Neoproterozoic fossils and oxygenation, NW Canada Oxygen, facies, and secular controls on the appearance of Cryogenian and Ediacaran body and trace fossils in the Mackenzie Mountains of northwestern Canada Erik A. Sperling1,†, Calla Carbone2, Justin V. Strauss1, David T. Johnston1, Guy M. Narbonne2,§, and Francis A. Macdonald1,§ 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA 2Department of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario K7L 3N6, Canada ABSTRACT need not apply to all areas of a heterogeneous recent years, particularly the temporal relation- Ediacaran ocean, and stably oxygenated ship between oxygenation of the oceans and the The causes behind the appearance of conditions on geological time scales were not first appearances of these fossils. In an extensive abundant macroscopic body and trace fos- required for the appearance of these Avalon- sedimentary geochemistry study of fossilifer- sils at the end of the Neoproterozoic Era re- assemblage Ediacaran organisms. At least in ous strata of the Conception Group of eastern main debated. Iron geochemical data from the Mackenzie Mountains, the appropriate Newfoundland, Canfield et al. (2007) used iron fossiliferous Ediacaran successions in New- facies for fossil preservation appears to be speciation data to demonstrate that the Mall foundland suggested that the first appear- the strongest control on the stratigraphic dis- Bay Formation was deposited under generally ances correlated with an oxygenation event. tribution of macrofossils. anoxic and ferruginous conditions, with more A similar relationship was claimed to exist in robust evidence for anoxia present near the the Mackenzie Mountains, Canada, although INTRODUCTION onset of the ca. 580 Ma Gaskiers glaciation. later stratigraphic studies indicated that the Anoxic conditions persisted through the deposi- sections analyzed for geochemistry were in- The first abundant macroscopic organisms in tion of glacial strata and then abruptly changed correctly correlated with those hosting the the fossil record make their appearance toward to oxygenated conditions immediately follow- fossils. To directly connect fossil occurrences the end of the Neoproterozoic Era. Ediacara- ing the glaciation. This redox change coincides with geochemistry in the Mackenzie Moun- type impressions, preceded by the appearance with the first appearance—both in the Concep- tains, we conducted a multiproxy iron, car- of the microscopic remains of multiple eukary- tion Group and globally—of macroscopic fos- bon, sulfur, and trace-element geochemical otic groups in the Tonian and early Cryogenian sils common to the Ediacara biota (Narbonne analysis of stratigraphic sections hosting both Periods (Knoll, 2014), were followed by the and Gehling, 2003). This was then taken as the Cryogenian “Twitya discs” at Bluefish first evidence for bilaterian burrows in the late potentially pointing to a causal link between an Creek as well as Ediacaran fossils and simple Ediacaran and ultimately the flowering of ani- oxygenation event (local or global) and diversi- bilaterian traces at Sekwi Brook. There is no mal life during the Cambrian radiation (Droser fication. Following from that work, other work- clear oxygenation event correlated with the et al., 1999; Maloof et al., 2010; Erwin et al., ers have claimed that an oxygenation event in appearance of macroscopic body fossils or 2011; Erwin and Valentine, 2013; Carbone and the Sheepbed Formation at the Shale Lake simple bilaterian burrows; however, some Narbonne, 2014). Although the phylogenetic locality in the Mackenzie Mountains of north- change in environment—a potential partial affinities of the Ediacara biota are contentious, western Canada (Fig. 1) correlates with the first oxygenation—is correlated with increasing the organisms are increasingly being viewed appearance of Ediacaran fossils at Sekwi Brook burrow width higher in the Blueflower For- as containing members of several disparate ~135 km farther south (Shen et al., 2008). mation. Data from Sekwi Brook suggest that eukaryotic groups, united by time and tapho- The first step in distinguishing coincidence these organisms were periodically colonizing nomic mode, rather than representing a single from correlation is determining whether tem- a predominantly anoxic and ferruginous ba- monophyletic clade (reviewed recently by Xiao poral linkages represent a global pattern, regional sin. This seemingly incongruent observation and Laflamme, 2009; Grazhdankin, 2014). The events, or simply the unrelated appearance is accommodated through accounting for dif- size and abundance of the Ediacara biota fossils, of organisms during a time interval charac- fering