Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage

Abstract

Deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques. Rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets. A variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating Amazon delta sediments indicates that both of these general assumptions are unjustified. Apparent lack of significant biogenic silica storage in deltaic environments, particularly in the tropics, may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica. Biogenic silica particles buried in suboxic Amazon delta deposits can be unaltered, partially dissolved, covered with aluminosilicate or metal-rich coatings, or completely reconstituted into authigenic K-Fe-rich aluminosilicate minerals. Pore water (K, Mg, F, Si) and solid-phase distributions, direct observations of particles, laboratory experiments, and depositional context indicate that authigenic clays form rapidly (<1 yr) in the seasonally reworked surface layer (∼ 0.5–2 m) of the delta topset and are disseminated during sediment remobilization. Fe, Al-oxide rich debris derived from the tropical drainage basin is an abundant reactant, and thus the supply of biogenic silica is a major control on the amount of clay formed. The mild 1% Na 2CO 3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0.1N HCl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution. Well-crystallized clays are not significantly affected by this modification nor is bulk Amazon River bed sediment. The two-step procedure indicates that ∼90% of the biogenic silica originally present in deposits is converted to clay or otherwise altered, raising the effective quantity of biogenic silica stored from ∼33 to ∼296 μmol Si g −1 (∼1.8% SiO 2). Biogenic Si stored in the delta increases away from the river mouth, across shelf and along the dispersal system where primary production is highest. The K/Si ratio of labile authigenic material is ∼0.19 mol mol −1, far higher than Amazon River suspended matter (∼0.07 mol mol −1). Diagenetic models indicate formation rates in the mobile sediment layer of ∼2.8 μmol K g −1 yr −1 (∼16 μmol Si g −1 yr −1). Inclusion of authigenic alteration products of biogenic silica in estimates of reactive Si burial increases the deltaic storage of riverine Si to ∼22% of the Amazon River input. The rapid formation of aluminosilicates from biogenic SiO 2, seawater solutes, and remobilized Fe, Al-oxides represents a form of reverse weathering. Rapid reverse weathering reactions in tropical muds and deltaic deposits, the largest sediment depocenters on Earth, confirms the general importance of these processes in oceanic elemental cycles.