http://lod.bco-dmo.org/id/dataset/786508
eng; USA
utf8
dataset
Highest level of data collection, from a common set of sensors or instrumentation, usually within the same research project
Biological and Chemical Oceanography Data Management Office (BCO-DMO)
Unavailable
508-289-2009
WHOI MS#36
Woods Hole
MA
02543
USA
info@bco-dmo.org
http://www.bco-dmo.org
Monday - Friday 8:00am - 5:00pm
For questions regarding this resource, please contact BCO-DMO via the email address provided.
pointOfContact
2020-01-08
ISO 19115-2 Geographic Information - Metadata - Part 2: Extensions for Imagery and Gridded Data
ISO 19115-2:2009(E)
Stable isotopes in reactive silica pools of Mississippi River plume sediments collected aboard the R/V Pelican in May 2017
2020-01-08
publication
2020-01-08
revision
Marine Biological Laboratory/Woods Hole Oceanographic Institution Library (MBLWHOI DLA)
2020-03-02
publication
https://doi.org/10.1575/1912/bco-dmo.786508.1
Rebecca A. Pickering
Dauphin Island Sea Lab
principalInvestigator
Jeffrey W. Krause
Dauphin Island Sea Lab
principalInvestigator
Kanchan Maiti
Louisiana State University Dept. of Oceanography and Coastal Science
principalInvestigator
Biological and Chemical Oceanography Data Management Office (BCO-DMO)
Unavailable
508-289-2009
WHOI MS#36
Woods Hole
MA
02543
USA
info@bco-dmo.org
http://www.bco-dmo.org
Monday - Friday 8:00am - 5:00pm
For questions regarding this resource, please contact BCO-DMO via the email address provided.
publisher
Cite this dataset as: Krause, J., Maiti, K. (2020) Stable isotopes in reactive silica pools of Mississippi River plume sediments collected aboard the R/V Pelican in May 2017. Biological and Chemical Oceanography Data Management Office (BCO-DMO). (Version 1) Version Date 2020-01-08 [if applicable, indicate subset used]. doi:10.1575/1912/bco-dmo.786508.1 [access date]
Dataset Description: <p>Stable isotopes in reactive silica pools of Mississippi River plume sediments collected aboard the R/V Pelican in May 2017</p> Methods and Sampling: Stable silicon isotopes (e.g. δ30Si) in sediment biogenic silica (bSi) are widely used as a paleoproxy for marine silicic acid usage by pelagic diatoms. Despite the growing body of work that uses bSi δ30Si signals, there are a lack of δ30Si data on other reactive pools of Si in sediments. This oversight misses valuable information on early diagenetic products and potentially biases existing sedimentary bSi δ30Si, which only quantified bSi fractions not altered by diagenesis. For the first time, we quantified δ30Si among operationally defined reactive Si pools (using a pre-leach of mild acid prior to alkaline digestion) in Mississippi River plume sediments. We compared the δ30Si signal within these reactive Si pools to a traditional alkaline-only digestion of sedimentary bSi. These data offer proof of concept that δ30Si is a higher throughput approach for quantifying isotopic properties among reactive Si pools marine sediments vs. the more laborious (albeit powerful) examination of natural silicon radioisotopes in these chemical leaches.
Core Sampling
Briefly, samples were acquired from the study area using an Ocean Instruments MC-900 Multi-corer, which preserved the sediment-water interface during recovery. Overlying bottom water was removed, cores were sectioned into 1cm slices, homogenized, packed under N2 gas and frozen at -20o C for further analysis.
Operational Definitions
Operational reactive Si pools have previously been defined by Rahman et al. (2016) but for consistency and clarity with previous literature (DeMaster, 1981; Michalopoulos and Aller, 2004; Qin et al., 2012; Wang et al., 2015; Rahman et al., 2016; Krause et al., 2017) it has been restated here. Therefore 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: 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, Maldonado et al., 2019).
4. T-bSi: Following the traditional definition of biogenic silica (DeMaster, 1981), with no acid pre-treatment.
Reactive Silica Pools
Frozen sediment samples were thawed to room temperature (22o C) and triplicate ~50-100 mg subsamples were immediately weighed into 50 mL polyethylene centrifuge tubes. Samples were never dried or ground before/during extractions. Procedural blanks were also prepared in triplicate. Additional subsamples of sediment were dried at 60o C to obtain correction for water content.
