ALBERT

Description: Zooplankton plays a notable role in ocean biogeochemical cycles. However, it is often simulated as one generic group and top closure term in ocean biogeochemical models. This study presents the description of three zooplankton functional types (zPFTs, micro-, meso- and macrozooplankton) in the ocean biogeochemical model FESOM-REcoM. In the presented model, microzooplankton is a fast-growing herbivore group, mesozooplankton is another major consumer of phytoplankton, and macrozooplankton is a slow-growing group with a low temperature optimum. Meso- and macrozooplankton produce fast-sinking fecal pellets. With three zPFTs, the annual mean zooplankton biomass increases threefold to 210 Tg C. The new food web structure leads to a 25% increase in net primary production and a 10% decrease in export production globally. Consequently, the export ratio decreases from 17% to 12% in the model. The description of three zPFTs reduces model mismatches with observed dissolved inorganic nitrogen and chlorophyll concentrations in the South Pacific and the Arctic Ocean, respectively. Representation of three zPFTs also strongly affects phytoplankton phenology: Fast nutrient recycling by zooplankton sustains higher chlorophyll concentrations in summer and autumn. Additional zooplankton grazing delays the start of the phytoplankton bloom by 3 weeks and controls the magnitude of the bloom peak in the Southern Ocean. As a result, the system switches from a light-controlled Sverdrup system to a dilution-controlled Behrenfeld system. Overall, the results suggest that representation of multiple zPFTs is important to capture underlying processes that may shape the response of ecosystems and ecosystem services to on-going and future environmental change in model projections.

Description: Mineral dust accumulated on the ocean floor is an important archive for reconstructing past atmospheric circulation changes and climatological conditions in the source areas. Dust emitted from Southern Hemisphere dust sources is widely deposited over the oceans. However, there are few records of dust deposition over the open ocean, and a large need for extended geographical coverage exists. We present a large data set (134 surface sediment samples) of Late Holocene dust deposition from seafloor surface sediments covering the entire South Atlantic Ocean. Polymodal grain-size distributions of the lithogenic fraction indicate that the sediments are composed of multiple sediment components. By using end-member modeling, we attempt to disentangle the dust signal from non-aeolian sediments. Combined with 230Th-normalized lithogenic fluxes, we quantified the specific deposition fluxes for mineral dust, crrent-sorted sediments and ice-rafted debris (IRD). Although the method could not completely separate the different components in every region, it shows that dust deposition off the most prominent dust source for the South Atlantic Ocean—southern South America—amounts up to approximately 0.7 g cm−2 Kyr−1 and decreases downwind. Bottom-current-sorted sediments and IRD are mostly concentrated around the continental margins. The ratio of the coarse to fine dust end members reveals input from north African dust sources to the South Atlantic. The majority of the observations are in good agreement with new model simulations. This extensive and relevant data set of dust grain size and deposition fluxes to the South Atlantic could be used to calibrate and validate further model simulations.