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Subsurface C dynamics

Wetlands Meeting 2025

June 20th, 2025

Susan Pit

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Outline

  • Background
  • Updates
  • Pristine Marsh: Yampah
    • Transport regime
    • Reactions
    • Preliminary model results
  • Future Plans

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Next Steps

Model Results

Yampah Marsh

Updates

Background

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Which reactions dominate subsurface C decomposition?

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Next Steps

Model Results

Yampah Marsh

Updates

Background

(O’Meara et al., 2024)

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Porter

North

Yampah

Hester

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Sample collection

Dry/Warm Season

Wet/Cold Season

High Tide

Low Tide

Vegetated

Mud

Vegetated

Mud

Vegetated

Mud

Vegetated

Mud

High Tide

Low Tide

x2

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Updates since June 2024

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Next Steps

Model Results

Yampah Marsh

Updates

Background

  1. Two field campaigns completed (Dry 24 and Wet 25)
  2. Lots of sample analysis
  3. New component added: Fe speciation in sediments
  4. Mathematica reactive transport model is largely running
  5. Started on PFLOTRAN reactive transport model

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Subsurface carbon dynamics in a pristine salt marsh

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Yampah sampling site

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Transport Regime

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Next Steps

Model Results

Yampah Marsh

Updates

Background

  • Assume no lateral transport
  • Diffusion or advection?
  • Evaporation
    • Seasonal shift
    • Diurnal cycle intensity?

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Respiration:

Nitrate,

Fe(II) and

Sulfate

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Next Steps

Model Results

Yampah Marsh

Updates

Background

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Respiration:

Nitrate,

Fe(II) and

Sulfate

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Next Steps

Model Results

Yampah Marsh

Updates

Background

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More respiration means vegetated cores have more dissolved C

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Next Steps

Model Results

Yampah Marsh

Updates

Background

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BSR/AOM at depth

​

S ox. at surface

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Next Steps

Model Results

Yampah Marsh

Updates

Background

AOM:

CH4 + SO42- + H+ β‡’ H2S + HCO3- + H2O

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BSR:

OM+ SO42- β‡’ H2S + 2HCO3- + 0.3NH4+

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Methanogenesis at low tide?

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AOM at high tide?

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Next Steps

Model Results

Yampah Marsh

Updates

Background

AOM

Methanogenesis

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Large interanual

differences

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Next Steps

Model Results

Yampah Marsh

Updates

Background

AOM

Methanogenesis

S Oxidation

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Ξ”TA:DIC ratios:

combination of denitrification, BSR, Fe reduction, and some aerobic respiration

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Methanogene

Vegetated Mud

Wet Season Dry Season

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Preliminary Conclusions

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Next Steps

Model Results

Yampah Marsh

Updates

Background

  1. Subsurface processes that control C cycling in tidal salt marshes are dynamic
    1. Methane has two distinct seasonally controlled patterns
  2. Anaerobic processes are dominant during the dry/warm season
  3. In the less reducing wet/cold season, both anaerobic and aerobic processes are relevant
  4. Depth correlated changes in sulfate isotopes indicate BSR and AOM are important.

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Mathematica RTM

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Next Steps

Model Results

Yampah Marsh

Updates

Background

  • Written by Pei Chuang (Chuang et al., 2016)
  • POC degradation constrained by sulfate depletion
  • Transport constrained by diffusion and burial velocity of sediments

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Assumption: each POCdegr produces 1 CH4 and 1 HCO3-.

Included reactions:

  • BSR
  • Methanogenesis
  • AOM by SO4
  • Fe(III) reduction
  • Multiple forms of pyrite precipitation

Depth [cm]

SO4 depletion [mM]

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Preliminary results

Vegetated core, Dry22

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Reaction Rates [umol cm-2 yr-1]

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High Tide

Low Tide

POC degradation

187

273

BSR

78

61

Methanogenesis

78

61

AOM

109

153

*Depth integrated

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Next Steps

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Next Steps

Model Results

Yampah Marsh

Updates

Background

  1. More sample analyses
  2. Send out remaining samples for external analysis
  3. Complete RTM for Yampah Marsh
  4. Compile and QAQC Hester, Porter, North data

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Questions?

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Wolf-Gladrow et al., (2007)

Liu et al., (2021)

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Next Steps

Model Results

Yampah Marsh

Updates

Background

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Next Steps

Model Results

Yampah Marsh

Updates

Background

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Next Steps

Model Results

Yampah Marsh

Updates

Background

Alongi (2020)