Description: The Apparent Oxygen Utilisation (AOU) is a classical measure of the amount of oxygen respired in the ocean's interior. We show that AOU systematically overestimates True Oxygen Utilisation (TOU) in 6 coupled circulation-biogeochemical ocean models. This is due to atmosphere–ocean oxygen disequilibria in the subduction regions, consistent with previous work. We develop a simple, new, observationally-based approach which we call Evaluated Oxygen Utilisation (EOU). In this approach, we take into account the impact of the upper ocean oxygen disequilibria into the interior, considering that transport takes place predominantly along isopycnal surfaces. The EOU approximates the TOU with less than half of the bias of AOU in all 6 models despite large differences in the physical and biological components of the models. Applying the EOU approach to a global observational dataset yields an oxygen consumption rate 25% lower than that derived from AOU-based estimates, for a given ventilation rate.

Description: The natural abundance of 14C in total CO2 dissolved in seawater is a property applied to evaluate the water age structure and circulation in the ocean and in ocean models. In this study we use three different representations of the global ocean circulation augmented with a suite of idealised tracers to study the potential and limitations of using natural 14C to determine water age, the time elapsed since a body of water had contact with the atmosphere. We find that, globally, bulk 14C-age is dominated by two equally important components, one associated with aging, i.e. the time component of circulation and one associated with a "preformed 14C-age". This latter quantity exists because of the slow and incomplete atmosphere/ocean equilibration of 14C in particular in high latitudes where many water masses form. The relative contribution of the preformed component to bulk 14C-age varies regionally within a given model, but also between models. Regional variability, e.g. in the Atlantic Ocean is associated with the mixing of waters with very different end members of preformed 14C-age. In the Atlantic, variations in the preformed component over space and time mask the circulation component to an extent that its patterns are not detectable from bulk 14C-age alone. Between models the variability of age can also be considerable (factor of 2), related to the combinations of physical model parameters, which influence circulation dynamics, and gas exchange in the models. The preformed component was found to be very sensitive to gas exchange and moderately sensitive to ice cover. In our model evaluation exercise, the choice of the gas exchange constant from within the current range of uncertainty had such a strong influence on preformed and bulk 14C-age that if model evaluation would be based on bulk 14C-age it could easily impair the evaluation and tuning of a models circulation on global and regional scales. Based on the results of this study, we propose that considering preformed 14C-age is critical for a correct assessment of circulation in ocean models.

Description: The marine nitrogen (N) inventory is controlled by the interplay of nitrogen loss processes, here referred to as denitrification, and nitrogen source processes, primarily nitrogen fixation. The apparent stability of the marine N inventory on time scales longer than the estimated N residence time, suggests some intimate balance between N sinks and sources. Such a balance may be perceived easier to achieve when N sinks and sources occur in close spatial proximity, and some studies have interpreted observational evidence for such a proximity as indication for a stabilizing feedback processes. Using a biogeochemical ocean circulation model, we here show instead that a close spatial association of N2 fixation and denitrification can, in fact, trigger destabilizing feedbacks on the N inventory and, because of stoichiometric constrains, lead to net N losses. Contrary to current notion, a balanced N inventory requires a regional separation of N sources and sinks. This can be brought about by factors that reduce the growth of diazotrophs, such as iron, or by factors that affect the fate of the fixed nitrogen remineralization, such as dissolved organic matter dynamics. In light of our findings we suggest that spatial arrangements of N sinks and sources have to be accounted for in addition to individual rate estimates for reconstructing past, evaluating present and predicting future marine N inventory imbalances.

Description: The marine CaCO3 cycle is an important component of the oceanic carbon system and directly affects the cycling of natural and the uptake of anthropogenic carbon. In numerical models of the marine carbon cycle, the CaCO3 cycle component is often evaluated against the observed distribution of alkalinity. Alkalinity varies in response to the formation and remineralization of CaCO3 and organic matter. However, it also has a large conservative component, which may strongly be affected by a deficient representation of ocean physics (circulation, evaporation, and precipitation) in models. Here we apply a global ocean biogeochemical model run into preindustrial steady state featuring a number of idealized tracers, explicitly capturing the model's CaCO3 dissolution, organic matter remineralization, and various preformed properties (alkalinity, oxygen, phosphate). We compare the suitability of a variety of measures related to the CaCO3 cycle, including alkalinity (TA), potential alkalinity and TA*, the latter being a measure of the time-integrated imprint of CaCO3 dissolution in the ocean. TA* can be diagnosed from any data set of TA, temperature, salinity, oxygen and phosphate. We demonstrate the sensitivity of total and potential alkalinity to the differences in model and ocean physics, which disqualifies them as accurate measures of biogeochemical processes. We show that an explicit treatment of preformed alkalinity (TA0) is necessary and possible. In our model simulations we implement explicit model tracers of TA0 and TA*. We find that the difference between modelled true TA* and diagnosed TA* was below 10% (25%) in 73% (81%) of the ocean's volume. In the Pacific (and Indian) Oceans the RMSE of A* is below 3 (4) mmol TA m−3, even when using a global rather than regional algorithms to estimate preformed alkalinity. Errors in the Atlantic Ocean are significantly larger and potential improvements of TA0 estimation are discussed. Applying the TA* approach to the output of three state-of-the-art ocean carbon cycle models, we demonstrate the advantage of explicitly taking preformed alkalinity into account for separating the effects of biogeochemical processes and circulation on the distribution of alkalinity. In particular, we suggest to use the TA* approach for CaCO3 cycle model evaluation.

