Preformed phosphate, soft tissue pump and atmospheric CO2

We develop a new theory relating atmospheric pCO2 and the efficiency of the soft tissue pump of CO2 in the ocean, measured by P*, a quasi-conservative tracer. P* is inversely correlated with preformed phosphate, and its global average represents the fraction of nutrients transported by the export and remineralization of organic material. This view is combined with global conservation constraints for carbon and nutrients leading to a theoretical prediction for the sensitivity of atmospheric pCO2 to changes in globally averaged P*. The theory is supported by sensitivity studies with a more complex, three-dimensional numerical simulations. The numerical experiments suggest that the ocean carbon cycle is unlikely to approach the theoretical limit where globally averaged P* = 1 (complete depletion of preformed phosphate) because the localized dynamics of deep water formation, which may be associated with rapid vertical mixing timescales, preclude the ventilation of strongly nutrient-depleted waters. Hence, in the large volume of the deep waters of the ocean, it is difficult to significantly reduce preformed nutrient (or increase P*) by increasing the efficiency of export production. This mechanism could ultimately control the efficiency of biological pumps in a climate with increased aeolian iron sources to the Southern Ocean. Using these concepts we can reconcile qualitative differences in the response of atmospheric pCO2 to surface nutrient draw down in highly idealized box models and more complex, general circulation models. We suggest that studies of carbon cycle dynamics in regions of deep water formation are the key to understanding the sensitivity of atmospheric pCO2 to biological pumps in the ocean

Upper ocean control on the solubility pump of CO2

We develop and test a theory for the relationship of atmospheric p CO2 and the solubility pump of CO2 in an abiotic ocean. The solubility pump depends on the hydrographic structure of the ocean and the degree of saturation of the waters. The depth of thermocline sets the relative volume of warm and cold waters, which sets the mean solubility of CO2 in the ocean. The degree of saturation depends on the surface residence time of the waters. We develop a theory describing how atmospheric CO2 varies with diapycnal diffusivity and wind stress in a simple, coupled atmosphere-ocean carbon cycle, which builds on established thermocline theory. We consider two limit cases for thermocline circulation: the diffusive thermocline and the ventilated thermocline. In the limit of a purely diffusive thermocline (no wind-driven gyres), atmospheric pCO2 increases in proportion to the depth of thermocline which scales as κ1/3, where κ is the diapycnal mixing rate coefficient. In the wind-driven, ventilated thermocline limit, the ventilated thermocline theory suggests the thickness of the thermocline varies as wek1/2. Moreover, surface residence times are shorter, and subducted waters are undersaturated. The degree of undersaturation is proportional to the Ekman pumping rate, wek, for moderate amplitudes of wek. Hence, atmospheric pCO2 varies as wek3/2 for moderate ranges of surface wind stress. Numerical experiments with an ocean circulation and abiotic carbon cycle model confirm these limit case scalings and illustrate their combined effect. The numerical experiments suggest that plausible variations in the wind forcing and diapycnal diffusivity could lead to changes in atmospheric pCO2 of as much as 30 ppmv. The deep ocean carbon reservoir is insensitive to changes in the wind, due to compensation between the degree of saturation and the equilibrium carbon concentration. Consequently, the sensitivity of atmospheric pCO2 to wind-stress forcing is dominated by the changes in the upper ocean, in direct contrast to the sensitivity to surface properties, such as temperature and alkalinity, which is controlled by the deep ocean reservoir

On the temperature dependence of oceanic export efficiency

Quantifying the fraction of primary production exported from the euphotic layer (termed the export efficiency ef) is a complicated matter. Studies have suggested empirical relationships with temperature which offer attractive potential for parameterization. Here we develop what is arguably the simplest mechanistic model relating the two, using established thermodynamic dependencies for primary production and respiration. It results in a single‐parameter curve that constrains the envelope of possible efficiencies, capturing the upper bounds of several ef‐T data sets. The approach provides a useful theoretical constraint on this relationship and extracts the variability in ef due to temperature but does not idealize out the remaining variability which evinces the substantial complexity of the system in question