Sequential Extractions
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).
Acid Leachable Silica (Si-HCl)
Sediment extractions occurred at room temperature (22o C) using Honeywell Fluka Trace SELECT 0.1 N HCl for 12 hrs, while keeping particles suspended via constant motion. Following centrifugation, the Si-HCl leachate was removed and neutralized. Remaining sediment was rinsed in triplicate with Milli-Q water to remove any residual acid (Michalopoulos and Aller, 2004). As it had previously been demonstrated by Rahman et al. (2016) that the rinses contained minor amounts of Si these rinses were discarded. A weak HCl molarity was purposely chosen to remove metal coatings, authigenic phases, and activate bSi surfaces while not affecting the sequential Si-Alk digestion (Michalopoulos and Aller, 2004).
Mild Alkaline Leachable Si (Si-Alk)
The remaining sediment from the acid pre-treatment was subsequently digested with 0.1 M Na2CO3 (Fisher Scientific Certified ACS) for 20 mins in a 85o C water bath. Following the 20 min timepoint, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, the Si-Alk leachate was removed and stored for further use. The process was stopped after 20 mins to ensure the absence of lithogenic material (DeMaster, 1981; Michalopoulos and Aller, 2004) and certify that the clear majority of solubilized silica present is biogenic. Fresh 0.1 M Na2CO3 was added to the samples and the digestions were continued for a total of 5 hrs (DeMaster, 1981) to completely remove the bSi phase. Concluding after 5 hrs, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, the leachate was removed and discarded. Remaining sediment was rinsed in triplicate with Milli-Q water to remove any residual Na2CO3 and again the rinses were discarded.
Harsh NaOH Digestion (Si-NaOH)
The remaining sediment from the Si-Alk treatment was subsequently digested with Honeywell Fluka 4 M NaOH for 2 hrs in a 85o C water bath. After 2 hrs, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, the Si- NaOH leachate was removed, the remaining sediment was rinsed with Milli-Q water to remove any residual leachate and this rinse was added to the Si-NaOH leachate and stored for further analysis (Rahman et al., 2016).
Traditional bSi Digestion (T-bSi)
Additionally, a second treatment following the traditional definition of biogenic silica (DeMaster, 1981), with no acid pre-treatment was used to derive δ30Si from traditional bSi measurements. New subsamples of sediment were weighed out. 0.1 M Na2CO3 was added to samples and heated in a 85o C water bath for 20 mins to remove the bSi phase. Following the 20 min timepoint, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, leachate was removed and stored for further use. Similar to the Si-Alk digestions, the process was stopped after 20 mins to ensure the absence of lithogenic material.
A 1 ml aliquot of each resulting liquid (Si-HCl, Si-Alk, Si-NaOH and T-bSi) was analyzed for dissolved SiOH4 concentration (dSi) as described by Brzezinski and Nelson (Brzezinski and Nelson, 1986) using the molybdate-blue method on a Genesys 10S UV-Vis Spectrophotometer. The remaining supernatants were concentrated via evaporation at 100o C and stored following DeMaster (1980) in preparation for stable isotope analysis.
Stable Isotope Analysis
Sample purification and isotope analysis were carried out at the University of Bristol Isotope Group laboratories. Concentrated sample fluids were purified via cation ion exchange chromatography (Bio-Rad AG50W-X12, 200-400 mesh cation exchange resin in H+ form). Purified solutions were analyzed in duplicate for Si isotopes (28Si, 29Si, 30Si) using a multi collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS, Finnigan Neptune s/n 1002), equipped with CETAC PFA spray chamber and PFA nebulizer (100ul/min). A standard-sample-standard bracketing procedure with Mg doping following Cardinal et al., (2003) was used to correct for both instrumental mass bias and matrix effects. Additionally, sample and standard solutions were both doped with 0.1 M H2SO4 (ROMIL UpA) and 1 M HCl (in-house distilled) to reduce any matrix effects from anion loading and guarantee matrix matching between sample and standard (Hughes et al., 2011). All isotopic composition results are expressed as δ30Si, corresponding to the silicon isotopic abundances in samples relative to the international reference standard NBS-28 (NIST RM8546, purified quartz sand). Reference standards Diatomite (Reynolds et al., 2007) and LMG08 (sponge) (Hendry et al., 2011) were run in tandem with samples to assess long- term reproducibility. Average measured values are reported as +1.27 ± 0.09‰ (n=75) and -3.47 ± 0.16‰ (n=27) (±SD) respectively, which are well within agreement with published values (Reynolds et al., 2007; Hendry et al., 2011). All samples and standards are consistent with the kinetic mass fractionation law (Reynolds et al., 2007) with the δ29Si = 0.518xδ30Si. Procedural blanks were lower than the detection limit and thus considered negligible on δ30Si of the samples.
