Aurélien Paulmier

Oxygen Minimum Zones (OMZs) are key components of the climate system, where uncertainties persist regarding the microbial processes controlling oxygen consumption and nitrogen cycling. The dynamics of the anaerobic remineralization process Dissimilatory Nitrate Reduction to Ammonium (DNRA) remain poorly understood in OMZs. Here, we implement DNRA in a simple biogeochemical model coupled to a 3 km resolution hydrodynamic model of the Chilean coastal OMZ. While the large-scale OMZ structure remains governed by physical processes, DNRA redistributes nitrogen toward recycling, away from pathways leading to fixed-N loss with negligible effects on mean oxygen. Unexpectedly, DNRA decreases nitrite and ammonium concentrations (by a factor of 5 and 2, respectively), due to a tight connection with nitrification oxidizing ammonium back to nitrate. This regenerated nitrate partly transported upward fosters growth of large phytoplankton and zooplankton. Results clarify DNRA contribution to both OMZ nitrogen cycling and surface ecosystem dynamics, and guide future observations.

The 2015–2016 El Niño was one of the most intense events in the Mexican Tropical Pacific and the Gulf of California, characterized by increased temperatures and shifts in water-mass proportions. To quantify these variations and their impact on carbon biogeochemistry, an Optimum Multiparameter (OMP) analysis was implemented using temperature, salinity, and dissolved oxygen (O2) data from three oceanographic cruises (June 2015, March 2016, and October 2017) and Argo float profiles. Results showed that between June 2015 and March 2016, Tropical Surface Water (TSW) dominated the upper 70 m (>50%), deepening both the oxycline and the dissolved inorganic carbon gradient (hereafter carboncline, defined as the depth of maximum DIC vertical gradient). This TSW intrusion reconfigured the vertical structure of the water column, directly affecting the biogeochemistry of O2 and dissolved inorganic carbon (DIC). Estimates of biogeochemical residuals (DIC and O2) revealed distinct processes: in June 2015, the distribution of residuals suggests a signal consistent with aerobic remineralization, located in the subsurface layer (70-140 m), driven by the mixing of Gulf of California Water (GCW) and Subtropical Subsurface Water (StSsW). In contrast, in March 2016, a cyclonic eddy acted as a localized productivity oasis, pumping nutrient-rich subsurface waters into the euphotic zone and driving a statistically significant photosynthetic signal (∆DIC < 0, ∆O₂ > 0) that challenged the expected regional oligotrophy of El Niño conditions. During the El Niño-Southern Oscillation (ENSO) neutral conditions in October 2017, physical water-mass mixing overrode any clearly distinguishable biogeochemical signal. These findings demonstrate that the use of biogeochemical residuals is a robust mechanism for assessing O2 and DIC biogeochemistry within the oxygen minimum zone (OMZ) of the Mexican Tropical Pacific under interannual fo

The Mexican Tropical Pacific (MTP) is a key component of the Eastern Tropical North Pacific Oxygen Minimum Zone, yet its carbonate system variability remains poorly constrained. This study examines wind-driven circulation effects on dissolved oxygen (DO) and the carbonate system —dissolved inorganic carbon (DIC), total alkalinity (TA), total-scale pH (pH_T), partial pressure of CO₂ in seawater (pCO_2w) and air–sea CO₂ fluxes (FCO₂)— in the Gulf of Tehuantepec (GT) and Tehuantepec Bowl (TB). Hydrographic data and discrete water samples were collected at 50 oceanographic stations during March 2020. Principal Component Analysis (PCA) identifies wind-driven circulation as the primary control of biogeochemical variability. Tehuano wind events and mesoscale eddies promoted upwelling of low-oxygen (DO < 20 µmol kg⁻¹) and high-DIC (>2200 µmol kg⁻¹) waters to 50 m depth in the central GT, while downwelling conditions prevailed in the TB. Stoichiometric analysis revealed DIC-DO coupling (slope = −1.39). Overall, the MTP acted as CO₂ source (FCO₂ ranging from −1.92 to 24.11 mmol m⁻² d⁻¹), with enhanced emissions linked to eddy-induced upwelling. This study provides the first integrated characterization of the carbonate system across both the GT and TB.

