Application of a chemical leach technique for estimating labile particulate aluminum, iron, and manganese in the Columbia River plume and coastal waters off Oregon and Washington

Abstract

[1] In order to determine the total concentration of bioavailable trace metals in seawater, measurement of both the dissolved and labile particulate fractions is necessary. Comparison of labile particulate metal concentrations from various researchers is limited because of differing definitions of the fraction that is potentially available to phytoplankton on a time frame of generations. A comparison experiment was conducted on coastal and riverine suspended particulate matter to determine the difference between several commonly used techniques that operationally define the labile particulate trace metal fraction. Furthermore, we compared two leach techniques for surface transect samples from within the Columbia River plume and water offshore of Oregon and Washington, United States. The particulate trace metal concentration in the leachate was determined by high-resolution inductively coupled plasma-mass spectrometry. From this comparison, one chemical leach was chosen to best define the labile particulate fraction of Al, Fe, and Mn: a weak acid leach (25% acetic acid at pH 2) with a mild reducing agent (0.02 M hydroxylamine hydrochloride) and a short heating step (10 min 90-95°C). This leach was applied to three surface transects within the Columbia River plume. These coastal waters were found to be rich in labile particulate trace metals that are directly delivered from the Columbia River and indirectly supplied via resuspension from upwelling over a broad continental shelf.

Stability of dissolved and soluble Fe(II) in shelf sediment pore waters and release to an oxic water column

“…The ascorbic acid leach extracts easily reducible ferrihydrite, which is the first amorphous Fe oxyhydroxide phase precipitated due to Fe(II) oxidation (Raiswell et al 2010). The hydroxylamine-HCl leach extracts ferrihydrite as well as other reactive Fe phases that have been argued to be bioavailable (Berger et al 2008). Manganese seems to be trapped preferentially by materials released by the reducing hydroxylamine-HCl leach.…”

“…In order to examine phase associations of Fe and Mn in solid sediment phases, two leaching schemes were applied to one core per season: (i) an ascorbic acid leach (Raiswell et al 2010) was applied, to extract the easily reducible oxide phases, such as amorphous ferrihydrite, but not the more crystalline oxide phases, and (ii) an acetic acid-hydroxylamineHCl (H-HCl) leach (Berger et al 2008), that extracts other amorphous oxide phases as well as ferrihydrite. Fe and Mn in the leach solutions were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP6000 Series, Thermo Scientific).…”

How well can we quantify dust deposition to the ocean?

“…For example, applying a sequential leach method to the aerosol samples used in this study, Shelley et al [54] found the Sol Al obtained using the acetic acid (pH 2) hydroxylamine hydrochloride ('acetate') leach of Berger et al [55] to be about an order of magnitude greater than for the same samples using the instantaneous deionized water ('DI') leach of Buck et al [56]. For the stations between Africa and the Cape Verde Islands, Sol Al obtained using the acetate leach averaged about 10% while Sol Al determined using the DI leach varied between 0.3% and 1.0%.…”

“…In this context, thorium has an advantage because one isotope ( 232 .3)) using measured dissolved 232 Th inventories and estimates of aerosol Th solubility based on (a) the instantaneous deionized water leach (DI, [56]) and (b) the acetate-reducing agent leach (acetate, [55]). In each case, the residence time of Th is derived from the 234 U-230 Th radioactive disequilibrium integrated from the surface to the depth to which the 232 Th inventory is integrated (either 50 m or 500 m).…”

“…For the stations between Africa and the Cape Verde Islands, Sol Al obtained using the acetate leach averaged about 10% while Sol Al determined using the DI leach varied between 0.3% and 1.0%. .2)) following the approach of Measures & Brown [46] using measured dissolved Al inventories in the upper 50 m [50] and estimates of aerosol Al solubility based on the instantaneous deionized water leach (DI, [56]) and the acetate-reducing agent leach (acetate, [55]). A residence time for dissolved Al in the upper ocean of 5 years was assumed for all stations.…”