Planktic foraminifer tissue and shell d15N from net tows on R/V S.A. Agulhas II cruises VOY016 and VOY019 in the Southern Ocean south of Africa during 2015-2016

Website: https://www.bco-dmo.org/dataset/805653
Data Type: Cruise Results
Version: 1
Version Date: 2020-03-10

Project
» High-resolution, Assemblage-specific Records of Diatom-bound N Isotopes from the Indian Sector of the Antarctic Ocean (Diatom-bound_N_Isotopes)
» Understanding the nitrogen isotopes of planktonic foraminifera: A modern Sargasso Sea study (N Isotopes Foraminifera)
» MRI: Acquisition of Stable Isotope Instrumentation for the Biogeosciences at Princeton University (stable isotope instrumentation)
ContributorsAffiliationRole
Sigman, Daniel M.Princeton UniversityPrincipal Investigator
Smart, Sandi M.Stellenbosch UniversityContact
Rauch, ShannonWoods Hole Oceanographic Institution (WHOI BCO-DMO)BCO-DMO Data Manager

Abstract
This dataset synthesizes net-tow data (foraminifera and zooplankton measurements) with tow-depth seawater measurements (physical and chemical properties) and surface measurements of bulk suspended particulate organic nitrogen (PON) collected from the underway intake (7 m depth).


Coverage

Spatial Extent: N:-40.9588 E:38.4761 S:-53.9942 W:0.0483
Temporal Extent: 2015-07-27 - 2016-05-09

Dataset Description

This dataset synthesizes net-tow data (foraminifera and zooplankton measurements) with tow-depth seawater measurements (physical and chemical properties) and surface measurements of bulk suspended particulate organic nitrogen (PON) collected from the underway intake (7 m depth).


Acquisition Description

Planktic foraminifera were collected by towing a 200-μm-mesh plankton net for 90 min at a target depth within the upper mixed layer. Approximately 90% of the tow material was preserved in a 5-10% pH-buffered formalin solution and stored at 4°C until sorting for foraminifera (Ren et al., 2012). The remaining 10% was size fractionated and frozen at −20°C for element and isotope analysis of zooplankton. During each tow, bulk (>0.3 µm) suspended PON and nitrate samples were collected from the underway intake (at 7 m depth) and frozen at −80°C and −20°C, respectively, until isotope analysis. In situ (i.e., tow-depth) nitrate samples and auxiliary hydrographic data come from CTD casts performed on station before the start of each net tow.

Foraminifera were separated by species, rinsed briefly with Milli-Q and crushed open to expose (non-shell-bound) tissue. Tissue N was converted to nitrate using the persulfate oxidation method (Nydahl, 1978; Knapp et al., 2005). Remnant shell material (having been oxidatively cleaned) was transferred, rinsed six times with Milli-Q and oven dried at 50°C. Shell samples were then dissolved with hydrochloric acid to release shell-bound organics for persulfate oxidation (Nydahl, 1978; Knapp et al., 2005). For both tissue- and shell-derived nitrate (from persulfate oxidation) as well as seawater nitrate, samples were measured for concentration by chemiluminescence (Braman & Hendrix, 1989) and N isotope composition using the denitrifier method and gas chromatography−isotope ratio mass spectrometry (GC-IRMS) (Sigman et al., 2001; Casciotti et al., 2002; Weigand et al., 2016). The N isotope composition of bulk PON and size-fractionated zooplankton were measured by elemental analyser-IRMS.

The isotopic composition of N₂O was measured either at Princeton University (USA) or the Max Planck Institute for Chemistry (MPIC; Germany) by GC-IRMS using a Thermo MAT 253 mass spectrometer with a purpose-built on-line N₂O extraction and purification system. Bulk PON and size-fractionated zooplankton samples were measured at the University of Cape Town using a Thermo Scientific FLASH 2000 elemental analyzer coupled to a Thermo Scientific Delta V Plus mass spectrometer. Hydrographic data were acquired by a Sea-Bird conductivity-temperature depth sensor during each cast.

See methods section of Smart et al. (2020) for more detail.


Processing Description

At Princeton and MPIC, N isotope measurements were referenced against automated injections of N₂O from a gas cylinder, and then calibrated to N₂ in air using international reference materials: IAEA-N3 and USGS-34 nitrate standards in the case of seawater samples, or USGS-40 and USGS-41 amino acid standards in the case of foraminifer (oxidized) samples. Oxidized samples were additionally corrected for the contribution of the N blank associated with the persulfate oxidizing reagent.

