Abstract
Abstract. It has been hypothesized that the evolution of animals during the Ediacaran–Cambrian transition stimulated the burial of phosphorus in marine sediments. This assumption is centrally based on data compilations from marine sediments deposited under oxic and anoxic bottom waters. Since anoxia excludes the presence of infauna and sediment reworking, the observed differences in P burial are assumed to be driven by the presence of bioturbators. This reasoning however ignores the potentially confounding impact of bottom-water oxygenation on phosphorus burial. Here, our goal is to test the idea that bioturbation increases the burial of organic and inorganic phosphorus (Porg and Pinorg, respectively) while accounting for bottom-water oxygenation. We present solid-phase phosphorus speciation data from salt marsh ponds with and without bioturbation (Blakeney salt marsh, Norfolk, UK). In both cases, the pond sediments are exposed to oxygenated bottom waters, and so the only difference is the presence or absence of bioturbating macrofauna. Our data reveal that the rate of Porg and Pinorg burial are indistinguishable between bioturbated and non-bioturbated sediments. A large terrestrial fraction of organic matter and higher sedimentation velocity than generally found in marine sediments (0.3 ± 0.1 cm yr−1) may partially impact these results. However, the absence of a clear effect of bioturbation on total P burial puts into question the presumed importance of bioturbation for phosphorus burial.
Highlights
The evolution of animals near the Ediacaran–Cambrian transition (∼ 542 Myr ago) was a major evolutionary event in Earth’s history (Mangano and Buatois, 2017; Meysman et al, 2006; Wood et al, 2019)
Values of Corg : Ntot range from 9 to 15, with an average value of 12 (Fig. 2k). These values are slightly higher than expected for sediments from fully marine settings (Corg : Ntot < 10; Burdige, 2006) but are consistent with sediments from temperate salt marshes, which generally have Corg : Ntot values of 10 or higher (Spivak et al, 2018)
Corg : P associated with organic matter (Porg) values averaged around 500 in both the non-bioturbated and bioturbated sediment cores (Fig. 2g). These Corg : Porg values are higher than expected for marine sediments with similar sedimentation rates underlying oxygenated waters (Corg : Porg of ∼ 200; Slomp and Van Cappellen, 2007) and are more representative of low-oxygen and anoxic marine environments (Corg : Porg = 300–700; Slomp and Van Cappellen, 2007)
Summary
The evolution of animals near the Ediacaran–Cambrian transition (∼ 542 Myr ago) was a major evolutionary event in Earth’s history (Mangano and Buatois, 2017; Meysman et al, 2006; Wood et al, 2019). Benthic animals developed the ability to burrow (the so-called “burrowing revolution”; Meysman et al, 2006), which profoundly changed the geochemical cycling and burial of elements in the seafloor (Aller, 1977; McIlroy and Logan, 1999; Meysman et al, 2006). Bio-irrigation can promote aerobic respiration by flushing oxygenated bottom waters into deeper anoxic horizons (Archer and Devol, 1992; van de Velde and Meysman, 2016), whereas bio-mixing transports fresh organic from the sediment–water interface into the anoxic zone, stimulating anaerobic mineralization pathways (Berner and Westrich, 1985; van de Velde and Meysman, 2016). Via bio-mixing, bioturbating organisms transport P that is adsorbed on iron oxides
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