Redox-Controlled Ammonium Storage and Overturn in Ediacaran Oceans

Christian Hallmann,David A Fike,Nathan D Shapiro,Yuichiro Kashiyama,Yoshito Chikaraishi,Naohiko Ohkouchi,Roger E Summons,Emmanuelle Grosjean

doi:10.3389/feart.2021.706144

Abstract

As a key nutrient, nitrogen can limit primary productivity and carbon cycle dynamics, but also evolutionary progress. Given strong redox-dependency of its molecular speciation, environmental conditions can control nitrogen localization and bioavailability. This particularly applies to periods in Earth history with strong and frequent redox fluctuations, such as the Neoproterozoic. We here report on chlorophyll-derived porphyrins and maleimides in Ediacaran sediments from Oman. Exceptionally light δ15N values (&lt; –10‰) in maleimides derived from anoxygenic phototrophs point towards ammonium assimilation at the chemocline, whereas the isotopic offset between kerogens and chlorophyll-derivatives indicates a variable regime of cyanobacterial and eukaryotic primary production in surface waters. Biomarker and maleimide mass balance considerations imply shallow euxinia during the terminal Ediacaran and a stronger contribution of anoxygenic phototrophs to primary productivity, possibly as a consequence of nutrient ‘lockup’ in a large anoxic ammonium reservoir. Synchronous δ13C and δ15N anomalies at the Ediacaran–Cambrian boundary may reflect one in a series of overturn events, mixing ammonium and isotopically-light DIC into oxic surface waters. By modulating access to nitrogen, environmental redox conditions may have periodically affected Ediacaran primary productivity, carbon cycle perturbations, and possibly played a role in the timing of the metazoan radiation across the terminal Ediacaran and early Cambrian.

Highlights

An increase in algal diversity during the Ediacaran (Knoll et al, 2006) and the advent of metazoan organismic complexity (Love et al, 2009) have traditionally been attributed to enhanced oxygen availability (e.g., Catling et al, 2005) or the demise of widespread marine sulfidic conditions (e.g. Cohen et al, 2009) toxic to complex eukaryotes
In terms of understanding the role that bioavailable nitrogen has played during the later Neoproterozoic, most studies have focused on measuring bulk δ15N values (e.g., Ader et al, 2014; Zhang et al, 2017; Wang et al, 2017; Wang et al, 2018a,b; Chen et al, 2019) yet there are limits to the amount of information that can be gained from bulk δ15N signals
Given the conversion of DPEP to Etio during thermal maturation (Baker et al, 1987), we conclude that the observed VO-porphyrin dominance throughout the Ediacaran South Oman Salt Basin (SOSB) sedimentary sequence (Table 1) is not due to thermal destruction of the generally more labile Ni-porphyrins (Baker et al, 1987) but rather points to environmental conditions: sulfidic bottom waters in the SOSB may have periodically removed any available nickel by precipitation as insoluble sulfides (Lewan, 1984) and left an excess of vanadium to react with labile organic matter

Summary

Introduction

An increase in algal diversity during the Ediacaran (Knoll et al, 2006) and the advent of metazoan organismic complexity (Love et al, 2009) have traditionally been attributed to enhanced oxygen availability (e.g., Catling et al, 2005) or the demise of widespread marine sulfidic conditions (e.g. Cohen et al, 2009) toxic to complex eukaryotes. Ammonium in Ediacaran Oceans grained sediments (Reinhard et al, 2017) and an abundance of late Neoproterozoic peritidal phosphate deposits (Brasier, 1990) suggest that phosphorus was likely widely bioavailable in the Ediacaran ocean. In terms of understanding the role that bioavailable nitrogen has played during the later Neoproterozoic, most studies have focused on measuring bulk δ15N values (e.g., Ader et al, 2014; Zhang et al, 2017; Wang et al, 2017; Wang et al, 2018a,b; Chen et al, 2019) yet there are limits to the amount of information that can be gained from bulk δ15N signals.

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