Proterozoic Ocean Chemistry and Evolution: A Bioinorganic Bridge?

Abstract

Recent data imply that for much of the Proterozoic Eon (2500 to 543 million years ago), Earth's oceans were moderately oxic at the surface and sulfidic at depth. Under these conditions, biologically important trace metals would have been scarce in most marine environments, potentially restricting the nitrogen cycle, affecting primary productivity, and limiting the ecological distribution of eukaryotic algae. Oceanic redox conditions and their bioinorganic consequences may thus help to explain observed patterns of Proterozoic evolution.

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

“…Although changes in water column stratification and/or global sulphate budgets 28 are, of course, relevant to euxinia, we argue that the 'electron tower' of respiratory-free energy yield 14 dictates that nitrate availability is a stronger first-order control. Similarly, the ongoing debate as to the significance of trace element limitation leading to stalls at various points in the nitrogen cycle 12,13,29,30 is not theoretically inconsistent with our arguments and may yet gain empirical validation. We suggest that as data become more available the empirical community should conduct a systematic cross-referencing exercise between the implicit occurrence of euxinia and the presence/absence of nitrogen fixation/nitrate-limited production.…”

“…Interestingly, some such OAEs appear on the basis of isotopic and biomarker evidence to have coincided with high rates of nitrogen fixation in the surface ocean 11 . A related link between Proterozoic euxinia and nitrogen cycle perturbation via trace element stress has been suggested 12 , but quantitative support for this 'bioinorganic bridge' hypothesis is presently lacking 13 .…”

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

“…With denitrification in the ocean interiors, N fixation in surface waters is needed to balance any P input to the oceans. Despite N isotopes having been used to argue for the antiquity of Mo-nitrogenase to a least 3.2 Ga (Stüeken et al, 2015a), prior to the Neoproterozoic rise in oxygen, the bulk marine Mo reservoir is likely to have been depressed owing to limited oxidative weathering, which in turn could have limited nitrogen fixation and overall primary productivity by oxygenic phototrophs in the Proterozoic (Anbar and Knoll, 2002;Scott et al, 2008;Reinhard et al, 2013). What role other biologically-essential nutrients played in ancient primary productivity is a current avenue of research (e.g., Robbins et al, 2013;Scott et al, 2013;Swanner et al, 2014).…”

Affinity, life cycle, and intracellular complexity of organic-walled microfossils from the Mesoproterozoic of Shanxi, China

“…All these factors might have driven the evolution of more complex eukaryotic cyst walls in the Mesoproterozoic. Environmentally induced light intensity, local nutrient supply, temperature changes, anoxia, and access to metabolically important metals (Anbar and Knoll, 2002;Klein et al, 1992;Lee, 2008;Parnell et al, 2012) could have driven the development of reproductive and resting stages. While the first true signs of micropredation (Porter et al, 2003) and biomineralized defense structures (Cohen et al, 2011) do not appear in the fossil record until Neoproterozoic, it is reasonable to assume that the first step in protection of early photosynthetic eukaryotes would have been the composite reinforcement of the organic cell wall, i.e.…”