Basin redox and primary productivity within the Mesoproterozoic Roper Seaway

Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum

“…In fact, Fe-S systematics and δ 34 S measurements throughout the Roper Group suggest inner and distal shelf depositional environments were likely oxic, while basinal shales record prolonged periods of euxinia (Shen et al, 2003). Trace metal concentrations from the same basinal shales corroborate basinal anoxia with extended periods of euxinia, but overlying surface waters were likely at least transiently oxic (Cox et al, 2016). The "bioinorganic bridge hypothesis" (Anbar and Knoll, 2002) links these micropaleontological and geochemical observations, as follows:…”

“…The facies-dependent trend in the Bangemall and Roper basins likely reflects primary nitrogen cycling rather than postdepositional alteration (see above), and so is interpreted as being consistent with cross-basin differences in nitrogen speciation, comparable to the Belt basin (Table 2; Stüeken, 2013). As noted in Section 1, Fe-S systematics, δ 34 S, and trace metal data also have facies-dependent trends in the Roper Group, providing evidence for an oxic near-shore water column, and anoxic/euxinic basinal deep waters overlain by at least transiently oxic surface waters (Shen et al, 2003;Cox et al, 2016). There are no equivalent data for the Bangemall Group, but a similar range of conditions appears to have been widespread in the Mesoproterozoic ocean (Sperling et al, 2015).…”

“…Sediments are predominantly siliciclastic, ranging from mudrock to sandstone, that were deposited in six progradational cycles (Abbott and Sweet, 2000). High degrees of pyritization, large fractionations in sulfur isotopes and relatively small molybdenum isotope fractionations in black shales indicate that anoxia was pervasive; euxinia was at least transient in deeper parts of the basin and was possibly common along continental margins globally at this time (Jackson and Raiswell, 1991;Shen et al, 2003;Arnold et al, 2004;Johnston et al, 2008;Kendall et al, 2009;Sperling et al, 2015;Cox et al, 2016). In many areas, the rocks are essentially unmetamorphosed, having never been exposed to temperatures above the oil window (Jackson et al, 1988).…”

“…1 shows a compilation of bulk nitrogen isotopic compositions from offshore marine environments, highlighting the decline between ~1.7 Ga and ~1.2 Ga or possibly later. This interval post-dates the proposed mid-Paleoproterozoic oxygen overshoot (~2.3-2.0 Ga, Bekker and Holland, 2012;Canfield et al, 2013;Partin et al, 2013;Hardisty et al, 2014) and has recently been identified as a time when atmospheric pO 2 may have dropped back to as little as 0.1% or as great as 4% (Zhang et al, 2016;Cox et al, 2016) of present atmospheric levels until a second, potentially protracted rise to nearer modern amounts across the Neoproterozoic/Paleozoic, possibly beginning at ~800 Ma Blamey et al, 2016). Statistical analysis of global Fe-speciation data indicates that while subsurface anoxia was widespread throughout the Proterozoic Eon, euxinia was disproportionately common in Mesoproterozoic oceans (Sperling et al, 2015), consistent with lower atmospheric oxygen levels.…”

The oxic degradation of sedimentary organic matter 1400 Ma constrains atmospheric oxygen levels

“…The carbon cycle of the Mesoproterozoic Era produced sedimentary organic carbon concentrations ranging from very low, nearly undetectable, to 20 wt % or more (e.g., Cox et al, 2016;Strauss et al, 1992;Zhang et al, 2015Zhang et al, , 2016 very similar to the range observed in modern sediments (e.g., Jahnke, 1996). However, in comparing organic carbon concentrations in modern and Mesoproterozoic Era sediments, one must consider the possibility that low concentrations of atmospheric oxygen could have inhibited the weathering of sedimentary organic carbon on land (e.g., Bolton et al, 2006;Daines et al, 2017), thus providing elevated concentrations of recycled ancient organic matter to marine sediments.…”