Respiration Patterns in the Dark Ocean

Abstract

In the dark ocean, respiring organisms are the main sink for dissolved oxygen. The respiration rate in a given seawater volume can be quantified through dissolved oxygen drawdown or organic matter consumption as a function of time. Estimates of dissolved oxygen utilization rates (OUR) abound in the literature, but are typically obtained using proxies of questionable accuracy, often with low vertical resolution, and neglecting key regions such as the Southern and Indian oceans. Respiration rates based on particulate (POC) or dissolved (DOC) organic carbon are also sparsely observed and for DOC unavailable in many regions. Consequently, the relative contributions of POC or DOC as a respiration substrate in the dark ocean are unknown. Here we use recent datasets of true oxygen utilization, seawater age, and DOC to derive OUR and DOC consumption‐rate profiles in 10 oceanic regions. We demonstrate that although DOC and POC consumption rates are globally consistent with OUR, they underestimate OUR in the deep, suggesting strong oxygen utilization at the seafloor. In the abyss, we find a negative correlation of DOC consumption rate with seawater age, suggesting that DOC reactivity decreases along the deep branch of the conveyor circulation. Our results highlight that benthic organisms are sensitive to perturbations in the surface production of organic matter and to large‐scale circulation changes that affect its supply to the abyss.This article is protected by copyright. All rights reserved.

Ocean Biogeochemical Fingerprints of Fast‐Sinking Tunicate and Fish Detritus

“…Indeed, even with the highest T eff_btm, modeled seafloor OUR was a factor of two lower than J22, though likely within uncertainty bounds. Reasons for this discrepancy (see also Andersson et al., 2004; Sulpis et al., 2023) could include biases in both the observations (uneven sampling and bias toward coasts and high productivity areas) and the models (coastal productivity in a coarse‐scale model is biased low despite broad skill in simulating NPP; Stock et al., 2014). The large‐scale benthic flux patterns between oligotrophic gyres and high latitudes as seen in J22 are better reproduced in the fast‐sinking detritus case, but fluxes in subtropical regions remain low.…”

“…In addition to direct estimates of detrital flux (sediment traps, Thorium‐234 isotopes; Buesseler et al., 2020), the oxygen and macronutrient (e.g., nitrate) concentrations deep in the water column and at the seafloor can also constrain estimates of the biological pump (Andersson et al., 2004; Sulpis et al., 2023). Organic matter remineralization consumes oxygen, but slows significantly in oxygen minimum zones (OMZs) as oxygen is depleted and anaerobic processes, such as denitrification, dominate (Devol & Hartnett, 2001; Van Mooy et al., 2002; Weber & Bianchi, 2020).…”

The Changing Biological Carbon Pump of the South Atlantic Ocean

“…Building on these observations, the depth-integrated increase in organic carbon export flux was between 0.004 ± 0.002 (C soft(AOU) ) and 0.006 ± 0.002 GtC yr -1 (C soft(NO3) ) for the study area. For comparison, a recent estimate of global export production accounting for both POC and DOC was 8.37 ± 1.57 GtC yr -1 (Sulpis et al, 2023), which amounts to 0.017 ± 0.003 GtC yr -1 when scaled down to the study area. This suggests that the increase in the BCP that we observed could represent an increase in the amount of carbon remineralised by 23% to 35% each year.…”

The Ocean's Meridional Oxygen Transport

“…Biological oxygen consumption falls off rapidly with depth, decreasing by a factor of 10–50 between 100 and 1,000 m (Feely et al., 2004). However, recent studies have revealed the existence of oxygen utilization near the seafloor (Davila et al., 2023; Holzer, 2022; Sulpis et al., 2023).…”