Modeling the coupling of ocean ecology and biogeochemistry

We examine the interplay between ecology and biogeochemical cycles in the context of a global three-dimensional ocean model where self-assembling phytoplankton communities emerge from a wide set of potentially viable cell types. We consider the complex model solutions in the light of resource competition theory. The emergent community structures and ecological regimes vary across different physical environments in the model ocean: Strongly seasonal, high-nutrient regions are dominated by fast growing bloom specialists, while stable, low-seasonality regions are dominated by organisms that can grow at low nutrient concentrations and are suited to oligotrophic conditions. In the latter regions, the framework of resource competition theory provides a useful qualitative and quantitative diagnostic tool with which to interpret the outcome of competition between model organisms, their regulation of the resource environment, and the sensitivity of the system to changes in key physiological characteristics of the cells.Gordon and Betty Moore FoundationNational Science Foundation (U.S.

Biogeographical controls on the marine nitrogen fixers

We interpret the environmental controls on the global ocean diazotroph biogeography in the context of a three-dimensional global model with a self-organizing phytoplankton community. As is observed, the model's total diazotroph population is distributed over most of the oligotrophic warm subtropical and tropical waters, with the exception of the southeastern Pacific Ocean. This biogeography broadly follows temperature and light constraints which are often used in both field-based and model studies to explain the distribution of diazotrophs. However, the model suggests that diazotroph habitat is not directly controlled by temperature and light, but is restricted to the ocean regions with low fixed nitrogen and sufficient dissolved iron and phosphate concentrations. We interpret this regulation by iron and phosphate using resource competition theory which provides an excellent qualitative and quantitative framework.Gordon and Betty Moore Foundation (Marine Microbiology Initiative)United States. National Oceanic and Atmospheric AdministrationUnited States. National Aeronautics and Space AdministrationNational Science Foundation (U.S.

Winners and losers: Ecological and biogeochemical changes in a warming ocean

We employ a marine ecosystem model, with diverse and flexible phytoplankton communities, coupled to an Earth system model of intermediate complexity to explore mechanisms that will alter the biogeography and productivity of phytoplankton populations in a warming world. Simple theoretical frameworks and sensitivity experiments reveal that ecological and biogeochemical changes are driven by a balance between two impacts of a warming climate: higher metabolic rates (the “direct” effect), and changes in the supply of limiting nutrients and altered light environments (the “indirect” effect). On globally integrated productivity, the two effects compensate to a large degree. Regionally, the competition between effects is more complicated; patterns of productivity changes are different between high and low latitudes and are also regulated by how the supply of the limiting nutrient changes. These complex regional patterns are also found in the changes to broad phytoplankton functional groups. On the finer ecological scale of diversity within functional groups, we find that ranges of some phytoplankton types are reduced, while those of others (potentially minor players in the present ocean) expand. Combined change in areal extent of range and in regionally available nutrients leads to global “winners and losers.” The model suggests that the strongest and most robust signal of the warming ocean is likely to be the large turnover in local phytoplankton community composition.United States. Dept. of Energy. Office of Science (Grant DE-FG02-94ER61937)United States. National Oceanic and Atmospheric AdministrationGordon and Betty Moore Foundatio

How have recent temperature changes affected the efficiency of ocean biological carbon export?

The ocean's large, microbially mediated reservoirs of carbon are intimately connected with atmospheric CO2 and climate, yet quantifying the feedbacks between them remains an unresolved challenge. Through an idealized mechanistic model, we consider the impact of documented climate change during the past few decades on the efficiency of biological carbon export out of the surface ocean. This model is grounded in universal metabolic phenomena, describing export efficiency's temperature dependence in terms of the differential temperature sensitivity of phototrophic and heterotrophic metabolism. Temperature changes are suggested to have caused a statistically significant decrease in export efficiency of 1.5% ± 0.4% over the past 33 yr. Larger changes are suggested in the midlatitudes and Arctic. This interpretation is robust across multiple sea surface temperature and net primary production data products. The same metabolic mechanism may have resulted in much larger changes e.g., in response to the large temperature shifts between glacial and interglacial time periods

