Pablo Lodeiro

Iron and nitrogen are the primary nutrients that limit productivity in the ocean. While nitrogen cycling is largely controlled by biology, iron cycling is strongly determined by chemistry because iron losses are driven by abiotic formation of authigenic mineral iron. Here, we apply a mechanistic approach to examine how organic matter across the dissolved-particulate size spectrum controls authigenic iron formation in subsurface waters (>250 m) of the South Pacific Ocean. We find that accounting for the chemical heterogeneity of organic matter is essential for predicting widespread authigenic iron formation. Predicted dissolved and particulate iron concentrations matched observations in the ocean interior, while discrepancies were linked to kinetic control of authigenic iron formation or inputs of particles from the seafloor. Our work shows the need to represent the complexity of abiotic interactions to better resolve the interplay of chemical and biological controls on ocean iron cycling.

Dissolved organic matter (DOM) exported from catchments regulates biogeochemical processing along river networks, but links between land use/land cover (LULC) and the composition of the operationally extractable DOM pool remain poorly constrained. We assessed how contrasting LULC patterns shape solid-phase extractable DOM along two Iberian rivers (Ebro and Mero) spanning forested headwaters, agricultural reaches, and urban zones. DOM isolates were characterized optically using synchronous fluorescence spectroscopy with Gaussian band fitting to quantify amino acid-, fulvic-, and humic-like fluorophores. Relative contributions shifted systematically downstream, and hierarchical clustering resolved three compositional groups consistent with the dominant LULC patterns. Forested headwaters were enriched in humic, aromatic material, consistent with preferential retention of hydrophobic, high-molecular-weight compounds by the styrene–divinylbenzene sorbent. Agricultural reaches, with higher nitrate and phosphate, showed stronger low-molecular-weight, microbially derived signatures that are less efficiently captured due to higher polarity and lower aromaticity. Urban sites displayed mixed optical fingerprints reflecting upstream inputs and local anthropogenic sources. Together, these results indicate that landscape setting and downstream hydrological processing jointly regulate extractable riverine DOM, emphasizing the need to account for LULC when interpreting basin-scale DOM dynamics.

Dissolved organic matter (DOM) shapes carbon cycling, nutrient availability, and contaminant transport in aquatic systems through its proton and metal binding properties. Here we investigate seasonal variation in DOM heterogeneity and proton binding affinity along the Ebro River, from its headwaters to the Mediterranean Sea, revealing clear spatial and temporal shifts in composition. River samples contained higher concentrations of low-affinity binding groups (94.9 ± 0.4 to 133.6 ± 0.4 mmol·mol C–1) compared with the marine endmember (84.9 ± 0.2 mmol·mol C–1). They also showed a broader range of high-affinity groups, though at lower concentrations (14.1 ± 0.2 to 33 ± 2 mmol·mol C–1), encompassing marine values (23 ± 2 mmol·mol C–1). Organic contributions to total alkalinity ranged from 2.5 to 15.6 μmol of binding groups per L of water sample. Seasonal patterns were linked to greater heterogeneity of proton binding sites in headwaters, while downstream processes, including groundwater inflows, terrestrial inputs, and degradation, reduced both the abundance and diversity of oxygen-rich groups. Overall, the findings are consistent with the River Continuum Concept in this watershed, capturing the downstream loss of acidic functional groups and the associated reduction in binding site diversity.

Iron and nitrogen are the primary nutrients that limit productivity in the ocean. While nitrogen cycling is largely controlled by biology, iron cycling is strongly determined by chemistry because iron losses are driven by abiotic formation of authigenic iron hydroxides (authFeOH). Here, we apply a mechanistic approach to examine how organic matter across the dissolved-particulate size spectrum controls authFeOH formation in subsurface waters (>250 m) of the South Pacific Ocean. We find that accounting for the chemical heterogeneity of organic matter is essential for predicting widespread authFeOH formation. Predicted dissolved and particulate iron concentrations matched observations in the ocean interior, while discrepancies were linked to kinetic control of authFeOH formation or inputs of particles from the seafloor. Our results highlight the need to represent complexity in abiotic interactions to better resolve the interplay of chemical and biological controls on ocean iron cycling.

