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Authigenic clay formation during early diagenesis of marine sediments, termed “reverse weathering,” is an important process for regulating ocean pH, seawater chemistry, and atmospheric CO₂over geologic time scales. Although the importance of reverse weathering has been increasingly recognized, the rates and mechanisms remain poorly constrained. This study investigated the mechanisms, kinetics, and mineral products derived from diatom biogenic silica. We show the formation of Fe(II)-bearing smectite and mica in 40 days, the most rapid process and first specific mineral phases reported to date. Unraveling the kinetics and mechanisms of authigenic clay formation suggests that reverse weathering is far more dynamic and responsive to changes in ocean chemistry than previously envisioned, with a potential to impact marine alkalinity cycling on a shorter timescale. 

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. 

Abstract Molecular clocks estimate that diatom microalgae, one of Earth’s foremost primary producers, originated near the Triassic–Jurassic boundary (200 Ma), which is close in age to the earliest, generally accepted diatom fossils of the genus Pyxidicula . During an extensive search for Jurassic diatoms from twenty-five sites worldwide, three sites yielded microfossils initially recognized as diatoms. After applying stringent safeguards and evaluation criteria, however, the fossils found at each of the three sites were rejected as new diatom records. This led us to systematically reexamine published evidence in support of Lower- and Middle-Jurassic Pyxidicula fossils . Although Pyxidicula resembles some extant radial centric diatoms and has character states that may have been similar to those of ancestral diatoms, we describe numerous sources of uncertainty regarding the reliability of these records. We conclude that the Lower Jurassic Pyxidicula fossils were most likely calcareous nannofossils, whereas the Middle Jurassic Pyxidicula species has been reassigned to the Lower Cretaceous and is likely a testate amoeba, not a diatom. Excluding the Pyxidicula fossils widens the gap between the estimated time of origin and the oldest abundant fossil diatom record to 75 million years. This study underscores the difficulties in discovering and validating ancient microfossils.