URLhttps://www.bco-dmo.org/dataset/748792
Download URLhttps://www.bco-dmo.org/dataset/748792/data/download
Media Typetext/tab-separated-values
CreatedOctober 25, 2018
ModifiedMarch 15, 2019
StateFinal no updates expected
Brief DescriptionPorewater measurements of nitrate and nitrite concentration and N and O isotopic ratios (d15N and d18O) from sites 3 and 10 on the North Atlantic Long Core Cruise R/V Knorr KN223

Acquisition Description

Samples were collected aboard the R/V Knorr using its long coring system in November of 2014 on cruise KN-223 in the North Atlantic. Samples used in this study came from two sediment coring sites located within 90m of each other at 50°37.25'W, 14°24.05'N, and 4455 m water depth. Porewaters were extracted at approximately 0.5m intervals from two long piston cores (30 and 34 m long) using Rhizon samplers (0.2 mm pore size) and either analyzed shipboard or frozen until analyses were conducted shore side.

Nitrate and nitrite concentrations were determined shipboard using ion chromatography with UV detection (D'Hondt et al., 2015). Isotopes were measured in the Wankel lab (Woods Hole Oceanographic Institution) using an Isoprime 100 isotope ratio mass spectrometer coupled to a modified TraceGas prep system similar to that described previously (McIlvin and Casciotti, 2011), which is used to flush, purify and cryogenically trap sample N2O from converted nitrate or nitrite samples. Nitrate isotopic composition was measured using the denitrifier method to convert nitrate to N2O, normalized to international reference materials (USGS 34, USGS 32, and USGS 35) (Sigman et al., 2001; Casciotti et al., 2002). Nitrite isotope measurements were made separately using the azide method for conversion of nitrite to N2O (McIlvin and Altabet, 2005), normalizing to previously calibrated Wankel isotope lab standards (WILIS 10, 11, and 20) (Buchwald et al., 2016). Where co-occurring nitrite concentrations were less than 5 times as high as nitrate, nitrite was removed by addition of sulfamic acid (Granger and Sigman, 2009) prior to the denitrifier method. In the deepest samples having measurable nitrate, where concentrations were very low, the N and O isotopic composition of nitrate was calculated by mass balance using analyses of the combined nitrate + nitrite pools by the denitrifier method, in which both nitrate and nitrite standards were also analyzed, together with nitrite isotope values from the azide-only measurements described previously (Casciotti and McIlvin, 2007).    

Processing Description

BCO-DMO Processing:
- modified parameter names to conform to BCO-DMO naming conventions (removed parentheses);
- converted original lat/lon values (degrees & decimal mins) to decimal degrees;
- replaced blank cells (no data) with "nd";
- combined data from both sites into one dataset.

Instruments

R/V Knorr long coring system [Piston Corer]
Details
The piston corer is a type of bottom sediment sampling device. A long, heavy tube is plunged into the seafloor to extract samples of mud sediment. A piston corer uses a "free fall" of the coring rig to achieve a greater initial force on impact than gravity coring. A sliding piston inside the core barrel reduces inside wall friction with the sediment and helps to evacuate displaced water from the top of the corer. A piston corer is capable of extracting core samples up to 90 feet in length.
Rhizon samplers [Sediment Porewater Sampler]
Details
Instance Description (Rhizon samplers)

Rhizon samplers extract small volumes of pore water from soil and sediments. See: https://www.rhizosphere.com/

A device that collects samples of pore water from various horizons below the seabed.

ion chromatography with UV detection [Ion Chromatograph]
Details

Ion chromatography is a form of liquid chromatography that measures concentrations of ionic species by separating them based on their interaction with a resin. Ionic species separate differently depending on species type and size. Ion chromatographs are able to measure concentrations of major anions, such as fluoride, chloride, nitrate, nitrite, and sulfate, as well as major cations such as lithium, sodium, ammonium, potassium, calcium, and magnesium in the parts-per-billion (ppb) range. (from http://serc.carleton.edu/microbelife/research_methods/biogeochemical/ic.html)

Isoprime 100 isotope ratio mass spectrometer [Isotope-ratio Mass Spectrometer]
Details
Instance Description (Isoprime 100 isotope ratio mass spectrometer)

Isoprime 100 isotope ratio mass spectrometer coupled to a modified TraceGas prep system

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).

Parameters

Site_Number [site]
Details
Site_Number

Site number where samples were collected

Sampling site identification.
Depth_for_O2 [depth_bsf]
Details
Depth_for_O2
Depth of sample for oxygen (O2) measurement

depth below seafloor.  Includes mbsf (meters below seafloor) and cmbsf (centimeters below seafloor).

Details
Oxygen
Oxygen concentration

dissolved oxygen concentration

Depth_for_Nitrate_and_Nitrate [depth_bsf]
Details
Depth_for_Nitrate_and_Nitrate
Depth of sample for nitrate and nitrite measurements

depth below seafloor.  Includes mbsf (meters below seafloor) and cmbsf (centimeters below seafloor).