Modeling the global ocean iron cycle

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 18 (2004): GB1002, doi:10.1029/2003GB002061.We describe a model of the ocean transport and biogeochemical cycling of iron and the subsequent control on export production and macronutrient distributions. Ocean transport of phosphorus and iron are represented by a highly idealized six-box ocean model. Export production is parameterized simply; it is limited by light, phosphate, and iron availability in the surface ocean. We prescribe the regional variations in aeolian deposition of iron and examine three parameterizations of iron cycling in the deep ocean: (1) net scavenging onto particles, the simplest model; (2) scavenging and desorption of iron to and from particles, analogous to thorium; and (3) complexation. Provided that some unknown parameter values can be set appropriately, all three biogeochemical models are capable of reproducing the broad features of the iron distribution observed in the modern ocean and explicitly lead to regions of elevated surface phosphate, particularly in the Southern Ocean. We compare the sensitivity of Southern Ocean surface macronutrient concentration to increased aeolian dust supply for each parameterization. Both scavenging-based representations respond to increasing dust supply with a drawdown of surface phosphate in an almost linear relationship. The complexation parameterization, however, asymptotes toward a limited drawdown of phosphate under the assumption that ligand production does not respond to increased dust flux. In the scavenging based models, deep water iron concentrations and, therefore, upwelled iron continually increase with greater dust supply. In contrast, the availability of complexing ligand provides an upper limit for the deep water iron concentration in the latter model.M. J. F. is grateful for funding from NOAA (NA16GP2988) and NSSF (OCE-336839). P. P. is grateful to the MIT Martin Fellowship and NASA Earth System Science Fellowship (NGT5- 30362) for funding

Interannual variability of the air-sea flux of oxygen in the North Atlantic

In studies using timeseries observations of atmospheric O[subscript 2]/N[subscript 2] to infer the fate of fossil fuel CO[subscript 2], it has been assumed that multi-year trends in observed O[subscript 2]/N[subscript 2] are insensitive to interannual variability in air-sea fluxes of oxygen. We begin to address the validity of this assumption by investigating the magnitude and mechanisms of interannual variability in the flux of oxygen across the sea surface using a North Atlantic biogeochemical model. The model, based on the MIT ocean general circulation model, captures the gross patterns and seasonal cycle of nutrients and oxygen within the basin. The air-sea oxygen flux exhibits significant interannual variability in the North Atlantic, with a standard deviation (0.36 mol m[superscript −2] y[superscript −1]) that is a large fraction of the mean (0.85 mol m[superscript −2] y[subscript −1]). This is primarily a consequence of variability in winter convection in the subpolar gyre.Goddard Space Flight Center (Grants NGTS-30189 and NCC5-244

The solubility pump of carbon in the subtropical gyre of the North Atlantic

The subduction of carbon is examined using abiotic models of the solubility pump in the subtropical gyre of the North Atlantic. The importance of the seasonal cycle of the mixed layer, and advection of carbon, is examined using sensitivity experiments with a Lagrangian model of the carbon system. The rate of subduction of carbon is found to be strongly influenced by the gradients in mixed-layer thickness over the gyre and, to a lesser extent, modified by the end of winter bias in the properties of subducted fluid. A seasonally-cycling geochemical model of the carbon system is then developed for the North Atlantic. The model is diagnosed to examine the seasonal exchange in carbon between the atmosphere and ocean induced by the seasonal warming and cooling. There is a net annual air-sea flux of carbon into the subtropical gyre of the model due to undersaturation of pco2 with respect to the local equilibrium with the atmosphere. The undersaturation is due to advection of carbon by the circulation. Along the path of the Gulf Stream, northward advection and cooling of the low latitude waters is so rapid that the surface waters are significantly undersaturated in carbon. Due to its long equilibration period, there is a resultant air-sea flux of carbon dioxide over the northern flank and interior of the subtropical gyre. Warm, low carbon water from the tropics is fluxed into the southern flank of the subtropical gyre in the Ekman layer, inducing an oceanic uptake of carbon there. The model experiments suggest that it is necessary to account for advection to close the carbon budget in the observed time-series measurements at Bermuda