Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene‐Oligocene

Abstract

Silicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23 Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from −3.16 to +0.18‰ and from −0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37 Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past.

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

“…Secondly, the role of radiolarians and sponges in modifying oceanic DSi concentrations has been questioned (Kidder and Tomescu, 2016) with shifts in the global distribution of radiolarians and sponges preserved in sediments related to changes in oceanic DSi concentrations. Finally, Fontorbe et al (2017) suggest that the superior competitive ability of diatoms for DSi relative to other siliceous plankton such as radiolarians, likely rapidly reduced DSi concentrations early in their evolution as they increased in abundance to the low levels of DSi observed in the oceans today (Tréguer and De La Rocha, 2013). These changes occurred due to the innovation of more sophisticated and versatile DSi uptake mechanisms especially by diatoms (Thamatrakoln and Hildebrand, 2008;Durkin et al, 2016;Marron et al, 2016).…”

Response to Referee #1

“…For comparison, Hendry et al (2011) and Reynolds et al (2007) report δ 30 Si = −3.37 ± 0.17 ‰ and δ 30 Si = 1.26 ± 0.20 ‰ for LMG-08 and Diatomite respectively. The new seawater standard ALOHA deep was analysed as an additional quality check and yielded values within error of those obtained during an interlaboratory study (Grasse et al, 2017) 0.12 ‰ (2 SD, n = 4). The δ 29 Si and δ 30 Si of all seawater and sponge samples are consistent with the kinetic mass fractionation law (Reynolds et al, 2007); i.e.…”

Assessing the Potential of Sponges (Porifera) as Indicators of Ocean Dissolved Si Concentrations

“…Sponge silicon isotope ratios are becoming more widely used as a proxy for pastdSi concentrations (e.g., Fontorbe et al, 2017). Though not widely used, "mutual-range" approaches (Sinka and Atkinson, 1999), modern analog techniques (MAT; Lytle and Wahl, 2005), or artificial neural networks (Racca et al, 2007) may prove fruitful palaeoecological complements.…”

“…To date, dSi reconstruction using sponges has been achieved by exploring morphometric relationships of fresh-water spicules through dSi gradients (Kratz et al, 1991). More recently, silicon isotope analysis of sponge spicules has exploited the observation that the fractionation of stable Si isotopes between ambient dSi and the biogenic Si in spicules is a function of dSi concentration in the surrounding environment (Hendry et al, 2010(Hendry et al, , 2011Wille et al, 2010;Hendry and Robinson, 2012;Fontorbe et al, 2016Fontorbe et al, , 2017. The development of a complementary palaeoecological approach based on taxa assemblages could greatly extend the understanding of dSi changes in past and present oceans.…”