At the University of Cape Town, N isotope measurements were referenced to atmospheric N₂ using three in-house organic standards: Choc, Merck Gel and Valine.


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Related Publications

Braman, R. S., & Hendrix, S. A. (1989). Nanogram nitrite and nitrate determination in environmental and biological materials by vanadium(III) reduction with chemiluminescence detection. Analytical Chemistry, 61(24), 2715–2718. doi:10.1021/ac00199a007
Methods
Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., & Hilkert, A. (2002). Measurement of the Oxygen Isotopic Composition of Nitrate in Seawater and Freshwater Using the Denitrifier Method. Analytical Chemistry, 74(19), 4905–4912. doi:10.1021/ac020113w
Methods
De Boyer Montégut, C. (2004). Mixed layer depth over the global ocean: An examination of profile data and a profile-based climatology. Journal of Geophysical Research, 109(C12). doi:10.1029/2004jc002378 https://doi.org/10.1029/2004JC002378
Methods
Knapp, A. N., Sigman, D. M., & Lipschultz, F. (2005). N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time-series Study site. Global Biogeochemical Cycles, 19(1). doi:10.1029/2004gb002320 https://doi.org/10.1029/2004GB002320
Methods
Nydahl, F. (1978). On the peroxodisulphate oxidation of total nitrogen in waters to nitrate. Water Research, 12(12), 1123–1130. doi:10.1016/0043-1354(78)90060-x https://doi.org/10.1016/0043-1354(78)90060-X
Methods
Ren, H., Sigman, D. M., Thunell, R. C., & Prokopenko, M. G. (2012). Nitrogen isotopic composition of planktonic foraminifera from the modern ocean and recent sediments. Limnology and Oceanography, 57(4), 1011–1024. doi:10.4319/lo.2012.57.4.1011
Methods
Sigman, D. M., Casciotti, K. L., Andreani, M., Barford, C., Galanter, M., & Böhlke, J. K. (2001). A Bacterial Method for the Nitrogen Isotopic Analysis of Nitrate in Seawater and Freshwater. Analytical Chemistry, 73(17), 4145–4153. doi:10.1021/ac010088e
Methods
Smart, S. M., Fawcett, S. E., Ren, H., Schiebel, R., Tompkins, E. M., Martínez‐García, A., … Sigman, D. M. (2020). The Nitrogen Isotopic Composition of Tissue and Shell‐Bound Organic Matter of Planktic Foraminifera in Southern Ocean Surface Waters. Geochemistry, Geophysics, Geosystems, 21(2). doi:10.1029/2019gc008440 https://doi.org/10.1029/2019GC008440
Results
Smart, S. M., Ren, H., Fawcett, S. E., Schiebel, R., Conte, M., Rafter, P. A., … Sigman, D. M. (2018). Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea. Geochimica et Cosmochimica Acta, 235, 463–482. doi:10.1016/j.gca.2018.05.023
Results
Weigand, M. A., Foriel, J., Barnett, B., Oleynik, S., & Sigman, D. M. (2016). Updates to instrumentation and protocols for isotopic analysis of nitrate by the denitrifier method. Rapid Communications in Mass Spectrometry, 30(12), 1365–1383. doi:10.1002/rcm.7570
Methods