Major Metal Compositions and Corrections
Additional thawed/wet sediment subsamples were used for duplicate sequential extractions and digestions (Si-HCl, Si-Alk, Si-NaOH and T-bSi) run as previously described. Supernatants were concentrated via evaporation at 100o C and fluids were reconstituted in 2% HNO3 (in-house distilled) to determine major ion concentrations on an Agilent 7700 Series ICP-MS. The instrument was calibrated using a blank and seven matrix-matched, mixed standards. Internal standardization during analysis was monitored via the addition of (50 μl, 10,000 ppb) 115In and 4Be to all standards and samples. Using Aluminum (Al):Si corrections (Kamatani and Oku, 2000; Ragueneau et al., 2005), both Si-Alk and T-bSi δ30Si signals (‰) and mass of Si released (μmol/g) were adjusted for bias from lithogenic material (however, this was more important for the mass of Si, as isotopic content was derived from 30-minute digestions, opposed to 5 hour digestions for the former).
Organic Matter
Sediment total organic carbon (TOC) and total organic nitrogen (TON) content were analyzed at the Dauphin Island Sea Lab using a Costech elemental combustion system (4010 ECS) following vapor phase acidification to remove carbonates. Briefly, dried sediment samples were placed in a glass desiccator and reacted with reagent-grade 12N HCl vapor for 24 hrs at room temperature. Samples were then dried at 60o C overnight to remove remaining HCl and water content before TOC/TON analyses (Yamamuro and Kayanne, 1995).
Funding provided by NSF Division of Ocean Sciences (NSF OCE) Award Number: OCE-1558957 Award URL: https://www.nsf.gov/awardsearch/show-award?AWD_ID=1558957
completed
Rebecca A. Pickering
Dauphin Island Sea Lab
+46 46 222 39 78
Department of Geology - Room 328 Sölvegatan 12, 223 62
Lund
Sweden
rebecca.pickering@geol.lu.se
pointOfContact
Jeffrey W. Krause
Dauphin Island Sea Lab
251-861-2141 x2289
101 Bienville Blvd
Dauphin Island
AL
36528
USA
jkrause@disl.edu
pointOfContact
Kanchan Maiti
Louisiana State University Dept. of Oceanography and Coastal Science
225-578-4531
1143 Energy Coast Env Bldg
Baton Rouge
LA
70803
USA
kmaiti@lsu.edu
pointOfContact
asNeeded
Dataset Version: 1
Unknown
Cruise_Collected
MultiCore
Station_Number
Bottom_Depth
Latitude_N
Longitude_W
Date_Collected
Time_Collected
Sample_Depth
Nominal_Depth
Porewater
Vapor_Phase_Carbonate
POC
POC_2_Stdev
PON
PON_2_Stdev
Reactive_Pool_Treatment
Avg_d30Si
Avg_d30Si_2_Stdev
Avg_Si_Released
Avg_Si_Released_2_Stdev
Mg
Al
K
V
Cr
Mn
Fe
Ni
Cu
Ti
ISO_DateTime_UTC
Finnigan Neptune s/n 1002 multi collector-inductively coupled plasma-mass spectrometer
Genesys 10S UV-Vis Spectrophotometer
theme
None, User defined
cruise name
core id
station
depth_bottom
latitude
longitude
date
time of day
depth range
depth core
No BCO-DMO term
particulate organic Carbon (POC)
standard deviation
particulate organic nitrogen
treatment
mean
Magnesium
Aluminum
potassium
Vanadium
trace metal concentration
Manganese
Iron
Copper
Titanium
ISO_DateTime_UTC
featureType
BCO-DMO Standard Parameters
Costech International Elemental Combustion System (ECS) 4010
Inductively Coupled Plasma Mass Spectrometer
Spectrophotometer
instrument
BCO-DMO Standard Instruments
PE17-20
service
Deployment Activity
Northern Gulf of Mexico Shelf
place
Locations
otherRestrictions
otherRestrictions
Access Constraints: none. Use Constraints: Please follow guidelines at: http://www.bco-dmo.org/terms-use Distribution liability: Under no circumstances shall BCO-DMO be liable for any direct, incidental, special, consequential, indirect, or punitive damages that result from the use of, or the inability to use, the materials in this data submission. If you are dissatisfied with any materials in this data submission your sole and exclusive remedy is to discontinue use.