Studies of the impact of hypoxic or anoxic environments on both climate and ecosystems rely on a detailed characterization of the oxygen (O₂) distribution along the water column. The former trivial separation between oxic and anoxic conditions is now often redefined as a blurry concentration range in which both aerobic and anaerobic processes might coexist, both in situ and during experimental incubations. The O₂ concentrations during such incubations have often been assumed to be equal to in situ levels, but the concentration was rarely measured. In order to evaluate the actual oxygen concentration in samples collected from low‐oxygen environments, a series of measurements were performed on samples collected in the Pacific oxygen minimum zones. Our results show a significant deviation from in situ anoxic conditions in samples collected by Niskin bottles where leakage from the bottle material resulted in O₂ concentrations of up to 1 μM. Subsequent sampling further increased the O₂ contamination. Sampling and analysis by Winkler method resulted in variable apparent concentrations of 2–4 μM. Two common procedures to avoid atmospheric contamination were also tested: allowing gentle overflow and keeping the sampling bottle submersed in a portion of the sampled water. Both procedures resulted in similar O₂ contamination with values of 0.5–1.5 μM when bottles were immediately closed and measurements performed with optical sensors, and 3–4 μM apparent concentration when analyzed by the Winkler method. Winkler titration is thus not suited for analysis of low‐O₂ samples. It can be concluded that incubation under anoxic conditions requires deoxygenation after conventional sampling.

Significance: Seawater elemental composition, namely Mg:Ca and Sr:Ca ratios are widely used in marine sciences. Our new single-laboratory global dataset, combined with past data of seawater Mg:Ca and Sr:Ca ratios, suggests that modern ocean variability is significant across different environments, being similar to the changes during the Neogene Period (20 Ma). Because there is large variability in seawater ratios, we cannot assume that secular changes derived from fossilized taxa with varied ecological niches and from diverse ecosystems provide an unbiased representation of past global seawater chemistry. We suggest that the problem of inconsistencies in previous studies of, for example, paleo conditions from different fossil taxa may be reduced by accounting for taxonomic differences, ecosystems, and abiotic processes to overcome the regional environmental effect.

We investigate the seasonal variability of the southern tip (30°–38°S) of the eastern South Pacific oxygen minimum zone (OMZ) based on a high horizontal resolution (1/12°) regional coupled physical‐biogeochemical model simulation. The simulation is validated by available in situ observations and the OMZ seasonal variability is documented. The model OMZ, bounded by the contour of 45 μM, occupies a large volume (4.5x10⁴ km³) during the beginning of austral winter and a minimum (3.5x10⁴ km³) at the end of spring, just 1 and 2 months after the southward transport of the Peru‐Chile Undercurrent (PCUC) is maximum and minimum, respectively. We showed that the PCUC significantly impacts the alongshore advection of dissolved oxygen (DO) modulating the OMZ seasonal variability. However, zonal transport of DO by meridionally alternating zonal jets and mesoscale eddy fluxes play also a major role in the seasonal and spatial variability of the OMZ. Consistently, a DO budget analysis reveals a significant contribution of advection terms to the rate of change of DO and the prominence of mesoscale variability within the seasonal cycle of these terms. Biogeochemical processes and horizontal and vertical mixing, associated with subgrid scale processes, play only a secondary role in the OMZ seasonal cycle. Overall, our study illustrates the interplay of mean and (mesoscale) eddy‐induced transports of DO in shaping the OMZ and its seasonal cycle off Central Chile.

The oxygen minimum zone (OMZ) of Peru is recognized as a source of CO₂ to the atmosphere due to upwelling that brings water with high concentrations of dissolved inorganic carbon (DIC) to the surface. However, the influence of OMZ dynamics on the carbonate system remains poorly understood given a lack of direct observations. This study examines the influence of a coastal Eastern South Pacific OMZ on carbonate system dynamics based on a multidisciplinary cruise that took place in 2014. During the cruise, onboard DIC and pH measurements were used to estimate pCO₂ and to calculate the calcium carbonate saturation state (Ω aragonite and calcite). South of Chimbote (9°S), water stratification decreased and both the oxycline and carbocline moved from 150 m depth to 20–50 m below the surface. The aragonite saturation depth was observed to be close to 50 m. However, values 2 (maximum 1,250 μatm), and high DIC concentrations (maximum 2,300 μmol kg⁻¹). These chemical characteristics are shown to be associated with Equatorial Subsurface Water (ESSW). Large spatial variability in surface values was also found. Part of this variability can be attributed to the influence of mesoscale eddies, which can modify the distribution of biogeochemical variables, such as the aragonite saturation horizon, in response to shallower (cyclonic eddies) or deeper (anticyclonic eddies) thermoclines. The analysis of a 21-year (1993–2014) data set of mean sea surface level anomalies (SSHa) derived from altimetry data indicated that a large variance associated with interannual timescales was present near the coast. However, 2014 was characterized by weak Kelvin activity, and physical forcing was more associated with eddy activity. Mesoscale activity modulates the position of the upper boundary of ESSW, which is associated with high DIC and influences the carbocline and aragonite saturation depths. Weighing the relative importance of each individual signal results in a better understanding of the biogeochemical processes present in the area.