We determine the global rates of marine N‐fixation and denitrification and their associated uncertainties by combining marine geochemical and physical data with a new two‐dimensional box model that ...separates the Atlantic from the IndoPacific basins. The uncertainties are estimated using a probabilistic approach on the basis of a suite of 2500 circulation configurations of this box model. N‐fixation and denitrification are diagnosed in an inverse manner for each of these configurations usingN*, P*, and the stable nitrogen isotope composition of nitrate as data constraints. Our approach yields a median water column denitrification rate of 52 TgN yr−1 (39 to 66 TgN yr−1, 5th to 95th percentile) and a median benthic denitrification rate of 93 TgN yr−1 (68 to 122 TgN yr−1). The resulting benthic‐to‐water column denitrification ratio of 1.8 confirms that the isotopic signature of water column denitrification has a limited influence on the global mean stable isotopic value of nitrate due to the dilution of the waters with a denitrification signal with the remainder of the ocean's nitrate pool. On the basis of two different approaches, we diagnose a global N‐fixation rate of between 94 TgN yr−1 and 175 TgN yr−1, with a best estimate of 131 TgN yr−1 and 134 TgN yr−1, respectively. Most of the N‐fixation occurs in the IndoPacific suggesting a relative close spatial coupling between sources and sinks in the ocean. Our N‐fixation and denitrification estimates plus updated estimates of atmospheric deposition and riverine input yield a pre‐industrial marine N cycle that is balanced to within 3 TgN yr−1 (−38 to 40 TgN yr−1). Our budget implies a median residence time for fixed N of 4,200 yr (3,500 to 5,000 yr).
Key Points
Probabilistic estimate of N‐fixation and denitrification
The pre‐industrial marine N cycle is balanced or close to balance
Magnitudes of denitrification are at the lower end compared to recent studies
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The reinvigoration of the Southern Ocean carbon sink Landschützer, Peter; Gruber, Nicolas; Haumann, F. Alexander ...
Science (American Association for the Advancement of Science),
09/2015, Volume:
349, Issue:
6253
Journal Article
Peer reviewed
Open access
Several studies have suggested that the carbon sink in the Southern Ocean—the ocean's strongest region for the uptake of anthropogenic CO2—has weakened in recent decades. We demonstrated, on the ...basis of multidecadal analyses of surface ocean CO2 observations, that this weakening trend stopped around 2002, and by 2012 the Southern Ocean had regained its expected strength based on the growth of atmospheric CO2. All three Southern Ocean sectors have contributed to this reinvigoration of the carbon sink, yet differences in the processes between sectors exist, related to a tendency toward a zonally more asymmetric atmospheric circulation. The large decadal variations in the Southern Ocean carbon sink suggest a rather dynamic ocean carbon cycle that varies more in time than previously recognized.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Ocean deoxygenation in a warming world Keeling, Ralph E; Körtzinger, Arne; Gruber, Nicolas
Annual review of marine science,
01/2010, Volume:
2
Journal Article
Peer reviewed
Ocean warming and increased stratification of the upper ocean caused by global climate change will likely lead to declines in dissolved O2 in the ocean interior (ocean deoxygenation) with ...implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitat. Ocean models predict declines of 1 to 7% in the global ocean O2 inventory over the next century, with declines continuing for a thousand years or more into the future. An important consequence may be an expansion in the area and volume of so-called oxygen minimum zones, where O2 levels are too low to support many macrofauna and profound changes in biogeochemical cycling occur. Significant deoxygenation has occurred over the past 50 years in the North Pacific and tropical oceans, suggesting larger changes are looming. The potential for larger O2 declines in the future suggests the need for an improved observing system for tracking ocean 02 changes.
Nearshore waters of the California Current System (California CS) already have a low carbonate saturation state, making them particularly susceptible to ocean acidification. We used eddy-resolving ...model simulations to study the potential development of ocean acidification in this system up to the year 2050 under the Special Report on Emissions Scenarios A2 and B1 scenarios. In both scenarios, the saturation state of aragonite Ω arag is projected to drop rapidly, with much of the nearshore region developing summer-long undersaturation in the top 60 meters within the next 30 years. By 2050, waters with Ω arag above 1.5 will have largely disappeared, and more than half of the waters will be undersaturated year-round. Habitats along the sea floor will become exposed to year-round undersaturation within the next 20 to 30 years. These projected events have potentially major implications for the rich and diverse ecosystem that characterizes the California CS.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Although mesoscale ocean eddies are ubiquitous in the Southern Ocean, their
average regional and seasonal association with phytoplankton has not been
quantified systematically yet. To this end, we ...identify over 100 000
mesoscale eddies with diameters of 50 km and more in the Southern Ocean and
determine the associated phytoplankton biomass anomalies using
satellite-based chlorophyll-a (Chl) as a proxy. The mean Chl anomalies,
δChl, associated with these eddies, comprising the upper echelon of
the oceanic mesoscale, exceed ±10 % over wide regions. The structure of
these anomalies is largely zonal, with cyclonic, thermocline lifted, eddies
having positive anomalies in the subtropical waters north of the Antarctic
Circumpolar Current (ACC) and negative anomalies along its main flow path.
