Global Ocean Carbon Cycle Research Articles

Effects of depth‐ and CO2‐dependent C:N ratios of particulate organic matter (POM) on the marine carbon cycle

According to a recent study, C:N ratios of sinking particulate organic matter (POM) in the ocean appear to be higher than Redfield (7.1 instead of 6.6) and depth dependent (increase +0.2/km). Here we investigate the effects of vertically variable C:N element ratios on marine carbon fluxes and the air‐sea exchange of CO2 using a global ocean carbon cycle model (AAMOCC). For a steady‐state ocean, the results show that models using the constant classical Redfield ratio underestimate both, total inventory and vertical gradients of dissolved inorganic carbon (DIC). While the amount of additional DIC (+150 Gt C) is negligible compared to the high marine carbon inventory, the C:N depth dependence can reduce the ambient atmospheric pCO2 by 20 ppm, permanently. Moreover, the simulation of a future scenario, estimating a possible effect of CO2‐dependent C:N ratios of POM on the marine carbon cycle, has shown that even a moderate rise in the C:N element ratio of sinking POM, which is on the order of magnitude of natural variability, yields a considerably higher oceanic uptake of anthropogenic CO2 on timescales of decades to centuries. The assumption is based on a predicted increase in the production of highly carbon enriched transparent exopolymer particles (TEP) caused by rising atmospheric CO2 concentrations and enhanced nutrient limitation. However, counteracting a predicted decrease of the physical (solubility) CO2 pump as a consequence of global change, the effect in our scenario will alleviate further rising atmospheric CO2 concentrations rather than compensate a reduced physical uptake.

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) sea surface distributions simulated from a global three‐dimensional ocean carbon cycle model

A global model for surface dimethylsulfide (DMS) and particulate dimethylsulfoniopropionate (DMSP) (pDMS) distributions is presented. The main goals of this work were to be able to predict the regional distribution of the air‐sea fluxes of DMS and to predict eventually their future evolution with climate change. Diagnostic relationships have been established from data sets obtained during the ALBATROSS and EUMELI cruises carried out in the Atlantic Ocean. These equations nonlinearly relate DMS and pDMSP concentrations to chlorophyll concentrations and to the trophic status of surface waters. This model has been embedded in the global ocean carbon cycle model Institut Pierre et Simon Laplace‐Ocean Carbon Cycle Model version 2 (ISPL‐OCCM2), a simple plankton model coupled to a global three‐dimensional ocean general circulation model. Predicted global distributions and seasonal variations of surface chlorophyll are in good agreement with the observations, except in the equatorial Pacific Ocean and, to a lesser extent, in the Southern Ocean. In these regions, simulated surface chlorophyll concentrations are strongly overestimated, most likely because limitations of the biological production by nutrients like iron or silicate are not considered. The model predicts surface DMS and pDMSP concentrations, which compare reasonably well with the observations. However, in the high latitudes, seasonal variations are underestimated, especially in the Ross and Weddell Seas where observed very elevated concentrations of DMS due to spring and summer blooms of Phaeocystis cannot be reproduced by the model. The global annual flux of DMS predicted by lPSL‐OCCM2 ranges from 17 to 26.7 Tg S yr−1 depending on the formulation for gas exchange coefficient. About one third of this flux is located in the subtropical/subpolar frontal zone of the Southern Ocean, which plays a critical role in the sulfur cycle. Furthermore, model results suggest that the Southern Ocean, south of the Polar Front, could be a rather modest source of DMS for the atmosphere.

European continental shelf as a significant sink for atmospheric carbon dioxide

The concentration of carbon dioxide was measured during 18 cruises in the surface waters of the North Atlantic European Shelf (Galician sea, Gulf of Biscay, Armonican Sea, Celtic Sea, English Channel, North Sea), covering all four seasons (9 out of 12 months) at interannual scale. This is the very first intensive field study of continental shelves, in terms of source/sink for atmospheric CO2, which allows to integrate fluxes on an annual basis and over a large surface area. Here we show that European continental shelves are a sink of 90–170 million tons of carbon per year, which is an additional appreciable fraction to the presently proposed flux for the open North Atlantic Ocean (∼ 45%). The air‐sea fluxes of CO2 we obtained are similar to those recently reported in the East China Sea, allowing us to conclude that the coastal ocean plays a considerable role in the global oceanic carbon cycle.

A model of the iron cycle in the ocean

The IRONEX II in situ fertilization experiment showed decisively that availability of iron limits the growth of phytoplankton in equatorial Pacific surface waters. High‐nutrient low‐chlorophyll (HNLC) surface waters, potentially iron limited, are also found in the North Pacific and the Southern Ocean. A model of seawater iron geochemistry has been incorporated into a global ocean circulation and carbon cycle model and tuned to match the observed ocean Fe distribution. The model reproduces the observed HNLC areas in the equatorial Pacific and Southern Ocean but predicts nutrient depletion in the North Pacific (a region of high dust fluxes from Asia). The model is based on the idea that the concentrations of organic iron‐binding ligands in seawater control the subsurface Fe concentration and predicts that a majority (70–80%) of the global carbon export production can be sustained by upwelling of dissolved iron in seawater rather than by atmospheric deposition. The ocean iron cycle increases the efficiency of iron utilization in the biological pump, buffers new production against interannual fluctuations in iron deposition, and contributes several new intrinsic timescales to the ocean carbon cycle: several hundred years for the ocean iron concentration and perhaps several thousand for the concentration of ligand in the deep sea. Ocean recycling of Fe appears to play a major role in determining the strength of the biological pump in the ocean and the pCO2 of the atmosphere.

