Triggering the Cambrian Explosion: carbon cycle reorganisation and the rise of Metazoans
Bowyer, Frederick Toby
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Numerous detailed geochemical studies of Ediacaran (~635 – 541 Ma) marine successions provide snapshots into the palaeoenvironmental redox conditions which accompanied examples of the earliest metazoans in the fossil record. Spatial heterogeneity with respect to palaeomarine redox is evident from reconstructions of geographically-widespread Ediacaran environments. This project provides new data of local-scale redox within a paleogeographic and sequence stratigraphic framework in order to explore the mechanisms which controlled water column redox variations and the potential impact on early macro-benthic ecosystems. Lower than present atmospheric and oceanic oxygen concentrations enabled some shallow marine settings to remain poised at iron reduction until well into the Cambrian and likely influenced regional-scale ecosystem structure and stability. Many basins had a shallow and highly dynamic chemocline above anoxic (ferruginous or euxinic) or low oxygen (manganous) waters. Regional differences in palaeoredox were likely controlled primarily by local detrital nutrient provision and organic matter remineralisation and the redox state of the global deep ocean was most likely similarly heterogeneous (but this remains uncertain). It is suggested that cratonic positioning and migration throughout the Ediacaran Period, in combination with gradually increasing dissolved oxygen loading, may have provided a long-term control on redox evolution through regulating circulation mechanisms in the Mirovian Ocean. Some unrestricted lower slope environments from mid-high latitudes benefited from sustained oxygenation via downwelling, whilst cratonic isolation or transit towards more equatorial positions stifled pervasive ventilation either through ineffective surface ocean mixing, Ekman-induced upwelling, elevated surface ocean productivity, or a combination of these processes. Co-preservation of largely-enigmatic fossil forms within sedimentary rocks of the late Ediacaran Nama Group of southern Namibia have allowed the four-dimensional reconstruction of local redox dynamics and associated biotic establishment. This has been made possible through collation of previously published fossil occurrence and geochemical information alongside new palaeoredox and palaeoproductivity estimates based on iron speciation, major element and carbonate-bound iodine data. This is further supplemented by the first detailed assessment of the paragenetic sequence and diagenetic relationships of carbonates which precipitated within the earliest metazoan reef framework. Skeletal invertebrate taxa in the Zaris Sub-Basin of the lower Nama Group (~550-547 Ma), grew above wave base where micritic carbonate sediment often shows evidence for early dolomitisation. Mid-ramp Cloudina reefs composed of open, highly porous structures formed multiple, successive assemblages. Thin layers of dolomitised sediment and dolomite cement terminate each assemblage. Reef cements show a paragenetic sequence from synsedimentary, early marine cement through to final burial, each of which were precipitated under dynamic redox conditions. These cements likely record a general shallow to deeper water transect, from oxic shallow waters to low oxygen manganous waters and finally to oxic, shallow burial conditions. Transient incursions of upwelled, anoxic, ferruginous and dolomitising waters may have occurred during short-term, transgressive cycles, although the timing for this is poorly constrained. Such incursions may have terminated Ediacaran benthic communities that grew close to the chemocline. Viewed in its entirety, the palaeoredox record of the Nama Group reveals evidence for a pronounced shift in the depth of the ferruginous redoxcline from shallow to deeper levels in the water column through time, which was accompanied by a reduced frequency of anoxic incursions onto the shallow shelf. This transition approximately coincided with the first appearance and subsequent diversification of novel sediment bioturbators in the Lower Urusis Formation (~547-542 Ma). It is proposed that the observed coevolution of palaeoredox and ichnofossil diversity may directly relate to the impact of bioturbation on phosphorus retention. In this way, the diversification of burrowing forms effectively oxygenated the sediment column, prevented efficient P recycling to the water column and limited the detrimental impact of productivity-induced anoxia in the local environment. However, this hypothesis remains to be tested and would benefit from a focused study of palaeoproductivity employing targeted analyses of total organic carbon and sedimentary phosphorus speciation. It is further proposed that the persistent spatial separation of anoxic deep waters from habitable ecospace, implied by the fossil distribution of phylogenetically-enigmatic soft-bodied forms, qualitatively supports the inference that at least intermittently oxic conditions (at or above EH typical of ferrous iron oxidation) were a metabolic requirement of these organisms. Finally, four new sections of the late Ediacaran, deposited approximately time-equivalent to aforementioned sediments of the Nama Group, are described and preliminary geochemical data reported. These include two shallow marine carbonate-dominated sections of the southeast Siberian Craton which correspond to the Yudoma Formation and two sections of the Dengying and lower Zhujiaqing (and correlative) Formations deposited on the Yangtze Block, South China. Integrated proxy methods are able to distinguish palaeoredox heterogeneity between and within early animal ecosystems and test the influence of anoxia on ecosystem structure. The first and last appearances of Treptichnus pedum and Cloudina respectively, which globally bracket the boundary between the Ediacaran and Cambrian Periods, show no identifiable range overlap in any sections analysed in this study. This suggests that the first appearance of the organism responsible for characteristic T. pedum may have lived approximately contemporaneous in oxic habitable refuges of all regions in this study, regardless of the dominance of reducing conditions that persisted in coeval deeper environments in many areas.