The palaeoecology of the lower Leintwardinian (Upper Silurian) of the Welsh borderlands and Wales

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How far did feedback between biodiversity and early diagenesis affect the nature of early Palaeozoic sea floors?

Latest Precambrian to Early Palaeozoic biosphere evolution triggered changes in early diagenesis and carbonate precipitation which fed back to biodiversity through colonization of hard substrates. Progressive increase in the depth and intensity of bioturbation and bio-irrigation lowered the zone of early carbonate cementation in the uppermost sediment column. This firstly led to a decline in the abundance of the flat-pebble conglomerates which had been a common feature of Cambrian and Early Ordovician successions, replaced by the peak and subsequent decline in the Palaeozoic abundance of submarine hardgrounds. The availability of very widespread lithified sea floors in shallow subtidal settings during the Ordovician promoted a rapid expansion in sclerobiont diversity and contributed to the Great Ordovician Biodiversification Event

Geometric and depositional responses of carbonate build-ups to Miocene sea level and regional tectonics offshore northwest Australia

The geometric and depositional responses of isolated carbonate build-ups to Miocene sea-level change and regional tectonics was investigated using a combination of 3D seismic and borehole data from the Browse Basin, North West Australia, and outcrop information from the Cariatiz Reef, southeast Spain. The interpreted seismic volume documents five (5) Miocene sequence boundaries and five (5) main seismic facies. Seismic attribute analyses proved a highly effective tool for interpreting carbonate facies but, when compared with outcrop information from southeast Spain, data are limited to large-scale features of scales beyond 16.4 m vertically and 18.75 m horizontally. Hence, this work clearly shows that estimations of reservoir potential are significantly underestimated if based on seismic data alone. As a corollary of the structural analysis in this work, growth patterns suggest Messinian structural partitioning across the Browse Basin, with deformation associated with plate collision focused in preferentially orientated faults - thus only influencing carbonate build-up evolution at a local scale

Pinnacle features at the base of isolated carbonate buildups marking point sources of fluid offshore Northwest Australia

We investigated pinnacle features at the base of late Oligocene–Miocene isolated carbonate buildups using three-dimensional seismic and borehole data from the Browse Basin, Northwest Australia. Brightened seismic reflections, dim spots, and other evidence of fluid accumulation occur below most pinnacle features. An important observation is that all pinnacles generated topography on successive late Oligocene–Miocene paleoseafloors, therefore forming preferential zones for the settlement of reef-building organisms by raising the paleo-seafloor into the photic zone. Their height ranges from 31 m to 174 m, for a volume varying from 33 km3 to 11,105 km3. Most of the pinnacles, however, are less than 2000 km3 in volume and present heights of 61–80 m. As a result of this work, pinnacles are explained as the first patch reefs formed in association with mud volcanoes or methanogenic carbonates, and they are considered as precluding the growth of the larger isolated carbonate buildups. We postulate that pinnacle features above fluidflow conduits demonstrate a valid seep-reef relationship, and we propose them to be refined diagnostic features for understanding fluid flow through geological time

Eocene and Oligocene chitons (Polyplacophora) from the Paris and Hampshire basins

Eocene and Oligocene chitons (Polyplacophora) from the Paris Basin of N France are described along with comparative material from the Hampshire Basin of the UK. The assemblages include eight species, five of which are new: Ischnochiton fehsei sp. nov., Stenoplax monila sp. nov., Chaetopleura gaasi sp. nov., C. abbessi sp. nov. and Tonicella lira sp. nov. Other taxa in the assemblages are Leptochiton cf. algesirensis, I. vectensis and S. anglica

The Mg/Ca–temperature relationship in brachiopod shells: calibrating a potential palaeoseasonality proxy

Brachiopods are long-lived, long-ranging, extant organisms, of which some groups precipitate a relatively diagenetically stable low magnesium calcite shell. Previous work has suggested that the incorporation of Mg into brachiopod calcite may be controlled by temperature (Brand et al., 2013). Here we build upon this work by using laser ablation sampling to define the intra-shell variations in two modern brachiopod species,Terebratulina retusa (Linnaeus, 1758) and Liothyrella neozelanica (Thomson, 1918). We studied three T. retusa shells collected live from the Firth of Lorne, Scotland, which witnessed annual temperature variations on the order of 7 °C, in addition to four L. neozelanica shells, which were dredged from a water depth transect (168–1488 m) off the north coast of New Zealand. The comparison of intra-shell Mg/Ca profiles with shell δ<sup>18</sup>O confirms a temperature control on brachiopod Mg/Ca and supports the use of brachiopod Mg/Ca as a palaeoseasonality indicator. Our preliminary temperature calibrations are Mg/Ca = 1.76 ± 0.27 e<sup>(0.16 ± 0.03)T</sup>, R<sup>2</sup> = 0.75, for T. retusa and Mg/Ca = 0.49 ± 1.27 e<sup>(0.2 ± 0.11)T</sup>, R<sup>2</sup> = 0.32, for L. neozelanica (errors are 95% confidence intervals)

