Dynamics of particulate organic carbon flux in a global ocean model

[摘要] The sinking of particulate organic carbon (POC) is a keycomponent of the ocean carbon cycle and plays an important role in theglobal climate system. However, the processes controlling the fraction ofprimary production that is exported from the euphotic zone (export ratio)and how much of it survives respiration in the mesopelagic to besequestered in the deep ocean (transfer efficiency) are not wellunderstood.In this study, we use a three-dimensional, coupledphysical–biogeochemical model (CCSM–BEC; Community Climate System Model–oceanBiogeochemical Elemental Cycle) to investigate the processescontrolling the export of particulate organic matter from the euphotic zoneand its flux to depth. We also compare model results with sediment trapdata and other parameterizations of POC flux to depth to evaluate modelskill and gain further insight into the causes of error and uncertainty inPOC flux estimates. In the model, export ratios are mainly a function ofdiatom relative abundance and temperature while absolute fluxes andtransfer efficiency are driven by mineral ballast composition of sinkingmaterial.The temperature dependence of the POC remineralization lengthscale is modulated by denitrification under low O₂ concentrationsand lithogenic (dust) fluxes. Lithogenic material is an important controlof transfer efficiency in the model, but its effect is restricted toregions of strong atmospheric dust deposition. In the remaining regions,CaCO₃ content of exported material is the main factor affectingtransfer efficiency. The fact that mineral ballast composition isinextricably linked to plankton community structure results in correlationsbetween export ratios and ballast minerals fluxes (opal and CaCO₃),and transfer efficiency and diatom relative abundance that do notnecessarily reflect ballast or direct ecosystem effects, respectively. Thissuggests that it might be difficult to differentiate between ecosystem andballast effects in observations. The model's skill in reproducing sedimenttrap observations is equal to or better than that of otherparameterizations. However, the sparseness and relatively largeuncertainties of sediment trap data makes it difficult to accuratelyevaluate the skill of the model and other parameterizations. More POC fluxobservations, over a wider range of ecological regimes, are necessary tothoroughly evaluate and test model results and better understand theprocesses controlling POC flux to depth in the ocean.

[学科分类] 地球化学与岩石