximate pathways of organic-carbon mineralization Middle Shelf 6 4 2 0 Off Shelf

0, 9 0, 8 0, 7 0, 6 0, 5 0, 4 0, 3 0, 2 0, 1 0 lf he fs he rs te Ou e dl id M Of f el sh ner rth No lf Aerobic Anaerobic f Fraction OC oxidation Regional variation in aerobic versus anaerobic respiration

What causes this pattern? • Variation in organic carbon supply • Variation in benthic communities and bioturbation Mean bioturbation rate from 234 Thxs 10 Db (cm 2 y-1) ± s. d. 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 8 6 4 2 he lf Of fs r te Ou e dl id M n No rth er f el sh r Of f te Ou e dl M id er n 0 rth No Deposit feeders m-2 ± s. d. Mean abundance of deposit feeders

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

te lfa Su ct io du re n n c n io n ct du -re Fe tio ct io du re n- M ca ifi itr n bi ro Ae io at xid 12 De n lo ta To Carbon Oxidation (mmol m-2 d-1) Organic carbon mineralization pathways Northern Shelf 10 8 6 4 2 0

Nitrogen cycle in marine sediment Water column Organic Rain NH 4 + NO 3 - NO 2 - N 2 O N 2 O 2 PON NH 4 + nitrification NO 2 dentirification NO 3 - NO 2 - N 2 O dentirification Sediment anammox N 2

Sedimentary denitrification rates (N 2/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−(PO 4− 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 Tg. N y-1 or about 16% of N uptake by phytoplankton • Denitrification exacerbates N limitation of primary productivity on the Bering shelf

Excess 234 Th (dpm g-1) Calculating rates of iron reduction 0 50 0 0, 5 Depth (cm) Bioturbation Sedimentation Reactions 1 1, 5 2 Bioturbation rate DB 2, 5 "Excess" Fe (μmol ml-1 sediment) Depth (cm) 0 0 1 2 3 4 5 6 7 8 9 10 10 20 Iron reduction rate 30

Rates and patterns of iron reduction 10 Fe reduction (mmol m-2 d-1) 10 8 6 4 2 0 5 2 1 5 0. 2 R 2 = 0. 265 p = 0. 003 0. 1. 05 Northern Shelf Middle Shelf Outer Shelf Off Shelf 0 5 10 15 Oxygen flux (mmol m-2 d-1) 20

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

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

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