The origin of ferruginous concretions on the Sunda Shelf (SE Asia) and its environmental implications

Marine concretions are an important source of information regarding resources, climate, and environment. However, the genesis of ferruginous concretions on the continental shelf remains unclear. In this study, we present the radiocarbon ages (AMS14C), elemental compositions, and mineral compositions of ferruginous concretions obtained from a gravity core on the central Sunda Shelf. The results indicate that the formation ages of ferruginous concretions range from 10.5 to 7.5 cal ka BP from the bottom to the top of the core. The predominant mineral identified in these concretions is goethite, which cements clay and other minerals in colloidal form. Notably, Fe, Si, and Al display distinct ring-band rhythmic variations. Our study suggests that ferruginous concretions are formed in subaerial and aqueous environments, with a weathered stiff clay layer serving as the main stratum for concretion development. During periods of low sea levels (before 10.5 cal ka BP), paleo-channels likely provided fluctuating wet and dry conditions for the formation of light and dark concentric layers within ferruginous concretions. Between 10.5 and 7.5 cal ka BP, the Sunda Shelf was gradually inundated, leading to the partial transport of concretions from nearby channels. Subsequently, after 7.5 cal ka BP, the paleo-channels became completely submerged, halting the formation of ferruginous concretions on the Sunda Shelf. Meanwhile, ferruginous concretions on the seafloor became mixed with sediments and marine organisms due to hydrodynamic sorting. The presence of ferruginous concretions can serve as an indicator of paleo-channel locations, aiding in the reconstruction of paleo-channel systems during periods of low sea level on the shallow continental shelf.