Table of Contents

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

Weekly sampling was conducted between December 2020 and November 2024 at the Ellen Browning Scripps Memorial Pier (Scripps Pier) in La Jolla, CA (32.8634 °N, 117.2546 °W). Total Hg (THg) and total methylated Hg (MeHg; not shown) were sampled at depths of < 0.5 m and 6 m using a peristaltic pump equipped with weighted sample tubing. Seawater was pumped into clean borosilicate glass bottles (I-Chem) and returned to the lab. MeHg and THg samples were acidified with 0.4% hydrochloric acid (HCl; Trace Metal Grade, J.T. Baker), and stored at 4°C until analysis within 6 months.

THg concentrations were measured following U.S. EPA Method 1631e (Lambourg et al., 2012). 30 mL samples were amended by bromine monochloride oxidation, hydroxylamine and tin (II) chloride reduction, and purged onto gold traps followed by cold-vapor atomic fluorescence spectrometry (CV-AFS) using a Tekran 2600 Automated Hg Analyzer. Calibration was based on an aqueous 1000 ppm mercuric chloride (HgCl2) standard (SPEX Certiprep®) and confirmed with a second 1000 ppm HgCl2 standard (Alfa Aesar). To validate concentrations, we analyzed the certified reference material ORMS-3 (National Research Council Canada, Ottawa, Canada) with a certified value of 12.6 ± 1.1 ng L-1 (Yang et al., 2005). We determined ORMS-3 concentrations to be 14.8 ± 2.44 ng L-1 (n = 12).  The detection limit was 0.17 pM (n = 75 reagent blanks) with ongoing precision and recovery of 101 ± 6% (n = 73). Average replicate precision was 6% (n = 130), and matrix spike recovery was 93 ± 12% (n = 36).

* Added lat/lon to dataset
* Added ISO_DateTime to dataset
* Converted numberic citation style to author citation style

NSF Award Abstract:
This project will study how dimethylmercury is formed and removed in the oceans. Dimethylmercury is a naturally occurring compound. It is thought to be formed when man-made mercury is converted into monomethylmercury, a toxin that accumulates in fish. Despite representing a large fraction of mercury in the oceans, the origin and fate of dimethylmercury is not known. This research will use state-of-the-art analytical, genomic and modeling tools to address this information gap. It will also train graduate and undergraduate students to use field, experimental, and modeling methods. The results will be used in predictive models to forecast future trends in the oceanic mercury cycle. These models are needed to evaluate the effectiveness of international actions that seek to reverse increasing trends in the bioaccumulation of monomethylmercury in fish.

Methylated mercury species in the ocean are formed in sediment and the water column from inorganic divalent mercury delivered from the atmosphere and rivers. The putative mechanism is a two-step process during which monomethylmercury is formed first, followed by slow methylation into dimethylmercury. The first step, biomethylation of divalent mercury into monomethylmercury, is relatively well-studied in sediment and known to be driven by sulfate- and iron-reducing bacteria and methanogens. The mechanism for monomethylmercury formation in the water column is less well understood, and the process of dimethylmercury formation in sediment or seawater is essentially unknown. Until recently, it was assumed that dimethylmercury represented a small enough fraction of the methylated and total mercury (the sum of all mercury species) pools to be insignificant in the global mercury cycle. Recent measurements, however, show that dimethylmercury levels in seawater can be in the same range as the other mercury species. This project will identify the biological and chemical methylating agents involved in the formation of dimethylmercury. Further, it will test the impact of varying biogeochemical conditions on dimethylmercury production. Results will be used to update the mercury module of the MIT General Circulation Model (MITgcm, a global biogeochemical model, and analyze the impacts of dimethylmercury production and degradation on monomethylmercury concentrations.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.