Surface carbon dioxide measurements between 1994 and 2004 at site Estoc

Seasonal patterns in hydrography, partial pressure of CO2, fCO2, pHt, total alkalinity, AT, total dissolved inorganic carbon, CT, nutrients, and chlorophyll a were measured in surface waters on monthly cruises at the European Station for Time Series in the Ocean at the Canary Islands (ESTOC) located in the northeast Atlantic subtropical gyre. With over 5 years of oceanographic data starting in 1996, seasonal and interannual trends of CO2 species and air-sea exchange of CO2 were determined. Net CO2 fluxes show this area acts as a minor source of CO2, with an average outgassing value of 179 mmol CO2/m**2 yr controlled by the dominant trade winds blowing from May to August. The effect of short-term wind variability on the CO2 flux has been addressed by increasing air-sea fluxes by 63% for 6-hourly sampling frequency. The processes governing the monthly variations of CT have been determined. From March to October, when CT decreases, mixing at the base of the mixed layer (11.5 ± 1.5 mmol/m**3) is compensated by air-sea exchange, and a net organic production of 25.5 ± 5.7 mmol/m**3 is estimated. On an annual scale, biological drawdown accounts for the decrease in inorganic carbon from March to October, while mixing processes control the CT increase from October to the end of autumn. After removing seasonality variability, fCO2sw increases at a rate of 0.71 ± 5.1 µatm/yr, and as a response to the atmospheric trend, inorganic carbon increases at a rate of 0.39 ± 1.6 µmol/kg yr

Ocean alkalinity enhancement using sodium carbonate salts does not lead to measurable changes in Fe dynamics in a mesocosm experiment

The addition of carbonate minerals to seawater through an artificial ocean alkalinity enhancement (OAE) process increases the concentrations of hydroxide, bicarbonate, and carbonate ions. This leads to changes in the pH and the buffering capacity of the seawater. Consequently, OAE could have relevant effects on marine organisms and in the speciation and concentration of trace metals that are essential for their physiology. During September and October 2021, a mesocosm experiment was carried out in the coastal waters of Gran Canaria (Spain), consisting on the controlled variation of total alkalinity (TA). Different concentrations of carbonate salts (NaHCO3 and Na2CO3) previously homogenized were added to each mesocosm to achieve an alkalinity gradient between Δ0 to Δ2400 µmol L−1. The lowest point of the gradient was 2400 µmol kg−1, being the natural alkalinity of the medium, and the highest point was 4800 µmol kg−1. Iron (Fe) speciation was monitored during this experiment to analyse total dissolved iron (TdFe, unfiltered samples), dissolved iron (dFe, filtered through a 0.2 µm pore size filter), soluble iron (sFe, filtered through a 0.02 µm pore size filter), dissolved labile iron (dFe′), iron-binding ligands (LFe), and their conditional stability constants () because of change due to OAE and the experimental conditions in each mesocosm. Observed iron concentrations were within the expected range for coastal waters, with no significant increases due to OAE. However, there were variations in Fe size fractionation during the experiment. This could potentially be due to chemical changes caused by OAE, but such an effect is masked by the stronger biological interactions. In terms of size fractionation, sFe was below 1.0 nmol L−1, dFe concentrations were within 0.5–4.0 nmol L−1, and TdFe was within 1.5–7.5 nmol L−1. Our results show that over 99 % of Fe was complexed, mainly by L1 and L2 ligands with ranging between 10.92 ± 0.11 and 12.68 ± 0.32, with LFe ranging from 1.51 ± 0.18 to 12.3 ± 1.8 nmol L−1. Our data on iron size fractionation, concentration, and iron-binding ligands substantiate that the introduction of sodium salts in this mesocosm experiment did not modify iron dynamics. As a consequence, phytoplankton remained unaffected by alterations in this crucial element

Redox interactions of Fe and Cu in seawater

The interaction between the redox chemistry of Fe and Cu at nanomolar has been studied in UV-treated seawater. The oxidation of Fe(II) was studied as a function of concentrations of Cu(II) and Cu(I) from 0 to 200nM. The effect of added H2O2 (0–500nM), pH (6.0–8.5) and NaHCO3 (2–9mM) on the Fe(II) rate constants was studied at Cu(II) levels (0–200nM). To understand the competition between Fe and Cu, the reduction of Cu(II) to Cu(I) was also studied as a function of oxygen (air-saturated and anoxic seawater), Fe(II) (0–200nM) and H2O2 (0–300nM). The Fe(II) oxidation was accelerated by the presence of Cu(II) and Cu(I). This acceleration has been explained by the redox coupling between Fe and Cu, competition for different inorganic species (hydroxyl and carbonate groups studied independently) and by the formation of Fe–Cu particles (cupric or cuprous ferrite). Superoxide played a key role in the oxidation rate of Fe(II) in the presence of Cu(II). The presence of Fe(II) caused a greater reduction of Cu(II) to Cu(I). This is directly related to the levels of oxygen, Fe(II) and H2O2 concentrations. The presence of Fe(II) produced a rapid formation of Cu(I) in the first 2–3min of reaction. The Cu(I) is oxidized reaching a steady-state around 20nM levels of Cu(I). These experimental results demonstrated that the presence of Fe and Cu strongly affected the inorganic redox chemistry of both metals in UV-treated seawater. [Display omitted] •The Fe(II) oxidation was accelerated by the presence of Cu(II) and Cu(I).•The Fe(II) and Cur interaction is explained by coupling between Fe and Cu, competition for inorganic species and the formation of Fe–Cu particles.•Superoxide played a key role in the oxidation rate of Fe(II) in the presence of Cu(II).•The presence of Fe(II) caused a greater reduction of Cu(II) to Cu(I).•The presence of Fe(II) produced a rapid formation of Cu(I) in the first 2–3min of reaction