Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Sediment cores, biomass, and dissolved organic matter were collected from two mangrove locations in Rookery Bay National Estuarine Research Reserve (Florida, USA) and Little Ambergris Caye (Turks and Caicos) during the time period of June 2022 through February 2024 using a 5-centimeter PVC push corer. Sediment cores were extruded in the field and kept at -20 degrees Celsius (°C) until analysis could be carried out back in the lab at UC Santa Barbara along with biogenic material. Dissolved organic matter was kept refrigerated in SPE cartridges until eluted back in the lab at UC Santa Barbara.

To isolate major carbon and sulfur pools, a ~1 gram (g) aliquot of the freeze-dried sample from each sediment interval was subjected to two sequential microwave extractions (MARS-6, CEM, 70°C for 15 minutes) using a 9:1 dichloromethane. Solvent-extracted solids were rinsed with 0.7N NaCl solution, ultra-pure water, freeze dried, and then acid fumigated to remove carbonates for 12 hours with a 6N HCl solution. A 500-milligram (mg) subsample of the same microwave-extracted solid residue was then subjected to a strong acid hydrolysis (6N HCl, 60°C, 2 hours) to extract acid-volatile sulfur (AVS, operationally defined as iron monosulfides) following Canfield et al. 1986 and Raven et al. 2019. The remaining solid residue underwent a chromium (II) chloride extraction at 60°C for 2 hours to isolate chromium reducible sulfur (CRS, operationally defined as pyrite) following Canfield et al. 1986. The leftover solid residue following the CRS extraction contains highly hydrolysis-resistant OM that we refer to as protokerogen (Burdige 2007; Raven et al. 2019). Biogenic material went under the same set of extractions while the dissolved organic matter was eluted with methanol and dried down in a fume hood (Phillips et al. 2022).

The oxidation state and bonding environment of organic sulfur in the post-CRS extraction, post-CRS extraction biogenic material, and dissolved organic matter were characterized using x-ray absorption spectroscopy (XAS). Spectra were obtained on Stanford Synchrotron Radiation Lightsource (SSRL) on beam line 14–3 using a spot size of 0.5 square millimeters (mm2) and a Si(111) (Φ = 90) double crystal monochromator calibrated to the thiol pre-edge peak of thiosulfate at 2472.02 electron volts (eV). Samples were adhered onto Saint Gobain M60 S-free polyester tape and covered with 5-micrometer (µm) SPEX 3520 polypropylene XRF film. Spectra were averaged and normalized in the SIXPACK (Webb 2005) software package using a K-edge E0 of 2473 and pre-edge and post-edge linear normalization ranges of -20 to -7 and 35 to 70 eV, respectively. The relative abundance (percent (%)) of individual sulfur species were determined in sediments and biomass samples using least squares fitting and a set of OS standards (Raven et al. 2021). The relative abundances of different sulfur species were then used to determine the percentage of reduced (disulfide, monosulfide, aromatic) and oxidized (sulfoxide, sulfone/sulfonate, sulfate ester) sulfur in samples.

Data were processed using R version 4.2.2 (2022-10-31) and SIXPACK (Webb 2005).

- Imported original file "988595_v2_mangrove_xas.xlsx" (internal version number) into the BCO-DMO system.
- Created "Year_month" column, containing date in YYYY-MM format.
- Saved the final file as "988595_v1_mangrove_xas.csv".

NSF Award Abstract:
Mangrove forest sediments are important hotspots of organic carbon preservation, and they have the potential to sequester substantial amounts of atmospheric CO2. Currently, however, is it not fully understood why these environments are able to bury so much organic carbon, or how they will respond to future changes in sea level, land use, and climate. This project will investigate a mechanism that may help explain this carbon burial: organic matter sulfurization, the transformation and effective ‘pickling’ of sedimentary organic matter by sulfide. Its central aim is to understand what controls the extent of sulfurization in mangrove sediments, and to estimate the contribution of organic matter sulfurization to sediment carbon storage in different parts of the environment. By providing some of the first constraints on how, when, and where organic matter sulfurization happens in mangroves, the results of this work will guide decisionmakers managing coastal watersheds and carbon stocks in the face of land use, climate and sea level change. As part of this work, four undergraduate students and one PhD student at UC Santa Barbara will gain field and research experience. And, in collaboration with local groups associated with the field site, the team will produce a season of ‘Ocean Solutions’ podcast episodes related to conservation and human impacts of Caribbean mangroves.

The overarching goal of this project is to understand how microbial sulfur cycling affects organic matter preservation in vegetated coastal sediments, which have substantial leverage to impact the global carbon cycle on decadal to millennial timescales. It specifically investigates organic matter sulfurization, which can transform fresh, easily respired organic matter into recalcitrant, polymerized carbon stocks with long-term preservation potential. Although organic matter sulfurization is known to occur in mangrove sediments, the scale of its impact is essentially unknown. A pair of field expeditions will be conducted at a mangrove forest on the southwestern coast of Florida. In the first field season, geochemical profiles will be used to quantify organic matter sulfurization in sediments and its relationships with carbon storage, iron mineralogy, and the characteristics of sedimentary organic matter inputs. In the second field season, cyclic voltammetry will be used to target redox dynamics at the millimeter scale. Laboratory experiments will be conducted to test the susceptibility of various local organic matter sources to sulfurization and characterize their sulfurized forms. Throughout, the project applies a holistic approach to sedimentary organic matter by characterizing the dissolved, lipid, protein/carbohydrate, and proto-kerogen pools with isotopic and spectroscopic techniques. This work will yield a first quantitative, mechanistic framework for predicting the extent of organic matter sulfurization in coastal vegetated habitats and its likely response to changes in ecology, land use, or sea level.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.