Table of Contents

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

I. Field sampling
Sample collection took place during R/V Pelican cruises on the Louisiana Shelf in the northern part of the Gulf of Mexico. At each site, an 8-spot Ocean Instruments Multi-corer collected sediment that was sectioned in 2 cm intervals and placed into 50mL metal clean, acid washed tubes, pre-weighed and labeled microcentrifuge tubes for porosity, and freezer-safe Ziploc bags. Tubes were centrifuged at 3500 rpm for 10 minutes to extract and filter supernatant. The filtered supernatant was placed into a metal clean/acid washed tube and frozen for later metal and dissolved silica analysis. The 50mL tube of sediment was frozen at -20°C for further analysis.

II. Porewater Dissolved Constituents
Where there was sufficient supernatant (porewater) from the centrifuged sediment sample, it was divided for three different analyses: dissolved silica analysis, nutrient analysis, and metal concentrations.  

1. DSi analysis: 50uL of porewater was analyzed for dissolved Si(OH)4 concentration using a spectrophotometric molybdate-blue method (Brzezinski & Nelson, 1986).

2. Skalar Analysis (N+N, NH4, PO4): Porewater was diluted with Milli‐Q (18.2 MΩ * cm) water and run through a Skalar Analyzer for Nitrate and Nitrite, Phosphorus, and Ammonia. For detection limits, please refer to the descriptions in the Parameters section below.

3. ICPMS (Metal Concentrations): Porewater was reconstituted in dilute HNO3 and diluted to minimize salt interferences before analysis on a Thermo Scientific Element XR High Resolution-ICP-MS housed at the University of Southern Mississippi at the Stennis Space Center. The following elements are reported in the data from the ICPMS analysis:  magnesium (Mg), sulfur (S), calcium (Ca), manganese (Mn), iron (Fe), potassium (K), cesium (Cs), uranium (U), lithium (Li), vanadium (V), cobalt (Co), nickel (Ni), copper (Cu), strontium (Sr), molybdenum (Mo), barium (Ba), phosphorus (P), and aluminum (Al), along with phosphorus (P) and silica (Si).  For each element's detection limit, please see the description in the Parameters section below.   

Sediments
Sediments were analyzed for physical properties, chemical properties, phases of silica, and isotopic composition. Each section below describes the specific methods in greater detail.  

III. Sediment properties
Porosity
For porosity measurements, microcentrifuge tubes were placed in a drying oven at 60°C until the sediment was dry. Once dried, tubes were weighed. Porosity was calculated as in Comeaux et al. (2012).

Loss on ignition (LOI)
For loss on ignition (LOI) analysis, dried sediment was ground into a fine consistency using a mortar and pestle. Then 1g of the ground sediment was weighed into pre-muffled and pre-weighed porcelain crucibles. To remove any carbon from the presence of calcium carbonate, samples were fumed by adding 1mL of mili-Q to each crucible and placing samples into a desiccator with 10 mL of 12M HCl for 6 hours (Harris et al. 2001, Ramnarine et al. 2011, Walthert et al. 2010). After 6 hours, samples were placed in a vacuum oven for roughly 16 hours, until dry (Ramnarine et al. 2011, Walthert et al. 2010). Once dry, samples were kept in the oven and weighed one at a time to keep moisture out of samples. If sample was still acidic (yellow in color), 1 mL of mili-Q was added and the sample was dried again. Sediment was ground again into a fine powder.

After the above preparation, fumed and ground sediment was weighed (100 mg) in triplicate into pre-weighed and muffled liquid scintillation (LSC) vials. Samples were combusted at 550°C for 6 hours (Kemp et al. 2021). After combustion, weights were recorded, and loss of organic matter was calculated (Kemp et al. 2021).

IV. Sediment Silica Pools  

Sequential Extractions  
Frozen samples were thawed, homogenized, and 50 mg were weighed in triplicates (per depth) into pre-labeled 50 mL centrifuge tubes (Krause et al., 2017; Pickering et al., 2020). Samples with procedural blanks (in triplicate) then went through the sequential extraction process. The sequential extraction methodology separates silica into operationally defined pools based on kinetics, reaction conditions and reaction sequence (DeMaster, 1981; Michalopoulos and Aller, 2004; Rahman et al., 2016; Pickering et al., 2020).

Operational Definitions
Based on prior studies, we use the following nomenclature:

1. Si-HCl: Mild acid-leachable pre-treatment; Highly reactive silica associated with authigenic clays and metal oxide coatings (Michalopoulos and Aller, 2004).