The biotic and abiotic controls on the Silicon cycle in the northern Gulf of Mexico
https://osprey.bco-dmo.org/project/712667
The biotic and abiotic controls on the Silicon cycle in the northern Gulf of Mexico
<p><em>NSF Award Abstract:</em><br />
The Louisiana Shelf system in the northern Gulf of Mexico is fed by the Mississippi River and its many tributaries which contribute large quantities of nutrients from agricultural fertilizer to the region. Input of these nutrients, especially nitrogen, has led to eutrophication. Eutrophication is the process wherein a body of water such as the Louisiana Shelf becomes enriched in dissolved nutrients that increase phytoplankton growth which eventually leads to decreased oxygen levels in bottom waters. This has certainly been observed in this area, and diatoms, a phytoplankton which represents the base of the food chain, have shown variable silicon/nitrogen (Si/N) ratios. Because diatoms create their shells from silicon, their growth is controlled not only by nitrogen inputs but the availability of silicon. Lower Si/N ratios are showing that silicon may be playing an increasingly important role in regulating diatom production in the system. For this reason, a scientist from the University of South Alabama will determine the biogeochemical processes controlling changes in Si/N ratios in the Louisiana Shelf system. One graduate student on their way to a doctorate degree and three undergraduate students will be supported and trained as part of this project. Also, four scholarships for low-income, high school students from Title 1 schools will get to participate in a month-long summer Marine Science course at the Dauphin Island Sea Laboratory and be included in the research project. The study has significant societal benefits given this is an area where $2.4 trillion gross domestic product revenue is tied up in coastal resources. Since diatoms are at the base of the food chain that is the biotic control on said coastal resources, the growth of diatoms in response to eutrophication is important to study.</p>
<p>Eutrophication of the Mississippi River and its tributaries has the potential to alter the biological landscape of the Louisiana Shelf system in the northern Gulf of Mexico by influencing the Si/N ratios below those that are optimal for diatom growth. A scientist from the University of South Alabama believes the observed changes in the Si/N ratio may indicate silicon now plays an important role in regulating diatom production in the system. As such, understanding the biotic and abiotic processes controlling the silicon cycle is crucial because diatoms dominate at the base of the food chain in this highly productive region. The study will focus on following issues: (1) the importance of recycled silicon sources on diatom production; (2) can heavily-silicified diatoms adapt to changing Si/N ratios more effectively than lightly-silicified diatoms; and (3) the role of reverse weathering in sequestering silicon thereby reducing diffusive pore-water transport. To attain these goals, a new analytical approach, the PDMPO method (compound 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylamino-carbamoyl)methoxy)phenyl)oxazole) that quantitatively measures taxa-specific silica production would be used.</p>
CLASiC
largerWorkCitation
project
eng; USA
oceans
Northern Gulf of Mexico Shelf
-90.83464
-89.75004
28.49884
28.94688
2017-05-05
2017-05-06
Northern Gulf of Mexico, specifically the Louisiana Shelf region dominated by the discharge of the Mississippi River on the western side of the delta
0
BCO-DMO catalogue of parameters from Stable isotopes in reactive silica pools of Mississippi River plume sediments collected aboard the R/V Pelican in May 2017
Biological and Chemical Oceanography Data Management Office (BCO-DMO)
Unavailable
508-289-2009
WHOI MS#36
Woods Hole
MA
02543
USA
info@bco-dmo.org
http://www.bco-dmo.org
Monday - Friday 8:00am - 5:00pm
For questions regarding this resource, please contact BCO-DMO via the email address provided.
