Quantifying biological carbon pump pathways with a data-constrained mechanistic model ensemble approach

[摘要] The ability to constrain the mechanisms that transport organiccarbon into the deep ocean is complicated by the multiple physical,chemical, and ecological processes that intersect to create, transform, andtransport particles in the ocean. In this paper we develop andparameterize a data-assimilative model of the multiple pathways of thebiological carbon pump (NEMURO BCP ). The mechanistic model is designedto represent sinking particle flux, active transport by vertically migratingzooplankton, and passive transport by subduction and vertical mixing, whilealso explicitly representing multiple biological and chemical propertiesmeasured directly in the field (including nutrients, phytoplankton andzooplankton taxa, carbon dioxide and oxygen, nitrogen isotopes, and 234 Thorium). Using 30 different data types (including standing stockand rate measurements related to nutrients, phytoplankton, zooplankton, andnon-living organic matter) from Lagrangian experiments conducted on 11cruises from four ocean regions, we conduct an objective statisticalparameterization of the model and generate 1 million different potentialparameter sets that are used for ensemble model simulations. The modelsimulates in situ parameters that were assimilated (net primary productionand gravitational particle flux) and parameters that were withheld( 234 Thorium and nitrogen isotopes) with reasonable accuracy. Modelresults show that gravitational flux of sinking particles and verticalmixing of organic matter from the euphotic zone are more importantbiological pump pathways than active transport by vertically migratingzooplankton. However, these processes are regionally variable, with sinkingparticles most important in oligotrophic areas of the Gulf of Mexico andCalifornia Current, sinking particles and vertical mixing roughly equivalentin productive coastal upwelling regions and the subtropical front in theSouthern Ocean, and active transport an important contributor in the easterntropical Pacific. We further find that mortality at depth is an importantcomponent of active transport when mesozooplankton biomass is high, but itis negligible in regions with low mesozooplankton biomass. Our results alsohighlight the high degree of uncertainty, particularly amongstmesozooplankton functional groups, that is derived from uncertainty in modelparameters. Indeed, variability in BCP pathways between simulations for aspecific location using different parameter sets (all with approximatelyequal misfit relative to observations) is comparable to variability in BCPpathways between regions. We discuss the implications of these results forother data-assimilation approaches and for studies that rely on non-ensemblemodel outputs.

[学科分类] 大气科学