time scales between the characteristic which are conspicuously absent from tapho- terized by broadly increasing oxygen levels. response time of sedimentary redox proxies nomically similar strata of older age, herald an Neo protero zoic oxygenation, if present, is versus that for ecological change. Thus, hy- important change in eukaryotic life on the eve of increasingly being recognized as regionally potheses directly connecting ocean oxygen- the Cambrian explosion. hetero geneous (Kah and Bartley, 2011). This is ation with the appearance of macrofossils This begs the question of why these organisms reflected in iron speciation data from the south- arose at this point in Earth’s history. The answer ern Canadian Cordillera showing an increased †Current address: Department of Geological Sci- need not require a single causal factor—a com- prevalence of anoxic conditions during the mid- ences, Stanford University, Stanford, California Ediacaran (Canfield et al., 2008), in contrast 94305, USA. bination of ecological, genomic, and environ- §E-mails: narbonne@ queensu .ca; fmacdon@ fas mental factors was likely at play. Environmen- to the Newfoundland data, and data from the .harvard .edu. tal factors have received considerable study in Wernecke Mountains of northwestern Canada, GSA Bulletin; Month/Month 2015; v. 1xx; no. X/X; p. 1–18; doi: 10.1130/B31329.1; 8 figures; 3 tables; Data Repository item 2015326.; published online XX Month 2015. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 © 2015 Geological Society of America Sperling et al. AB129° W 128° W C 65° N Franklin Mountain/ Sekwi formations n r ve Backbone Ranges/ aleozoic Ingta/Vampire tain Ri Cambria n P formations 1 Norman Mou Wells SL Risky 0.5 p GT. 0 June beds km Ediacaran Sheepbed upper grou RT. 632.3±6.3 Ma ver Keele/Ice Brook Twitya Ri 500 km Twitya 662.4±3.9 Ma BFC Hay Crk. 64° N Shezal Neoproterozoic (Cryogenian-Ediacaran) Twitya, Keele, Ice Brook, Sheepbed, June Sayunei n beds, Gametrail and Blueower Formations. Mt. Berg Rapitan Gp Neoproterozoic Latest Neoproterozoic and Early Cambrian . Backbone Ranges, Vampire, and Coppercap 716.5±0.25 Ma Sekwi Formations; also equivalent map units Windermere Supergroup ryogenia 732.2±3.9 Ma Proterozoic and Phanerozoic units undivided C June Redstone Lake River r Rivers ve e Ri oates Lake Gp SBN C Thundercloud Keel Canol Heritage Trail Little Dal Basalt 777.7 +2.5/–1.8 Ma SL Shale Lake Conglomerate Evaporite BFC Bluesh Creek Sandstone Diamictite SBN Sekwi Brook North Shale/Siltstone Basalt 050 Carbonate Iron Formation km 63° N Re-Os ORR Age U-Pb Igneous Age Figure 1. Locality map of investigated Cryogenian and Ediacaran strata in NW Canada. (A) Outline of study region in the Northwest Terri- tories of NW Canada. (B) Distribution of Neoproterozoic–Cambrian strata in the Mackenzie Mountains, Northwest Territories. Sampling localities in this study are Bluefish Creek (BFC) and Sekwi Brook North (SBN). The sedimentary geochemistry of the Ediacaran Sheepbed Formation was sampled at Shale Lake (SL) by Shen et al. (2008) and Canfield et al. (2008). The Ediacaran succession analyzed by Johnston et al. (2013) at the Goz A locality is located ~170 km northwest of Shale Lake (off map) in the Wernecke Mountains, Yukon. Map is modified from MacNaughton et al. (2008). (C) Generalized stratigraphic column of the Windermere Supergroup, Mackenzie Mountains, Northwest Territories. Stratigraphic thicknesses are schematic and vary widely. Italics indicate informally named units. Column is modified from Narbonne and Aitken (1995); Martel et al. (2011); Macdonald et al. (2013); Rooney et al. (2014, 2015). U-Pb age on Little Dal Basalt is from Jefferson and Parish (1989), U-Pb ages from base of Rapitan Group are from Macdonald et al. (2010), and Re-Os ages are from Rooney et al. (2014, 2015). RT.—Ravensthroat, GT.—Gametrail Formation, ORR—organic-rich rock, N.W.T.—Northwest Territories. which show no change at all (Johnston et al., of such a change to much less than is normally sections of the Sheepbed Formation at the Shale 2013). Other regions such as Namibia also show depicted (e.g., Holland, 2006). Lake locality (Fig. 1). These data were corre- hetero geneous but generally anoxic and ferru- Poor age constraints further complicate the lated with the appearance of Ediacaran fossils ginous conditions through the late Ediacaran integration of geochemical and ecological at Sekwi Brook, in strata that were originally (Wood et al., 2015). Analyzed collectively and patterns, and thus, in many sedimentary suc- assigned to the Sheepbed Formation (Aitken, statistically, a global database of Proterozoic and cessions, inferring the relationship between 1989).