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Parameters

ParameterDescriptionUnits
cruiseCruise identifier unitless
shipShip name unitless
towTow number unitless
dateDate; format: yyyy-mm-dd unitless
time_starttowTime at start of tow (UTC); format: HH:MM unitless
time_endtowTime at end of tow (UTC); format: HH:MM unitless
speciesThe planktic foraminifer species measured with the following abbreviations: bul = G. bulloides; glu = G. glutinata; hir = G. hirsuta; inc = N. incompta; inf = G. inflata; pac = N. pachyderma; qui = T. quinqueloba; tru = G. truncatulinoides; uni = O. universa unitless
typeThe kind of foraminiferal organic matter (i.e., tissue or shell-bound) measured for N isotope composition unitless
analysis_yearThe year in which foraminifer samples were measured for N isotope composition. The measurement protocol differed slightly between the three analysis sessions (see Methods Section and Supplemental Text S1 of Smart et al. (2020) for more detail). unitless
latitudeLatitude; positive values = North decimal degrees
longitudeLongitude; positive values = East decimal degrees
depth_towDepth of the tow meters (m)
d15N_foram_avgAverage d15N. d15N is the nitrogen isotopic composition of a sample expressed in delta notation (d15N in units of per mil, ‰) relative to atmospheric N2, where d15N = {[(15N/14N)sample/(15N/14N)atmN2] – 1} × 1000 per mil vs AIR
d15N_foram_sdStandard deviation of d15N_foram_avg per mil vs AIR
d15N_foram_nNumber of replicate measurements of d15N_foram_avg unitless
indiv_avgAverage number of individual foraminifer specimens combined to make a measurement unitless
indiv_sdStandard deviation of indiv_avg unitless
indiv_nNumber of replicate measurements of indiv_avg unitless
Ni_avgAverage nitrogen contents of foraminifera per individual nanomoles per individual (nmol/indiv)
Ni_sdStandard deviation of Ni_avg nanomoles per individual (nmol/indiv)
Ni_nNumber of replicate measurements of Ni_avg unitless
Nw_avgAverage nitrogen contents of foraminifera per milligram nanomoles per milligram (nmol/mg)
Nw_sdStandard deviation of Nw_avg nanomoles per milligram (nmol/mg)
Nw_nNumber of replicate measurements of Nw_avg unitless
d15N_avg_nitrateAverage d15N. d15N is the nitrogen isotopic composition of a sample (nitrate only) expressed in delta notation (d15N in units of per mil, ‰) relative to atmospheric N2, where d15N = {[(15N/14N)sample/(15N/14N)atmN2] – 1} × 1000 per mil vs AIR
conc_avg_nitrateConcentration of nitrate in a seawater sample micromolar (uM)
d15N_avg_nitrate_nitriteAverage d15N. d15N is the nitrogen isotopic composition of a sample (nitrate+nitrite) expressed in delta notation (d15N in units of per mil, ‰) relative to atmospheric N2, where d15N = {[(15N/14N)sample/(15N/14N)atmN2] – 1} × 1000 per mil vs AIR
conc_avg_nitrate_nitriteConcentration of nitrate+nitrite in a seawater sample micromolar (uM)
temperature_ctdTemperature measured by CTD degrees Celsius
salinity_ctdSalinity measured by CTD psu
oxygen_ctdOxygen measured by CTD milliliters per liter (ml/l)
fluorescence_ctdFluorescence measured by CTD milligrams per cubic meter (mg/m3)
potdens_calcPotential density, calculated from temperature and salinity kilograms per cubic meter (kg/m3)
MLD_calcCalculated mixed layer depth at each profile station, defined as the closest depth to the surface at which potential density is greater by greater than or equal to 0.03 kg/m3 than the value at a reference depth of 11 m (the shallowest depth common to every CTD station), based on the criterion of de Boyer Montégut et al. (2004). meters (m)
conc_PON_avgAverage concentration of PON. PON is the bulk (> 0.3 um) particulate organic nitrogen in surface waters filtered from the ship’s underway intake during the net tow. micromolar (uM)
conc_PON_sdStandard deviation of PON concentration micromolar (uM)
conc_PON_nNumber of replicate measurements of PON concentration unitless
d15N_PON_avgAverage d15N of PON. d15N is the nitrogen isotopic composition of a sample (nitrate only) expressed in delta notation (d15N in units of per mil, ‰) relative to atmospheric N2, where d15N = {[(15N/14N)sample/(15N/14N)atmN2] – 1} × 1000 per mil vs AIR
d15N_PON_sdStandard deviation of d15N of PON per mil vs AIR
d15N_PON_nNumber of replicate measurements of d15N of PON unitless
d15N_zoop_avgAverage d15N of zooplankton. zoop refers to the bulk (i.e., per mil vs AIR
d15N_zoop_seStandard error of the d15N of zooplankton per mil vs AIR
d15N_zoop_nNumber of replicate measurements of the d15N of zooplankton unitless
ISO_DateTime_UTC_starttowDate and time (UTC) of start of tow formatted to ISO 8601 standard; format: yyyy-mm-ddTHH:MM:SSZ yyyy-MM-dd'T'HH:mm:ss'Z'
ISO_DateTime_UTC_endtowDate and time (UTC) of end of tow formatted to ISO 8601 standard; format: yyyy-mm-ddTHH:MM:SSZ unitless


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Instruments

Dataset-specific Instrument Name
Sea-Bird conductivity-temperature depth sensor
Generic Instrument Name
CTD Sea-Bird
Generic Instrument Description
Conductivity, Temperature, Depth (CTD) sensor package from SeaBird Electronics, no specific unit identified. This instrument designation is used when specific make and model are not known. See also other SeaBird instruments listed under CTD. More information from Sea-Bird Electronics.