2. Si-Alk: Mild alkaline-leachable digestion completed after acid pretreatment; Frees reactive silica associated with the biogenic silica pool (Michalopoulos and Aller, 2004).

3. Si-NaOH (Alk): Harsh NaOH digestion done after Si-HCl and Si-Alk (Rahman et al., 2016; Rahman et al., 2017); Associated with the reactive lithogenic Si (LSi) pool and the comparatively refractory “dark bSiO2” (e.g. sponge spicules and Rhizaria).

4. tbSi: Following the traditional definition of biogenic silica (DeMaster, 1981), with no acid pre-treatment.

5. Si-NaOH (TbSi): Harsh NaOH digestion done after T-bSi.

V. Sediment Organic matter

Preparation
Same preparation as that listed above for Loss on Ignition

Particulate Organic Carbon and Nitrogen content and isotopes (δ13C and δ15N)
After fumigation (explained above), ~60 to 70 mg of sediment were packed in 5x9mm silver capsules, which were then packed in tin 5x9mm capsules in triplicate (UC Davis protocol recommendation). Samples were placed in a 96 well plate and kept in a desiccator until shipped to UC Davis for isotopic (δ13C and δ15N) analysis (Krause et al. 2017).

Results from UC Davis had an absolute accuracy for calibrated reference materials of ±0.04 ‰ (±0.05 ‰ SD) for δ13C and ±0.05 ‰ (±0.05 ‰ SD) for δ15N. Core depth 24-26cm was the only sample below detection (9.7ug) for δ15N, which is represented as 0 on the data sheet.

Raw data were input into Microsoft Excel (Version 2302) to calculate final reported values.

• Porosity was calculated as in Comeaux et al. (2012).
• Loss of organic matter was calculated as in Kemp et al. (2021).

- Imported data from source file "Extreme_Si_MASTER_BCODMO.xlsx" into the BCO-DMO data system.
- Added R/V Pelican official cruise IDs corresponding to the submitted cruise name.
- Combined separate date and time columns into a single datetime column
- Kept local datetime but added a column for ISO8601 formatted UTC datetime
- Removed percent (%) signs from the column values
- Modified parameter (column) names to conform with BCO-DMO naming conventions. The only allowed characters are A-Z,a-z,0-9, and underscores. Replaced spaces with underscores.

NSF Award Abstract:
This project will take advantage of the passage of Hurricane Ida across the northern Gulf of Mexico shelf in August, 2021 to study important aspects of the cycling of silica in coastal sediments. In coastal systems, water column primary productivity is dominated by diatoms, a group of phytoplankton which produce a shell of amorphous biogenic silica. This biogenic silica can either be buried in its original unaltered form or undergo chemical reactions that convert it to aluminosilicate minerals (e.g. marine clays). This latter process is important in global chemical budgets for many elements, including carbon. One of the factors that influences whether silicon is buried as biogenic silica or converted to aluminosilicates may be the amount of oxygen in the sediments. Storms mix the ocean waters and can add oxygen to sediments in shallow water, potentially changing the silica balance. The investigators collected sediment samples in early August, 2021, two weeks before Hurricane Ida. Sampling through the year after the storm will allow them to test whether storms affect silica cycling. This project will support an early-career investigator and undergraduate student researchers.

Tropical and subtropical coastal deposition systems sequester 25-40% of the global silica sink, a disproportionately large impact relative to their area. In the northern Gulf of Mexico seasonal water column stratification may impact how sedimentary biogenic silica is processed and preserved by limiting oxygen injections into the benthos, seasonally driving biogenic silica burial efficiency. Hurricane Ida moved through the Mississippi Delta and adjacent Louisiana shelf at the end of August 2021. Thirteen days prior to Hurricane Ida’s landfall in Port Fourchon, LA, in August 2021, the team collected sediment cores at 3 sites in the topset delta sediment between the Southwest Pass (major Mississippi River distributary) and Cocodrie, LA; all sites experienced Category 4 hurricane conditions from Ida with maximum sustained winds of 130 knots (67 m/s). Ida, a much stronger storm than either Hurricanes Harvey or Nate in 2017, likely introduced oxygen into the sediment of the proximal coastal zone, perhaps enough to turn most of the top meter or so of the sediment column oxic. The investigators hypothesize that the storm-induced change in redox and diagenetic conditions initially favored burial of biogenic silica rather than an authigenic aluminosilicate. Using a multi-proxy approach, this project will analyze how this major storm event altered biogenic silica burial over the course of ten months.

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.