Table of Contents

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

This dataset contains the mercury stable isotope ratios collected in zooplankton during R/V Kilo Moana cruises around Station ALOHA. For more information about the ALOHA observatory see: http://aco-ssds.soest.hawaii.edu/. These data were published in Motta et al., (2019) with supporting information.

Zooplankton was collected using a multiple opening-closing net and environmental sensing system (MOCNESS); cod ends were fitted with a 200 micron mesh. Onboard, zooplankton were wet-sieved in filter seawater using different size mesh sieves and frozen at -20 (Degree-Celsius).

The zooplankton was collected at Station ALOHA during the KM1418 and KM1506 research cruises. In the summer cruise, each tow collected material from 9 depth intervals spanning the ocean surface to 1500 m (0-50 m, 50-100 m, 100-150 m, 150-200 m, 200-300 m, 300-500 m, 500-700 m, 700-1000 m, 1000-1500 m). In the spring, four separate net tows were conducted to collected additional samples from a single depth interval (500-700 m). This was done to provide enough sample material.

For total Hg (THg) determination about 10 mg of zooplankton was digested in reverse aqua regis overnight. For THg isotope determination zooplankton samples were combusted in a two-stage combustion furnace and Hg(0) g was trapped in a 1% KMnO4 solution. The 1% KMnO4 solution was analyzed for Hg stable isotope composition using a multiple collector inductively coupled plasma mass spectrometer.

All the methods are detailed in Motta et al., (2019).

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