Atmospheric and Oceanographic Characteristics of the BGR Exploration Area for Polymetallic Nodules in the Northeastern Tropical Pacific Ocean and Model Simulations of Dredge-Induced Suspended Sediment Transport

This chapter provides an overview on atmospheric and hydrographic conditions, and temporal variations of bottom water currents in the exploration area for polymetallic nodules of the German Federal Institute for Geosciences and Natural Resources (BGR) in the northeastern tropical Pacific Ocean. Furthermore, we compare in situ resuspension experiments with numerical model results on transport pathways and deposition patterns of the suspended sediments to assess the reliability of model predictions with respect to future deep-sea mining. The BGR area underlies seasonal climate variations due to meridional migrations of the Intertropical Convergence Zone (ITCZ) and changes in the subtropical high. In March, when the ITCZ is weakest near the equator, sea level pressure is high and precipitation and northeast winds are low, while in August and September, when the ITCZ is most northerly, pressure is low, precipitation increases, and winds are variable. The frequency of tropical storms and hurricanes peaks in June through August. Tropical storm magnitude is usually reached once a year, and hurricanes occur every other year. The two parts of the exploration area are located at the equatorward edge of the westward flowing North Equatorial Current. Water temperatures reach as high as 27 °C and salinity as low as 33.9 psu in the mixed layer with an average depth of 30 m. The thermo- and haloclines are sharp and separate the surface layers from cooler, saltier waters that are devoid of oxygen to a depth of 700 m to 900 m. These water masses overlie Pacific Intermediate waters, the deeper Antarctic Intermediate water, and Lower Circumpolar waters closest to the ocean bottom. Analysis of near-bottom current data recorded by moored ADCPs between 2013 and 2019 indicates low velocities of 1–5 cm s−1 on average and maximum values of 8–14 cm s−1. Current directions are strongly influenced by semidiurnal tidal cycles and near-inertial oscillations, with predominant southeasterly to southwesterly flow. The bottom currents can be amplified and change their predominant direction by mesoscale eddies that form under the influence of Central American Gap Winds and travel several 1000 km westward. Coupled ocean-circulation and sediment-transport models, taking into account local sediment particle properties, were used to simulate the near-field and far-field transport and settling of suspension plumes on the seafloor generated by dredging experiments in the BGR exploration area and by an epibenthic sledge tow in the Peru Basin. Comparison of the observed and predicted deposition patterns showed satisfactory agreement and increased confidence in the model’s ability to adequately predict the spread of suspension plumes and particle redeposition resulting from future industrial-scale deep-sea mining.