Redox-dependent distribution of early macro-organisms: Evidence from the terminal Ediacaran Khatyspyt Formation in Arctic Siberia
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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- 1
Current address: United States Geological Survey, Menlo Park, CA 94025, USA.
- 2
Current address: Department of Geological Sciences, School of Earth, Energy & Environmental Sciences, Stanford University, CA 94305, USA.