In-situ geotechnical properties of abyssal sediments and implications for environmental impacts of polymetallic nodule mining

Polymetallic nodules on the global seabed are an enormous resource for critical metals, while environmental impacts caused by mining activities such as sediment disturbance and plume diffusion are under evaluation. Geotechnical properties of surface sediments hosting nodules will critically impact mining operations. However, the spatial variation in geotechnical properties and sediment transport and deposition dynamics are poorly known. We carried out in-situ penetration strength tests at 11 stations in the China Minmetals Contract Area, western Clarion-Clipperton Fracture zone. By establishing a quantitative relationship between sediment penetration strength (indicative of substrate hardness) and the bathymetric position index (BPI, a measure of topographic relief) and sediment grain size, we observed that topographic highs (highlands) exhibit coarser sediments and harder substrates compared to topographic lows (depressions). This observation aligns well with a lateral sediment redistribution model, where sediment is eroded from elevated areas and redeposited in depressions. Our findings suggest that a harder substrate on highlands would lead to reduced mining vehicle subsidence and sediment disturbance, thereby mitigating sediment plumes. Consequently, preferential nodule mining on highlands is likely to yield lower environmental impact and faster ecosystem recovery compared to mining in depressions.