Separating Si phases from diagenetically-modified sediments through sequential leaching

Silicon (Si) phases such as biogenic silica, lithogenic silicate and authigenic silica/silicate in marine sediments provide valuable information about past Si cycling. Wet-chemical sequential leaching methods are often applied to extract different Si phases from marine sediments to study Si diagenetic processes in shallow subsurface. The potential of this method to separate Si phases from deeply-buried and diagenetically-modified sediments has not been systematically examined. We applied a sequential leaching protocol to drill core sediments retrieved from the Ulleung Basin, East/Japan Sea. We performed geochemical (elemental abundance and stable Si isotopes, 830Si) and microscopic (X-ray diffraction and scanning electron microscope) analyses to monitor leaching efficiency in separating different Si phases. We show that, prior to alkaline leaching, applying weak acid is able to remove metal oxide and/or clay-like phases. The following Na2CO3 leaching, based on a commonly-adopted protocol, is able to dissolve some but not all diatoms. The results of elemental contents and 830Si values of leachates suggest that, in diagenetically-modified sediments, either a longer digesting time or a harsher alkaline leaching is needed to dissolve all diatoms. This is attributed to increased resistance of diatoms to Na2CO3 leaching as a result of reduced surface area and/or improved SiO2 tetrahedron ordering during diagenetic processes over time and burial depths. Lithogenic silicate minerals can be dissolved by NaOH and potentially separated from diatoms if the latter is completely removed in the preceding leaching steps. Even if a trace amount of diatom is left undissolved in the NaOH leaching, it is still possible to separate the two through a mass balance calculation given the knowledge of composition for the two end-members. We conclude that a successful separation of Si phases in diagenetically modified sediments relies on the knowledge of elemental abundance and even 830Si values of the leachates, as well as information such as species of Si-skeleton organisms, contents and maturation degree of biogenic silica.

Automated summary

Silicon phases in marine sediments, including biogenic silica, lithogenic silicate, and authigenic silica/silicate, provide valuable information about past Si cycling. Wet-chemical sequential leaching methods are commonly used to extract different Si phases, but their efficiency in deeply-buried and diagenetically-modified sediments is not well studied. This study applied a sequential leaching protocol to drill core sediments and found that a longer digesting time or harsher alkaline leaching is needed to dissolve all diatoms in diagenetically-modified sediments. The success of separating Si phases relies on the knowledge of elemental abundance, stable Si isotopes, and information about the composition and maturation degree of biogenic silica.