Redox-dependent distribution of early macro-organisms: Evidence from the terminal Ediacaran Khatyspyt Formation in Arctic Siberia

https://doi.org/10.1016/j.palaeo.2016.08.015Get rights and content

Highlights

  • Chemostratigraphy of the terminal Ediacaran Khatyspyt Formation

  • Pyrite sulfur isotope excursion from ca. –20‰ to ca. + 50‰

  • Euxinia in the terminal Ediacaran ocean

  • Coupling between seawater redox and fossil distribution

Abstract

The Ediacaran Period witnessed the first appearance of macroscopic animal life in Earth's history. However, the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain, in part due to the dearth of integrated paleobiological and chemostratigraphic datasets. In this study, a comprehensive geochemical analysis was conducted on the fossiliferous Khatyspyt Formation in Arctic Siberia, in order to gain insights into the Ediacaran paleoenvironments. This study was designed to specifically address the relationship between paleoredox conditions and Ediacaran fossil occurrences in the Khatyspyt Formation. Our data reveal a dramatic shift in pyrite sulfur isotope compositions (δ34Spyrite) from ca. − 20‰ to ca. 55‰, and this shift is intriguingly associated with the first occurrence of Ediacara-type macrofossils at the studied section, suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms. Based on multiple lines of sedimentological and geochemical evidence, we propose that the development of oceanic euxinia — which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal Ediacaran Period — may have locally prohibited the colonization of Ediacara-type organisms and resulted in low δ34Spyrite values in the lower Khatyspyt Formation. In the middle and upper Khatyspyt Formation, progressive secular transition from euxinic to non-euxinic and more habitable conditions may have allowed for the colonization of Ediacara-type and other macro-organisms.

Introduction

The Ediacaran Period (ca. 635–541 Ma) holds the answers to key questions related to the origins of the modern Earth system. In particular, Ediacaran strata contain the planet's first unambiguous evidence of macroscopic metazoans and assemblages of “Ediacara-type” fossils (Xiao and Laflamme, 2009). Our present understanding of the origin of animals depends critically on the ability to interpret fossil impressions in siliciclastic sediments made by the Ediacara-type macro-organisms (Gehling, 1999, Narbonne, 2005, Fedonkin et al., 2007) and to document their spatial and temporal distribution (Grazhdankin, 2011, Grazhdankin, 2014, Narbonne et al., 2014). However, siliciclastic rocks, particularly coarse-grained sandstones, offer limited opportunities for biogeochemical reconstructions of the deep-time record. Fortunately, Ediacara-type macrofossils can also be preserved in carbonate rocks, including the Khatyspyt Formation in Siberia (Fedonkin, 1990, Nagovitsin et al., 2015, Rogov et al., 2015) and the Dengying Formation in South China (Ding and Chen, 1981, Sun, 1986, Xiao et al., 2005, Chen et al., 2014). Chemostratigraphic investigation of these fossiliferous carbonate successions can provide critical geochemical data that complement our understanding from siliciclastic successions.

Ediacaran animals and perhaps Ediacara-type macro-organisms are believed to be oxygen-breathing life forms (Cloud, 1968, Cloud, 1976, Xiao, 2014), therefore a putative rise in atmospheric oxygen during the Ediacaran Period (Derry et al., 1992, Kaufman et al., 1993) may have dictated their evolutionary trajectories and their environmental distribution. Compilations of redox-sensitive proxies at broad timescale suggest a general pattern of rising atmospheric oxygen levels during the late Proterozoic (Kah et al., 2004, Canfield et al., 2007, Kump, 2008, Kah and Bartley, 2011, Shields-Zhou and Och, 2011, Lyons et al., 2014, Planavsky et al., 2014, Liu et al., 2016), although oceanic anoxia has also been argued to remain persistent in many parts of the ocean even in the Ediacaran Period (Canfield et al., 2008, Sperling et al., 2015c, Reinhard et al., 2016, Sahoo et al., 2016). In addition, individual case studies on integrative chemostratigraphy and biostratigraphy of the terminal Ediacaran strata, including the Blueflower Formation in NW Canada (Johnston et al., 2013, Macdonald et al., 2013, Sperling et al., 2015a), the Nama Group in Namibia (Hall et al., 2013, Darroch et al., 2015, Wood et al., 2015), and the Dengying Formation in South China (Duda et al., 2014, Cui et al., 2016a) reveal dynamic redox histories in these depositional basins, suggesting a complex relationship between the emergence of macrometazoans and the putative Ediacaran oxygenation. To further test the various hypotheses about the relationship between the rise of animals and atmospheric oxygen levels, we carried out an integrative investigation of the terminal Ediacaran Khatyspyt Formation in northern Siberia, which contains a moderate diversity of Ediacara-type macrofossils and abundant carbonate rocks for chemostratigraphic analysis (Knoll et al., 1995, Pelechaty et al., 1996a). The goal of this study is to assess the effect of redox conditions on the distribution of early macro-organisms — particularly Ediacara-type macro-organisms — in the Khatyspyt Formation, using carbon, oxygen, sulfur, and strontium isotopes, as well as trace element concentrations.

