Tracking Euxinia in the Ancient Ocean: A Multiproxy Perspective and Proterozoic Case Study

Abstract

The evolution and extinction of life are tied intimately to the oxygen state of the ocean, and particularly to the presence of anoxic and H2S-containing (euxinic) water on a global scale. Anoxia and euxinia were more common in the past, relative to today's <0.5% euxinic seafloor. We are able to constrain the distributions of these conditions through a combination of indirect numerical modeling methods and more direct geochemical proxies, with particular emphasis on Fe-S-Mo analysis of fine-grained siliciclastic rocks for the latter. Establishing the spatiotemporal pattern of oceanic redox is more difficult with very old rocks because of the limited availability of well-dated, well-preserved materials that span shallow and deep environments across time lines. Despite these difficulties, the multiple approaches synthesized in our case study point to global oxygen-deficiency in the deep ocean and perhaps euxinia during most, if not all, of the Proterozoic and likely extending into the early Paleozoic.

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

“…Unless inequality (18) is true (that is, unless there is a bioavailable nitrogen deficit in comparison to Redfield stoichiometry with phosphate) we set export production to zero for the fixation case. When this is realized, export production scales with phosphate upwelling into the photic zone:…”

“…This is responsible for the (rare) sulphide build-up that occasionally occurs during coastal upwelling events 7,8,16 . Here we explore the implications of these simple feedbacks in the context of the Proterozoic ocean, specifically in terms of the ongoing discussion 17,18 about what caused significant euxinia to occur against the backdrop of largely ferruginous conditions.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…Although there are a few instances of locally developed mid-Proterozoic euxinia (e.g., Shen et al, 2003;Brocks et al, 2005;Lyons et al, 2009), the idea of global-scale deep euxinia has also recently fallen out of favour. All available data that constrain the marine landscape in the midProterozoic ocean are consistent with only locally developed euxinic conditions along productive continental margins (e.g., in oxygen-minimum zones) or within intracratonic basins (Scott et al, 2008;Poulton et al, 2010;Planavsky et al, 2011;Lyons et al, 2012;Reinhard et al, 2013).…”

Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

“…In the modern oxygenated ocean, Mo is the most abundant transition metal (∼105 nM) (Collier, 1985;Lyons et al, 2009) and exists primarily as molybdate (MoO 4 2− ), with a residence time of ∼0.8 Myr (Emerson and Huested, 1991;Morford and Emerson, 1999). The major source of Mo to the ocean is oxidative weathering of crustal sulfides and black shales (Bertine and Turekian, 1973).…”