Dataset-specific Instrument Name
Thermo MAT 253 mass spectrometer
Generic Instrument Name
Isotope-ratio Mass Spectrometer
Generic Instrument Description
The Isotope-ratio Mass Spectrometer is a particular type of mass spectrometer used to measure the relative abundance of isotopes in a given sample (e.g. VG Prism II Isotope Ratio Mass-Spectrometer).

Dataset-specific Instrument Name
Thermo Scientific Delta V Plus
Generic Instrument Name
Isotope-ratio Mass Spectrometer
Generic Instrument Description
The Isotope-ratio Mass Spectrometer is a particular type of mass spectrometer used to measure the relative abundance of isotopes in a given sample (e.g. VG Prism II Isotope Ratio Mass-Spectrometer).

Dataset-specific Instrument Name
200-um-mesh plankton net
Generic Instrument Name
Plankton Net
Generic Instrument Description
A Plankton Net is a generic term for a sampling net that is used to collect plankton. It is used only when detailed instrument documentation is not available.

Dataset-specific Instrument Name
Thermo Scientific FLASH 2000
Generic Instrument Name
Elemental Analyzer
Generic Instrument Description
Instruments that quantify carbon, nitrogen and sometimes other elements by combusting the sample at very high temperature and assaying the resulting gaseous oxides. Usually used for samples including organic material.


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Deployments

VOY016

Website
Platform
R/V S.A. Agulhas II
Start Date
2015-07-22
End Date
2015-08-15
Description
VOY016: Winter Cruise 2015, Good Hope Line. Subantarctic Atlantic, between South Africa and Antarctic winter sea-ice edge (at 56.4°S, 0.3°E).

VOY019

Website
Platform
R/V S.A. Agulhas II
Start Date
2016-04-07
Description
VOY019: Marion Cruise 2016. Subantarctic Indian, between South Africa and Marion/Prince Edward Islands (at 46.9°S, 37.7°E).


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Project Information

High-resolution, Assemblage-specific Records of Diatom-bound N Isotopes from the Indian Sector of the Antarctic Ocean (Diatom-bound_N_Isotopes)

Coverage: Kerguelen Plateau


Description from NSF award abstract:
The high concentration of the major nutrients nitrate and phosphate is a fundamental characteristic of the Antarctic Zone in the Southern Ocean and is central to its role in global ocean fertility and the global carbon cycle. The isotopic composition of diatom-bound organic nitrogen is one of the best hopes for reconstructing the nutrient status of polar surface waters over glacial cycles, which in turn may hold the explanation for the decline in atmospheric carbon dioxide during ice ages. The PIs propose to generate detailed diatom-bound nitrogen isotope (δ15Ndb) records from high sedimentation rate cores from the Kerguelen Plateau. Because the cores were collected at relatively shallow seafloor depths, they have adequate planktonic and benthic foraminifera to develop accurate age models. The resulting data could be compared with climate records from Antarctic ice cores and other archives to investigate climate-related changes, including the major steps into and out of ice ages and the millennial-scale events that occur during ice ages and at their ends. The records generated in this project will provide a critical test of hypotheses for the cause of lower ice age CO2.

This study will contribute to the goal of understanding ice ages and past CO2 changes, which both have broad implications for future climate. Undergraduates will undertake summer internships, with the possibility of extending their work into junior year projects and senior theses. In addition, the PI will lead modules for two Princeton programs for middle school teachers and will host a teacher for a six-week summer research project.


Understanding the nitrogen isotopes of planktonic foraminifera: A modern Sargasso Sea study (N Isotopes Foraminifera)

Coverage: Sargasso Sea


NSF Award Abstract:
Nitrogen (N) and phosphorus are the two nutrients required in large quantity by phytoplankton in the ocean, and together they limit productivity throughout most of the tropical, subtropical, and temperate ocean. Both the cycling of N and its input/output budget have been argued to control the fertility of the ocean and the ocean's role in setting atmospheric CO2. The CaCO3 tests of foraminifera can represent a substantial fraction of marine sediments and have been used extensively in paleoceanography; they are an obvious target for isotopic analysis of microfossil-bound organic matter.