Description: Recent suggestions to slow down the increase in atmospheric carbon dioxide have included ocean fertilization by addition of the micronutrient iron to Southern Ocean surface waters, where a number of natural and artificial iron fertilization experiments have shown that low ambient iron concentrations limit phytoplankton growth. Using a coupled carbon-climate model with the marine biology's response to iron addition calibrated against data from natural iron fertilization experiments, we examine biogeochemical side effects of a hypothetical large-scale Southern Ocean Iron Fertilization (OIF) that need to be considered when attempting to account for possible OIF-induced carbon offsets. In agreement with earlier studies our model simulates an OIF-induced increase in local air-sea CO2 fluxes by about 60 GtC over a 100-year period, which amounts to about 40% of the OIF-induced increase in organic carbon export. Offsetting CO2 return fluxes outside the region and after stopping the fertilization at 1, 7, 10, 50, and 100 years are quantified for a typical accounting period of 100 years. For continuous Southern Ocean iron fertilization, the return flux outside the fertilized area cancels about 8% of the fertilization-induced CO2 air-sea flux within the fertilized area on a 100-yr timescale. This "leakage" effect has a similar radiative impact as the simulated enhancement of marine N2O emissions. Other side effects not yet discussed in terms of accounting schemes include a decrease in Southern Ocean oxygen levels and a simultaneous shrinking of tropical suboxic areas, and accelerated ocean acidification in the entire water column in the Southern Ocean on the expense of reduced globally averaged surface water acidification. A prudent approach to account for the OIF-induced carbon sequestration would account for global air-sea CO2 fluxes rather than for local fluxes into the fertilized area only. However, according to our model, this would underestimate the potential for offsetting CO2 emissions by about 20% on a 100 year accounting timescale. We suggest that a fair accounting scheme applicable to both terrestrial and marine carbon sequestration has to be based on emission offsets rather than on changes in individual carbon pools.

Description: Phosphate distributions simulated by seven state-of-the-art biogeochemical ocean circulation models are evaluated against observations of global ocean nutrient distributions. The biogeochemical models exhibit different structural complexities, ranging from simple nutrient-restoring to multi-nutrient NPZD type models. We evaluate the simulations using the observed volume distribution of phosphate. The errors in these simulated volume class distributions are significantly larger when preformed phosphate (or regenerated phosphate) rather than total phosphate is considered. Our analysis reveals that models can achieve similarly good fits to observed total phosphate distributions for a very different partitioning into preformed and regenerated nutrient components. This has implications for the strength and potential climate sensitivity of the simulated biological carbon pump. We suggest complementing the use of total nutrient distributions for assessing model skill by an evaluation of the respective preformed and regenerated nutrient components.

Description: The Beta Triangle, a region of the oligotrophic subtropical eastern North Atlantic Ocean, is notorious for its enigmatic oxygen, carbon, and nitrogen balances, in which nutrient supply is said to explain only a fraction of production necessary for estimated carbon export. Rates of dissolved organic carbon accumulation and dissolved organic nitrogen utilization in surface water and an assessment of oxygen utilized, organic matter consumed, and nitrate and phosphate regenerated in subsurface water, show that conventional production estimates miss substantial shares of biotic production. The shallow export of total organic carbon, predominantly dissolved (DOC), by subduction is responsible for about 50–70% of apparent oxygen utilization in subsurface water between the base of the surface layer at ca. 140 m and ca. 195 m depth, but it is insignificant below. Additionally, there is an estimated accumulation of 1.0 to 1.75 mol DOC m−2 a−1 in surface water. Including DOC dynamics in its carbon balance reveals the surface of this ultra-oligotrophic part of the ocean to be net autotrophic. Increasing subsurface values of excess nitrogen (DINxs) imply the export of nitrogen from surface water stemming from production not exclusively fuelled by new nitrate supplied from below. Total organic nitrogen (almost exclusively dissolved, DON) is consumed in the surface layer at a rate estimated at 0.13 to 0.23 mol m−2 a−1. There is no variation in dissolved organic phosphorus (DOP) in the same direction. DON utilization thus contributes to the pronounced subsurface DINxs signature. DOC export and accumulation are important in the carbon balance in surface and near-surface water. DON utilization and, probably, N2 fixation contribute significant amounts to the nitrogen supply of surface water. These processes can close part of the enigmatic carbon and nitrogen balances in the Beta Triangle. There are, however, no comparable processes which can explain the equally enigmatic situation concerning phosphorus supply in this area.

