An emerging picture of Neoproterozoic ocean chemistry: Insights from the Chuar Group, Grand Canyon, USA

Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

“…(Because fixers will be outcompeted by non-fixers where NO 3 À is available, we only consider model results from the nitrogen-fixation case when photic zone nitrate is lower in concentration than the value required for Redfield stoichiometry with phosphate). We include the nitrification of aerobically remineralized organic nitrogen (anaerobically remineralized organic nitrogen is assumed to remain as NH 4 þ ), which acts as an input of NO 3 À to the upwelling waters, and (via eddy diffusion) to the photic zone. Consequently, oxic conditions below the photic zone can, by regenerating and oxidizing organic nitrogen, cause NO 3 À concentration to rise in the photic zone, meaning that the occurrence of nitrogen fixation is no longer The results we show below constitute an analysis of the sensitivity that the upwelling zone's nutrient composition has to the rate of advective upwelling from intermediate depths B (including the other advective upwelling parameter A in the analysis does not qualitatively alter the results).…”

“…E nrichments in reactive iron are a consistent feature of marine sediments throughout most of the Proterozoic [1][2][3][4] , indicating persistent ocean anoxia well after Earth's first major rise in atmospheric oxygen (the Great Oxidation Event) at B2.32 Ga (ref. 5).…”

“…2), in which we resolve export production and the apportioning of its respiration between various potential electron acceptors (full model description given in the Supplementary Methods and in the original paper by Canfield 8 ). We focus on the nutrient composition and mass balance of the upwelling zone, into which newly fixed organic carbon sinks from the photic zone, and is subsequently respired in a manner dictated by electron-acceptor availability, according to the hierarchy in free energy yield O 2 4NO 3 À 4SO 4 2 À . (In the context of the minimalist approach taken here, we neglect Fe(III) respiration, primarily because it is difficult to envisage any unambiguous difference in its magnitude between the N 2 fixation and non-fixation cases on which we focus).…”

“…S1), we include the explicit possibility of electron-acceptor limitation of denitrification, and separate new production into components based on reductive assimilation of NO 3 À and on direct assimilation of NH 4 þ . Our results constitute a 'snapshot' of a simple, chemically continuous coastal upwelling region in a context representative of the Proterozoic ocean.…”

Dynamic redox conditions control late Ediacaran metazoan ecosystems in the Nama Group, Namibia

“…This has variously been interpreted as being due to oxidation of a substantial reservoir of organic carbon dissolved in the deep ocean (Rothman et al, 2003;Fike et al, 2006), to a large flux of methane released from clathrates (Bjerrum and Canfield, 2011), or to diagenetic phenomena (Derry, 2010). The models of Bristow and Kennedy (2008), however, suggest that there were not enough oxidants available for the model proposed by Fike et al (2006), and thus that the Shuram could not have represented a large scale oxidation event.Indeed, the global response of ocean redox chemistry to rising oxygen levels through this period has been shown to be complex (Fike et al, 2006;Canfield et al, 2008;Johnston et al, 2010Johnston et al, , 2012bSperling et al, 2013a) A c c e p t e d M a n u s c r i p t 6 state existed until at least ~580 Ma, and beyond in certain areas (Canfield et al, 2008;Planavsky et al, 2011;Poulton and Canfield, 2011), whereas surface-water oxygenation is thought to be a near-continuous feature throughout the latter half of the Ediacaran (Canfield et al, 2008). Indeed some have argued that pervasive and persistent oxygenation of the deep ocean did not occur until the later Palaeozoic (e.g.…”