Biologically induced initiation of Neoproterozoic snowball-Earth events

Abstract

The glaciations of the Neoproterozoic Era (1,000 to 542 MyBP) were preceded by dramatically light C isotopic excursions preserved in preglacial deposits. Standard explanations of these excursions involve remineralization of isotopically light organic matter and imply strong enhancement of atmospheric CO 2 greenhouse gas concentration, apparently inconsistent with the glaciations that followed. We examine a scenario in which the isotopic signal, as well as the global glaciation, result from enhanced export of organic matter from the upper ocean into anoxic subsurface waters and sediments. The organic matter undergoes anoxic remineralization at depth via either sulfate-or iron-reducing bacteria. In both cases, this can lead to changes in carbonate alkalinity and dissolved inorganic pool that efficiently lower the atmospheric CO 2 concentration, possibly plunging Earth into an ice age. This scenario predicts enhanced deposition of calcium carbonate, the formation of siderite, and an increase in ocean pH, all of which are consistent with recent observations. Late Neoproterozoic diversification of marine eukaryotes may have facilitated the episodic enhancement of export of organic matter from the upper ocean, by causing a greater proportion of organic matter to be partitioned as particulate aggregates that can sink more efficiently, via increased cell size, biomineralization or increased C∶N of eukaryotic phytoplankton. The scenario explains isotopic excursions that are correlated or uncorrelated with snowball initiation, and suggests that increasing atmospheric oxygen concentrations and a progressive oxygenation of the subsurface ocean helped to prevent snowball glaciation on the Phanerozoic Earth.carbon isotopes | CO2

Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth

“…The Re-Os geochronology presented here suggests that the Islay anomaly returns to positive δ 13 C carb values by ∼732 Mya, well before the onset of glaciation at ∼717 Mya (4). Therefore, this anomaly cannot be mechanistically linked to the onset of glaciation as has been previously proposed (54)(55)(56). Moreover, none of these successions host any evidence for glaciation before the Islay anomaly.…”

Uncovering the Neoproterozoic carbon cycle

“…3c-d 30 , which is difficult to reconcile with the close stratigraphic association of at least some of the excursions and glacial deposits 27 . Another possibility is that the oxidation of reduced carbon is accomplished with SO 4 2-or Fe-oxides as an electron acceptor 31 , rather than O 2 , which may allow for some stabilization of pCO 2 through the effect on alkalinity 25 . If the duration of these events is found to be longer, then carbon mass input arguments would be modified accordingly, however the relevant reservoirs interacting with the surface C-cycle remain unchanged.…”

The Proterozoic Record of Eukaryotes

“…Was the diversification of crown group eukaryotes and origin of biomineralization (Parfrey et al 2011 driven by tectonically modulated changes in ocean chemistry (e.g., Halverson et al 2010;Squire et al 2006)? Did increased sinking of newly evolved mineralized tests drive changes in the biogeochemical cycles and climate (Tziperman et al 2011)? What were the effects of global glaciation (a.k.a.…”