pointOfContact
http://lod.bco-dmo.org/id/dataset-parameter/792246.rdf
Name: Cruise_Collected
Units: dimensionless
Description: <p>local name chosen by project investigators for a research expedition on a vessel as opposed to the formal/official cruise ID</p>
http://lod.bco-dmo.org/id/dataset-parameter/792247.rdf
Name: MultiCore
Units: number/identification
Description: <p>core number/identification</p>
http://lod.bco-dmo.org/id/dataset-parameter/792248.rdf
Name: Station_Number
Units: dimensionless
Description: <p>station identifier</p>
http://lod.bco-dmo.org/id/dataset-parameter/792249.rdf
Name: Bottom_Depth
Units: meters
Description: <p>bottom depth in meters</p>
http://lod.bco-dmo.org/id/dataset-parameter/792250.rdf
Name: Latitude_N
Units: decimal degrees
Description: <p>latitude in decimal degrees</p>
http://lod.bco-dmo.org/id/dataset-parameter/792251.rdf
Name: Longitude_W
Units: decimal degrees
Description: <p>longitude in decimal degrees</p>
http://lod.bco-dmo.org/id/dataset-parameter/792252.rdf
Name: Date_Collected
Units: unitless
Description: <p>date when core was collected in the format mmddyyyy</p>
http://lod.bco-dmo.org/id/dataset-parameter/792253.rdf
Name: Time_Collected
Units: HHMM
Description: <p>time GMT when core was collected</p>
http://lod.bco-dmo.org/id/dataset-parameter/792254.rdf
Name: Sample_Depth
Units: centimeters
Description: <p>subsection used for analysis</p>
http://lod.bco-dmo.org/id/dataset-parameter/792255.rdf
Name: Nominal_Depth
Units: centimeters
Description: <p>depth used for data plots</p>
http://lod.bco-dmo.org/id/dataset-parameter/792256.rdf
Name: Porewater
Units: uM
Description: <p>concentration of dissolved silica acid Si(OH)4 in porewater collected</p>
http://lod.bco-dmo.org/id/dataset-parameter/792257.rdf
Name: Vapor_Phase_Carbonate
Units: %
Description: <p>% of carbonates present in the sediment sample via vapor phase acidification</p>
http://lod.bco-dmo.org/id/dataset-parameter/792258.rdf
Name: POC
Units: %
Description: <p>particular organic carbon</p>
http://lod.bco-dmo.org/id/dataset-parameter/792259.rdf
Name: POC_2_Stdev
Units: dimensionless
Description: <p>2 standard deviations of sample variation</p>
http://lod.bco-dmo.org/id/dataset-parameter/792260.rdf
Name: PON
Units: %
Description: <p>particular organic nitrogen</p>
http://lod.bco-dmo.org/id/dataset-parameter/792261.rdf
Name: PON_2_Stdev
Units: dimensionless
Description: <p>2 standard deviations of sample variation</p>
http://lod.bco-dmo.org/id/dataset-parameter/792262.rdf
Name: Reactive_Pool_Treatment
Units: unitless
Description: <p>which reactive pool the following data is for</p>
http://lod.bco-dmo.org/id/dataset-parameter/792263.rdf
Name: Avg_d30Si
Units: 0/00
Description: <p>Average d30Si (n=3) for each corresponding reactive Si Pool (- per mille)</p>
http://lod.bco-dmo.org/id/dataset-parameter/792264.rdf
Name: Avg_d30Si_2_Stdev
Units: dimensionless
Description: <p>Average d30Si (n=3) for each corresponding reactive Si Pool including 2 standard deviations of sample variation</p>
http://lod.bco-dmo.org/id/dataset-parameter/792265.rdf
Name: Avg_Si_Released
Units: umol/g
Description: <p>Average Si released (n=3) for each corresponding reactive Si Pool (- micromoles per gram dry sediment)</p>
http://lod.bco-dmo.org/id/dataset-parameter/792266.rdf
Name: Avg_Si_Released_2_Stdev
Units: dimensionless
Description: <p>Average Si released (n=3) for each corresponding reactive Si Pool including 2 standard deviations of sample variation</p>
http://lod.bco-dmo.org/id/dataset-parameter/792267.rdf
Name: Mg
Units: ppm
Description: <p>magnesium concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792268.rdf
Name: Al
Units: ppm
Description: <p>aluminum concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792269.rdf
Name: K
Units: ppm
Description: <p>potassium concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792271.rdf
Name: V
Units: ppm
Description: <p>vanadium concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792272.rdf
Name: Cr
Units: ppm
Description: <p>chromium concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792273.rdf
Name: Mn
Units: ppm
Description: <p>manganese concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792274.rdf
Name: Fe
Units: ppm
Description: <p>iron concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792275.rdf
