Continental erosion and the Cenozoic rise of marine diatoms

Abstract

Marine diatoms are silica-precipitating microalgae that account for over half of organic carbon burial in marine sediments and thus they play a key role in the global carbon cycle. Their evolutionary expansion during the Cenozoic era (66 Ma to present) has been associated with a superior competitive ability for silicic acid relative to other siliceous plankton such as radiolarians, which evolved by reducing the weight of their silica test. Here we use a mathematical model in which diatoms and radiolarians compete for silicic acid to show that the observed reduction in the weight of radiolarian tests is insufficient to explain the rise of diatoms. Using the lithium isotope record of seawater as a proxy of silicate rock weathering and erosion, we calculate changes in the input flux of silicic acid to the oceans. Our results indicate that the long-term massive erosion of continental silicates was critical to the subsequent success of diatoms in marine ecosystems over the last 40 My and suggest an increase in the strength and efficiency of the oceanic biological pump over this period.

Changes in the Si / P weathering ratio and their effect on the selection of coccolithophores and diatoms

“…The precise value of the weathering ratio was dependent on estimates of modern river fluxes of reactive P and dissolved Si, and their associated uncertainties. Overall, P and Si weathering fluxes increased over the past 40 My (Follmi, 1995;Cermeño et al, 2015). The Si/P weathering ratio increased 20 primarily across the Eocene/Oligocene (E/O) (~35-31 Ma), early Miocene (~23-17 Ma) and late Miocene (~12-5 Ma), and remained relatively high until the present (Fig.…”

“…The Summed Common Species Occurrence Rate (SCOR) of coccolithophores and diatoms was used as a proxy of plankton functional group dominance Cermeño et al, 2015). The SCOR is based on the assumption 5 that the more globally abundant a species is, the more likely it is to occur in a greater number of sampling sites.…”

“…Diatom dominate phytoplankton community biomass in temperate and polar seas, and coastal environments, where nutrient-rich deep waters episodically enter the euphotic zone (Margalef, 1978). When diatom blooms collapse, they sink rapidly into the deep ocean increasing the concentration of silica and C in the bottom waters and sediments (Tréguer and De La Rocha, 2013;Cermeño et al, 2015;Smetacek, 1999). This is why diatoms are considered to be important players in the cycling of silica and C (Cortese et 5 al., 2004;Falkowski et al, 2004).…”

“…To address this question, we used two complementary approaches. First, we analyzed sedimentary records of P burial rate and lithium (Li) isotopes, which 10 have been previously used to infer continental weathering fluxes of P and Si, respectively (Follmi, 1995;Cermeño et al, 2015). Second, our sediment-based data analysis was substantiated by an Earth system biogeochemical model (COPSE) (Bergman et al, 2004), which allows P and Si weathering fluxes to be simulated through time.…”

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

“…Both transitions are ascribed to biological innovations-the first to the evolution of silicification by radiolarians and sponges, and the second to the expansion of diatoms-and therefore biology's progressive ability to remove dSi from seawater. Superimposed on this long-term decline in ocean dSi, transient excursions in the ocean dSi inventory have been proposed in response to periods of extensive volcanism (particularly the emplacement of large igneous provinces, LIPs) (Ritterbush et al, 2014Kidder and Tomescu, 2016), or orogenesis (e.g., Cermeño et al, 2015). One of the lines of evidence supporting these interpretations is a change in the sedimentary facies associated with sponges in the rock or sediment record (e.g., Kidder and Tomescu, 2016).…”