Passive acoustic monitoring is a promising tool for long-term ocean observations, offering a unique means to capture physical and biological processes. This study explores its potential as a source of fine temporal scale in-situ wind speed product by assembling a unique corpus of acoustic datasets co-located with or near in-situ weather stations. This study offers two key contributions: i) setting up a benchmarking framework for the development and evaluation of models in acoustic meteorology, and ii) applying this framework to assess the performance of various models, comparing parameters from the literature with those trained on datasets from this study's corpus. Regarding the latter point, results show that most untrained models fail to generalize due to the intrinsic variability of soundscape in different basins and environmental conditions, as well as calibration inaccuracies. However, all models can achieve satisfactory performance on specific datasets after training. Incorporating diverse observational sources, such as gliders and BGC-Argo floats, could enhance model robustness, and improved acoustic-based estimates will help refine satellite-derived wind products and numerical weather predictions, ultimately advancing global wind field modeling and air-sea interaction research.

Abstract. One important aspect of understanding ocean acidification is the nature and drivers of pH variability in surface waters on smaller spatial (i.e. areas up to 100 km2) and temporal (i.e. days) scales. However, there has been a lack of high-quality pH data at sufficiently high resolution. Here, we describe a simple optical system for continuous high-resolution surface seawater pH measurements. The system includes a PyroScience pH optode placed in a flow-through cell directly connected to the underway supply of a ship through which near-surface seawater is constantly pumped. Seawater pH is measured at a rate of 2 to 4 measurements min−1 and is cross-calibrated using discrete carbonate system observations (total alkalinity, dissolved inorganic carbon, and nutrients). This setup was used during two research cruises in different oceanographic conditions: the North Atlantic Ocean (December 2020–January 2021) and the South Pacific Ocean (February–April 2022). By leveraging this novel high-frequency measurement approach, our findings reveal fine-scale fluctuations in surface seawater pH across the North Atlantic and South Pacific oceans. While temperature is a significant abiotic factor driving these variations, it does not account for all observed changes. Instead, our results highlight the interplay between temperature, biological activity, and waters with distinct temperature–salinity properties and their impact on pH. Notably, the variability differed between the two regions, suggesting differences in the dominant factors influencing pH. In the South Pacific, biological processes appeared to be mostly responsible for pH variability, while in the North Atlantic, additional abiotic and biotic factors complicated the correlation between expected and observed pH changes. While our findings indicate that broader ocean-basin-scale analyses based on lower-resolution datasets can effectively capture surface ocean CO2 variability at a global scale, they also highlight the necessity of fine-scale observations for resolving regional processes and their drivers, which is essential for improving predictive models of ocean acidification and air–sea CO2 exchange.

Abstract Global marine anthropogenic CO 2 inventories have traditionally emphasized the North Atlantic's role in the carbon cycle, while Southern hemisphere processes are less understood. The South Subtropical Convergence (SSTC) in the South Atlantic, a juncture of distinct nutrient‐rich waters, offers a valuable study area for discerning the potential impacts of climate change on the ocean's biological carbon pump (C soft ). Using discrete observations from GLODAPv2.2022 and BGC‐Argo at 40°S in the Atlantic Ocean from 1972 to 2023, an increase in dissolved inorganic carbon (DIC) of +1.44 ± 0.11 μmol kg −1 yr −1 in surface waters was observed. While anthropogenic CO 2 played a role, variations in the contribution of C soft were observed. Discrepancies emerged in assessing C soft based on the tracers employed: when using AOU, C soft(AOU) recorded an increase of +0.20 ± 0.03 μmol kg −1 yr −1 , while using nitrate as the reference, C soft(NO3) displayed an increase of +0.85 ± 0.07 μmol kg −1 yr −1 . Key processes such as water mass composition shifts, changes in oxygenation, remineralization in the Southern Ocean, and the challenges they pose in accurately representing the evolving C soft are discussed. These findings highlight that while global studies primarily attribute DIC increase to anthropogenic CO 2 , observations at 40°S reveal an intensified biological carbon pump, showing that regional DIC changes are more complex than previously thought and emphasizing the need for better parameterizations to compute the BCP in the marine carbon budget.

Abstract<p>Carbon dioxide entering and acidifying the ocean can be neutralized by the dissolution of calcium carbonate, which is mainly found in two mineral forms. Calcite is the more stable form and is often found in deep‐sea sediments, whilst aragonite is more soluble and therefore rarely preserved. Recent research shows aragonite may account for a much larger portion of marine calcium carbonate export to the ocean interior via the biological pump than previously thought, and that aragonite does reach the deep sea and seafloor despite rarely being buried. If aragonite is present and dissolving at the seafloor it will raise local pH and calcium and carbonate concentrations, potentially enough to inhibit calcite dissolution, representing a deep‐sea, carbonate version of galvanization. Here, we test this hypothesis by simulating aragonite dissolution at the sediment‐water interface in the laboratory and measuring its effects on pH using microsensors. We show that the addition of aragonite to calcite sediment, overlain by seawater undersaturated with respect to both minerals, results in an unchanged alkalinity flux out of the dissolving sediment, suggesting a decrease the net dissolution rate of calcite. In combination with a diagenetic model, we show that aragonite dissolution can suppress calcite dissolution in the top millimeters of the seabed, locally leading to calcite precipitation within 1 day. Future research efforts should quantify this galvanization effect in situ, as this process may represent an important component of the marine carbon cycle, assigning a key role to aragonite producers in controlling ocean alkalinity and preserving climatic archives.</p>

Abstract. The Saguenay Fjord is a major tributary of the St. Lawrence Estuary and is strongly stratified. A 6–8 m wedge of brackish water typically overlies up to 270 m of seawater. Relative to the St. Lawrence River, the surface waters of the Saguenay Fjord are less alkaline and host higher dissolved organic carbon (DOC) concentrations. In view of the latter, surface waters of the fjord are expected to be a net source of CO2 to the atmosphere, as they partly originate from the flushing of organic-rich soil porewaters. Nonetheless, the CO2 dynamics in the fjord are modulated with the rising tide by the intrusion, at the surface, of brackish water from the Upper St. Lawrence Estuary, as well as an overflow of mixed seawater over the shallow sill from the Lower St. Lawrence Estuary. Using geochemical and isotopic tracers, in combination with an optimization multiparameter algorithm (OMP), we determined the relative contribution of known source waters to the water column in the Saguenay Fjord, including waters that originate from the Lower St. Lawrence Estuary and replenish the fjord's deep basins. These results, when included in a conservative mixing model and compared to field measurements, serve to identify the dominant factors, other than physical mixing, such as biological activity (photosynthesis, respiration) and gas exchange at the air–water interface, that impact the water properties (e.g., pH, pCO2) of the fjord. Results indicate that the fjord's surface waters are a net source of CO2 to the atmosphere during periods of high freshwater discharge (e.g., spring freshet), whereas they serve as a net sink of atmospheric CO2 when their practical salinity exceeds ∼5–10.