Name: Ni
Units: ppm
Description: <p>nickel concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792276.rdf
Name: Cu
Units: ppm
Description: <p>copper concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/792327.rdf
Name: Ti
Units: ppm
Description: <p>titanium concentration for each corresponding reactive Si pool</p>
http://lod.bco-dmo.org/id/dataset-parameter/804980.rdf
Name: ISO_DateTime_UTC
Units: YYYY-MM-DDTHH:MM:SS[.xx]Z
Description: <p>Date/Time (UTC) ISO formatted</p>
GB/NERC/BODC > British Oceanographic Data Centre, Natural Environment Research Council, United Kingdom
Biological and Chemical Oceanography Data Management Office (BCO-DMO)
Unavailable
508-289-2009
WHOI MS#36
Woods Hole
MA
02543
USA
info@bco-dmo.org
http://www.bco-dmo.org
Monday - Friday 8:00am - 5:00pm
For questions regarding this resource, please contact BCO-DMO via the email address provided.
pointOfContact
13433
https://darchive.mblwhoilibrary.org/bitstream/1912/25457/1/dataset-786508_stable-isotopes-reactive-si-pools__v1.tsv
download
https://doi.org/10.1575/1912/bco-dmo.786508.1
download
onLine
dataset
Stable silicon isotopes (e.g. δ30Si) in sediment biogenic silica (bSi) are widely used as a paleoproxy for marine silicic acid usage by pelagic diatoms. Despite the growing body of work that uses bSi δ30Si signals, there are a lack of δ30Si data on other reactive pools of Si in sediments. This oversight misses valuable information on early diagenetic products and potentially biases existing sedimentary bSi δ30Si, which only quantified bSi fractions not altered by diagenesis. For the first time, we quantified δ30Si among operationally defined reactive Si pools (using a pre-leach of mild acid prior to alkaline digestion) in Mississippi River plume sediments. We compared the δ30Si signal within these reactive Si pools to a traditional alkaline-only digestion of sedimentary bSi. These data offer proof of concept that δ30Si is a higher throughput approach for quantifying isotopic properties among reactive Si pools marine sediments vs. the more laborious (albeit powerful) examination of natural silicon radioisotopes in these chemical leaches.
Core Sampling
Briefly, samples were acquired from the study area using an Ocean Instruments MC-900 Multi-corer, which preserved the sediment-water interface during recovery. Overlying bottom water was removed, cores were sectioned into 1cm slices, homogenized, packed under N2 gas and frozen at -20o C for further analysis.
Operational Definitions
Operational reactive Si pools have previously been defined by Rahman et al. (2016) but for consistency and clarity with previous literature (DeMaster, 1981; Michalopoulos and Aller, 2004; Qin et al., 2012; Wang et al., 2015; Rahman et al., 2016; Krause et al., 2017) it has been restated here. Therefore 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: 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, Maldonado et al., 2019).
4. T-bSi: Following the traditional definition of biogenic silica (DeMaster, 1981), with no acid pre-treatment.
Reactive Silica Pools
Frozen sediment samples were thawed to room temperature (22o C) and triplicate ~50-100 mg subsamples were immediately weighed into 50 mL polyethylene centrifuge tubes. Samples were never dried or ground before/during extractions. Procedural blanks were also prepared in triplicate. Additional subsamples of sediment were dried at 60o C to obtain correction for water content.
Sequential Extractions
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).
Acid Leachable Silica (Si-HCl)
Sediment extractions occurred at room temperature (22o C) using Honeywell Fluka Trace SELECT 0.1 N HCl for 12 hrs, while keeping particles suspended via constant motion. Following centrifugation, the Si-HCl leachate was removed and neutralized. Remaining sediment was rinsed in triplicate with Milli-Q water to remove any residual acid (Michalopoulos and Aller, 2004). As it had previously been demonstrated by Rahman et al. (2016) that the rinses contained minor amounts of Si these rinses were discarded. A weak HCl molarity was purposely chosen to remove metal coatings, authigenic phases, and activate bSi surfaces while not affecting the sequential Si-Alk digestion (Michalopoulos and Aller, 2004).