The pattern is similar, but reversed for anticyclonic, thermocline deepened
eddies. The seasonality of δChl is weak in subtropical waters, but
pronounced along the ACC, featuring a seasonal sign switch. The spatial
structure and seasonality of the mesoscale δChl can be explained
largely by lateral advection, especially local eddy-stirring. A
prominent exception is the ACC region in winter, where δChl is
consistent with a modulation of phytoplankton light exposure caused by an
eddy-induced modification of the mixed layer depth. The clear impact of
mesoscale eddies on phytoplankton may implicate a downstream effect on
Southern Ocean biogeochemical properties, such as mode water nutrient
contents.
The Mg/Ca of planktic foraminifera Globigerinoides ruber (white) is a widely applied proxy for tropical and sub-tropical sea-surface temperature. The accuracy with which temperature can be ...reconstructed depends on how accurately relationships between Mg/Ca and temperature and the multiple secondary controls on Mg/Ca are known; however, these relationships remain poorly quantified under oceanic conditions. Here, we present new calibrations based on 440 sediment trap/plankton tow samples from the Atlantic, Pacific and Indian Oceans, including 130 new samples from the Bay of Bengal/Arabian Sea and the tropical Atlantic Ocean. Our results indicate temperature, salinity and the carbonate system all significantly influence Mg/Ca in G. ruber (white). We propose two calibration models: The first model assumes pH is the controlling carbonate system parameter. In this model, Mg/Ca has a temperature sensitivity of 6.0±0.8%/°C (2σ), a salinity sensitivity of 3.3±2.2%/PSU and a pH sensitivity of −8.3±7.7%/0.1 pH units; The second model assumes carbonate ion concentration (CO32−) is the controlling carbonate system parameter. In this model, Mg/Ca has a temperature sensitivity of 6.7±0.8%/°C, a salinity sensitivity of 5.0±3.0%/PSU and a CO32− sensitivity of −0.24±0.11%/μmol kg−1. In both models, the temperature sensitivity is significantly lower than the widely-applied sensitivity of 9.0±0.6%/°C. Application of our new calibrations to down-core data from the Last Glacial Maximum, considering whole ocean changes in salinity and carbonate chemistry, indicate a cooling of 2.4±1.6°C in the tropical oceans if pH is the controlling parameter and 1.5±1.4°C if CO32− is the controlling parameter.
•New Mg/Ca calibration based on 440 sediment trap/plankton tow samples.•The sensitivity of Mg/Ca to temperature is ∼6/°C.•The sensitivity of Mg/Ca to salinity is ∼3%.•The carbonate system significantly influences Mg/Ca.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
It has been speculated that the partial pressure of carbon dioxide (pCO
) in shelf waters may lag the rise in atmospheric CO
. Here, we show that this is the case across many shelf regions, implying ...a tendency for enhanced shelf uptake of atmospheric CO
. This result is based on analysis of long-term trends in the air-sea pCO
gradient (ΔpCO
) using a global surface ocean pCO
database spanning a period of up to 35 years. Using wintertime data only, we find that ΔpCO
increased in 653 of the 825 0.5° cells for which a trend could be calculated, with 325 of these cells showing a significant increase in excess of +0.5 μatm yr
(p < 0.05). Although noisier, the deseasonalized annual data suggest similar results. If this were a global trend, it would support the idea that shelves might have switched from a source to a sink of CO
during the last century.
Several studies in upwelling regions have suggested that mesoscale structures, such as eddies and filaments, contribute substantially to the long-range transport of the organic carbon from the ...nearshore region of production to the offshore region of remineralization. Yet a comprehensive analysis of this mesoscale flux and of its impact across the Canary Upwelling System (CanUS) has not been provided. Here, we fill this gap using simulations with the Regional Oceanic Modeling System (ROMS) coupled to a Nutrient, Phytoplankton, Zooplankton and Detritus (NPZD) ecosystem model. We run climatological simulations on an Atlantic telescopic grid with an eddy-resolving resolution in the CanUS. Using both a Reynolds flux decomposition and structure-identification algorithms, we quantify and characterize the organic carbon fluxes driven by filaments and eddies within the upper 100 m and put them in relationship to the total offshore transport. Our analysis reveals that both coastal filaments and eddies enhance the offshore flux of organic carbon, but that their contribution is very different. Upwelling filaments, with their high speeds and high concentrations, transport the organic carbon offshore in a very intense, but coastally confined manner, contributing nearly 80 % to the total flux of organic carbon at 100 km offshore. The filament contribution tapers off quickly to near zero values at 1000 km off the coast, leading to a strong offshore flux divergence that is the main lateral source of organic carbon in the coastal waters up to 1000 km offshore. Some of this divergence is also due to the filaments inducing a substantial vertical subduction of the organic carbon below 100 m. Owing to the temporal persistence and spatial recurrence of filaments, the filament transport largely constitutes a time-mean flux, while the time-varying component, i.e., the turbulent flux, is comparatively small. At distances beyond 500 km from the coast, eddies dominate the mesoscale offshore transport. Although their contribution represents only 20 % of the total offshore flux and its divergence, eddies, especially cyclones, transport organic carbon offshore to distances as great as 2000 km from the coast. The eddy transport largely represents a turbulent flux, but striations in this transport highlight the existence of typical formation spots and recurrent offshore propagation pathways. While they propagate slowly, eddies are an important organic carbon reservoir for the open waters, as they contain, on average, a third of the organic carbon in this region, two thirds of which is found in cyclones. Our analysis confirms the importance of mesoscale processes for the offshore organic carbon transport and the fueling of the heterotrophic activity in the eastern subtropical North Atlantic, and highlights the need to consider the mesoscale flux in order to fully resolve the three-dimensionality of the marine organic carbon cycle.
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