Ocean Carbon and Biogeochemistry Data System

VERTIGO KM0414 cruise
PI notes for d15N isotopic analysis of sinking 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

Sinking particles were collected in replicate Neutrally-Buoyant Sediment Traps (NBST's; (Buesseler, Steinberg et al. 2000; Valdes and Price 2000) deployed at nominal depths of 150 m (3 traps), 300 m (2 traps), and 500 m (2 traps). The traps were each deployed for two 3-5 day collections during the cruise. Each trap held five replicate tubes containing 500 mL brine (freeze-condensed filtered Sargasso seawater) poisoned with 180 µM HgCl₂ or 2% formalin (Lee, Hedges et al. 1992; Buesseler, Lamborg et al. 2007). The brine (with particles) from the replicate tubes were combined, pre-screened through 350-µm mesh to remove "swimmers", and split into 8 equal volumes on board after recovery of the traps. Sample splits for δ¹⁵N_PN measurements from the first set of NBST trap deployments at ALOHA were filtered onto pre-combusted 25 mm GF/F filters, dried, and stored frozen until analysis. Only HgCl₂-treated samples were analyzed here for δ¹⁵N_PN.

Analytic Protocols

From splits of sinking material, whole 25 mm filters were used, which contained approximately 2-20 µg N per filter. 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

tube_num is taken from NBST tube (trap cylinder) numbers and could be cross referenced with other measured properties from the same tube through the VERTIGO event number.

preservative is the type of chemical preservative in the trap brine. "HgCl2" refers to 180 µM mercuric chloride in the brine.

filter_ID is the designation of the particular sample split from the NBST traps and could be cross referenced with other measured properties from the same tube through the VERTIGO sampling event number.

d15N_PN (permil vs. AIR) is defined as: ((¹⁵N/¹⁴N)_PN/(¹⁵N/¹⁴N)_AIR -1)*1000 in units of permil (‰). For sinking material, particulate nitrogen, 'PN', is defined as material passing through 350 µM mesh and caught on a GF/F filter.

References

Buesseler, K. O., C. H. Lamborg, et al. (2007). "Revisiting carbon flux through the ocean's twilight zone." Science 316(5824): 567-570.

Buesseler, K. O., D. K. Steinberg, et al. (2000). "A comparison of the quantity and composition of material caught in a neutrally buoyant versus surface-tethered sediment trap." Deep-Sea Research Part I-Oceanographic Research Papers 47(2): 277-294.

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.

Lee, C., J. I. Hedges, et al. (1992). "Effectiveness of Various Treatments in Retarding Microbial Activity in Sediment Trap Material and Their Effects on the Collection of Swimmers." Limnology and Oceanography 37(1): 117-130.

Valdes, J. R. and J. F. Price (2000). "A neutrally buoyant, upper ocean sediment trap." Journal of Atmospheric and Oceanic Technology 17(1): 62-68.