Identifying Appropriate Locations for the Accelerated Weathering of Limestone to Reduce CO2 Emissions

The reduction in CO2 emissions is a major task for the coming decades. Accelerated weathering of limestone (AWL) can be used to capture CO2 from effluent gas streams and store it as bicarbonate in marine environments. We give an overview of the fundamental aspects of AWL, including associated CO2 emissions during the operation of AWL, characteristics of the accumulating bicarbonate-rich product water, and factors influencing the outgassing of CO2 from the ocean back into the atmosphere. Based on these aspects, we identify locations where AWL could be carried out favorably. The energy demand for AWL reduces the theoretical CO2 sequestration potential, for example, by only 5% in the case of a 100 km transport of limestone on roads. AWL-derived product water is characterized by high alkalinity but low pH values and, once in contact with the atmosphere, passive outgassing of CO2 from AWL-derived water occurs. This process is mainly driven by the difference between the fCO2 in the atmosphere and the oceanic surface layer, as well as the sea surface temperature at the discharge site. Promising sites for AWL may be in Florida or around the Mediterranean Sea, where outgassing could be prevented by injections into deep water layers.

Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.

Effects of Ocean Acidification on Calcification of the Sub-Antarctic Pteropod Limacina retroversa

Ocean acidification is expected to impact the high latitude oceans first, as CO2 dissolves more easily in colder waters. At the current rate of anthropogenic CO2 emissions, the sub-Antarctic Zone will start to experience undersaturated conditions with respect to aragonite within the next few decades, which will affect marine calcifying organisms. Shelled pteropods, a group of calcifying zooplankton, are considered to be especially sensitive to changes in carbonate chemistry because of their thin aragonite shells. Limacina retroversa is the most abundant pteropod in sub-Antarctic waters, and plays an important role in the carbonate pump. However, not much is known about its response to ocean acidification. In this study, we investigated differences in calcification between L. retroversa individuals exposed to ocean carbonate chemistry conditions of the past (pH 8.19; mid-1880s), present (pH 8.06), and near-future (pH 7.93; predicted for 2050) in the sub-Antarctic. After 3 days of exposure, calcification responses were quantified by calcein staining, shell weighing, and Micro-CT scanning. In pteropods exposed to past conditions, calcification occurred over the entire shell and the leading edge of the last whorl, whilst individuals incubated under present and near-future conditions mostly invested in extending their shells, rather than calcifying over their entire shell. Moreover, individuals exposed to past conditions formed larger shell volumes compared to present and future conditions, suggesting that calcification is already decreased in today’s sub-Antarctic waters. Shells of individuals incubated under near-future conditions did not increase in shell weight during the incubation, and had a lower density compared to past and present conditions, suggesting that calcification will be further compromised in the future. This demonstrates the high sensitivity of L. retroversa to relatively small and short-term changes in carbonate chemistry. A reduction in calcification of L. retroversa in the rapidly acidifying waters of the sub-Antarctic will have a major impact on aragonite-CaCO3 export from oceanic surface waters to the deep sea.

Plankton Community Metabolism in Western Australia: Estuarine, Coastal and Oceanic Surface Waters

Net community production (NCP) is a community level process informing on the balance between production and consumption, determining the role of plankton communities in carbon and nutrient balances fueling the marine food web. An assessment of net and gross community production (NCP, GPP) and community respiration (CR) in 86 surface plankton communities sampled between 15° and 36° South along coastal Western Australia (WA) revealed a prevalence of net autotrophic metabolism (GPP/CR > 1), comprising 81% of the communities sampled. NCP, GPP, and CR decreased with decreasing nutrient and chlorophyll-a concentrations, from estuarine, to coastal and oceanic waters. CR, standardized per unit chlorophyll-a, increased with temperature, with higher activation energies (Ea) than GPP per unit chlorophyll-a (Ea 1.07 ± 0.18 eV and 0.65 ± 0.15 eV, respectively) either across ecosystem types and for coastal and estuary communities alone, indicating plankton CR to increase much faster with warming than GPP. These results characterize surface plankton communities across Western Australia as CO2 sinks, the stronger thermal-dependence of respiration that gross primary production rates suggests that their role may weaken with future warming.