Nitrate [NO3]
Details
Nitrate
Nitrate concentration

Nitrate concentration in the water column

Nitrite [NO2]
Details
Nitrite
Nitrite concentration

Nitrite

Depth_for_Nitrite_isotopes [depth_bsf]
Details
Depth_for_Nitrite_isotopes
Depth of sample for nitrite isotope measurements

depth below seafloor.  Includes mbsf (meters below seafloor) and cmbsf (centimeters below seafloor).

d15N_Nitrite [NO2]
Details
d15N_Nitrite
d15N nitrite

Nitrite

d18O_Nitrite [NO2]
Details
d18O_Nitrite
d18O nitrite

Nitrite

Depth_for_Nitrate_isotopes [depth_bsf]
Details
Depth_for_Nitrate_isotopes
Depth of sample for nitrate isotope measurements

depth below seafloor.  Includes mbsf (meters below seafloor) and cmbsf (centimeters below seafloor).

d15N_Nitrate [dN15_NO3]
Details
d15N_Nitrate

d15N nitrate

delta 15N of dissolved nitrate relative to atmospheric N2
d18O_Nitrate [d18O_NO3]
Details
d18O_Nitrate
d18O nitrate

Oxygen isotopic composition (18O/16O) of nitrate. This property is reported as the relative quantity delta18O in units of per mil vs. VSMOW (Vienna Standard Mean Ocean Water)

Depth_for_Ammonium [depth_bsf]
Details
Depth_for_Ammonium
Depth of sample for ammonium measurements

depth below seafloor.  Includes mbsf (meters below seafloor) and cmbsf (centimeters below seafloor).

Ammonium [Ammonium]
Details
Ammonium
Ammonium concentration

Ammonium and ammonia concentration parameters in any body of fresh or salt water.

Dataset Maintainers

NameAffiliationContact
Carolyn BuchwaldWoods Hole Oceanographic Institution (WHOI)
Scott D. WankelWoods Hole Oceanographic Institution (WHOI)
Arthur J. SpivackUniversity of Rhode Island (URI-GSO)
Shannon RauchWoods Hole Oceanographic Institution (WHOI BCO-DMO)

BCO-DMO Project Info

Project TitleDetermining the rates of denitrification, nitrification, and nitrogen fixation using natural abundance isotope profiles in North Atlantic sediments
AcronymNorth Atlantic Nitrate and Nitrite
URLhttps://www.bco-dmo.org/project/748767
CreatedOctober 23, 2018
ModifiedOctober 26, 2018
Project Description

Project Abstract:
Deep-sea sediments in the oligotrophic ocean host a diverse array of microbes that are involved in multiple processes within the nitrogen cycle. Using measurements of nitrate and nitrite, and their stable isotopes (d15N and d18O) in sedimentary pore fluids, we have been developing approaches for determining the distribution and magnitude of key processes in the oligotrophic sediments of the North Atlantic. While concentration profiles alone indicate the production of nitrate through nitrification in the surface sediments and the reduction of the nitrate deeper in the absence of oxygen, the dual stable isotope profiles of NO3- and NO2- demonstrate clear evidence of further complexity; specifically, that nitrite oxidation occurs deeper in the sediments as well, apparently in the absence of O2. A number of lines of evidence contribute to this refined understanding of the distribution of N cycling processes in these environments, including large differences in the nitrate and nitrite d15N, as well as the evolution of a greater than 1:1 relationship between the d15N and d18O of nitrate. We used a 1D inverse model that predicts the distribution and rates of different oxidative and reductive nitrogen cycling processes throughout these vertical profiles. Our analysis reveals that nitrate reduction and nitrite oxidation co-occur between 0 and 10 meters, and that the ratio of these processes changes in relation to the abundance of porewater oxygen. In the upper profile where dissolved oxygen is more abundant oxidative processes (e.g., nitrite re-oxidation) play an exceptionally large role, as reflected in the very high slope for the evolving relationship between d15N and d18O nitrate. Below the depth of oxygen penetration, while nitrate reduction becomes a substantially more important processes, a clear indication of oxidation remains – as reflected in the large difference between nitrate and nitrite d15N. All rates were predicted to be slow on the order of 0.1 mM per year, which was substantiated by d18O values of nitrite reflecting complete isotopic equilibration with water.

This project was funded by a C-DEBI Postdoctoral Fellowship to Carolyn Buchwald (advisor: Scott Wankel).

Project Maintainers
NameAffiliationRoleContact
Carolyn BuchwaldWoods Hole Oceanographic Institution (WHOI)Principal Investigator
Scott D. WankelWoods Hole Oceanographic Institution (WHOI)Co-Principal Investigator
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