Jong-Mi Lee

Analysis of trace metals (Cu, Cd, Pb, and Fe) in seawater using single batch nitrilotriacetate resin extraction and isotope dilution inductively coupled plasma mass spectrometry.

A simple and accurate low-blank method has been developed for the analysis of total dissolved copper, cadmium, lead, and iron in a small volume (1.3-1.5 mL per element) of seawater. Pre-concentration and salt-separation of a stable isotope spiked sample are achieved by single batch extraction onto nitrilotriacetate (NTA)-type Superflow(®) chelating resin beads (100-2400 beads depending on the element). Metals are released into 0.1-0.5 M HNO(3), and trace metal isotope ratios are determined by ICPMS. The benefit of this method compared to our previous Mg(OH)(2) coprecipitation method is that the final matrix is very dilute so cone-clogging and matrix sensitivity suppression are minimal, while still retaining the high accuracy of the isotope dilution technique. Recovery efficiencies are sensitive to sample pH, number of resin beads added, and the length of time allowed for sample-resin binding and elution; these factors are optimized for each element to yield the highest recovery. The method has a low procedural blank and high sensitivity sufficient for the analysis of pM-nM open-ocean trace metal concentrations. Application of this method to samples from the Bermuda Atlantic Time-Series Study station provides oceanographically consistent Cu, Cd, Pb, and Fe profiles that are in good agreement with other reliable data for this site. In addition, the method can potentially be modified for the simultaneous analysis of multiple elements, which will be beneficial for the analysis of large number of samples.

Dissolved iron and iron isotopes in the southeastern Pacific Ocean

The Southeast Pacific Ocean is a severely understudied yet dynamic region for trace metals such as iron, since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters. In this study, we report the dissolved iron (dFe) distribution from seven stations and Fe isotope ratios (δ56Fe) from three of these stations across a near-zonal transect from 20 to 27°S. We found elevated dFe concentrations associated with the oxygen-deficient zone (ODZ), with light δ56Fe implicating porewater fluxes of reduced Fe. However, temporal dFe variability and rapid δ56Fe shifts with depth suggest gradients in ODZ Fe source and/or redox processes vary over short-depth/spatial scales. The dFe concentrations decreased rapidly offshore, and in the upper ocean dFe was controlled by biological processes, resulting in an Fe:C ratio of 4.2 µmol/mol. Calculated vertical diffusive Fe fluxes were greater than published dust inputs to surface waters, but both were orders of magnitude lower than horizontal diffusive fluxes, which dominate dFe delivery to the gyre. The δ56Fe data in the deep sea showed evidence for a −0.2‰ Antarctic Intermediate Water end-member and a heavy δ56Fe of +0.55‰ for distally transported hydrothermal dissolved Fe from the East Pacific Rise. These heavy δ56Fe values were contrasted with the near-crustal δ56Fe recorded in the hydrothermal plume reaching Station ALOHA in the North Pacific. The heavy hydrothermal δ56Fe precludes a nanopyrite composition of hydrothermal dFe and instead suggests the presence of oxides or, more likely, binding of hydrothermal dFe by organic ligands in the distal plume.

Recent distribution of lead in the Indian Ocean reflects the impact of regional emissions

Significance Humans have altered the earth surface environment by massive injection of certain chemicals into our air and water. In some cases these injections are detrimental to environmental health and must be monitored to limit the damage (e.g., freons and the ozone layer); in other cases (e.g., freon dissolving into the ocean) there are no harmful consequences, but the chemicals are useful as tracers of ocean circulation patterns. Lead remains a major hazard when it is proximate to humans (e.g., plumbing, housepaint, and contaminated soils); in the open ocean, lead serves as an inadvertent experiment demonstrating how metals move through the marine environment. To our knowledge, this study is the first to examine the fate of human-injected lead in the Indian Ocean. Humans have injected lead (Pb) massively into the earth surface environment in a temporally and spatially evolving pattern. A significant fraction is transported by the atmosphere into the surface ocean where we can observe its transport by ocean currents and sinking particles. This study of the Indian Ocean documents high Pb concentrations in the northern and tropical surface waters and extremely low Pb levels in the deep water. North of 20°S, dissolved Pb concentrations decrease from 42 to 82 pmol/kg in surface waters to 1.5–3.3 pmol/kg in deep waters. South of 20°S, surface water Pb concentrations decrease from 21 pmol/kg at 31°S to 7 pmol/kg at 62°S. This surface Pb concentration gradient reflects a southward decrease in anthropogenic Pb emissions. The upper waters of the north and central Indian Ocean have high Pb concentrations resulting from recent regional rapid industrialization and a late phase-out of leaded gasoline, and these concentrations are now higher than currently seen in the central North Pacific and North Atlantic oceans. The Antarctic sector of the Indian Ocean shows very low concentrations due to limited regional anthropogenic Pb emissions, high scavenging rates, and rapid vertical mixing, but Pb still occurs at higher levels than would have existed centuries ago. Penetration of Pb into the northern and central Indian Ocean thermocline waters is minimized by limited ventilation. Pb concentrations in the deep Indian Ocean are comparable to the other oceans at the same latitude, and deep waters of the central Indian Ocean match the lowest observed oceanic Pb concentrations.

Lead isotope exchange between dissolved and fluvial particulate matter: a laboratory study from the Johor River estuary

Atmospheric aerosols are the dominant source of Pb to the modern marine environment, and as a result, in most regions of the ocean the Pb isotopic composition of dissolved Pb in the surface ocean (and in corals) matches that of the regional aerosols. In the Singapore Strait, however, there is a large offset between seawater dissolved and coral Pb isotopes and that of the regional aerosols. We propose that this difference results from isotope exchange between dissolved Pb supplied by anthropogenic aerosol deposition and adsorbed natural crustal Pb on weathered particles delivered to the ocean by coastal rivers. To investigate this issue, Pb isotope exchange was assessed through a closed-system exchange experiment using estuarine waters collected at the Johor River mouth (which discharges to the Singapore Strait). During the experiment, a known amount of dissolved Pb with the isotopic composition of NBS-981 (206Pb/207Pb = 1.093) was spiked into the unfiltered Johor water (dissolved and particulate 206Pb/207Pb = 1.199) and the changing isotopic composition of the dissolved Pb was monitored. The mixing ratio of the estuarine and spike Pb should have produced a dissolved 206Pb/207Pb isotopic composition of 1.161, but within a week, the 206Pb/207Pb in the water increased to 1.190 and continued to increase to 1.197 during the next two months without significant changes of the dissolved Pb concentration. The kinetics of isotope exchange was assessed using a simple Kd model, which assumes multiple sub-reservoirs within the particulate matter with different exchange rate constants. The Kd model reproduced 56% of the observed Pb isotope variance. Both the closed-system experiment and field measurements imply that isotope exchange can be an important mechanism for controlling Pb and Pb isotopes in coastal waters. A similar process may occur for other trace elements. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.