Abstract
The remineralization depth of sinking organic particles controls the efficiency of the biological carbon pump by setting the sequestration timescale of remineralized carbon in the ocean interior. Oxygen minimum zones (OMZs) have been identified as regions of elevated particle transfer and efficient carbon sequestration at depth, but direct measurements remain sparse in these regions and only provide snapshots of the particle flux. Here, we use remineralization tracers to reconstruct time-mean particle flux profiles in the OMZs of the Eastern Tropical Pacific and the Arabian Sea. Compared to the surrounding tropical waters, both OMZs exhibit slow flux attenuation between 100-1000m where suboxic waters reside, and sequester carbon beneath 1000m more than twice as efficiently. Using a mechanistic model of particle sinking, remineralization, and disaggregation, we show that three different mechanisms might explain the shape of the OMZ flux profiles: (i) a significant slow-down of remineralization when carbon oxidation transitions from aerobic to anaerobic respiration (e.g. denitrification); (ii) the exclusion of zooplankton that mediate disaggregation of large particles from suboxic waters (iii) the limitation of remineralization by the diffusive supply of oxidants (oxygen and nitrate) into large particles. We show that each mechanism leaves a unique signature in the size distribution of particles, suggesting that observations with optical instruments such as Underwater Vision Profilers hold great promise for understanding the drivers of efficient carbon transfer though suboxic water columns. In turn, this will allow more accurate prediction of future changes in carbon sequestration as the ocean loses oxygen in a warming climate.
Highlights
The biological pump sequesters carbon out of contact with the atmosphere in deep ocean waters, owing to the formation of organic particles in the surface ocean followed by their sinking and remineralization at depth (Passow and Carlson, 2012)
The key contribution of this study is the new constraints we have placed on the efficiency of the biological pump in oxygen minimum zones (OMZs) regions, by reconstructing time-mean particle flux profiles for the Eastern Tropical North Pacific and the Arabian Sea from geochemical tracer data
These reconstructions revealed slow particle flux attenuation over depth in the suboxic water column, consistent with previous evidence from sediment trap “snapshots” (Devol and Hartnett, 2001; Keil et al, 2016) and confirming that this is a systematic feature of OMZ particle fluxes
Summary
The biological pump sequesters carbon out of contact with the atmosphere in deep ocean waters, owing to the formation of organic particles in the surface ocean followed by their sinking and remineralization at depth (Passow and Carlson, 2012). In Model 1, our optimization process selects εd of ∼0.19 and ∼0.17 in ETNP and AS, suggesting that carbon oxidation must slow down more than 80% following the transition from aerobic respiration to denitrification, in order to best explain the reconstructed fluxes This is not consistent with the difference in free energy yield between the two process, which is only ∼1% (Froelich et al, 1979), but might be explained by lag time associated with particle colonization by the denitrifying microbial community, which remains poorly quantified to date (Bristow, 2018), or other processes that limit the efficiency of anaerobic metabolisms. A compilation 6 UVP size spectrum profiles from the ETSP (Bianchi et al, 2018), found that the spectral slope increased slightly between 100 and 1000 m, largely driven by increased abundance of large particles relative to an oxic water column, rather than decreased abundance of small particles This is not consistent with Hypothesis 1 (slow remineralization during denitrification) and is most consistent with the predictions of Hypothesis 3 (diffusion limitation of carbon oxidation in large particles). These processes should be considered in future modeling of OMZ particle fluxes
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