In recent years, researchers at Princeton have developed a protocol for the isotopic analysis of foraminiferal shell-bound N. The current protocol is at least 100 times more sensitive than typical on-line combustion, allowing for rapid progress with a N isotope archive that was previously not feasible to measure. Measurements on surface sediments and a downcore record from the Caribbean show the promise of foraminifera-bound del15N (fb-del15N) to provide both a robust N isotope archive for paleoceanography, and one with a unique potential of richness, given the existence of multiple foraminiferal species with different depth habitats and behaviors. Moreover, the finding from the Caribbean Sea record -- reduced N fixation in ice age Atlantic -- has changed the scientific conversation about the nature of the input/output budget of oceanic fixed N and its potential to change ocean fertility and atmospheric CO2. However, the controls on fb-del15N have not yet been adequately studied.

In this project, as a first major step in developing a foundation for the paleoceanographic application of fb-del15N, the same Princeton University team will study its genesis in the water column, transport to the seafloor, and early diagenesis. They will undertake this study in the Sargasso Sea south of Bermuda. This is one of the best studied regions of the ocean, in general and with respect to foraminifera, and a region that has been has been a focus of the N isotope research of the PI for the last decade and others previously. Moreover, its significant seasonality -- in physical oceanography, biogeochemistry, and foraminiferal species abundance -- will facilitate the effort to understand the controls on fb-del15N at a mechanistic level. The research team will participate in six Bermuda Atlantic Time-series Study (BATS) cruises over two years, collecting foraminifera and other N forms likely to provide insight into the controls on fb-del15N. From the nearby Oceanic Flux Program (OFP) moored sediment traps and from shallow sediments collected in the region, they will pick foraminifera shells and again make relevant ancillary measurements. This work will establish the relationship of foraminiferal biomass to shell-bound del15N for different species, and comparison of the foraminiferal isotope data with the upper ocean N pools will yield empirical isotopic relationships and work toward a mechanistic insight of fb-del15N (e.g., the importance of different N pools to the diets of different foraminifera; the role of algal symbionts). The sediment trap and surface sediment data will support the plankton tow data by integrating over longer time scales and will also address questions regarding late stage (e.g., gametogenic) calcification and the early diagenesis of fb-del15N and fb-N content.

Broader Impacts: This study will yield an improved understanding of the nutrient dynamics of foraminifera, a class of organisms whose shells are a central tool in micropaleontology and paleoclimatology. The project will also build on the principal investigator's involvement in the Bermuda Institute of Ocean Sciences as an asset for integrating ocean-related education and research at both the undergraduate and graduate levels.


MRI: Acquisition of Stable Isotope Instrumentation for the Biogeosciences at Princeton University (stable isotope instrumentation)


NSF Award Abstract:

Intellectual Merit:
The PIs are requesting funds to acquire a suite of instruments for stable isotope (N) research, including a denitrifier-based, natural abundance isotope system, a tracer isotope system, and general purpose natural abundance peripherals for an existing mass spectrometer. Specific instruments requested include a Thermo Electron MAT 253 stable isotope mass spectrometer, a Thermo DeltaVAdvantage stable isotope mass spectrometer with peripherals, an EA peripheral for combustion-based isotope analysis, and a GasBench II carbon dioxide preparation and purification system. The proposed instrumentation would enable research efforts examining natural stable isotope abundances that include ocean biogeochemistry and paleoceanography, terrestrial biogeochemistry, carbon isotopes in Precambrian Earth history. Stable isotope tracer research would include nitrification, nitrogen fixation, and carbon partitioning during biostimulation.

Broader Impacts:
The PIs state that the proposed instrumentation would expand the research capabilities of the institution and specifically the Departments of Geosciences and Ecology and Evolutionary Biology. The improvements to the denitrifier method will also serve to expand the capabilities of the ocean research community at large. Additionally, the proposed instrumentation will enable method development and provide training for undergraduate students at the institution and those participating in a collaborative effort with the University of Cape Town.



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Funding

Funding SourceAward
NSF Division of Polar Programs (NSF PLR)
NSF Division of Ocean Sciences (NSF OCE)
NSF Division of Ocean Sciences (NSF OCE)

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