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 marine particles 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.

Small (1-53 µm) and large (>53 µm) marine particles were sampled on all three cruises using in situ pumps (WTS-LV, standard, 8 L min⁻¹; McLane Research Laboratories, East Falmouth); water was passed sequentially through 53 μm pore-size nylon mesh and 1 μm pore-size quartz microfiber (QMA) filters with 142 mm diameter using a mini-MULVFS filter holder (Bishop et al., 2012). On the spring cruise, one pump was equipped with a pump head and motor with a maximum flow rate of 30 L min⁻¹, and high volume samples of particles >53 µm were collected. The large particles collected on the nylon mesh were sonicated and concentrated into a pre-combusted 47 mm QMA filter.

Marine particles were collected in two size fractions, small particles in combusted quartz filters (1-53 μm) and large particles in a nylon mesh (> 53 μm) using in situ McLane pumps during the three research cruises.

The marine particles were analyzed for total Hg concentrations by acid assisted microwave digestions and aliquots were analyzed by cold vapor atomic fluorescence spectrophotometry.

For THg isotope determination 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