Sinking organic particles transport organic matter to the deep ocean, where aerobic (i.e., based on oxygen) microbial respiration returns carbon and nutrients to the dissolved components. When oxygen is absent, microbial respiration proceeds anaerobically, i.e., in using chemicals such as nitrate (denitrification) or sulfate (sulfate reduction). In the ocean, anaerobic respiration should be rare, and confined to low oxygen waters found in OMZ. However, observations have pointed to a much broader distribution. In this project, we tested multiple hypotheses related to the distribution and consequences of anaerobic respiration in the ocean. In a first line of investigation (Bianchi, Weber et al., 2018, Nature Geosciences), we tested the hypothesis that anaerobic respiration occurs in oxygen-deprived microenvironments that develop inside sinking organic particles. We developed a new size-resolved model of sinking particles to predict the occurrence of anaerobic respiration from particle size and seawater chemistry (Figure 1). We combined this model with observations of particle size distribution from Underwater Vision Profilers (UVP), to show that denitrification and sulfate reduction can be sustained throughout vast, low-oxygen expanses of the ocean (Figure 2), changing our understanding of the marine nitrogen cycle in a changing climate. In a second line of investigation, we explored the effect of microenvironment formation on the depth at which sinking particles are decomposed, which in turn controls how efficiently carbon is sequestered in the deep ocean (Weber and Bianchi, 2020, Frontiers in Earth Science). We showed that, compared to the surrounding tropical waters, OMZ consistently exhibit higher particle fluxes where low-oxygen waters reside, sequestering carbon more than twice as efficiently. Using our particle model, we showed that three different mechanisms might explain the shape of particle fluxes in OMZ (Figure 3), each leaving a unique signature in the size distribution of particles, and that UVP observations hold great promise for understanding the drivers of efficient carbon sequestration in OMZs, allowing more accurate prediction of future changes in carbon sequestration as the ocean loses oxygen in a warming climate. We further continued analysis of UVP observations (Clements et al., in preparation), with the goal of understanding what controls the abundance, size, and downward flux of organic particles in the ocean. To this end, we used machine learning techniques to reconstruct global particle size distributions from sparse UVP observations. We combined these reconstructions to in-situ observations to extrapolate carbon fluxes globally, providing a novel estimate of carbon sequestration by sinking particles (Figure 4), and demonstrating the utility of UVP observations for three-dimensional reconstructions of the oceanic biological "carbon pump". In a last line of research, we investigated the role of physical circulation in sustaining "pockets" of low-oxygen that could support anaerobic respiration in otherwise well-oxygenated waters. We focused on "subsurface eddies", i.e. swirl-like currents, usually smaller than 100km in diameter, that trap oceanic water for months to years, transporting it from coastal OMZ into the open ocean (Figure 5). In a first paper (Frenger, Bianchi et al., 2018, Global Biogeochemical Cycles), we analyzed a high-resolution global ocean model to show that low-oxygen subsurface eddies occur broadly, for example offshore California, Peru, and West Africa, expanding the regions where anaerobic respiration can be found. In a second paper (McCoy et al., 2020, Progress in Oceanography) we analyzed the global array of observations with Argo floats, which measure temperature and salinity in the upper 2,000 m of the ocean, to provide the first global detection and characterization of these subsurface eddies (Figure 6), revealing many previously unknown eddy populations, and laying the basis to estimate their importance for ocean biogeochemistry. Last Modified: 12/28/2020 Submitted by: Daniele Bianchi
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