Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

This dataset contains amino acid compound specific nitrogen isotope ratios in particles collected during R/V Kilo Moana cruises around Station ALOHA (KM1407 and KM1418).

For more information about the ALOHA observatory see: http://aco-ssds.soest.hawaii.edu/

These data were published in Gloeckler et al (2018), Supporting Information file lno10762-sup-0002-suppinfo2.xlsx

Amino acid-specific stable N isotope composition was determined on samples that were hydrolyzed, derivatized, and analyzed according to Popp et al. (2007) and Hannides et al. (2009).  Briefly, size-fractioned zooplankton material and target zooplankton taxa were hydrolyzed using trace metal-grade 6 M HCl and the resulting AAs purified using cation exchange chromatography.  The samples were then esterified using 4:1 isopropanol:acetyl chloride and derivatized using 3:1 methylene chloride:trifluoroacetyl anhydride.  The resulting trifluoroacetyl and isopropyl ester (TFA) derivatives were purified using chloroform extraction and stored at -20°C for up to 1 month before analysis.  This method yielded information for the following AAs: alanine (Ala), glycine (Gly), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), tyrosine (Tyr), and valine (Val).  During acid hydrolysis asparagine (Asn) is converted to aspartic acid (Asp) and glutamine (Gln) is converted to glutamic acid (Glu), thus we also report information on the combined pools, termed Asx (Asn+Asp) and Glx (Gln+Glu), respectively.

TFA derivatives of AAs were analyzed for stable N isotope composition (d15NAA values) following Hannides et al. (2013).  AAs were analyzed using a Thermo Scientific Delta V Plus IRMS interfaced to a trace gas chromatograph (GC) fitted with a 60 m BPx5 capillary column through a GC-C III combustion furnace (980°C), reduction furnace (680°C) and liquid nitrogen cold trap. d15NAA values were measured on 3 – 5 replicate injections with norleucine and aminoadipic acid with known d15N values as internal reference materials co-injected on each run.

A composite source d15NAA value was calculated by a weighted averaging a suite of AAs (e.g., d15NSrc-AA = average of Gly, Lys, Phe, and Ser d15N values). Weighting was based the analytical uncertainty calculated from at least triplicate analysis of each sample.

BCO-DMO Processing Notes:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions
- added comment column

NSF award abstract:
Mercury is a pervasive trace element that exists in several states in the marine environment, including monomethylmercury (MMHg), a neurotoxin that bioaccumulates in marine organisms and poses a human health threat. Understanding the fate of mercury in the ocean and resulting impacts on ocean food webs requires understanding the mechanisms controlling the depths at which mercury chemical transformations occur. Preliminary mercury analyses on nine species of marine fish from the North Pacific Ocean indicated that intermediate waters are an important entry point for MMHg into open ocean food webs. To elucidate the process controlling this, researchers will examine mercury dynamics in regions with differing vertical dissolved oxygen profiles, which should influence depths of mercury transformation. Results of the study will aid in a better understanding of the pathways by which mercury enters the marine food chain and can ultimately impact humans. This project will provide training for graduate and undergraduate students, and spread awareness on oceanic mercury through public outreach and informal science programs.

Mercury isotopic variations can provide insight into a wide variety of environmental processes. Isotopic compositions of mercury display mass-dependent fractionation (MDF) during most biotic and abiotic chemical reactions and mass-independent fractionation (MIF) during photochemical radical pair reactions. The unusual combination of MDF and MIF can provide information on reaction pathways and the biogeochemical history of mercury. Results from preliminary research provide strong evidence that net MMHg formation occurred below the surface mixed layer in the pycnocline and suggested that MMHg in low oxygen intermediate waters is an important entry point for mercury into open ocean food webs. These findings highlight the critical need to understand how MMHg levels in marine biota will respond to changes in atmospheric mercury emissions, deposition of inorganic mercury to the surface ocean, and hypothesized future expansion of oxygen minimum zones. Using field collections across ecosystems with contrasting biogeochemistry and mercury isotope fractionation experiments researchers will fill key knowledge gaps in mercury biogeochemistry. Results of the proposed research will enable scientists to assess the biogeochemical controls on where in the water column mercury methylation and demethylation likely occur.

Related background publication with supplemental data section:
Joel D. Blum, Brian N. Popp, Jeffrey C. Drazen, C. Anela Choy & Marcus W. Johnson. 2013. Methylmercury production below the mixed layer in the North Pacific Ocean. Nature Geoscience 6, 879–884. doi:10.1038/ngeo1918