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

Abstract

Understanding the delayed rise of eukaryotic life on Earth is one of the most fundamental questions about biological evolution. Numerous studies have presented evidence for oxygen and nutrient limitations in seawater during the Mesoproterozoic era, indicating that open marine settings may not have been able to sustain a eukaryotic biosphere with complex, multicellular organisms. However, many of these data sets represent restricted marine basins, which may bias our view of habitability. Furthermore, it remains untested whether rivers could have supplied significant nutrient fluxes to coastal habitats. To better characterize the sources of the major nutrients nitrogen and phosphorus, we turned to the late Mesoproterozoic Paranoá Group in Brazil (~1.1 Ga), which was deposited on a passive margin of the São Francisco craton. We present carbon, nitrogen and sulphur isotope data from an open shelf setting (Fazenda Funil) and from a brackish-water environment with significant riverine input (São Gabriel). Our results show that waters were well-oxygenated and nitrate was bioavailable in the open ocean setting at Fazenda Funil; the redoxcline appears to have been deeper and further offshore compared to restricted marine basins elsewhere in the Mesoproterozoic. In contrast, the brackish site at São Gabriel received only limited input of marine nitrate and sulphate. Nevertheless, previous reports of acritarchs reveal that this brackish-water setting was habitable to eukaryotic life. Paired with previously published cadmium isotope data, which can be used as a proxy for phosphorus cycling, our results suggest that complex organisms were perhaps not strictly dependent on marine nutrient supplies. Riverine influxes of P and possibly other nutrients likely rendered coastal waters perhaps equally habitable to the Mesoproterozoic open ocean. This conclusion supports the notion that eukaryotic organisms may have thrived in brackish or perhaps even freshwater environments.

Exploring the Effects of Residence Time on the Utility of Stable Isotopes and S/C Ratios as Proxies for Ocean Connectivity

“…For example, sulfate concentrations in the Archean ocean may have been over 3 orders of magnitude lower than today, and nitrate was probably limited to rare oxygen oases in surface waters. , At that time, sulfate and nitrate would likely have acted as nonconservative species in seawater with markedly shorter residence times and response times to nonmarine conditions. Both nitrate and sulfate concentrations increased in seawater around the time of the Paleoproterozoic Great Oxidation Event , and probably reached modern levels in the Neoproterozoic or Phanerozoic during the second rise of oxygen. , In a previous study, we showed that late Proterozoic marine sulfate levels (at ∼1.1 Ga) were high enough to leave a significant mark in brackish environments, where trace elements and nitrogen isotopes revealed a nonmarine influence . Hence the modern hierarchy of proxy response time was perhaps established by that point; however, care is nevertheless required in deep-time studies at variable stratigraphic resolution.…”

“… 54 , 55 In a previous study, we showed that late Proterozoic marine sulfate levels (at ∼1.1 Ga) were high enough to leave a significant mark in brackish environments, where trace elements and nitrogen isotopes revealed a nonmarine influence. 56 Hence the modern hierarchy of proxy response time was perhaps established by that point; however, care is nevertheless required in deep-time studies at variable stratigraphic resolution.…”

A stable and moderate nitrate pool in largely anoxic Mesoproterozoic oceans and implications for eukaryote evolution

“…7. Compilation of sedimentary δ 15 N data through the Mesoproterozoic with data from ~1560Ma, ~1460 Ma and ~1380 Ma Yanliao Basin, China(Luo et al, 2015;Wang et al, 2020b;Shi et al, 2021; Zhang et al, 2021 and this study), ~1500Ma Bangemall basin and ~1450 Ma Roper Basin, Australia(Koehler et al, 2017), ~1400 Ma Belt Basin, USA(Stüeken, 2013), ~1200 Ma Vindhyan Basin, India(Gilleaudeau et al, 2020), ~1100 Ma Paranoá Group, Brazil(Stüeken et al, 2021) and~1048 Ma Borden Basin, Canada (Hodgskiss et al, 2020. The orange area represents the typical range of nitrogen fixation.…”

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

“…LOWESS analysis of the lowest quartile data revealed a stepwise increase in δ 15 N values around 800 Ma, from less than 2 to ~3‰. The deviation in the 1.1- to 1.2-Ga time bin is probably related to sampling bias: data in this time bin come from only one section that was deposited in a well-oxygenated basin ( 67 ). Because of the scarcity of data in the 800- to 900-Ma time bin, the bootstrapped LOWESS result indicates that the rise in δ 15 N could have started as early as 900 Ma, but a changepoint detection analysis revealed that the most significant change in the mean and variance of the lowest 25% δ 15 N data occurred at ca.…”