Description: The conversion of fixed nitrogen to N2 in suboxic waters is estimated to contribute roughly a third to total oceanic losses of fixed nitrogen and is hence understood to be of major importance to global oceanic production and, therefore, to the role of the ocean as a sink of atmospheric CO2. At present heterotrophic denitrification and autotrophic anammox are considered the dominant sinks of fixed nitrogen. Recently, it has been suggested that the trophic nature of pelagic N2-production may have additional, "collateral" effects on the carbon cycle, where heterotrophic denitrification provides a shallow source of CO2 and autotrophic anammox a shallow sink. Here, we analyse the stoichiometries of nitrogen and associated carbon conversions in marine oxygen minimum zones (OMZ) focusing on heterotrophic denitrification, autotrophic anammox, and dissimilatory nitrate reduction to nitrite and ammonium in order to test this hypothesis quantitatively. For open ocean OMZs the combined effects of these processes turn out to be clearly heterotrophic, even with high shares of the autotrophic anammox reaction in total N2-production and including various combinations of dissimilatory processes which provide the substrates to anammox. In such systems, the degree of heterotrophy (ΔCO2:ΔN2), varying between 1.7 and 6, is a function of the efficiency of nitrogen conversion. On the contrary, in systems like the Black Sea, where suboxic N-conversions are supported by diffusive fluxes of NH4+ originating from neighbouring waters with sulphate reduction, much lower values of ΔCO2:ΔN2 can be found. However, accounting for concomitant diffusive fluxes of CO2, ratios approach higher values similar to those computed for open ocean OMZs. Based on our analysis, we question the significance of collateral effects concerning the trophic nature of suboxic N-conversions on the marine carbon cycle.

Description: The internal consistency of measurements and computations of components of the CO2-system, namely total alkalinity (AT), total dissolved carbon dioxide (CT), CO2 fugacity (fCO2), and pH, has been confirmed repeatedly in open ocean studies when the CO2 system had been over determined. Differences between measured and computed properties, such as ΔfCO2 (=fCO2(measured) – fCO2(computed from AT and CT))/ fCO2(measured)× 100), there are usually below 5%. Recently, Hoppe et al. (2010) provided evidence of significantly larger ΔfCO2 in experimental setups. These observations are currently not well understood. Here we discuss a case from a series of phytoplankton culture experiments with ΔfCO2 of up to about 25%. ΔfCO2 varied systematically during the course of these experiments and showed a clear correlation with the accumulation of dissolved organic carbon (DOC). Culture and mesocosm experiments are often carried out under very high initial nutrient concentrations, yielding high biomass concentrations that in turn often lead to a substantial build-up of DOC. DOC can reach concentrations much higher than typically observed in the open ocean. To the extent that DOC includes organic acids and bases, it will contribute to the alkalinity of the seawater contained in the experimental device. Our analysis suggests that whenever substantial amounts of DOC are produced during the experiment, standard computer programs used to compute CO2 fugacity can underestimate true fCO2 significantly when the computation is based on AT and CT. Alternative explanations for large ΔfCO2, e.g. uncertainties of pKs, are explored as well, but are found to be of minor importance. Unless the effect of DOC-alkalinity is accounted for, this might lead to significant errors in the interpretation of the system under consideration to the experimentally applied CO2 perturbation, which could misguide the development of parameterisations used in simulations with global carbon cycle models in future CO2-scenarios.

Description: Global biogeochemical ocean models are often tuned to match the observed distributions and fluxes of inorganic and organic quantities. This tuning is typically carried out “by hand”. However, this rather subjective approach might not yield the best fit to observations, is closely linked to the circulation employed and is thus influenced by its specific features and even its faults. We here investigate the effect of model tuning, via objective optimisation, of one biogeochemical model of intermediate complexity when simulated in five different offline circulations. For each circulation, three of six model parameters have been adjusted to characteristic features of the respective circulation. The values of these three parameters – namely, the oxygen utilisation of remineralisation, the particle flux parameter and potential nitrogen fixation rate – correlate significantly with deep mixing and ideal age of North Atlantic Deep Water (NADW) and the outcrop area of Antarctic Intermediate Waters (AAIW) and Subantarctic Mode Water (SAMW) in the Southern Ocean. The clear relationship between these parameters and circulation characteristics, which can be easily diagnosed from global models, can provide guidance when tuning global biogeochemistry within any new circulation model. The results from 20 global cross-validation experiments show that parameter sets optimised for a specific circulation can be transferred between similar circulations without losing too much of the model's fit to observed quantities. When compared to model intercomparisons of subjectively tuned, global coupled biogeochemistry–circulation models, each with different circulation and/or biogeochemistry, our results show a much lower range of oxygen inventory, oxygen minimum zone (OMZ) volume and global biogeochemical fluxes. Export production depends to a large extent on the circulation applied, while deep particle flux is mostly determined by the particle flux parameter. Oxygen inventory, OMZ volume, primary production and fixed-nitrogen turnover depend more or less equally on both factors, with OMZ volume showing the highest sensitivity, and residual variability. These results show a beneficial effect of optimisation, even when a biogeochemical model is first optimised in a relatively coarse circulation and then transferred to a different finer-resolution circulation model.