Pore-water chemistry: A proxy for tracking the signature of ongoing silica diagenesis

Abstract

Silica diagenesis leads to dramatic petrophysical variations in the host sediment across the depth of an opal-A to opal-CT transition zone. Predicting the present-day diagenetic status of opal-A to opal-CT transition zones, i.e., active versus fossilized fronts, is essential to constraining the drivers that control abrupt changes in the physical state of sediment. This study assesses whether there are modern signatures of ongoing silica diagenesis in the sediment pore water, and demonstrates the potential for pore-water-chemistry profiles for distinguishing between active opal-CT precipitation and fossil transition zones. Pore-water chemistry, mineralogy, and thermodynamic analyses of the Ocean Drilling Program Wells 794 and 795 indicate that solubility equilibrium has been reached with respect to opal-CT in the transition zones captured by the Neogene biosilica in the Sea of Japan. Even though silica dissolution might be triggering a reverse-weathering process, the equilibrium reached with respect to diagenetic opal strongly suggests that the silica drop across the transition zones is mainly influenced by active opal-A to opal-CT transformation. Owing to abrupt petrophysical variations associated with opal-CT formation, other interstitial profiles—major ions and primary parameters—have been influenced by silica diagenesis. The extremely low silica diffusion fluxes in the sediment, the low permeability of host sediment, and the occurrence of considerable pore-water loss at the depth of the transition zone all support this conclusion that the dissolved species have not been diffused in the sediment at rates comparable to those by pore-water advection. Advection and diffusion, however, appear to have ceased recently because they have failed to smooth the signature of ongoing silica diagenesis. The porosity drop during opal-A to opal-CT diagenesis at Sites 794 and 795 is principally attributed to chemically induced anomalous compaction, causing the sediment framework to lose its strength under fragmentation and extensive opal-A dissolution.

Arrested versus active silica diagenesis reaction boundaries—A review of seismic diagnostic criteria

“…Accordingly, the systematic upward decrease in the fold relief is consistent with a significant reduction in differential porosity collapse (Figure 11) which is essentially impacted by the rate of silica diagenetic reaction (e.g. Mizutani, 1970, 1971, 1977; Varkouhi, Cartwright, et al, 2020, Varkouhi, Tosca, et al, 2020). As a conclusion, association with this mode of deformation cannot independently provide strong supporting evidence that the silica transition zones accommodated within the Neogene deposits of the Faeroe‐Shetland basin and mid‐Norwegian margin are currently arrested reaction fronts.…”

“…The contradictory interpretation for current state of the regionally discordant‐to‐seabed TZ A/CT from the Sea of Okhotsk proposed by Meadows and Davies (2007) was challenged here, given that the reaction boundary has been more highly complicated by structural deformations than by the geometry of its overburden. In contrast, a BSR attitude can equally represent either active or arrested status of a TZ A/CT as indicated by Varkouhi, Tosca, et al (2020) for the likely active characteristic of the transition zone penetrated by the ODP Wells 794 and 795, but the arrested nature of the BSR TZ A/CT drilled by the ODP Site 800. From this, a case can be made that although a non‐BSR feature strongly supports arrested state of a TZ A/CT , a BSR geometry is not an independent reliable proof of ongoing silica diagenesis.…”

“…Based on seismic, thermodynamic and petrographic analysis of the BSR TZ A/CT accommodated within the Middle Miocene sediments of the Ocean Drilling Program (ODP) Sites 794 and 795 in the Sea of Japan, Varkouhi, Tosca, et al (2020) compiled a set of observations that indicate the opal‐A to opal‐CT transition zones with a BSR geometry possibly represent an actively migrating diagenetic front: They are found in continuous stratigraphic sections with no major breaks or unconformities in deposition; The front is clearly identifiable on seismic profiles; Though parallel to the present‐day seabed, the TZ A/CT lies discordant to host sediments (Hein et al, 1978; Figure 4a through c); The front is hosted in silica‐rich young sediments (Late Miocene and younger). The age of depositional horizons hosting the TZ A/CT was constrained by diatom biostratigraphy (Koizumi, 1992; Shipboard Scientific Party, 1990a, 1990b); The geothermal gradient of the sediment accommodating the TZ A/CT is very high (e.g.…”

“…This transformation comprises a complex set of dissolution–reprecipitation reactions (Kastner et al, 1977; Leeder, 1982; Stein & Kirkpatrick, 1976; Williams & Crerar, 1985; Williams et al, 1985; Wrona, Jakson, et al, 2017), which lead to marked changes in the host sediment petrophysics, particularly a porosity drop (Chaika & Dvorkin, 1997, 2000; Isaacs, 1981; Meadows & Davies, 2007, 2009; Nobes et al, 1992; Tada, 1991; Weller & Behl, 2015). These petrophysical variations are essentially due to extensive dissolution of opal‐A and lesser impacts from precipitation of pore‐filling opal‐CT (Varkouhi, Cartwright, et al, 2020, Varkouhi, Tosca, et al, 2020; Wrona, Jackson, et al, 2017; Wrona, Magee, et al, 2017; Wrona, Taylor, et al, 2017). A marked reduction in the content of biosilica frustules following dissolution significantly reduces the sediment stability, which makes its framework susceptible to abrupt collapse, sharp reduction in intergranular and intragranular porosities, and an appreciable interstitial‐water expulsion (Varkouhi, Cartwright, et al, 2020, Varkouhi, Tosca, et al, 2020).…”

“…A lesser role for subsequent formation of diagenetic opal on physical‐property response than pioneer dissolution of biogenic opal is commonly due to the restraining influences of authigenic phases other than opal‐CT (e.g. authigenic clays) that precipitate mostly concomitant with silica diagenesis and restrict substantial formation of opal‐CT through affecting the solubility and chemical kinetics of pore‐water silica (Emerson & Hedges, 2008; Loucaides et al, 2010; Varkouhi, Tosca, et al, 2020, Varkouhi et al, 2021; Varkouhi & Wells, 2020). The prominent increase in sediment bulk density over a vertical extent of a few metres results from porosity reduction, and compressional velocity increases owe to cementation of pore space by opal‐CT (Varkouhi, Cartwright, et al, 2020).…”