Cosmogenic ³²Si as a tracer of biogenic silica burial and diagenesis: Major deltaic sinks in the silica cycle

Abstract

Measurements of natural cosmogenic 32Si (t1/2 ~ 140 years) in tropical deltaic sediments demonstrate for the first time that most 32Si is present in rapidly formed authigenic clays and not biogenic opaline silica (bSi). The burial of bSi in deltaic and continental margin sediments has likely been greatly underestimated because of diagenetic alteration of bSi to clay, little of which dissolves in the classically used operational bSi leach. Rapid reverse weathering reactions during early diagenesis must be considered as a significant pathway of reactive Si storage in deltaic deposits. Based on 32Si, actual storage may be 2–3 times the best recent estimates extrapolated from diagenetic models or attempts to modify operational bSi methods to include authigenic clay (~900 µmol/g versus ~250 µmol/g). Measurements of natural 32Si inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…Thus the conservative estimates for the net storage of altered bSiO 2 and authigenic silicates in deltaic sediments range from 2-3x in the Mississippi (Presti and Michalopoulos, 2008) to 5-10x in the Amazon (Michalopoulos and Aller, 2004). Based on 32 Si, actual storage in the Amazon delta may be 2-3x greater (i.e., 3-4.5 Tmol yr −1 ) than the best recent conservative estimates (Rahman et al, 2016).…”

“…It has been proposed that reactive bSiO 2 that has undergone reconstitution/dissolution/reprecipitation transformations remains partially unaccounted for (Michalopoulos and Aller, 2004;Presti and Michalopoulos, 2008) using the traditional operational leach method of DeMaster (1981). However, measurements of cosmogenic 32 Si incorporated in reactive SiO 2 products indicate that even the modified alkaline leach methods used to date in high sediment accumulation rates underestimate the amount of reconstituted bSiO 2 and authigenic silicates stored in deltaic sediments (Rahman et al, 2016). Thus the conservative estimates for the net storage of altered bSiO 2 and authigenic silicates in deltaic sediments range from 2-3x in the Mississippi (Presti and Michalopoulos, 2008) to 5-10x in the Amazon (Michalopoulos and Aller, 2004).…”

The Missing Silica Sink: Revisiting the Marine Sedimentary Si Cycle Using Cosmogenic 32Si

“…Since then diatom growth rates, sponge ages, and silica production rates in different areas have been estimated using this radionuclide as an introduced tracer (Blain et al, ; Brzezinski & Phillips, ; Smith et al, ; Kristiansen et al, , ; Brzezinski et al, ; Nelson et al, ; Rousseau et al, ; Sigmon et al, ; Ragueneau et al, ; Ellwood et al, ; Kemp & Villareal, ; Krause et al, ). Cosmogenic 32 Si at natural activity levels has recently been used to determine bSi opal and bSi clay storage (bSi clay ~0.4 Tmol/yr) in the Amazon delta‐Guianas mudbank dispersal system, suggesting that burial of bSi clay has been substantially underestimated in continental margins (Rahman et al, ).…”

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…The sources enter the ocean from rivers, meltwaters, groundwaters, as hydrothermal fluids and from the atmosphere (Tréguer and De La Rocha, 2013). The sinks include biosilicification and burial (Knoll, 2003) and the authigenesis of clay minerals (Mackenzie et al, 1967;Rahman et al, 2016).…”