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Mud banks of Kerala (southwest coast of India) are renowned for immense fish harvesting during the southwest monsoon season. Although inclement weather and turbulent waves during monsoon prevent vessels sailing into the sea, the tranquil mud banks provide a natural platform for artisanal fishing. Despite several attempts over the past 6 decades, the formation and sustenance of these mud banks remain scientifically unexplained. In this study, we analyzed the geochemical properties of sediments and fluid mud collected from Alappuzha mud banks. The sediments were mostly silty clay (relative abundance of kaolinite [35%–67%] and montmorillonite [25%–45%]), whereas the fluid mud was purely fine clay (montmorillonite [47%–67%] and kaolinite [29%–45%]). Phosphorus content was exceptionally high in the fluid mud (2240–2634 μ g/g) and sediments (1559–3013 μ g/g). Laboratory experiments showed an increase in volume and a decrease in the settling rate of sediments spiked with varying proportions of montmorillonite and phosphorus. Natural conditions such as winds, waves, currents, or terrestrial factors dislodged the sediments into the water column. Natural sedimentation of clay particles results in the rapid flocculation of large particles (kaolinite and illite), whereas light particles (montmorillonite) remain suspended in the water column for a longer time. During this period, phosphate anions (or any oxyanions) geochemically interact with montmorillonite as well as with its interlayer cations in the following ways: (i) the ligand exchange reaction between the edge hydroxyl groups of mineral with phosphate anions increase the size and negative charge of the clay particles and (ii) phosphate anions effectively adsorb to the interlayer cations. The resultant strong interparticle repulsive force stabilizes the clay suspension, and thereafter, (iii) the intercalation of organic moieties stably converts the hydrophilic clay suspension into a hydrophobic suspension. Results of bulk geochemical parameters along the southwest coast of India show that montmorillonite and phosphorus-rich sediments are the unique properties of Alappuzha mud banks, which lead to their recurrent formation.

The southwest coast of India is known to experience two concurrent processes during the southwest monsoon, namely coastal upwelling and mud banks at selected stretches such as that off Alappuzha. The mud bank sediments were found to contain very high concentrations of phosphorus (ave. 2255 ± 278 μ g/g), and the present study measured changes in concentration of elements such as carbon, phosphorus, iron, and manganese within the Alappuzha mud banks, to understand the influence of upwelling on the cycling of these elements. The results showed that coastal upwelling led to hypoxic conditions within the mud bank region during the peak southwest monsoon, which induced significant changes in the phosphorus fractions in the sediments. There was an enrichment of iron, caused by the efflux of reactive Fe from the subsurface sediment layers and depletion of manganese, due to reductive dissolution of Mn oxides and subsequent release of Mn. The low molar ratios of total organic carbon to total nitrogen (9 ± 1) and total organic carbon to organic phosphorus (118 ± 57) generally indicated that the organic matter was relatively fresh and derived from marine plankton. Sedimentary phosphorus also exhibited wide fluctuations in its geochemical forms during hypoxia. Five geochemical forms of P such as exchangeable P (Ex-P), Fe and Mn bound P (Fe-P), P from Al oxides, non-reducible Fe bound P and organic P (Non Red-P), calcium bound P (Ca-P) and refractory organic P (ROP) were extracted, adopting sequential extraction procedure. The percentage contribution of various P forms were 2%–11% Ex-P, 4%–25% Fe-P, 4%–10% Non Red-P, 60%–78% Ca-P and 4%–12% ROP. The fractionation study revealed that precipitation of apatite and vivianite are the significant pathways of phosphorus enrichment in mud bank sediments. The study indicates that the enrichment of labile P fractions could be mediating the enhancement in biological production in the Alappuzha mud banks during the southwest monsoon.