Abstract
Abstract. The sinking of particulate organic carbon (POC) is a key component of the ocean carbon cycle and plays an important role in the global climate system. However, the processes controlling the fraction of primary production that is exported from the euphotic zone (export ratio) and how much of it survives respiration in the mesopelagic to be sequestered in the deep ocean (transfer efficiency) are not well understood. In this study, we use a three-dimensional, coupled physical–biogeochemical model (CCSM–BEC; Community Climate System Model–ocean Biogeochemical Elemental Cycle) to investigate the processes controlling the export of particulate organic matter from the euphotic zone and its flux to depth. We also compare model results with sediment trap data and other parameterizations of POC flux to depth to evaluate model skill and gain further insight into the causes of error and uncertainty in POC flux estimates. In the model, export ratios are mainly a function of diatom relative abundance and temperature while absolute fluxes and transfer efficiency are driven by mineral ballast composition of sinking material. The temperature dependence of the POC remineralization length scale is modulated by denitrification under low O2 concentrations and lithogenic (dust) fluxes. Lithogenic material is an important control of transfer efficiency in the model, but its effect is restricted to regions of strong atmospheric dust deposition. In the remaining regions, CaCO3 content of exported material is the main factor affecting transfer efficiency. The fact that mineral ballast composition is inextricably linked to plankton community structure results in correlations between export ratios and ballast minerals fluxes (opal and CaCO3), and transfer efficiency and diatom relative abundance that do not necessarily reflect ballast or direct ecosystem effects, respectively. This suggests that it might be difficult to differentiate between ecosystem and ballast effects in observations. The model's skill in reproducing sediment trap observations is equal to or better than that of other parameterizations. However, the sparseness and relatively large uncertainties of sediment trap data makes it difficult to accurately evaluate the skill of the model and other parameterizations. More POC flux observations, over a wider range of ecological regimes, are necessary to thoroughly evaluate and test model results and better understand the processes controlling POC flux to depth in the ocean.
Highlights
The transfer of carbon from the upper ocean to deep waters through the sinking of particulate biogenic material is a major component of the ocean carbon cycle and plays an important role in regulating atmospheric CO2 levels (Archer et al, 2000; Siegenthaler et al, 2005)
The processes controlling the fraction of primary production that is exported from the euphotic zone and how much of it survives respiration in the mesopelagic to be sequestered in the deep ocean are not well understood (Boyd and Trull, 2007)
A type II linear regression of log10-transformed particulate organic carbon (POC) flux estimates from CCSM–BEC against log10-transformed observations results in a correlation coefficient of 0.65 and a slope of 0.91 (Fig. 2a)
Summary
The transfer of carbon from the upper ocean to deep waters through the sinking of particulate biogenic material (biological pump, Volk and Hoffert, 1985) is a major component of the ocean carbon cycle and plays an important role in regulating atmospheric CO2 levels (Archer et al, 2000; Siegenthaler et al, 2005). Biogenic minerals (CaCO3 and opal) are thought to be important controlling factors (ballast hypothesis) by increasing the density of sinking particles and/or providing protection against remineralization. Ecosystem structure is thought to play an important role (Guidi et al, 2009; Lam et al, 2011; Henson et al, 2012a, b; Wilson et al, 2012). According to this view, diatom-dominated phytoplankton communities in productive areas, such as high-latitude environments and upwelling regions, produce large, dense and relatively labile aggregates
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