Shift in limiting nutrients in the late Ediacaran–early Cambrian marine systems of South China
Introduction
The Ediacaran to Cambrian periods are critical periods in the history of the Earth. Fossil and sedimentary records in this interval have suggested that the rapid diversification and mass extinction of metazoans and carbon-cycle perturbation is closely related to long-term ocean oxygenation with spatiotemporal redox fluctuation (Ding et al., 1996; Och and Shields-Zhou, 2012; Mángano and Buatois, 2014; Tahata et al., 2015). Ocean nutrient cycles are key factors to control ocean primary production and productivity-driven shallow water anoxia as suggested for the Cretaceous oceanic anoxic events (Monteiro et al., 2012). Thus, ocean nutrient cycles and their relations to the redox state and primary production in the ocean must be reconstructed during the Ediacaran to Cambrian periods.
Nitrogen is a major component of cellular material and, together with phosphorous, controls primary production in the ocean over different time scales (Tyrrell, 1999). The nitrogen isotopic composition (δ15N) of sedimentary rocks is a common tool for the reconstruction of past nitrogen cycles. Apart from in oceanic regions in which active nitrogen fixation occurs, nitrate supplied from subsurface waters is a major substrate for new production in the modern ocean (e.g., Eppley and Peterson, 1979). This nitrate (collected at depths from 100 m to 500 m) reflects both the mean ocean nitrate δ15N value (5‰) and the regional isotopic imprint of water-column denitrification and/or nitrogen fixation (Rafter et al., 2012; Mollier-Vogel et al., 2012; Yoshikawa et al., 2018). In the equatorial Pacific and the Indian Ocean, δ15N values of surface sediments are negatively correlated with the nitrate content in the surface waters and, presumably, records relative nitrate utilization in the surface waters (Altabet and Francois, 1994). Coastal sites having high export production and/or good organic matter preservation, δ15N values of surface sediments are almost similar to (with some offset) that of the sub-euphotic zone nitrate transported via ocean current from the open ocean (Galbraith et al., 2008). Based on these observations, sedimentary δ15N records in the Phanerozoic have been used to reconstruct spatial and temporal variations in (1) relative nitrate utilization (or the nitrate content) in the surface ocean (Francois et al., 1997; Rafter and Charles, 2012), (2) intensity of water-column denitrification (Ganeshram et al., 1995; Galbraith et al., 2013; Saitoh et al., 2014b), or (3) global average of δ15N value of nitrate (Kienast, 2000; Algeo et al., 2014).
The Ediacaran to Cambrian successions are fossiliferous and developed along the Yangtze Platform, South China, providing an excellent opportunity to conduct paleoceanographical and paleontological studies (Fig. 1). Over the past six years, nitrogen isotope ratios of sedimentary rocks are reported from different depositional settings in the Yangtze Platform (Cremonese et al., 2013, Cremonese et al., 2014; Ader et al., 2014; Kikumoto et al., 2014; Wang et al., 2013, Wang et al., 2018b; Wang et al., 2015, Wang et al., 2018a; Wang et al., 2017; Zhang et al., 2017; Xiang et al., 2018; Lan et al., 2019). The δ15N values are ~5‰ in the Ediacaran strata before the largest negative carbonate carbon isotope excursions (known as the Shuram δ13Ccarb Excursion) and −5–2‰ in Cambrian Stage 2. With these profiles and other geochemical (organic and carbonate carbon isotopes) and paleontological records, some studies have suggested potential links among nitrogen cycle, ocean redox state, as well as ecological and biological evolution during the Ediacaran to Cambrian transition (the E/C transition) (Kikumoto et al., 2014; Wang et al., 2018a). However, the underlying mechanisms to drive the difference in δ15N values between the early–middle Ediacaran and the early Cambrian strata remains unexplored (Cremonese et al., 2014; Kikumoto et al., 2014; Johnson et al., 2017; Wang et al., 2018a). To enhance our understanding of this issue, it is important to investigate the relations among stratigraphic profiles of 1) nitrogen isotopes, 2) geochemical proxies of P and Mo availability in seawater, as well as ocean redox state. Comparison of these profiles allows us to infer limiting nutrients of ocean primary production during the Ediacaran to Cambrian periods.
This study reports the stratigraphic profiles of nitrogen and organic carbon isotopes for three sections of sediments in South China that were deposited in the shelf, continental slope, and deep basin environments from the Ediacaran to early Cambrian periods. The data include high-resolution nitrogen isotope profiles of the shelf section at the beginning of Cambrian Stage 3 in the Cambrian Series 2 when the third phase of metazoan diversification (the most prominent phase of “Cambrian Explosion”; Shu et al., 2014) occurred in the Yangtze shelf environments (Zhao et al., 2012). Combining this data with previously reported data (δ15N, C, P, Mo), we discuss shift in limiting nutrients in the late Ediacaran–early Cambrian marine systems and its role in shallow water anoxia in the early Cambrian with implications for the extinction and diversification of metazoans around the Terreneuvian/Cambrian Series 2 boundary.