Climatic changes and astrochronology: an Ordovician perspective

Review of current progress in Mid to Late Ordovician astrochronological studies exposes some important issues related to cyclostratigraphical studies, including the completeness and correlation of successions, and the connection between inferred astronomical cycles and geological events recorded in the sedimentary record. While bulk, low-field, mass specific magnetic susceptibility methods are widely applied in studies of high resolution cyclostratigraphy, they require close support from sequence stratigraphy and biostratigraphy, and should be linked back to outcrop patterns. Otherwise they risk distortion in the calibration against geological time, through lack of anchoring to well-defined biostratigraphical horizons and unrecognised condensed intervals and larger hiatuses. A significant limitation currently is that few high-resolution radio-isotope ages are linked to well-defined biostratigraphical boundaries. Nevertheless, fourth order sedimentary sequences linked to 405 kyr orbital eccentricity cycles, and longer orbital cyclicity impressed in third-order sequences, represent good grounds for development of a reliable astrochronological scale. The astrochronologically calibrated sequence-stratigraphical record documented from high latitude Gondwana shows significant impact from orbital forcing on the Mid to Late Ordovician global climate

Facilitating corals in an early Silurian deep‐water assemblage

Corals are powerful ecosystem engineers and can form reef communities with extraordinary biodiversity through time. Understanding the processes underlying the spatial distribution of corals allows us to identify the key biological and physical processes that structure coral communities and how these processes and interactions have evolved. However, few spatial ecology studies have been conducted on coral assemblages in the fossil record. Here we use spatial point process analysis (SPPA) to investigate the ecological interactions of an in situ tabulate and rugose coral community (n = 199), preserved under volcanic ash in the Silurian of Ireland. SPPA is able to identify many different sorts of interactions including dispersal limitation and competition within and between taxa. Our SPPA found that the spatial distribution of rugose corals were best modelled by Thomas clusters (pd = 0.834), indicating a single dispersal episode and that the tabulate corals were best modelled by double Thomas clusters (pd = 0.820), indicating two dispersal episodes. Further, the bivariate distribution was best modelled by linked double clusters (pd = 0.970), giving significant evidence of facilitation between the tabulate and rugose populations, and identifying the facilitators in this community to be the tabulate corals. This interaction could be an important ecological driver for enabling the aggregation of sessile organisms over long temporal periods and facilitation may help to explain trends in reef diversity and abundance during the Ordovician biodiversification and in the early Silurian

Testing whether early diagenesis of skeletal carbonate is different in non-marine settings: contrasting styles of molluscan preservation in the Upper Jurassic of Portugal

This study tests the hypothesis that the early diagenesis of aragonite shells should differ fundamentally between marine and freshwater environments. This is predicted to be the case because aragonite is highly susceptible to dissolution in the TAZ (Taphonomically Active Zone) in low energy marine settings due to acidity caused largely by the oxidation of H2S generated by sulphate-reducing bacteria, but reduced sulphide activity in freshwater settings should result in less early dissolution of aragonite. To test this hypothesis a range of fresh-brackish-hypersaline and marine limestones were sampled from the Upper Jurassic (mid Oxfordian) Cabaços Formation of central western Portugal. In these freshwater and brackish deposits, molluscs are preserved mostly as sparite shell replacements indicating that the original aragonite was preserved through the TAZ and was later replaced during subsequent burial by calcite cement. In limestones deposited in more marine to hypersaline settings, molluscan remains mostly consist of the calcitic layers of bimineralic bivalves, as shell where the original was wholly calcitic, or as gastropod steinkerns. Exceptions occur and reflect other factors such as higher energy conditions during deposition whereby organic matter, as the drive for microbial decay processes, was removed. The implications for molluscan preservation including some hydrocarbon reservoirs are discussed

Geomorphological evidence of carbonate build-up demise on equatorial margins: A case study from offshore northwest Australia

The demise of Miocene carbonate build-ups in the Browse Basin, Northwest Australia, has been explained as relating to geological and oceanographic processes. These include accelerated tectonic subsidence driven by subduction, ocean cooling following the mid-Miocene climate optimum, nutrient excess, poisoning by sediment drifts and local erosion driven by current winnowing, occurring discretely or simultaneously. Here, we critically assess the evidence for these different mechanisms using a combination of high-resolution 3-D seismic data, regional 2-D seismic profiles, and numerical stratigraphic forward modelling. Seismic interpretation and numerical modelling found that the proposed uniform subsidence rate of 125 m/Ma between 16.5 Ma and 5.33 Ma for the northern Browse Basin (Belde et al., 2017), when combined with the published estimate of eustatic sea level in Miller et al. (2005), was insufficient to drown the Miocene carbonate sequence and generate the geomorphological changes (barrier reef to isolated carbonate build-ups) observed on seismic data. Instead, a subsidence profile comprising pulses of rapid and slow subsidence is required. Significantly, our results suggest that subsidence rates exceeded 400 m/Ma in the northern Browse Basin, and that parts of the basin record the accumulation of sediment drifts. These sediment drifts are interpreted to have buried some carbonate build-ups, while suspended sediment reduced light transmissivity, inhibiting carbonate production. Thus, we postulate that current activity and excess nutrient supply are key, but often overlooked, oceanographic processes that lead to the demise of carbonate build-ups