Section snippets

Lithostratigraphy

The fossiliferous Khatyspyt Formation is well exposed along the right-hand side tributaries of the Olenek River (i.e., the Khorbusuonka and Kersyuke rivers) that drains the Olenek Uplift in the northeastern part of the Siberian platform, Republic of Sakha, Russia (Fig. 1, Fig. 2A) (Nagovitsin et al., 2015). Sedimentological observations from the studied Khatyspyt interval (0–130 m in Fig. 3) suggest an overall shelf marine environment with relatively deeper water depth compared with the Maastakh

Samples and analytical methods

The samples were collected from the Khatyspyt Formation at the 0601 (GPS: 71° 08′ 28.80″ N, 123° 52′ 23.97″ E) and 0605 (GPS: 71° 12′ 17.20″ N, 123° 39′ 35.43″ E) sections. The 0601 section is of particular interest because it has yielded an exceptionally preserved assemblage of diverse Ediacara-type macrofossils, representing the first appearance of these fossils in the Khatyspyt Formation (Fig. 3). Using regionally consistent occurrence of thick-bedded limestones and volcanic tuffs as marker

δ13Ccarb and δ18Ocarb data

Chemostratigraphic profiles of the Khatyspyt Formation reveal strong fluctuations in carbon and sulfur isotope compositions (Fig. 4, Fig. 5). The percent carbonate values of most limestone samples are high (approaching 100%) except the shaly interval in the 0605 section (Fig. 4A). Carbonate carbon isotope (δ13Ccarb) data show a positive excursion (up to ca. 5‰) in the intraclastic limestone interval of the 0601B section, and then decrease to lower values (Fig. 4B). The lower 45 m of the measured

Diagenetic evaluation

Multiple lines of evidence (Fig. 6) suggest that the measured samples from the Khatyspyt Formation are well preserved, and experienced little diagenetic alteration. In contrast with the proposed diagenetic alteration trend (Arthur, 2009, Knauth and Kennedy, 2009, Derry, 2010, Oehlert and Swart, 2014), which typically shows a positive correlation between δ13Ccarb and δ18Ocarb resulting from meteoric water alteration and organic carbon oxidation during fluid percolation, the Khatyspyt samples

Conclusions

Based on integrated litho-, bio-, and chemo-stratigraphy of the terminal Ediacaran Khatyspyt Formation in Arctic Siberia, a large positive δ34Spyrite excursion with a magnitude of ca. 70‰ is reported here for the first time in this time interval. This shift of δ34Spyrite values from − 20‰ to + 55‰ is closely coupled with the local appearance of Ediacara-type macrofossils in the studied section, suggesting an intriguing geobiological response of early metazoans to dynamic redox conditions. Based

Acknowledgments

We thank Konstantin Nagovitsin and Boris Kochnev for their assistance in the field; Mike Evans, Rebecca Ohly, Courtney Ray, Rebecca E. Plummer and Yongbo Peng for lab assistance in the UMD Paleoclimate CoLaboratory; and Richard Walker, Igor Puchtel, Jingao Liu and Katherine Bermingham for the guidance on strontium isotope analysis in the UMD TIMS Laboratory. We also thank James Farquhar, Roberta Rudnick, Xianguo Lang, Chuanming Zhou and Jon Husson for helpful discussion in the course of this

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      The age of the Khatyspyt Formation also has significant implications for the evolution and morphological changes in macroalgae during the late Ediacaran (Bykova et al., 2020). The Khatyspyt Formation has long been assumed to record deposition between ca. 560 and 550 Ma, approximately contemporaneously with the Miaohe Member (South China) and fossiliferous deposits of the White Sea area (e.g. Cui et al., 2016a). In fact, the only radiometric constraint available is a maximum age for intrusion of the volcanic breccia of the Tas-Yuryakh volcanic complex within the lower part of the Syhargalakh Formation (lower Kessyusa Group), which unconformably overlies the Khatyspyt and overlying Turkut formations.

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    Current address: United States Geological Survey, Menlo Park, CA 94025, USA.

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    Current address: Department of Geological Sciences, School of Earth, Energy & Environmental Sciences, Stanford University, CA 94305, USA.

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