Our understanding of the global silicon (Si) cycle has changed progressively over the past several decades. In the late 1970’s, the silica (SiO2) produced by weathering on the continents and delivered in dissolved form from the world’s rivers to the ocean was thought to eventually deposit largely in the Southern Ocean and other regions of the deep sea where biologically formed siliceous oozes are found.
It is now known that the marine silicon cycle is far more nuanced and that significant quantities can be removed from the ocean not only in the form of biologically formed opal such as diatom frustules (bSi) but also minerals such as clay that can form rapidly during reactions (termed reverse weathering) in surface sediment. Silica is also supplied to the oceans from hydrothermal regions. Unfortunately because clay is a very common class of mineral that is also eroded from the continents, it has been very difficult to determine exactly how much new clay (termed “authigenic”) is actually being formed and buried in different environments.
In a recent article published in Geophysical Research Letters (“Cosmogenic 32Si as a tracer of silica burial and diagenesis: major deltaic sinks in the silica cycle” GRL, 43, 7124-7132), SoMAS PhD graduate Shaily Rahman and her advisors, Robert Aller and J. Kirk Cochran, showed for the first time that naturally occurring cosmogenic 32Si, which has a half- life of ~ 140 years, can be used to determine the different modes of silica burial in recently deposited sediments. They demonstrate that in tropical deltaic environments such as the massive Amazon delta and coastal muds derived from it along French Guiana, virtually all biologically-formed silica such as diatom frustules (bSi) carrying 32Si is rapidly converted to clay. Because there is less than ~ 1 kg of cosmogenic 32Si on Earth, its measurement is extremely difficult. Nevertheless, Rahman, Aller, and Cochran show that it is possible to independently and quantitatively constrain the sinks for silicon in the ocean using 32Si as a new tool, and to much more accurately resolve the global Si cycle, particularly the role of the continental margins where most sediment in the oceans is deposited. Indeed, these results suggest that deltaic and associated dispersal systems may rival or exceed the Southern Ocean in importance as sites of reactive silica (bSi + altered products of bSi) storage.