Iron redox dynamics in the surface waters of the Gulf of Aqaba, Red Sea

Abstract

Redox transformations of iron in the surface waters of the Gulf of Aqaba, Red Sea, were studied on recurrent cruises from September 2006 to May 2007. Fe(II) concentrations and oxidation kinetics were measured in situ using luminol chemiluminescence. High Fe(II) concentrations of 200–400 pM were recorded in the autumn, followed by low concentrations of 20–130 pM in the winter–spring. A distinct diurnal pattern in Fe(II) concentrations was observed in the autumn with maximum values coinciding with maximum solar irradiance. In situ and in vitro Fe(II) oxidation rates showed temporal and spatial variability that was accounted for by changes in water temperature and pH. Dissolved oxygen was found to be the dominant oxidant in all but one cruise. In situ photoreduction rates (deduced from oxidation rates) were linearly correlated with solar irradiance during the autumn, suggesting that the reducible iron pool was not exhausted even at the strongest irradiances and that it was kept constant throughout the season. Phytoplankton had no discernible influence on Fe(II) production, consumption, or oxidation kinetics. Given the fast oxidation and photoreduction rates of up to 180 pM min⁻¹, the turn-over rates of iron were estimated at 10–30 per day. Such a dynamic Fe redox cycle probably influences the chemical reactivity and bioavailability of iron and may enhance the solubility of the abundant aerosol dust.

Introduction

Oxidation–reduction (redox) reactions of iron in near surface waters significantly affect its biogeochemical cycling. Light- and/or biologically-induced redox transformations of iron have been shown to increase its solubility and bioavailability in a variety of seawater and fresh water environments (Pehkonen et al., 1992, Johnson et al., 1994, Miller and Kester, 1994, Waite et al., 1995). Changes in the chemical speciation of iron, and thus its biological availability, have been predominantly attributed to reductive dissolution of iron minerals and the reductive release of organically bound iron (Sulzberger et al., 1988, Barbeau et al., 2001). The cycling of iron between its reduced (Fe(II)) and oxidized (Fe(III)) forms has a number of other environmental consequences, such as degradation of dissolved organic matter, release of reactive oxygen species, and mobilization or alteration of organic and inorganic pollutants (Voelker et al., 1997, Epp et al., 2007).

In oxygenated surface water with circumneutral pH, the thermodynamically unstable Fe(II) oxidizes back to Fe(III) within a few minutes (King et al., 1995, Rose and Waite, 2002, Santana-Casiano et al., 2005). Consequently, a continuous supply of both energy and electrons is required to drive the redox cycle and maintain measurable Fe(II) concentrations. To date, Fe(II) concentrations ranging from a few picomolars to several nanomolars have been reported in a number of aquatic environments (Emmenegger et al., 2001, Shaked et al., 2002, Boyé et al., 2006, Croot et al., 2007, Hopkinson and Barbeau, 2007, Moffett et al., 2007, Ussher et al., 2007). In near surface waters, solar irradiance drives photochemical reduction of iron by direct photolysis and/or by indirect photolysis of chromophoric dissolved organic matter (CDOM) to form Fe(III)-reducing radicals (Barbeau, 2006). The mechanisms and rates of photoreduction in natural water are thought to vary as a function of the sun spectrum and intensity, the concentration and composition of dissolved organic matter and the chemical speciation (and to lesser extent concentrations) of iron (King et al., 1993, Rijkenberg et al., 2005, Laglera and Van den Berg, 2007). In oligotrophic and/or Fe-limited regions, where CDOM concentrations are low and the vast majority of dissolved Fe is bound to strong organic complexes or forms part of strong organic complexes, the direct photochemical mechanism is likely to be more important (Meunier et al., 2005). In contrast, in high-CDOM high-iron coastal and freshwater systems, indirect iron reduction by photo-generated superoxides probably plays a significant role (Voelker and Sedlak, 1995, Rose and Waite, 2006).

Fe(II) oxidation has been thoroughly investigated in a number of natural and artificial solutions. The reaction mechanism has been resolved, individual rate constants evaluated and detailed kinetic models constructed (Millero et al., 1987, Millero, 1989, King et al., 1995, Rose and Waite, 2002, Rose and Waite, 2003a, Santana-Casiano et al., 2006). The influence of pH, temperature, and ionic strength on the kinetics of nanomolar level Fe(II) oxidation by oxygen, hydrogen peroxide and to some extent superoxide in seawater is now well-documented (Millero and Sotolongo, 1989, King et al., 1995, Gonzalez-Davila et al., 2005, Rose and Waite, 2006). Various organic ligands and colloids were shown to strongly influence Fe(II) oxidation kinetics, either by accelerating or decelerating the reaction (Rose and Waite, 2003b, Rijkenberg et al., 2006, Ussher et al., 2005). Several field studies have reported marked deviations in the rate of Fe(II) oxidation compared to laboratory rates, possibly as a result of organic complexation of the Fe(II) and/or the presence of photo-generated radicals (Emmenegger et al., 2001, Croot and Laan, 2002, Croot et al., 2005, Croot et al., 2007, Hopkinson and Barbeau, 2007, Laglera and Van den Berg, 2007, Moffett et al., 2007, Roy et al., 2008). This extensive knowledge, coupled with recent analytical developments in real-time Fe(II) measurements, enables us to closely monitor and interpret the oxidation kinetics of Fe(II) in situ. This information, in turn, can facilitate more analytically challenging investigations of the rates and mechanisms of iron photoreduction in ambient conditions (as opposed to solar-simulated experiments).

The northern Gulf of Aqaba, enclosed by a desert landmass (Fig. 1), is an atypical coastal environment. It is subject to elevated aerosol–iron input, but the lack of runoff and the water circulation pattern make it a low-CDOM, oligotrophic ecosystem (Genin et al., 1995, Chen et al., 2007). It bears some similarity (although at higher iron concentrations) to the oligotrophic Mediterranean Sea, which is influenced by Saharan dust (Bonnet and Guieu, 2006). This unique condition of low-CDOM high-iron waters may become more prevalent in a global warming scenario of increased aerosol dust flux to the ocean as a result of the drying up of subtropical regions (Jickells et al., 2005).

This study is the first step in ongoing research on the effects of photoreduction on the solubility and subsequent bioavailability of aerosol–dust iron that reaches the ocean’s surface. This is entirely field-based research that takes advantage of the high dust input, strong solar irradiance, and unique hydrographic conditions of the northern Gulf of Aqaba. A detailed examination of diurnal changes in Fe(II) concentrations in surface waters and the rate of Fe(II) oxidation in situ was conducted on recurrent cruises. These data were combined with meteorological, hydrographic and chemical measurements to elucidate the governing reactions and reactants in the iron redox cycle.