Mild Alkaline Leachable Si (Si-Alk)
The remaining sediment from the acid pre-treatment was subsequently digested with 0.1 M Na2CO3 (Fisher Scientific Certified ACS) for 20 mins in a 85o C water bath. Following the 20 min timepoint, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, the Si-Alk leachate was removed and stored for further use. The process was stopped after 20 mins to ensure the absence of lithogenic material (DeMaster, 1981; Michalopoulos and Aller, 2004) and certify that the clear majority of solubilized silica present is biogenic. Fresh 0.1 M Na2CO3 was added to the samples and the digestions were continued for a total of 5 hrs (DeMaster, 1981) to completely remove the bSi phase. Concluding after 5 hrs, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, the leachate was removed and discarded. Remaining sediment was rinsed in triplicate with Milli-Q water to remove any residual Na2CO3 and again the rinses were discarded.
Harsh NaOH Digestion (Si-NaOH)
The remaining sediment from the Si-Alk treatment was subsequently digested with Honeywell Fluka 4 M NaOH for 2 hrs in a 85o C water bath. After 2 hrs, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, the Si- NaOH leachate was removed, the remaining sediment was rinsed with Milli-Q water to remove any residual leachate and this rinse was added to the Si-NaOH leachate and stored for further analysis (Rahman et al., 2016).
Traditional bSi Digestion (T-bSi)
Additionally, a second treatment following the traditional definition of biogenic silica (DeMaster, 1981), with no acid pre-treatment was used to derive δ30Si from traditional bSi measurements. New subsamples of sediment were weighed out. 0.1 M Na2CO3 was added to samples and heated in a 85o C water bath for 20 mins to remove the bSi phase. Following the 20 min timepoint, samples were placed on ice and neutralized to stop the digestion. Following centrifugation, leachate was removed and stored for further use. Similar to the Si-Alk digestions, the process was stopped after 20 mins to ensure the absence of lithogenic material.
A 1 ml aliquot of each resulting liquid (Si-HCl, Si-Alk, Si-NaOH and T-bSi) was analyzed for dissolved SiOH4 concentration (dSi) as described by Brzezinski and Nelson (Brzezinski and Nelson, 1986) using the molybdate-blue method on a Genesys 10S UV-Vis Spectrophotometer. The remaining supernatants were concentrated via evaporation at 100o C and stored following DeMaster (1980) in preparation for stable isotope analysis.
Stable Isotope Analysis
Sample purification and isotope analysis were carried out at the University of Bristol Isotope Group laboratories. Concentrated sample fluids were purified via cation ion exchange chromatography (Bio-Rad AG50W-X12, 200-400 mesh cation exchange resin in H+ form). Purified solutions were analyzed in duplicate for Si isotopes (28Si, 29Si, 30Si) using a multi collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS, Finnigan Neptune s/n 1002), equipped with CETAC PFA spray chamber and PFA nebulizer (100ul/min). A standard-sample-standard bracketing procedure with Mg doping following Cardinal et al., (2003) was used to correct for both instrumental mass bias and matrix effects. Additionally, sample and standard solutions were both doped with 0.1 M H2SO4 (ROMIL UpA) and 1 M HCl (in-house distilled) to reduce any matrix effects from anion loading and guarantee matrix matching between sample and standard (Hughes et al., 2011). All isotopic composition results are expressed as δ30Si, corresponding to the silicon isotopic abundances in samples relative to the international reference standard NBS-28 (NIST RM8546, purified quartz sand). Reference standards Diatomite (Reynolds et al., 2007) and LMG08 (sponge) (Hendry et al., 2011) were run in tandem with samples to assess long- term reproducibility. Average measured values are reported as +1.27 ± 0.09‰ (n=75) and -3.47 ± 0.16‰ (n=27) (±SD) respectively, which are well within agreement with published values (Reynolds et al., 2007; Hendry et al., 2011). All samples and standards are consistent with the kinetic mass fractionation law (Reynolds et al., 2007) with the δ29Si = 0.518xδ30Si. Procedural blanks were lower than the detection limit and thus considered negligible on δ30Si of the samples.