Section snippets
Geological outline
Neoproterozoic to Cambrian strata are widely distributed in South China (Fig. 1A) and were deposited in a rift basin between the Yangtze and Cathaysia blocks (Li, 1998). The depositional environments in the Ediacaran and Early Cambrian periods ranged from shallow marine to deep-sea environments (Jiang et al., 2003; Zhu et al., 2003).
Analytical methods
Rock powders were prepared by micro-drilling small holes, 1 cm across, on fresh surfaces of the drill core samples obtained at the Three Gorges section and of outcrop samples collected at the Fengtan and Wuhe sections, to avoid secondary alterations including weathering, veins, and visible recrystallization. To completely eliminate carbonate minerals from the samples, about 500 mg of the powdery samples was first digested overnight in 10–15 mL of concentrated hydrochloric acid (conc. HCl) at
Chemostratigraphic profiles of the Three Gorges section
Stratigraphic profiles of δ13Corg and δ15N values, TOC and TN contents, and C/N ratios from the Shuijingtuo to Shipai Formations at the Three Gorges section are shown in Fig. 2. Black shales and argillaceous dolostones in the samples from the Shuijingtuo Formation display higher TOC contents compared with the samples from the Shipai Formation. TOC contents in middle black shale sequence and upper carbonate rock sequence of the Shuijingtuo Formation show upward decreasing trends from 1.5% to
Effects of acid treatment on C/N ratio and δ15N values
HCl treatment did not change C/N ratio beyond 20% for 14 of 15 samples (Table 4). For comparison, acidification of modern samples leads to change in C/N ratio by −10 to −15% for sediments and <±10% for soils (Kennedy et al., 2005; Brodie et al., 2011). HCl treatment did not change δ15N values beyond 1‰ for 14 of 15 samples. Therefore, Section 5.2 focuses on the δ15N value and C/N ratio of HCl-treated samples to extract palaeoceanographic signals assuming that HCl treatment may not likely cause
Conclusions
The following conclusions have been newly obtained by combining the nitrogen isotope profiles from this study with previously reported nutrient profiles (δ15N, P, and Mo).
- 1)
The relatively invariable positive δ15N values (5–6‰) of the lower to middle Doushantuo Formation across the shelf-basin transection suggest the existence of a stable nitrate reservoir with a δ15N value of 5‰ in the early–middle Ediacaran ocean (635 Ma–ca. 560 Ma), as for the modern ocean. Therefore, the ratio of water-column
Acknowledgments
Authors would like to thank Prof. Shigenori Maruyama at Tokyo Institute of Technology for helping discussion and sample collecting. Authors thank Enago (www.enago.jp) for the English language review. Comments by two anonymous reviewers and Prof. Thomas Algeo greatly improved the manuscript. This study was supported by JSPS/MEXT (JP23340152, JP23654176, JP25610148), the Mitsubishi Foundation (T.K.), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB26000000),
References (118)
- et al.
A multilayered water column in the Ediacaran Yangtze platform? Insights from carbonate and organic matter paired δ13C
Earth Planet. Sci. Lett.
(2009) - et al.
Ocean redox structure across the Late Neoproterozoic oxygenation event: a nitrogen isotope perspective
Earth Planet. Sci. Lett.
(2014) - et al.
Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmosphere chemistry?
Precambrian Res.
(1999) - et al.
Nitrogen isotopic fractionation in the synthesis of photosynthetic pigments in Rhodobacter capsulatus and Anabaena cylindrica
Org. Geochem.
(2000) - et al.
Biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end-Permian mass extinction event
Earth Planet. Sci. Lett.
(2009) - et al.
Marine biogeochemical cycling during the early Cambrian constrained by a nitrogen and organic carbon isotope study of the Xiaotan section, South China
Precambrian Res.
(2013) - et al.
Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran-Cambrian transition in South China
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2014) - et al.
Early diagenesis of organic matter from sediments of the eastern subtropical Atlantic: evidence from stable nitrogen and carbon isotopes
Geochim. Cosmochim. Acta
(2001) - et al.
Concentration and natural stable isotope profiles of nitrogen species in the Black Sea
Mar. Chem.
(2008) - et al.
Diagenetic overprint to a negative carbon isotope anomaly associated with the Gaskiers glaciation of the Ediacaran Doushantuo Formation in South China
Precambrian Res.
(2016)
Nitrogen in Past Marine Environments
Carbon isotope chemostratigraphy of a Precambrian/Cambrian boundary section in the Three Gorge area, South China: prominent global-scale isotope excursions just before the Cambrian Explosion
Gondwana Res.
Irreversible change of the oceanic carbon cycle in the earliest Cambrian: High-resolution organic and inorganic carbon chemostratigraphy in the Three Gorges area, South China
Precambrian Res.
