The Great Ordovician Biodiversification Event (GOBE) is regarded as one of the most significant evolutionary events in the history of Phanerozoic life. The present study integrates palynological, petrographic, molecular and stable isotopic (&delta;¹³C of biomarkers) analyses of cores from four boreholes that intersected the Goldwyer Formation, Canning Basin, Western Australia, to determine depositional environments and microbial diversity within a Middle Ordovician epicontinental, tropical sea. Data from this study indicate lateral and temporal variations in lipid biomarker assemblages extracted from Goldwyer Formation rock samples. These variations likely reflect changing redox conditions between the upper (Unit 4) and lower (Units 1 + 2) Goldwyer, which is largely consistent with existing depositional models for the Goldwyer Formation. Cryptospores were identified in Unit 4 in the Theia-1 well and are most likely derived from bryophyte-like plants, making this is the oldest record of land plants in Australian Middle Ordovician strata. Biomarkers in several samples from Unit 4 that also support derivation from terrestrial organic matter include benzonaphthofurans and δ¹³C-depleted mid-chain n-alkanes. Typical Ordovician marine organisms including acritarchs, chitinozoans, conodonts and graptolites were present in the lower and upper Goldwyer Formation, whereas the enigmatic organism <i>Gloeocapsomorpha prisca </i>(<i>G. prisca</i>) was only detected in Unit 4. The correlation of a strong <i>G. prisca</i> biosignature with high 3-methylhopane indices and ¹³C depleted <i>G. prisca</i>–derived chemical fossils (biomarkers) is interpreted to suggest an ecological relationship between methanotrophs and <i>G. prisca</i>. This research contributes to a greater understanding of Ordovician marine environments from a molecular perspective since few biomarker studies have been undertaken on age-equivalent sections. Furthermore, the identification of the oldest cryptospores in Australia and their corresponding terrestrial biomarkers provides further insight into the geographical distribution and evolution of early land plants. <b>Citation:</b> Gemma Spaak, Dianne S. Edwards, Clinton B. Foster, Anais Pagès, Roger E. Summons, Neil Sherwood, Kliti Grice, Environmental conditions and microbial community structure during the Great Ordovician Biodiversification Event; a multi-disciplinary study from the Canning Basin, Western Australia, <i>Global and Planetary Change</i>, Volume 159, 2017, Pages 93-112, ISSN 0921-8181 https://doi.org/10.1016/j.gloplacha.2017.10.010.

The Ordovician is an important period in Earth’s history with exceptionally high sea levels that facilitated the Great Ordovician Biodiversification Event. This crucial biological event is regarded as the second most significant evolutionary event in the history of Paleozoic life, after the Cambrian radiation. The present study integrates palynological, petrographic, molecular and stable isotopic (δ13C of biomarkers) analyses of cores from five boreholes that intersected the Goldwyer Formation, Canning Basin, Western Australia, to determine depositional environments and microbial diversity within a Middle Ordovician epicontinental, tropical sea. A major transgression was detected in the laminated shales of the lower Goldwyer Formation (Units 1+2) which were deposited in anoxic bottom waters, as confirmed by low (<1) Pristane/Phytane ratios, and elevated dibenzothiophene and gammacerane indices. A second, less extensive, flooding event is recorded by shallow marine sediments of the upper Goldwyer Formation (Unit 4). Cores of these sediments, from two wells (Solanum-1 and Santalum-1A) are bioturbated and biomarkers indicate relatively oxygenated conditions, as well as the presence of methanotrophic bacteria, as determined from the high 3-methylhopane indices. Typical Ordovician marine organisms including acritarchs, chitinozoans, conodonts and graptolites were present in the lower and upper Goldwyer Formation, whereas the enigmatic organism Gloeocapsomorpha prisca (G. prisca) was only detected in Unit 4. The presence of G. prisca was based on microfossils and specific biosignatures presenting an odd-over-even predominance in the C15 to C19 n-alkane range. Cryptospores were identified in Unit 4 in the Theia-1 well and are most likely derived from bryophytes, making this is the oldest record of land plants in Australian Middle Ordovician strata. Biomarkers in some samples from Unit 4 that also support derivation from terrestrial organic matter include retene, benzonaphthofurans and δ13C-depleted mid-chain n-alkanes. This research contributes to understanding Ordovician marine environments from a molecular perspective since few biomarker studies have been undertaken on age-equivalent sections. Furthermore, the identification of the oldest cryptospores in Australia and their corresponding terrestrial biomarkers contributes to understanding the geographical evolution of early land plants.