Major Metal Compositions and Corrections
Additional thawed/wet sediment subsamples were used for duplicate sequential extractions and digestions (Si-HCl, Si-Alk, Si-NaOH and T-bSi) run as previously described. Supernatants were concentrated via evaporation at 100o C and fluids were reconstituted in 2% HNO3 (in-house distilled) to determine major ion concentrations on an Agilent 7700 Series ICP-MS. The instrument was calibrated using a blank and seven matrix-matched, mixed standards. Internal standardization during analysis was monitored via the addition of (50 μl, 10,000 ppb) 115In and 4Be to all standards and samples. Using Aluminum (Al):Si corrections (Kamatani and Oku, 2000; Ragueneau et al., 2005), both Si-Alk and T-bSi δ30Si signals (‰) and mass of Si released (μmol/g) were adjusted for bias from lithogenic material (however, this was more important for the mass of Si, as isotopic content was derived from 30-minute digestions, opposed to 5 hour digestions for the former).
Organic Matter
Sediment total organic carbon (TOC) and total organic nitrogen (TON) content were analyzed at the Dauphin Island Sea Lab using a Costech elemental combustion system (4010 ECS) following vapor phase acidification to remove carbonates. Briefly, dried sediment samples were placed in a glass desiccator and reacted with reagent-grade 12N HCl vapor for 24 hrs at room temperature. Samples were then dried at 60o C overnight to remove remaining HCl and water content before TOC/TON analyses (Yamamuro and Kayanne, 1995).
Specified by the Principal Investigator(s)
<p>BCO-DMO Data Manager Processing Notes:<br />
* added a conventional header with dataset name, PI name, version date<br />
* modified parameter names to conform with BCO-DMO naming conventions<br />
* blank values in this dataset are displayed as "nd" for "no data."&nbsp; nd is the default missing data identifier in the BCO-DMO system. Added ND as a missing data identifier.<br />
* removed all spaces in headers and replaced with underscores<br />
* removed all units from headers<br />
* created an ISO_DateTime_UTC column from the Date_Collected and Time_Collected columns<br />
* set Types for each data column&nbsp;</p>
Specified by the Principal Investigator(s)
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7.x-1.1
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PI Supplied Instrument Name: Instrument Name: Costech International Elemental Combustion System (ECS) 4010 Instrument Short Name:Costech ECS 4010 Instrument Description: The ECS 4010 Nitrogen / Protein Analyzer is an elemental combustion analyser for CHNSO elemental analysis and Nitrogen / Protein determination. The GC oven and separation column have a temperature range of 30-110 degC, with control of +/- 0.1 degC.
Finnigan Neptune s/n 1002 multi collector-inductively coupled plasma-mass spectrometer
Finnigan Neptune s/n 1002 multi collector-inductively coupled plasma-mass spectrometer
PI Supplied Instrument Name: Finnigan Neptune s/n 1002 multi collector-inductively coupled plasma-mass spectrometer Instrument Name: Inductively Coupled Plasma Mass Spectrometer Instrument Short Name:ICP Mass Spec Instrument Description: An ICP Mass Spec is an instrument that passes nebulized samples into an inductively-coupled gas plasma (8-10000 K) where they are atomized and ionized. Ions of specific mass-to-charge ratios are quantified in a quadrupole mass spectrometer. Community Standard Description: http://vocab.nerc.ac.uk/collection/L05/current/LAB15/
Genesys 10S UV-Vis Spectrophotometer
Genesys 10S UV-Vis Spectrophotometer
PI Supplied Instrument Name: Genesys 10S UV-Vis Spectrophotometer Instrument Name: Spectrophotometer Instrument Short Name:Spectrophotometer Instrument Description: An instrument used to measure the relative absorption of electromagnetic radiation of different wavelengths in the near infra-red, visible and ultraviolet wavebands by samples. Community Standard Description: http://vocab.nerc.ac.uk/collection/L05/current/LAB20/
Cruise: PE17-20
PE17-20
R/V Pelican
Community Standard Description
International Council for the Exploration of the Sea
R/V Pelican
vessel
PE17-20
Jeffrey W. Krause
Dauphin Island Sea Lab
R/V Pelican
Community Standard Description
International Council for the Exploration of the Sea
R/V Pelican
vessel