Search for: All records

Abstract. The oceanic silicon cycle has undergone a profound transformation from an abiotic system in the Precambrian to a biologically regulated cycle driven by siliceous organisms such as diatoms, Rhizaria, and sponges. These organisms actively uptake silicon using specialized proteins to transport and polymerize it into amorphous silica through the process of biosilification. This biological control varies depending on environmental conditions, influencing both the rate of silicification and its ecological function, including structural support, defence, and stress mitigation. Evidence suggests that silicification has evolved multiple times independently across different taxa, each developing distinct molecular mechanisms for silicon handling. This review identifies major gaps in our understanding of biosilicification, particularly among lesser-known silicifiers beyond traditional model organisms like diatoms. It emphasizes the ecological significance of these underexplored taxa and synthesizes current knowledge of molecular pathways involved in silicon uptake and polymerization. By comparing biosilicification strategies across taxa, this review calls for expanding the repertoire of model organisms and leveraging new advanced tools to uncover silicon transport mechanisms, efflux regulation, and environmental responses. It also emphasizes the need to integrate biological and geological perspectives, both to refine palaeoceanographic proxies and to improve the interpretation of microfossil records and present-day biogeochemical models. On a global scale, silicon enters the ocean primarily via terrestrial weathering and is removed through burial in sediments and/or authigenic clay formation. While open-ocean processes are relatively well studied, dynamic boundary zones – where land, sediments, and ice interact with seawater – are nowadays recognized as key regulators of silicon fluxes, though they remain poorly understood. Therefore, special attention is given to the role of dynamic boundary zones such as the interfaces between land and ocean, the benthic zone, and the cryosphere, which are often overlooked yet play critical roles in controlling silicon cycling. By bringing together cross-discipline insights, this review proposes a new integrated framework for understanding the complex biological and biogeochemical dimensions of the oceanic silicon cycle. This integrated perspective is essential for improving global silicon budget estimates, predicting climate-driven changes in marine productivity, and assessing the role of silicon in modulating Earth’s long-term carbon balance. 

The oxidation of organic carbon contained within sedimentary rocks (“petrogenic” carbon, or hereafter OC_petro) emits nearly as much CO₂as is released by volcanism, thereby playing a key role in the long-term global C budget. High erosion rates in mountains have been shown to increase OC_petrooxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OC_petrooxidation in mountains versus floodplains remains difficult to assess as disparate methods have been used in the different environments. Here, we investigate the sources and fluxes of rhenium (Re) in the Rio Madre de Dios to quantify OC_petrooxidation from the Andes to the Amazon floodplain using a common approach. Dissolved rhenium concentrations (n = 131) range from 0.01 to 63 pmol L⁻¹and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OC_petrooxidation and that this proportion increases downstream. We estimate that in the Andes, OC_petrooxidation releases 11.2^+4.5/_−2.8tC km⁻²y⁻¹of CO₂, which corresponds to ~41% of the total OC_petrodenudation (sum of oxidized and solid OC_petro). A Re mass balance across the Rio Madre de Dios shows that 46% of OC_petrooxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OC_petrooxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OC_petrooxidation and CO₂release.