Testing global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO ≈ O2 + CO2), which is conservative with respect to terrestrial photosynthesis and respiration. We then compare observations of APO to the simulations of an atmospheric transport model which uses ocean‐model air‐sea fluxes and fossil fuel combustion estimates as lower boundary conditions. We present observations of the annual‐average concentrations of CO2, O2, and APO at 10 stations in a north‐south transect. The observations of APO show a significant interhemispheric gradient decreasing towards the north. We use air‐sea CO2, O2, and N2 fluxes from the Princeton ocean biogeochemistry model, the Hamburg model of the ocean carbon cycle, and the Lawrence Livermore ocean biogeochemistry model to drive the TM2 atmospheric transport model. The latitudinal variations in annual‐average APO predicted by the combined models are distinctly different from the observations. All three models significantly underestimate the interhemispheric difference in APO, suggesting that they underestimate the net southward transport of the sum of O2 and CO2 in the oceans. Uncertainties in the model‐observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the physical and biological components of the ocean models. Potential deficiencies in the physical components of the ocean models, which have previously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the APO observations. These deficiencies include the inadequate parameterization of subgrid‐scale isopycnal eddy mixing, a lack of subgrid‐scale vertical convection, too much Antarctic sea‐ice formation, and an overestimation of vertical diffusivities in the main thermocline.

Temporal variations of bomb radiocarbon inventory in the Pacific Ocean

Abstract The natural and anthropogenic components of the radiocarbon measurements from seawater samples can be successfully separated by an improved method, which is based on a very well-defined relationship between natural radiocarbon and dissolved silica observed mainly during the GEOSECS survey for waters beneath 1000 m depth. This relationship is further reconfirmed by the 14 C measurements from large volume samples taken in the deep waters in the Pacific Ocean during the recent WOCE survey program. Analysis of upper ocean 14 C measurements made along 152°W, and north of 20°N, in the northeastern Pacific Ocean during the NOAA's CGC91 cruise, which is a part of the WOCE survey program, indicates that the bomb 14 C inventory in this part of the ocean has increased by 22% since the GEOSECS measurements made in 1974. This increase is consistent with the model prediction of 25% for the northern hemisphere ocean. Change of the surface water bomb Δ 14 C values during this period is insignificant. This feature is also consistent with the model simulation. Results of this new analysis will provide useful information of the temporal variations of bomb 14 C inventory in the ocean, in addition to the spatial distribution, which can be used as powerful constraints in calibrating the global ocean carbon cycle models, especially those based on three-dimensional ocean general circulation models, for estimating the uptake of CO2 by the ocean.

Latest in oceanographic research vessels under way

Its inaugural cruise brought Atlantis , the newest of the U.S.'s 29 academic research ships, to a dock site on the Potomac River in Alexandria, Va., last week. Members of Congress, government officials, and selected student groups toured the ship, as did representatives from 45 nations participating in a conference on protecting the sea being held—not just by chance—in nearby Washington, D.C. Built for the Navy at a cost of $50 million, the 274-foot diesel-powered ship was launched on March 25—the third ship since 1930 to be named Atlantis and operated by Woods Hole Oceanographie Institution, Woods Hole, Mass. It is expected to function as a general-purpose oceanographie research vessel for at least 30 years, studying, for example, the global ocean carbon cycle and climate change and natural and artificial radioactivity in the sea. Included in the cost of Atlantis is $3 million to outfit the ship with a hydraulic A-frame used to launch the ...

Sensitivity of a global oceanic carbon cycle model to the circulation and to the fate of organic matter: Preliminary results

We examine the effect of the circulation on the resulting distribution of tracers in the ocean and the sensitivity of surface CO 2 to the remineralization cycle of organic matter. For this purpose we built a 3-dimensional global model of the oceanic carbon cycle. The processes considered are air-sea CO 2 exchanges, export production of organic matter and its remineralization, CaCO 3 precipitation and dissolution or sedimentation. The annual mean fields needed to drive the model are provided by two different OGCM's. The sensitivity of the tracer distribution to the circulation is evaluated by examining the results obtained with each set of hydrodynamic fields driving the geochemical model with identical parameterizations of the biochemical processes in both cases. The model is then applied to investigate the consequences on the surface Δ p co 2 of various behavior of the dead organic material. The consideration of fast sinking particles leads to a distribution which is in better agreement with measurements than the one obtained under the assumption that the organic matter is mainly under dissolved form.