Ocean Carbon and Biogeochemistry Data System

VERTIGO KM0414 cruise
PI notes for d15N and d13C isotopic analysis of MULVFS sample suspended particles

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

Suspended particles were collected from the Multiple Unit Large Volume Filtration System (MULVFS; (Bishop, Schupack et al. 1985) at depths of 30, 54, 79, 104, 153, 202, 326, 474, 572, and 770 m. Particles were collected from 3,840 to 12,790 L of seawater (J. Bishop, personal communication), depending on the depth, by filtering in situ over a four hour period through a 53 µm nylon mesh and then two layers of (pre-combusted) QMA (quartz micro-fiber) filter (1 µm nominal pore size). The MULVFS QMA filters were subsampled upon retrieval in a HEPA-filtered laminar flow hood by excising 47 mm discs, which were then frozen at -20 °C until isotopic analysis.

Analytic Protocols

Frozen MULVFS subsamples were thawed, then dried at 60 °C. Once dried, ten to twenty punches (5 mm each) from the top QMA filter were combined to achieve approximately 5 µg N for δ¹⁵N_PN analyses. Filters were packed into silver capsules, treated with 1N HCl to remove carbonates, and dried overnight at 60°C. The capsules were then crimped shut and analyzed using a Fisions 1500 Elemental Analyzer and ConFlo-II interface inline with a Finnigan Delta^PLUS isotope ratio mass spectrometer. A small-bore combustion tube (Karsh, Trull et al. 2003) was also used to increase sensitivity (signal amplitude) for the small quantities of sinking and suspended material collected in this study. Individual analyses were normalized to N₂ working gases and to AIR reference scales by parallel analyses of IAEA-N1 (ammonium sulfate,δ¹⁵N = +0.4 ‰) and IAEA-N3 (potassium nitrate, δ¹⁵N = +4.7 ‰). Precision for δ¹⁵N standard analyses was 0.11 ‰ and 0.12 ‰ during runs for suspended and sinking δ¹⁵N_PN, respectively.

Parameter Descriptions

Vnum refers to a sample ID which should allow cross-reference to other data provided for these filter samples.

layer refers to whether the suspended sample was taken from the 'top' layer MULVFS (Bishop, Schupack et al. 1985) QMA or the 'bottom' layer MULFVS QMA.

pore_sz is nominal pore size (in µm).

d13C_susp (permil vs. PDB) is defined as:
((¹³C/¹²C)Susp/(¹³C/¹²C)_PDB -1)*1000 in units of permil (‰). Suspended particulate carbon is defined as organic carbon passing through 53 µm mesh and caught on the top layer QMA filter.

d15N_susp (permil vs. AIR) is defined as:
((¹⁵N/¹⁴N)_Susp/(¹⁵N/¹⁴N)_AIR -1)*1000 in units of permil (‰). Suspended particulate nitrogen is defined as organic nitrogen passing through 53 µm mesh and caught on the top layer QMA filter.

References

Bishop, J. K. B., D. Schupack, et al. (1985). A Multiple Unit Large Volume in-situ Filtration System (MULVFS) for sampling oceanic particulate matter in mesoscale environments. Mapping Strategies in Chemical Oceanography. A. Zirino, American Chemical Society. 209: 155-175.

Karsh, K. L., T. W. Trull, et al. (2003). "Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)." Limnology and Oceanography 48(3): 1058-1068.