LONG-RANGE TIME DOMAIN CORRELATION: AN EMERGING BIOGEOCHEMICAL SYSTEM MEASUREMENT ELUCIDATES CHANGES IN DOMINANT GEOCHEMICAL PROCESSES

Correlation analysis of environmental electrochemical time series is an emerging biogeochemical technique. Analyzing a suite of time series provides insight into the nature of the geochemical environment. Four time series from an oxic-anoxic cyclical bioreactor experiment was analyzed using detrended fluctuation analysis. A sediment suspension from a hyper-eutrophic freshwater marsh was exposed to alternating 7-day periods of purging with air and N₂ gas for 74 days¹. Differences in strength of temporal scaling in each geochemical setting serve to distinguish individual environments on the basis of electrochemical response. pH, redox potential (E_h), dissolved oxygen (DO), and carbon dioxide (CO₂) were measured every 20 min. Scaling exponents for three different geochemical environments: aerobic respiration, NO₃^- reduction, and mixed Fe(III), Mn²⁺, SO₄^2- reduction were calculated. Correlation strength ranges from 0 – 2; 0 – 0.5 is anticorrelation, 0.5 is no correlation, and 0.5 – 2 is progressively stronger correlation.

During anoxic-oxic transitions, E_h increases >300 mV and O₂ saturation increases to 68.5% within 1 h of N₂ – air transition (95% within 5 h). pH increases by 0.3 within 6 h; such that steady state conditions are reestablished within 6 h. This contrasts the observed scaling behavior during aerobic respiration, characterized as follows: very strong correlation of pH (1.74 ± 0.04), and E_h (1.72 ± 0.04), strong correlation of DO (1.35 + 0.04), and moderate correlation of CO₂ (0.94 + 0.05). During oxic-anoxic transitions; following N₂ influx, O₂ is depleted within 2 h, E_h decreases by 100 mV within 1 h, and CO₂ levels increase for 6 h then plateau, as does pH. During NO₃^- reduction, exponents indicate very strong correlation of CO₂ (1.68), and strong correlation of pH (1.22) and E_h (1.38), and weak correlation of DO (0.82). Periods of mixed Fe(III), Mn²⁺, and SO₄^2- reduction are characterized by very strong correlation of pH (1.67 ± 0.02) and E_h (1.77 ± 0.02), uncorrelated DO (0.59 ± 0.01), and moderately correlated CO₂ (0.94 ± 0.02). Scaling exponents >1.60 have been linked to active biological cycling of electrolytes. Collectively, the scaling behavior makes it possible to distinguish the three geochemical states.

¹Parsons CT, Rezanezhad F, O’Connell DW, Van Cappellen P (2017) Biogeosciences 14, 3585-3602.