Le rôle climatique des fronts océaniques de fine échelle en profondeur (prix Prud'homme 2020)

L'océan est le plus grand réservoir d'énergie de notre planète. La quantité de chaleur qu'il est capable de stocker est modulée par sa circulation complexe, opérant sur des échelles allant du centimètre à la dizaine de milliers de kilomètres. Les découvertes scientifiques des deux dernières décennies ont révélé l'existence de fronts de fine échelle (d'environ 1 à 50 km), analogues aux fronts atmosphériques, dans la couche de mélange océanique de surface. Ces fronts agissent comme des conduits entre l'océan et l'atmosphère, contrôlant les échanges de gaz et de chaleur. Combinant observation et modélisation, nous démontrons pour la première fois le rôle capital de ces fronts jusqu'à 1000 m de profondeur. Ils génèrent d'importants flux de chaleur dirigés de l'intérieur de l'océan vers la surface, pouvant modifier la capacité de stockage de chaleur de l'océan, avec des répercussions potentiellement majeures pour les systèmes biogéochimique et climatique. The ocean is the largest solar energy collector on Earth. The amount of heat it can store is modulated by its complex circulation, which spans a broad range of spatial scales, from centimeters to thousands of kilometers. Scientific discoveries of the past two decades revealed the existence of fine-scale fronts (≈ 10-20 km wide), analogous to atmospheric fronts, in the oceanic surface mixed layer. These fronts control the exchanges between the ocean and the atmosphere just as the capillary vessels of our pulmonary alveoli facilitate the exchange of gas when breathing. Combining observation and modeling, we demonstrate for the first time the crucial role played by these fronts in the ocean interior. These fine-scale fronts drive an anomalous upward heat transport from the ocean interior back to the surface. This can alter the ocean heat storage capacity, with potential major implications for the biogeochemical and climate systems.

Evaluating Monin-Obukhov Scaling in the Unstable Oceanic Surface Layer

Monin-Obukhov Similarity Theory (MOST) provides important scaling laws for flow properties in the surface layer of the atmosphere and has contributed to most of our understanding of the near-surface turbulence. The prediction of near-surface vertical mixing in most operational ocean models is largely built upon this theory. However, the validity of MOST in the upper ocean is questionable due to the demonstrated importance of surface waves in the region. Here we examine the validity of MOST in the statically unstable oceanic surface layer, using data collected from two open ocean sites with different wave conditions. The observed vertical temperature gradients are found to be about half of those predicted by MOST. We hypothesize this is attributable to either the breaking of surface waves, or Langmuir turbulence generated by the wave-current interaction. Existing turbulence closure models for surface wave breaking and for Langmuir turbulence are simplified to test these two hypotheses. Although both models predict reduced temperature gradients, the simplified Langmuir turbulence model matches observations more closely, when appropriately tuned.

Is the Ocean Speeding Up? Ocean Surface Energy Trends

A recent paper by Hu et al. (https://doi.org/10.1126/sciadv.aax7727) has raised the interesting question of whether the ocean circulation has been “speeding up” in the last decades. Their result contrasts with some estimates of the lack of major trends in oceanic surface gravity waves and wind stress. In general, both the increased energy and implied power inputs of the calculated circulation correspond to a small fraction of the very noisy background values. An example is the implied power increase of about 3 × 108 W, as compared to wind energy inputs of order 1012 W. Here the problem is reexamined using a state estimate that has the virtue of being energy, mass, etc. conserving. Because it is an estimate over an entire recent 26-yr interval, it is less sensitive to the strong changes in observational data density and distribution, and it does not rely upon nonconservative “reanalyses.” The focus is on the energy lying in the surface layers of the ocean. A potential energy increase is found, but it is almost completely unavailable—arising from the increase in mean sea level. A weak increase in kinetic energy in the top layer (10 m) is confirmed, corresponding to an increase of order 1 cm s−1 yr−1 over 26 years. An estimate of kinetic energy in the full water column shows no monotonic trend, but the changes in the corresponding available potential energy are not calculated here.