Breaking up is easy to do: Sinking particles transport carbon to the seafloor, where they are buried in sediments and either provide food for benthic organisms or sequester the carbon they contain. However, only ∼30% of the maximum flux reaches depths of a kilometer. This loss cannot be fully accounted for by current measurements. Briggs et al. used data collected by robotic Biogeochemical-Argo floats to quantify total mesopelagic fragmentation and found that this process accounts for roughly half of the observed flux loss (see the Perspective by Nayak and Twardowski). Fragmentation is thus perhaps the most important process controlling the remineralization of sinking organic carbon. Science , this issue p. 791 ; see also p. 738

Life at ocean depths below ∼100 m requires organic carbon from the upper ocean. Analyses of satellite and Argo-float data reveal that seasonal changes in mixed-layer depth supply substantial amounts of carbon to this deep and dark ecosystem. The ocean region known as the mesopelagic zone, which is at depths of about 100–1,000 m, harbours one of the largest ecosystems and fish stocks on the planet1,2. Life in this region is believed to rely on particulate organic carbon supplied by the biological carbon pump3. Yet this supply appears insufficient to meet mesopelagic metabolic demands4,5,6. An additional organic carbon source to the mesopelagic zone could be provided by the seasonal entrainment of surface waters in deeper layers, a process known as the mixed-layer pump7,8,9,10,11. Little is known about the magnitude and spatial distribution of this process globally or its potential to transport carbon to the mesopelagic zone. Here we combine mixed-layer depth data from Argo floats with satellite estimates of particulate organic carbon concentrations to show that the mixed-layer pump supplies an important seasonal flux of organic carbon to the mesopelagic zone. We estimate that this process is responsible for a global flux of 0.1–0.5 Pg C yr−1. In high-latitude regions where the mixed layer is usually deep, this flux amounts on average to 23% of the carbon supplied by fast sinking particles, but it can be greater than 100%. We conclude that the seasonal mixed-layer pump is an important source of organic carbon for the mesopelagic zone.

Despite its fundamental role in controlling the Earth's climate, present estimates of global organic carbon export to the deep sea are affected by relatively large uncertainties. These uncertainties are due to lack of observations as well as disagreement among methods and assumptions used to estimate carbon export. Complementary observations are thus needed to reduce these uncertainties. Here we show that optical backscattering measured by Bio‐Argo floats can detect a seasonal carbon export flux in the Norwegian Sea. This export was most likely due to small particles (i.e., 0.2–20 μm), was comparable to published export values, and contributed to long‐term carbon sequestration. Our findings highlight the importance of small particles and of physical mixing in the biological carbon pump and support the use of autonomous platforms as tools to improve our mechanistic understanding of the ocean carbon cycle.

The detrainment of organic matter from the mixed layer, a process known as the mixed layer pump (ML pump), has long been overlooked in carbon export budgets. Recently, the ML pump has been investigated at seasonal scale and appeared to contribute significantly to particulate organic carbon export to the mesopelagic zone, especially at high latitudes where seasonal variations of the mixed layer depth are large. However, the dynamics of the ML pump at intraseasonal scales remains poorly known, mainly because the lack of observational tools suited to studying such dynamics. In the present study, using a dense network of autonomous profiling floats equipped with bio‐optical sensors, we captured widespread episodic ML pump‐driven export events, during the winter and early spring period, in a large part of the subpolar North Atlantic Ocean. The intraseasonal dynamics of the ML pump exports fresh organic material to depth (basin‐scale average up to 55 mg C·m⁻²·day⁻¹), providing a significant source of energy to the mesopelagic food web before the spring bloom period. This mechanism may sustain the seasonal development of overwintering organisms such as copepods with potential impact on the characteristics of the forthcoming spring phytoplankton bloom through predator‐prey interactions.

Marine microscopic particles profoundly impact global biogeochemical cycles, but our understanding of their dynamics is hindered by lack of observations. To fill this gap, optical backscattering measured by satellite sensors and in-situ autonomous platforms can be exploited. Unfortunately, these observations remain critically limited by an incomplete mechanistic understanding of what particles generate the backscattering signal. To achieve this understanding, optical models are employed. The simplest of these models—the homogeneous sphere—severely underestimates the measured backscattering and the missing signal has been attributed to submicron particles. This issue is known as the missing backscattering enigma. Here we show that a slightly more complex optical model—the coated sphere—can predict the measured backscattering and suggests that most of the signal comes from particles >1 µm. These findings were confirmed by independent size-fractionation experiments. Our results demonstrate that the structural complexity of particles is critical to understand open-ocean backscattering and contribute to solving the enigma.

The particulate optical backscattering coefficient (b_bp) is a fundamental optical property that allows monitoring of marine suspended particles both in situ and from space. Backscattering measurements in the open ocean are still scarce, however, especially in oligotrophic regions. Consequently, uncertainties remain in b_bp parameterizations as well as in satellite estimates of b_bp. In an effort to reduce these uncertainties, we present and analyze a dataset collected in surface waters during the 19^th Atlantic Meridional Transect. Results show that the relationship between particulate beam-attenuation coefficient (c_p) and chlorophyll-a concentration was consistent with published bio-optical models. In contrast, the particulate backscattering per unit of chlorophyll-a and per unit of c_p were higher than in previous studies employing the same sampling methodology. These anomalies could be due to a bias smaller than the current uncertainties in b_bp. If that was the case, then the AMT19 dataset would confirm that b_bp:c_p is remarkably constant over the surface open ocean. A second-order decoupling between b_bp and c_p was, however, evident in the spectral slopes of these coefficients, as well as during diel cycles. Overall, these results emphasize the current difficulties in obtaining accurate b_bp measurements in the oligotrophic ocean and suggest that, to first order, b_bp and c_p are coupled in the surface open ocean, but they are also affected by other geographical and temporal variations.