The binding properties of dissolved organic matter (DOM) play a crucial role in the biogeochemical cycles of trace metals and carbon. The composition of DOM is anticipated to exert influence over the magnitude and distribution of the intrinsic ion binding affinities that occur over a continuum of values, referred to as the affinity spectra. These spectra encompass many different organic acid groups that contribute to the nuanced binding characteristics of DOM. The total proton binding capacity represents the maximum sites available for other chemical species that may compete with protons for the same DOM binding sites, particularly in the case of metals. Consequently, the study of proton binding by DOM becomes the initial step to investigate deeper into metal binding mechanisms. Here, we research the variability in proton binding exhibited by DOM extracted from the Ebro and Mero Rivers (NNE and NW of Spain), and at the Atlantic Ocean and Mediterranean Sea. Our approach combines the non-ideal competitive adsorption (NICA) isotherm, offering insights into chemical binding on heterogeneous ligands, with the Donnan electrostatic model, which accounts for polyelectrolytic effects, i.e., the non-specific binding. This methodology enables us to pinpoint potential shifts in DOM binding affinities and derive a comprehensive set of intrinsic binding parameters for DOM. Importantly, these parameters are thermodynamically consistent and remain independent of the specific conditions of the samples, enhancing the extrapolation to future environmental changes. Acknowledgements: Authors thank Agencia Española de Investigación for the financial support through the research projects PID2020-117910GB-C21 and -C22/AEI/10.13039/501100011033. P.L. acknowledges current support from the Ministerio de Ciencia, Innovación y Universidades of Spain and University of Lleida (Beatriz Galindo Senior award number BG20/00104)References:[1] Lodeiro, P., Rey-Castro, C., David, C., Humphreys, M. H., Gledhill, M., 2023. Proton Binding Characteristics of Dissolved Organic Matter Extracted from the North Atlantic. Environmental Science & Technology 57, 21136–21144.[2] Waska, H., Brumsack, H.-J., Massmann, G., Koschinsky, A., Schnetger, B., Simon, H., Dittmar, T., 2019. Inorganic and organic iron and copper species of the subterranean estuary: Origins and fate. Geochimica et Cosmochimica Acta 259, 211–232.[3] Heerah, K. M., Reade, H. E., 2023. Towards the identification of humic ligands associated with iron transport through a salinity gradient. Scientific Reports 12:15545.

Marine dissolved organic matter (DOM) presents key thermodynamic properties that are not yet fully constrained. Here, we report the distribution of binding sites occupied by protons (i.e., proton affinity spectra) and parametrize the median intrinsic proton binding affinities (log K̅_H) and heterogeneities (m), for DOM samples extracted from the North Atlantic. We estimate that 11.4 ± 0.6% of C atoms in the extracted marine DOM have a functional group with a binding site for ionic species. The log K̅_H of the most acidic groups was larger (4.01-4.02 ± 0.02) than that observed in DOM from coastal waters (3.82 ± 0.02), while the chemical binding heterogeneity parameter increased with depth to values (m₁= 0.666 ± 0.009) ca. 10% higher than those observed in surface open ocean or coastal samples. On the contrary, the log K̅_H for the less acidic groups shows a difference between the surface (10.01 ± 0.08) and deep (9.22 ± 0.35) samples. The latter chemical groups were more heterogeneous for marine than for terrestrial DOM, and m₂ decreased with depth to values of 0.28 ± 0.03. Binding heterogeneity reflects aromatic carbon compounds' persistence and accumulation in diverse, low-abundance chemical forms, while easily degradable low-affinity groups accumulate more uniformly in the deep ocean.

The article is difficult to read and contains grammatical errors. The writting sould be improved. Most of the citations in the manuscript are inappropriate. It is unethical and unacceptable that 13 out of 24 citations in the manuscript are self-cita...

We present high‐resolution profiles of dissolved, labile, and total particulate trace metals (TMs) on the Northeast Greenland shelf from GEOTRACES cruise GN05 in August 2016. Combined with radium isotopes, stable oxygen isotopes, and noble gas measurements, elemental distributions suggest that TM dynamics were mainly regulated by the mixing between North Atlantic‐derived Intermediate Water, enriched in labile particulate TMs (LpTMs), and Arctic surface waters, enriched in Siberian shelf‐derived dissolved TMs (dTMs; Co, Cu, Fe, Mn, and Ni) carried by the Transpolar Drift. These two distinct sources were delineated by salinity‐dependent variations of dTM and LpTM concentrations and the proportion of dTMs relative to the total dissolved and labile particulate ratios. Locally produced meltwater from the Nioghalvfjerdsbræ (79NG) glacier cavity, distinguished from other freshwater sources using helium excess, contributed a large pool of dTMs to the shelf inventory. Localized peaks in labile and total particulate Cd, Co, Fe, Mn, Ni, Cu, Al, V, and Ti in the cavity outflow, however, were not directly contributed by submarine melting. Instead, these particulate TMs were mainly supplied by the re‐suspension of cavity sediment particles. Currently, Arctic Ocean outflows are the most important source of dFe, dCu, and dNi on the shelf, while LpTMs and up to 60% of dMn and dCo are mainly supplied by subglacial discharge from the 79NG cavity. Therefore, changes in the cavity‐overturning dynamics of 79NG induced by glacial retreat, and alterations in the transport of Siberian shelf‐derived materials with the Transport Drift may shift the shelf dTM‐LpTM stoichiometry in the future.

Concentrations of the toxic element lead (Pb) are elevated in seawater due to historical emissions. While anthropogenic atmospheric emissions are the dominant source of dissolved Pb (dPb) to the Atlantic Ocean, evidence is emerging of a natural source associated with subglacial discharge into the ocean but this has yet to be constrained around Greenland. Here, we show subglacial discharge from the cavity underneath Nioghalvfjerdsbræ floating ice tongue, is a previously unrecognized source of dPb to the NE Greenland Shelf. Contrasting cavity‐inflowing and cavity‐outflowing waters, we constrain the associated net‐dPb flux as 2.2 ± 1.4 Mg·yr⁻¹, of which ∼90% originates from dissolution of glacial bedrock and cavity sediments. We propose that the retreat of the floating ice tongue, the ongoing retreat of many glaciers on Greenland, associated shifts in sediment dynamics, and enhanced meltwater discharges into shelf waters may result in pronounced changes, possibly increases, in net‐dPb fluxes to coastal waters.