Profiling float‐based observations of net respiration beneath the mixed layer

Hennon, Tyler D.; Riser, Stephen C.; Mecking, Stefan

doi:10.1002/2016gb005380

Cited by 30 publications

(60 citation statements)

References 42 publications

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“…R stands for the mean respiration rate of organic matter in the mesopelagic zone and is equal to 10 mmol C m −3 yr −1 , the average respiration rate between 100 and 400 m estimated from BGCArgo floats in Southern Ocean open waters (Hennon et al, ). The definition of <w sub > in equation was based on four strong assumptions: POC surf was representative of the POC ev concentration in the water parcel before being subducted. The difference between POC surf and POC ev was only due to respiration of organic matter since the water parcel left the surface mixed layer. The distance traveled by the water parcel since leaving the mixed layer until being detected was equivalent to the difference between the depth of detection and the MLD estimated for each profile. The average respiration rate in the water parcel was constant in the water parcel. …”

Section: Resultsmentioning

confidence: 88%

Evaluating Southern Ocean Carbon Eddy‐Pump From Biogeochemical‐Argo Floats

et al. 2018

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The vertical transport of surface water and carbon into ocean's interior, known as subduction, is one of the main mechanisms through which the ocean influences Earth's climate. New instrumental approaches have shown the occurrence of localized and intermittent subduction episodes associated with small‐scale ocean circulation features. These studies also revealed the importance of such events for the export of organic matter, the so‐called eddy‐pump. However, the transient and localized nature of episodic subduction hindered its large‐scale evaluation to date. In this work, we present an approach to detect subduction events at the scale of the Southern Ocean using measurements collected by biogeochemical autonomous floats (BGCArgo). We show how subduction events can be automatically identified as anomalies of spiciness and Apparent Oxygen Utilization (AOU) below the mixed layer. Using this methodology over more than 4,000 profiles, we detected 40 subduction events unevenly distributed across the Sothern Ocean. Events were more likely found in hot spots of eddy kinetic energy (EKE), downstream major bathymetric features. Moreover, the bio‐optical measurements provided by BGCArgo allowed measuring the amount of Particulate Organic Carbon (POC) being subducted and assessing the contribution of these events to the total downward carbon flux at 100 m (EP100). We estimated that the eddy‐pump represents less than 19% to the EP100 in the Southern Ocean, although we observed particularly strong events able to locally duplicate the EP100. This approach provides a novel perspective on where episodic subduction occurs that will be naturally improved as BGCArgo observations continue to increase.

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Section: Resultsmentioning

confidence: 88%

Evaluating Southern Ocean Carbon Eddy‐Pump From Biogeochemical‐Argo Floats

et al. 2018

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“…ANCP reaches a maximum in the latitude band between 40° and 50° S. This pattern is consistent with a variety of independent measurements using several techniques. In particular, the measurements of oxygen respiration rates in the mesopelagic zone with profiling floats [ Martz et al ., ; Hennon et al ., ], which are measures of Export Production, match closely with our estimated ANCP derived from annual nitrate cycles in the euphotic zone. The float‐based measurement indicate that the cycle of NCP in the upper ocean and EP are closely balanced, as required to maintain low organic nitrogen stocks in the upper ocean.…”

Section: Discussionmentioning

confidence: 84%

“…[], Hennon et al . [], Lourey and Trull [], McNeil and Tilbrook [], Shadwick et al . [], Riser and Johnson [], and Bender and Jönsson [] are shown for comparison.…”

Section: Resultsmentioning

confidence: 99%

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Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production

et al. 2017

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Annual nitrate cycles have been measured throughout the pelagic waters of the Southern Ocean, including regions with seasonal ice cover and southern hemisphere subtropical zones. Vertically resolved nitrate measurements were made using in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) optical nitrate sensors deployed on profiling floats. Thirty‐one floats returned 40 complete annual cycles. The mean nitrate profile from the month with the highest winter nitrate minus the mean profile from the month with the lowest nitrate yields the annual nitrate drawdown. This quantity was integrated to 200 m depth and converted to carbon using the Redfield ratio to estimate annual net community production (ANCP) throughout the Southern Ocean south of 30°S. A well‐defined, zonal mean distribution is found with highest values (3–4 mol C m−2 yr−1) from 40 to 50°S. Lowest values are found in the subtropics and in the seasonal ice zone. The area weighted mean was 2.9 mol C m−2 yr−1 for all regions south of 40°S. Cumulative ANCP south of 50°S is 1.3 Pg C yr−1. This represents about 13% of global ANCP in about 14% of the global ocean area.

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“…This is followed by a shallowing of the mixed layer above the community compensation depth (depth at which gross organic matter production is equivalent to community respiration) in the spring–summer period, during which (mesopelagic) oxygen is consumed via respiration of sinking organic matter (Najjar & Keeling, ). Based on this seasonality in oxygen levels, respiration rates have been estimated from the temporal evolution of oxygen measured by autonomous profiling floats in the Southern Ocean (Martz et al, ), as well as other ocean regions (Hennon et al, ; Riser & Johnson, ). Similarly, the net production of organic matter can be assessed from the seasonal drawdown of the upper ocean nitrate reservoir (Johnson et al, ).…”

Section: Introductionmentioning

confidence: 99%

Nutrient Controls on Export Production in the Southern Ocean

Arteaga

Pahlow

Bushinsky

et al. 2019

Global Biogeochemical Cycles

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We use observations from novel biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program to estimate annual net community production (ANCP; associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (∼3 mol C m−2 y−1), compared to lower rates in the subtropical zone (≤ 1 mol C m−2 y−1) and the seasonal ice zone (<2 mol C m−2 y−1). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of particulate organic carbon and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.

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Profiling float‐based observations of net respiration beneath the mixed layer

Cited by 30 publications

References 42 publications

Evaluating Southern Ocean Carbon Eddy‐Pump From Biogeochemical‐Argo Floats

Evaluating Southern Ocean Carbon Eddy‐Pump From Biogeochemical‐Argo Floats

Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production

Nutrient Controls on Export Production in the Southern Ocean

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