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Recently, new spatiotemporal-scale particle observations by autonomous profiling floats equipped with bio-optical sensors have revealed that, in addition to gravitational particle sinking, the downward transport of surface particles by physical mixing events, which has been overlooked, contributes to particulate organic carbon export. However, the subsequent behavior of these exported particles in the mesopelagic zone (e.g., particle fragmentation and degradation) remains unclear, although it may influence the efficiency of carbon transport to further depths. This study successfully depicted the new annual mean mesopelagic particulate nitrogen (PN) dynamics with multi-layer, steady-state suspended PN pools by reanalyzing seasonal data on the stable nitrogen isotopic compositions of both suspended and sinking particles, each with different profiles, from subarctic station K2 and subtropical station S1 in the North Pacific, which are both CO2 sinks but in different oceanic settings. As analytical conditions, we assumed that the net loss of sinking PN was entirely due to abiotic fragmentation of particle aggregates to non-sinking particles and that the apparent 15N enrichment associated with heterotrophic degradation in the suspended PN pools was vertically constant. The 15N mass balance for the PN supply to the uppermost mesopelagic pool, derived from such constraints, allowed estimating the PN export by the mixed-layer pump, which was 1.6 times greater at K2 than at S1. However, its contribution to the total export (including gravitational PN sinking) from the surface layer was approximately 20% at both stations. Moreover, the ratio of PN supplied to the uppermost pool by the mixed-layer pump and by the fragmentation of particle aggregates was also similar at both stations, approximately 1:1. Using these ratios, together with separate observations of the mixed-layer pump-driven flux, it may be possible to estimate the efficiency of the particulate organic carbon transport due to the biological gravitational pump responsible for carbon sequestration in the deep sea.

Key Points: Synoptic‐scale winter blooms and mixed‐layer pump events were observed by a BGC‐Argo float in the midlatitude Northwest Pacific OceanThe repetitive mixed‐layer pump episodes generated a very high particulate organic carbon export of ~110 mg C m−2 day−1 in wintertimeThe current BGC‐Argo protocols captured only 20%, or entirely missed, substantial pulsed carbon export driven by mixed‐layer pump events The winter‐ and spring‐time mixed‐layer pump (MLP) significantly augments the global carbon transport from the surface mixed layer to deeper waters when ephemeral surface phytoplankton blooms are repeatedly mixed to depth. Exploiting unusual 190 + daily BGC‐Argo profiling measurements within a recirculation gyre, we show repetitive MLP episodes generating a January–March averaged particulate organic carbon (POC) export of ~110 mg C m−2 day−1 in the midlatitude (31°N) Northwest Pacific. Subsampling this dataset on a 5‐ or 10‐day cycle yielded an order of magnitude less export, or even totally missed all the MLP events. The evidence here supports the need for new strategies if the BGC‐Argo program is to adequately quantify ocean carbon cycling and its effects on biological systems. We propose that a handful of floats be tasked with daily profiling, and machine learning strategies be used to link these data with satellite derived measurements to estimate the synoptic‐scale MLP export. Plain Language Summary: The Biogeochemical‐Argo program (BGC‐Argo) is an essential tool for quantifying carbon export from surface waters to the ocean interior, which is critical for understanding and forecasting changes in atmospheric CO2. Using novel, high‐frequency (1‐day cycle during 9‐ month period) BGC‐Argo data, we estimated the carbon export in the midlatitude Northwest Pacific Ocean stemming from unexpectedly continuous mixed‐layer pump (MLP) events. Such events occur when wintertime phytoplankton blooms developed during calm wind periods then are injected to depth by storms. This process generated remarkably high carbon export over winter, with values ranging from 18% to 30% of that from the massive North Atlantic spring bloom. These events, however, are largely (80%) or entirely missed using the longer standard float cycling intervals (5–10 days) accepted by the BGC‐Argo program. The findings here suggest that some high‐frequency BGC‐Argo observations are necessary for more accurately estimating the global carbon export. [ABSTRACT FROM AUTHOR]

Estimates of the ocean biological carbon pump are limited by uncertainties in the magnitude of the physical injection of particulate and dissolved organic carbon to the ocean interior. A major challenge is to evaluate the contribution of these physical pumps at small spatial and temporal scales (<100 km and <1 month). Here, we use a submesoscale permitting biophysical model covering a large domain representative of a subpolar and a subtropical gyre to quantify the impact of small‐scale physical carbon pumps.The model successfully simulates intense eddy‐driven subduction hot spots with a magnitude comparable to what has been observed in nature (1,000–6,000 mg C·m−2·day−1). These eddy‐driven subduction events are able to transfer carbon below the mixed‐layer, down to 500‐ to 1,000‐m depth. However, they contribute <5% to the annual flux at the scale of the basin, due to strong compensation between upward and downward fluxes. The model also simulates hot spots of export associated with small‐scale heterogeneity of the mixed layer, which intermittently export large amounts of suspended particulate and dissolved organic carbon. The mixed‐layer pump contributes ∼20% to the annual flux. High‐resolution measurements of export flux are needed to test models such as this one and to improve our mechanistic understanding of the biological pump and how it will respond to climate change. Key Points: A high‐resolution model simulates realistic hot spots of organic carbon export by physical subductionEddy‐driven subduction contributes little to annual export due to compensation between upward and downward fluxesEddy‐driven spatiotemporal variations in the mixed‐layer efficiently export dissolved and particulate organic carbon [ABSTRACT FROM AUTHOR]

The biological carbon pump exports carbon fixed by photosynthesis out of the surface ocean and transfers it to the deep, mostly in the form of sinking particles. Despite the importance of the pump in regulating the air‐sea CO2 balance, the magnitude of global carbon export remains unclear, as do its controlling mechanisms. A possible sinking flux of carbon to the mesopelagic zone may be via the mixed‐layer pump: a seasonal net detrainment of particulate organic carbon (POC)‐rich surface waters, caused by sequential deepening and shoaling of the mixed layer. In this study, we present a full year of daily small‐particle POC concentrations derived from glider optical backscatter data, to study export variability at the Porcupine Abyssal Plain (PAP) sustained observatory in the Northeast Atlantic. We observe a strong seasonality in small‐particle transfer efficiency, with a maximum in winter and early spring. By calculating daily POC export driven by mixed‐layer variations, we find that the mixed‐layer pump supplies an annual flux of at least 3.0 ± 0.9 g POC·m−2·year−1 to the mesopelagic zone, contributing between 5% and 25% of the total annual export flux and likely contributing to closing a gap in the mesopelagic carbon budget found by other studies. These are, to our best knowledge, the first high‐frequency observations of export variability over the course of a full year. Our results support the deployment of bio‐optical sensors on gliders to improve our understanding of the ocean carbon cycle on temporal scales from daily to annual. Key Points: Gliders captured a full annual cycle of high‐frequency POC variability over the upper mesopelagic zoneSmall‐particle transfer efficiency displayed a strong seasonality, peaking in winter and early springThe data indicate a significant annual carbon supply to the mesopelagic zone by small particles through the mixed‐layer pump [ABSTRACT FROM AUTHOR]