The δ13C excursions spanning the Cambrian explosion to the Canglangpuian mass extinction in the three Gorges area, South China
Gondwana Res.
Carbon isotope variability across the Ediacaran Yangtze platform in South China: implications for a large surface-to-deep ocean δ13C gradient
Earth Planet. Sci. Lett.
Organic carbon isotope constraints on the dissolved organic carbon (DOC) reservoir at the Cryogenian-Ediacaran transition
Earth Planet. Sci. Lett.
Stratigraphy and paleogeography of the Ediacaran Doushantuo Formation (ca. 635–551 Ma) in South China
Gondwana Res.
The origin of decoupled carbonate and organic carbon isotope signatures in the early Cambrian (ca. 542–520Ma) Yangtze platform
Earth Planet. Sci. Lett.
A highly redox-heterogeneous ocean in South China during the early Cambrian (?529-514 Ma): Implications for biota-environment co-evolution
Earth Planet. Sci. Lett.
Nitrogen isotope chemostratigraphy of the Ediacaran and early Cambrian platform sequence at Three Gorges, South China
Gondwana Res.
Evolution of the composition of seawater through geologic time, and its influence on the evolution of life
Gondwana Res.
Ediacaran chemostratigraphy and paleoceanography at a shallow marine setting in northwestern Hunan Province, South China
Precambrian Res.
An integrated chemostratigraphic (δ13C-δ18O-87Sr/86Sr-δ15N) study of the Doushantuo Formation in western Hubei Province, South China
Precambrian Res.
Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis
Geochim. Cosmochim. Acta
Increased productivity as a primary driver of marine anoxia in the Lower Cambrian
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Nitrogen isotope ratios of red tide organisms in the East China Sea. A characterization of biological nitrogen fixation
Mar. Chem.
Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency
Deep-Sea Res. I Oceanogr. Res. Pap.
Speciation and isotope ratios of nitrogen in fluid inclusions from seafloor hydrothermal deposits at 3.5 Ga
Earth Planet. Sci. Lett.
Physiological and isotopic characteristics of nitrogen fixation by hyperthermophilic methanogens: Key insights into nitrogen anabolism of the microbial communities in Archean hydrothermal systems
Geochim. Cosmochim. Acta
The Neoproterozoic oxygenation event: Environmental perturbations and biogeochemical cycling
Earth Sci. Rev.
Organic nitrogen: sources, fates, and chemistry
New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China
Gondwana Res.
Chapter 19 - the Cambrian period
Nitrogen isotope chemostratigraphy across the Permian-Triassic boundary at Chaotian, Sichuan, South China
J. Asian Earth Sci.
The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area, South China
Precambrian Res.
In-situ analyses of phosphorus contents of carbonate minerals: reconstruction of phosphorus contents of seawater from the Ediacaran to early Cambrian
Gondwana Res.
Birth and early evolution of metazoans
Gondwana Res.
A euxinic southern North Atlantic Ocean during the Cenomanian/Turonian oceanic anoxic event
Earth Planet. Sci. Lett.
Carbon and oxygen isotope chemostratigraphies of the Yangtze platform, South China: decoding temperature and environmental changes through the Ediacaran
Gondwana Res.
Three-step modernization of the ocean: Modeling of carbon cycles and the revolution of ecological systems in the Ediacaran/Cambrian periods
Geosci. Front.
Paleobathymetric influence on the development of the late Ediacaran Yangtze platform (Hubei, Hunan, and Guizhou provinces, China)
Sediment. Geol.
Microbial ecology of the stratified water column of the Black Sea as revealed by a comprehensive biomarker study
Org. Geochem.
History of Neoproterozoic rift basins in South China: implications for Rodinia break-up
Precambrian Res.
Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: evidence from nitrogen and organic carbon isotopes
Precambrian Res.
Mo-total organic carbon covariation in modern anoxic marine environments: implications for analysis of paleoredox and paleohydrographic conditions
Paleoceanography
Icehouse-greenhouse variations in marine denitrification
Biogeosciences
Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization
Glob. Biogeochem. Cycles
Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman
Am. J. Sci.
Ammonium and nitrite oxidation at nanomolar oxygen concentrations in oxygen minimum zone waters
Proc. Natl. Acad. Sci.
Evidence for bias in measured δ15N values of terrestrial and aquatic organic materials due to pre-analysis acid treatment methods
Rapid Commun. Mass Spectrom.
Cited by (17)
Marine nitrogen cycling in the aftermath of the Marinoan Snowball Earth
2024, Palaeogeography, Palaeoclimatology, PalaeoecologyEarly Cambrian sedimentary rocks in South China: A link between oceanic oxygenation and biological explosion
2024, Earth-Science ReviewsOxygen increase and the pacing of early animal evolution
2024, Global and Planetary Change