Redox and nutrient cycling in the Mesoproterozoic Taoudeni Basin

Abstract

Following the Great Oxidation Event, current evidence suggests that much of the ocean remained anoxic throughout the Proterozoic, with oxygenation in deeper waters only becoming expansive at the end of the Precambrian. Previous models have suggested that the mid-Proterozoic, often known as the “boring billion” owing to an apparent stasis in environmental and evolutionary history, was characterized by pervasive euxinia, in contrast to dominantly ferruginous conditions found both before and after this period. However, more recent studies indicate that ocean redox was highly heterogenous during this “boring billion”, with dynamic cycling between oxic, ferruginous and sulphidic states, though data for the mid-Proterozoic remains relatively scarce. Ocean redox conditions are believed to exert a strong control on nutrient cycling, so influencing organic carbon production and burial, and, in turn, environmental oxygen levels. Therefore, in order to understand controls on environmental conditions during a potentially dynamic boring billion, and thus better understand the progression towards a biosphere more suited for animal evolution, detailed studies into oceanic redox chemistry and its influence on nutrient cycling are vital. This study of Fe-S-C systematics in 3 well preserved cores (S2, S3 and S4) from the 1.1 Ga Taoudeni Basin of Mauritania provides a rare glimpse into evolving redox chemistry during the second half of the “boring billion”, for which redox data is currently sparse. Earlier in the succession, where data is limited to S4, euxinia was prevalent in shallow coastal waters. Further up the succession, as sea level rises, euxinia persists in this part of the basin, with fluctuating, mostly anoxic, conditions in the shallower waters of S2. At the highest sea levels encountered in this study ferruginous conditions dominate in S2, while the mid-depths of S3 are oxic, and the likely deeper S4 appears mostly oxic, with possible ferruginous incursions. Following a drop in sea level limited data suggest oxic conditions across the shallower part of the basin. High organic C concentrations, at times exceptionally so, in the middle of the succession in S2 suggest this may have been an area of high productivity. High TOC contents in a fourth core, S1, suggest that, if correct, this area of high productivity could potentially have extended over 200km, or shifted in locus over this distance. However, probable metamorphic alteration associated with a dolerite sill evidenced by presence of AVS and high trace metal concentrations has rendered this core unsuitable for redox analysis. Enhanced organic C burial in S2 is associated with both ferruginous and euxinic conditions, suggesting that the development of euxinic conditions was not simply driven by organic C availability. P speciation is utilized to provide insight into redox-driven nutrient feedbacks. Results of P speciation suggest extensive drawdown of P in association with organic matter, although a fairly large proportion of total P was not extracted as part of the reactive (that assumed to have been biologically available) fraction. Comparison of TOC/reactive P to the Redfield ratio suggests efficient recycling of P back to the water column under both euxinic and ferruginous conditions, allowing continued high productivity and thus burial of organic C, especially in S2, where recycling of P also appears to have been efficient under oxic conditons. However, in S3 and S4, TOC/reactive P ratios are lower than the Redfield ratio, suggesting efficient trapping of P in the sediment, and suggesting that while very little P was extracted with the Fe oxide fraction, some P drawdown, later retained in other mineral phases, must have been associated with Fe oxides.