Pyrite in the Coastal Everglades, It’s more than Fool’s Gold

\(^3\) Department of Biological Sciences, Florida International University, Miami, FL, USA.

Introduction

Soil pyrite forms where sulfate reduction and associated fermentation reactions occur during anaerobic decomposition of organic matter. Pyrite is a dynamic soil constituent and is sensitive to oxidation-reduction and oxygen conditions. Temporal changes in the quantity and timing of surface water flow and tidal exchange in the coastal zones influence redox conditions within wetland soils and affect iron (Fe) and sulfur (S) concentrations and interactions.

Schematic of pyrite formation in wetland soil. Adapted from Reddy and DeLaune (2009).

Prior studies suggest that the Everglades systems is a pyrite preferred geochemical system. Other metastable Fe-S minerals include mackinawite and greigite which are precursors to the thermodynamically favorable Pyrite.

Objectives

Characterize Fe-S interactions via pyrite formation within the Everglades ecosystem
Evaluate trends in the degree of pyritization (DOP) along regional transects
Investigate DOP response due to sea-level rise within sites in ecotone regions of each transect

Materials and Methods

Data sources

Fig 1. Map of the study area with monitoring locations and ecosystems identified from stations within the Florida Coastal Everglades Long Term Ecological Research network in Everglades National Park.

Soil samples were collected annually during wet season from 17 monitoring sites within ENP (Fig 1) between 2004 and 2015 by syringe core to a depth of 10 cm. Soil samples were analyzed for bulk density, soil organic matter, acid volatile sulfide (AVS), chromium reducible sulfur (CRS) and acid extractable iron (FeHCl) (Chambers and Russell 2017).
Surface water samples were collected approximately monthly via grab sampling and analyzed for total nitrogen and total phosphorus. Salinity data were collected during sampling events (Briceno 2016; Gaiser and Childers 2016; Troxler 2016; Fourqurean 2017).

Data Analysis

Degree of pyritization (DOP) was calculated as:

\[DOP = \frac{0.5\times [CRS]}{0.5 \times [CRS]+[Fe_{HCl}]}\]

Site conditions were classified as:
- Aerobic Conditions: DOP< 0.46
- Anoxic Conditions: 0.46>DOP<0.75
- Anaerobic Conditions: DOP>0.75
All statistical analyses were performed in R (Ver 3.1.2).
Annual mean DOP values for each region and ecosystem were analyzed using Kendall’s Tau trend analysis (base package) and Thiel-Sen slope estimator (zyp package).
Surface water salinity were analyzed using linear model. Model assumptions were tested using global linear model validation (gvlma package).

Results

Fig 2. Annual trends in Degree of Pyritization for each flow path and ecosystem. Values expressed as arithmetic mean ± standard error. Grey line indicated by Theil-Sen slope estimate. Trend analysis was performed for all regions and ecosystems, howerver SRS ecotone was the only statistically significant trend (\(\tau\) = 0.60, \(\rho\) <0.01).

Fig 3.Mosaic plot of degree of pyritization site condition categories by flow path (left; \(\chi^2\) = 34.7,df = 6,\(\rho\) <0.01) and ecosystem (right; \(\chi^2\) = 22.0,df = 6, \(\rho\) <0.01) for data collected between 2004 and 2014 (excluding 2008) within Everglades National Park.

Fig 4. Annual mean salinity trend for Shark River Slough, Taylor Slough and Florida Bay between water year 2005 and 2016. Values expressed as arithmetic mean ± standard error. Site SRS5 significantly increased over the period of record (\(\tau\) = 0.45,\(\rho\) <0.05) at a rate of 0.29 PSU Yr\(^{-1}\). All other sites did not exhibit a significant trend.

Fig 5. Water Year mean surface water salinity by degree of pyritization for Shark River Slough, Taylor Slough and Florida Bay. Linear models were developed for all station however SRS5 was the only statistically significant model (R\(^2\) = 0.56; F\(_{(1,10)}\) = 13.0 \(\rho\) <0.01).

Conclusions

Pyrite formation is dependent upon the availability of Fe, free sulfide (S\(^{-2}\)) and organic matter mediated by microbial dynamics and landscape scale processes such as sea-level rise and upstream water management as apparent by distribution of DOP values between each region and ecosystem (Fig 2 and Fig 3).
For estuarine portions of SRS (i.e. SRS5), annual mean salinity has significantly increased during the period of record (Fig 4). However due to limited data not all regions have been compared.
A concurrent increase in DOP and annual mean salinity in estuarine portions of the SRS was observed (Fig 5). While not statistically significant, general increases in DOP and salinity have been observed in ecotone regions (Fig 2 and Fig 4).
The DOP and salinity relationship is more variable in TS and FB regions.
Based on these results DOP could potentially be used as an indicator of sea-level rise. However more work is needed to understand driving factors related to pyrite formation in TS, Ph and FB regions.

Post-hoc conceptual gradient model for degree of pyritization across the ecosystem gradient within Everglades National Park. Based on median DOP values for the period of record aggregated by ecosystem.

References

Briceno, H. 2016. Surface Water Quality Monitoring Data collected in South Florida Coastal Waters (FCE) from June 1989 to Present.
Chambers, R, and T Russell. 2017. Physical and Chemical Characteristics of Soil Sediments from the Shark River Slough and Taylor Slough, Everglades National Park (FCE) from August 2004 to Present.
Fourqurean, J. 2017. Florida Bay Physical Data, Everglades National Park (FCE), South Florida from September 2000 to Present.
Gaiser, E, and D Childers. 2016. Water Quality Data (Grab Samples) from the Shark River Slough, Everglades National Park (FCE), from May 2001 to Present.
Reddy, K. R, and R. D DeLaune. 2008. Biogeochemistry of wetlands: science and applications. Boca Raton, FL: CRC Press.
Troxler, T. 2016. Water Quality Data (Grab Samples) from the Shark River Slough, Everglades National Park (FCE), from May 2001 to Present.