The Exploring for the Future (EFTF) program is an initiative by the Australian Government dedicated to boosting investment in resource exploration in Australia. Geoscience Australia’s EFTF Energy program is aimed at improving the understanding of the petroleum resource potential of Australia. A key to understanding resource potential and basin evolution is a reliable time frame to correlate rock units. This palynological reconnaissance study focusses on the acid-resistant organic-walled microfossils (or palynomorphs) recovered from 42 samples taken within the fully cored Lower Ordovician Nambeet Formation (1354.80–2435.04 mRT) in the Barnicarndy 1 stratigraphic well, located in the Barnicarndy Graben, Canning Basin. The lack of palynomorph recovery from the Barnicarndy Formation, Yapukarninjarra Formation, and Neoproterozoic Yeneena Basin, also intersected in this well, means that the age of those units remain undated using micropalaeontological methods. The purpose of this study is to assess the yield and preservation of recovered palynomorphs, and determine their utility for regional, and international, correlation of the Lower Ordovician sedimentary section. Although the total organic matter content of the sampled Lower Ordovician core is typically low (average ≤0.2 wt%), reflecting sediment deposition in an oxidising, open marine environment, a diverse suite of palynomorphs has been identified and includes: acritarchs (of probable algal origin); other algal microfossils (including green algae, or prasinophytes); probable cyanobacteria; cryptospores (derived from the earliest land plants); graptolites and chitinozoans (both from extinct marine groups); scolecodonts (detached elements of worm jaws); and organic-walled tubes, some of which are of either probable fungal or cyanobacterial origin. Digital images accompany this record and include examples of all of these aforementioned microfossils. Microfossil yield per sample is, mostly, low; and preservation ranges from poor, where specimens are either fragmentary and/or distorted by pyrite crystal growth, to good; and commonly both preservation states occur together within the same sample. As with the admixture of preservation states per sample, palynomorph colour, typically used as an indicator of thermal maturity of organic matter, commonly ranges from thermally mature (brown) to over mature (black), often within the same Lower Ordovician core sample. This is tempered by the fact that these observations are based, mostly, on oxidised kerogen preparations, but, the relative maturity indicators remain valid. The occurrence of acritarchs assigned to the Rhopaliophora pilata–R. palmata complex, together with Athabascaella playfordii, and Aryballomorpha grootaertii, allows correlation with assemblages previously recovered from the Nambeet Formation intersected in two petroleum exploration wells in the Canning Basin (Samphire Marsh 1, type section; and Acacia 2). These species also occur globally, with A. grootaertii recovered from sedimentary rocks in southern China and Canada that have been independently dated as Early Ordovician, late Tremadocian–early Floian (about 475–482 Ma). Conodont faunas from cores in Barnicarndy 1 record the same (late Tremadocian–early Floian) age, which enhances the utility of A. grootaertii for age dating. The dates also demonstrate that the Barnicarndy 1 well intersects some of the oldest Paleozoic sedimentary rocks in the Canning Basin. There are compositional differences between the palynological assemblages from the younger Samphire Marsh Member and underlying Fly Flat Member of the Nambeet Formation which, despite difficulties in sample processing, are genuine and reflect changes in the depositional environment. Most obvious is the record of Gloeocapsomorpha prisca and ?Eomerismopedia maureeniae, both of probable cyanobacterial affinity, with in situ occurrences in the Lower Ordovician Samphire Marsh Member. Earlier studies suggested that G. prisca was confined to younger (Middle) Ordovician palynological assemblages in the Canning Basin, and its common abundance was used as a biozone marker, but the occurrences reported here and in unpublished studies, have shown that this is no longer applicable. In younger Ordovician formations in the Canning Basin (notably the upper Goldwyer Formation), and globally, G. prisca is an important organism contributing to the hydrocarbon potential of Paleozoic marine source rocks. If present in greater abundance elsewhere in the basin, it could increase the petroleum prospectivity of the Nambeet Formation. A distinctively shaped acritarch, of probable algal origin, assigned to the genus Dactylofusa is restricted to an assemblage from the Fly Flat Member, and may be useful for future basinal biozone correlation. Most samples from the Samphire Marsh Member contain early land-plant spores, of probable bryophyte affinity, that sometimes occur together with irregularly-shaped spore clusters, likely derived from aeroterrestrial charophyte algae; both of which are collectively known as cryptospores. In addition, Grododowon orthagonalis, superficially similar to E. maureeniae and recorded in some samples from the Samphire Marsh Member, is also considered to be of charophyte algal origin. The cryptospores include the species Dyadospora murusattenuata, Tetraplanarisporites sp., and Laevolancis divellomedium. Collectively, these cryptospores are important as they herald the first emergence of plants onto wetlands during the Early Ordovician; and being of late Tremadocian–early Floian age, they are amongst the oldest land-plant spores known in Australia and globally. The record of cryptospores from Barnicarndy 1 enhances those recently reported from the Nambeet Formation in Samphire Marsh 1, and from the lower Goldwyer Formation in Theia 1. Locally, the cryptospore record demonstrates a supply of terrestrial material into the marine environment during deposition of the Samphire Marsh Member. Globally, records of these cryptospores contribute to the understanding of the evolution and geographic distribution of the earliest land flora. Inevitably, there are microfossils found in this study that could be described as new species, and a detailed systematic study of all fossil groups is recommended to realise their utility for zonal correlation and age dating. The palynological data presented here provide complementary information to the conodont age dating, organic petrological, and organic geochemical studies conducted on the Barnicarndy 1 core. Collectively, these studies contribute to a better understanding of the depositional history and hydrocarbon prospectivity of the Canning Basin.