Ocean Carbon and Biogeochemistry Data System

VERTIGO KM0414 cruise
PI notes for d15N and d18O isotopic analysis of Niskin bottle nitrate samples

PI: Karen Casciotti (WHOI) and Tom Trull (UTAS)

22 May 2008: Prepared for OCB data system by Cyndy Chandler, OCB DMO (WHOI) from documentation contributed by data set PI.

All samples were collected within 18 nautical miles of station ALOHA (22° 45´ N, 158° W) in the North Pacific Subtropical Gyre during June 20 - July 11, 2004. Original data were contributed by PI as a single multi sheet Excel file (Casciottietal_VERTIGOdata_4.xls).

Contact: Karen Casciotti (WHOI)

Associated Publication:

K. L. Casciotti, T. W. Trull, D. M. Glover, and D. Davies. (in press) "Constraints on Nitrogen Cycling at the Subtropical North Pacific Station ALOHA from Isotopic Measurements of Nitrate and Particulate Nitrogen". Deep-Sea Research II.

to be published in "Understanding the Ocean’s Biological Pump: results from VERTIGO (VERtical Transport In the Global Ocean)", Guest editors Ken Buesseler and Richard Lampitt

Sampling Methodology

Samples for δ¹⁵N_NO3 and δ¹⁸O_NO3 measurements were collected from 11 profiles (cast number and event ID provided for each cast). Samples were collected from 10 L Niskin bottles into 60 mL HDPE Nalgene bottles, rinsed three times with sample before final volumes were collected. Samples were frozen immediately on board, transported frozen, and stored at -20 °C until isotopic analysis.

Analytic Protocols

Nitrate (plus nitrite) δ¹⁵N and δ¹⁸O analyses were performed using the denitrifier method, as previously described (Sigman, Casciotti et al. 2001; Casciotti, Sigman et al. 2002). Based on nitrate (plus nitrite) concentration analyses, sample volumes were calculated to achieve either 5 nmoles (samples from 150-200 m) or 20 nmoles (samples from >200 m) for a given batch of analyses. At either target sample size, aliquots of nitrate isotopic reference materials USGS32, USGS34, and USGS35 (Bohlke, Mroczkowski et al. 2003) were analyzed in parallel, with an aliquot of each standard analyzed after every 9 samples. The use of USGS32, USGS34, and USGS35 provides a 2-3 point calibration curve for δ¹⁵N_NO3 and δ¹⁸O_NO3 measurements. Calibration to the most recently reported δ¹⁵N and δ¹⁸O values for these materials (Bohlke, Mroczkowski et al. 2003) yields δ¹⁵N_NO3 values of 5.5 ‰ and δ¹⁸O_NO3 values of 1.6-2.0 ‰ for the deep (>3000 m) Pacific (this study; Casciotti, unpublished data). Average δ¹⁵N_NO3 and δ¹⁸O_NO3 values and standard deviations reported here are based on replicate analyses (n=2-4) of each sample, with analyses conducted on multiple days representing independent batches of bacteria and standard calibrations. Nitrite was not removed from these samples or analyzed separately for isotopic composition.

Parameter Descriptions

Please note that all *_HOT parameters were obtained from the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS) data server. HOT-DOGS » http://hahana.soest.hawaii.edu/hot/hot-dogs/interface.html

NO3_HOT (umol/kg) is estimated from nitrate data obtained from the HOT-DOGS data server for HOT cruises 160 and 161. HOT data were used to determine the profile of nitrate concentrations (µmol kg^-1) at depths sampled for δ¹⁵N_NO3 and δ¹⁸O_NO3. Where necessary, available data were interpolated linearly between HOT standard depths to obtain nitrate concentrations at the appropriate depths. The interpolated nitrate concentrations obtained in this way matched well with the nitrate concentrations estimated from integrated ion beam amplitudes during the nitrate isotopic analyses.

PO4_HOT (umol/kg) is estimated from phosphate data obtained from the HOT-DOGS data server for HOT cruises 160 and 161. HOT data were used to determine the profile of phosphate concentrations (µmol kg^-1) at depths sampled for δ¹⁵N_NO3 and δ¹⁸O_NO3. Where necessary, available data were interpolated linearly between HOT standard depths to obtain nitrate concentrations at the appropriate depths.

d15NO3_AIR (permil vs. AIR) is defined as:
((¹⁵N/¹⁴N)_NO3/(¹⁵N/¹⁴N)_AIR -1)*1000 in units of permil (‰).

d18NO3_VSMOW (permil vs. VSMOW) is defined as:
((¹⁸O/¹⁶O)_NO3/(¹⁸O/¹⁶O)_VSMOW -1)*1000 in units of permil (‰).

d15N_18O (permil) is defined as:
(δ¹⁵N_NO3 - δ¹⁵N_NO3deep) - ¹⁵Σ/¹⁸Σ*(δ¹⁸O_NO3 - δ¹⁸O_NO3deep), where δ¹⁵N_NO3deep and δ¹⁸O_NO3deep are the isotopic values of deep ocean nitrate and 15Σ/18Σ is the ratio of isotope effects for δ¹⁵N_NO3 and δ¹⁸O_NO3 fractionation, respectively (Sigman, Granger et al. 2005). For the purposes of this study, δ¹⁵N_NO3deep and δ¹⁸O_NO3deep have been assumed to be +5.4 ‰ and +1.6 ‰ respectively. Reported in units of permil (‰).

References

Bohlke, J. K., S. J. Mroczkowski, et al. (2003). "Oxygen isotopes in nitrate: new reference materials for O-18 : O-17 : O-16 measurements and observations on nitrate-water equilibration." Rapid Communications in Mass Spectrometry 17(16): 1835-1846.

Casciotti, K. L., D. M. Sigman, et al. (2002). "Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method." Analytical Chemistry 74: 4905-4912.

Sigman, D. M., K. L. Casciotti, et al. (2001). "A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater." Analytical Chemistry 73(17): 4145-4153.

Sigman, D. M., J. Granger, et al. (2005). "Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin." Global Biogeochemical Cycles 19(4): GB4022.