Coupled Nitrate and Phosphate Availability Facilitated the Expansion of Eukaryotic Life at Circa 1.56 Ga

Abstract

Recent geochemical and paleontological studies have revealed a significant ocean oxygenation episode and an evolutionary leap of eukaryotes at the onset of the Mesoproterozoic. However, the potential role of nitrogen availability and its interaction with other nutrients in these environmental and biological events have not been investigated. Here we present an integrated study of nitrogen isotopes (δ 15 N), organic carbon isotopes (δ 13 C org ), and major and trace element concentrations from Member III of the Gaoyuzhuang Formation in the central North China Craton where the earliest macroscopic multicellular eukaryotic fossils were reported. The enrichments of redox-sensitive elements (Mo, U, and V), coupled with Mo-U covariations, δ 13 C org , and I/(Ca + Mg), indicate that the Gaoyuzhuang Member III in the study area was deposited in largely suboxic-anoxic environments with ephemeral occurrences of euxinia. These data reinforce previous inferences of a strongly redox stratified ocean during the early Mesoproterozoic, but a pulsed oxygenation event may have resulted in deepening of the chemocline. The high δ 15 N values from the study section are interpreted as a result of aerobic N cycling and the presence of a fairly stable nitrate pool in the surface oxic layer, possibly due to the combined effects of oxygenation and low primary productivity. Increased availability of nitrate could have contributed to the expansion of eukaryotic life at this time. However, our data also suggest that nitrate alone was not the only trigger. Instead, this evolutionary leap was likely facilitated by multiple environmental factors, including a rise in O 2 levels and increasing supplies of phosphorus and other bio-essential trace elements.Plain Language Summary The oldest macroscopic eukaryotes (algae) first appeared in ancient oceans about 1.56 billion years ago. Before that, only simple and microscopic fossils have been discovered. The reason behind this evolutionary leap is, however, still puzzling scientists. Here we use multiple proxies to study the change of nutrient and oxygen levels during this critical period. Our results demonstrate the availability of the nutrient nitrogen (in form of nitrate) in the oxic surface ocean owing largely to restrictive consumption by organic carbon decomposition, providing favorable N substance for bio-utilization. Furthermore, we also found evidence for the joint increase of O 2 level, seawater phosphorus, and some bio-essential trace elements along with the occurrence of these earliest algae. Our study provides important clues for untangling the triggers of biological innovation at circa 1.56 Ga.

Nitrate limitation in early Neoproterozoic oceans delayed the ecological rise of eukaryotes

“…S6 and S7). Considering that oxic oases may have been present in Mesoproterozoic oceans and nitrate supplies may have been locally abundant ( 63 ), such isotopic heterogeneity was likely accentuated. Because the focus of this study was on the ecological rise of eukaryotes on a global scale, we are more interested in time intervals and regions where nitrate availability could have limited eukaryote success, rather than local environments where eukaryotes may have managed to survive.…”

A Pulsed Oxygenation in Terminal Paleoproterozoic Ocean: Evidence From the Transition Between the Chuanlinggou and Tuanshanzi Formations, North China

“…1.56 Ga). These Gaoyuzhuang macrofossils are also associated with a short marine oxygenation evidenced by multiple redox proxies (Shang et al., 2019; Wang et al., 2020; Zhang et al., 2018). Therefore, pulsed oxygenation may have been a significant evolutionary driver for early eukaryotes in the mid‐Proterozoic ocean, whereby increased oxygen levels provided a more hospitable environment for the development of macroscopic form and multicellularity in early eukaryotes (Mukherjee et al., 2018).…”

Contrasting nutrient availability between marine and brackish waters in the late Mesoproterozoic: Evidence from the Paranoá Group, Brazil

“…Therefore, this basinal gradient of nitrogen isotope ratios has been interpreted as evidence that nitrate bioavailability was restricted to shallow waters during the mid-Proterozoic, imposing an additional throttle on the radiation of eukaryotic life (Koehler et al, 2017;Stüeken, 2013). The relatively high values of +7‰ on average from the subtidal setting at Fazenda Funil in the Paranoá Group represent an outlier and are more akin to observations from the Jixian Group (1.56 Ga) (Wang et al, 2020). In the latter case, these values were interpreted as evidence for low biological productivity, which allowed nitrate to accumulate in surface waters further offshore than in other basins.…”