Dataset: Sediment porewater and particulates of Louisiana Shelf (pre-Ida)

I. Field sampling
Sample collection took place during R/V Pelican cruise PE22-05 in the northern part of the Gulf of Mexicao, specifically the Louisiana Shelf region dominated by the discharge of the Mississippi River plume. 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 analysis of metal and dissolved silica. 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 (DSi), nutrient analysis (Skalar), and metal concentrations (ICPMS).  

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.