Carbonate rocks form a substantial part of the ancient geological record, yet their reliability as archives of past environments and ecosystems has been questioned because of diagenetic alteration. This review investigates the potential of carbonate depositional surfaces – irregular or planar interfaces formed at the sediment–water or sediment–air boundary during deposition – to serve as records of ancient environments and ecosystems. These surfaces, commonly preserved in carbonate strata, provide valuable insights into sedimentary processes and ecosystems over varying timescales. Some carbonate depositional surfaces have the same features as siliciclastic examples, such as desiccation cracks, ripples, dunes and surfaces with trace and body fossils. Surfaces offer a glimpse into ordinary day-to-day deposition, such as ripples moving with the tide or mudcracks forming on an exposed tidal flat. Other depositional surfaces are unique to carbonate sediment, including hardgrounds, tepees, beachrock, reefs and microbialites. These surfaces represent cemented and/or condensed intervals of deposition, preserving significant differences in the duration of exposure and environmental conditions. Sedimentary factors, original mineralogy, biological and ocean chemical changes over Earth history and tectonic/burial history all impact the likelihood of preserving carbonate depositional surfaces.

A widespread system of breccias and neptunian dykes is present within the Paleoproterozoic Teena Dolomite of the McArthur Basin. Most breccia types within the unit are infilled by early fibrous marine cements and dark grey sediments. We interpret the dominant breccia types as in-situ synsedimentary fracture systems (neptunian dykes) while some matrix-supported breccias are likely to be mass flow deposits. Fibrous marine cements filling the neptunian dykes have several different textures and all now consist of dolomite. Petrographic evidence indicates the cements were likely precipitated as calcite and dolomitized during early diagenesis. The trace and rare earth element chemistry of the marine cements is typical of Paleoproterozoic marine cements and indicate relatively anoxic marine conditions. The breccias in the Teena Dolomite appear to be the result of a basin-wide tectonic event that occurred during Teena Dolomite to early Barney Creek time at similar to 1640 Ma. Early marine-cemented neptunian dykes likely formed by tectonic fracturing and gravitational collapse on tectonic highs. Many large stratiform Zn-Pb deposits in northern Australia (HYC, Teena, Lady Loretta, Mt Isa) are hosted by shales that are coeval with this tectonic event, suggesting a genetic link between tectonism and mineralization. We suggest that this similar to 1640 Ma tectonic event is responsible for both the release of mineralization fluids (via faulting and/or dewatering), and the deposition of deeper water organic-rich fine-grained sediments that host the mineralization (via tectonic subsidence). This tectonic event appears to be an important component of the sedimentary exhalative mineral system in the Carpentaria Zinc Belt of northern Australia.

The tropical North West Shelf of Australia hosts a diverse range of modern reefs. Six shelf edge isolated atolls are present north of 18 degrees S including: Ashmore Reef, Scott Reef and Seringapatam Reef, and three Rowley Shoals. The Ningaloo Reef is a fringing reef around the North West Cape at 22 degrees S. All of these reefs are the remnants of a vast 2000 km long barrier reef that drowned during the Late Miocene (similar to 10 Ma). Despite extensive hydrocarbon exploration in the region, the history of these isolated reefs is not well known. Seismic analyses combined with stratigraphic analyses of International Ocean Discovery Site U1464 near the Rowley Shoals has revealed that these modern isolated atolls have a complex evolution related to climate and tectonism as they managed to survive on their Miocene barrier reef foundation. The first Miocene reefs (similar to 17 Ma) near the Rowley Shoals were isolated small, mound-shaped features. These evolved into a barrier reef by the Middle Miocene (similar to 16 Ma). However, by the Late Miocene (similar to 10 Ma) this barrier reef backstepped landward, evolved into isolated mounds/atolls and drowned prior to the Miocene-Pliocene boundary (similar to 6 Ma) largely due regional tectonic subsidence. Early Pliocene reef expansion (similar to 4.6 Ma) led to the growth of four isolated atolls (the Rowley Shoals) related to local faulting and Early Pliocene warmth. Subsequently a second Pliocene reef growth phase occurred from similar to 3.5 to 3 Ma when eastern Indian Ocean sea surface temperatures cooled by similar to 4 degrees C due to Indonesian Gateway constriction and a reduced Leeuwin Current. By the Pleistocene (similar to 2.4 Ma) one the four Rowley Shoals had drowned. Strong sea level variability, together with Indonesian Throughflow constriction and reduction in intensity of the Leeuwin Current after 2.4 Ma may have led to enhanced cooling and regional upwelling. These factors may have been sufficient to cause local drowning of the southerly fourth Rowley Shoal while the more northerly three Shoals survived until present.

Earth's surface underwent stepwise oxygenation before persistently reaching modern levels late in its history1-5, but the details of this transition remain unclear5-16. Here we present a high-resolution 2.5-Gyr record of mass-independent oxygen isotopes in sedimentary sulfate (Δ'17Osulfate), a proxy linked to the atmospheric partial pressure of O2 ( p O 2 )17-19. This record, together with existing sedimentary Δ33S data20-22, demonstrates a 2-Gyr transition characterized by generally low, fluctuating p O 2 between an O2-free state before 2.4 billion years ago (Ga) and a modern p O 2 state after 0.41 Ga, with relatively elevated levels after 1.0 Ga. Our data also show coupled declines in Δ'17Osulfate and sulfate-δ34S during major negative carbonate-δ13C excursions in the Neoproterozoic. Quantitative biogeochemical modelling indicates that these isotopic couplings reflect the increasing p O 2 , which may have driven episodic ocean oxygenation through an increased atmospheric O2 influx. This process intensified the oxidation of marine organics and reduced-sulfur species, while triggering temporary p O 2 drawdowns as negative feedback15. These findings support a dynamic, lengthy co-oxygenation history for the atmosphere and oceans-marked by long-term positive coupling and short-term negative feedbacks-offering a coherent explanation for the anomalous Neoproterozoic carbon cycles23,24 and the protracted, episodic rise of complex life25-27.

The first appearance of animals during the Ediacaran is arguably related to an increase in oceanic oxygenation during this time. However, there is considerable ambiguity in the global record of Ediacaran oxygenation, making it difficult to assess the potential links between oxygen and metazoan evolution. Here, we examine the earliest Ediacaran Nuccaleena Formation cap dolomite and basal Brachina Formation of the Adelaide Superbasin, South Australia, to determine the redox landscape in which these units were deposited. Red shales are present at the base of the Brachina Formation (lower Moolooloo Siltstone Member) over much of the Adelaide Superbasin but these transition laterally into green shales in the north, correlating with a facies transition into a deeper water setting.Fibrous dolomite cements within sheet cavities of the Nuccaleena Formation cap dolomite display evidence of a primary marine origin. Cathodoluminescence microscopy and laser ablation ICP-MS trace element analysis of these marine dolomite cements indicates a transition from an oxic environment in the south (with low Fe, Mn, and a Ce anomaly) to an anoxic (ferruginous) setting in the north (with high Fe, Mn and no Ce anomaly). This cap carbonate trace element geochemistry is spatially consistent with the overlying red to green shale transition in the basal Brachina Formation. Together, these data suggest the existence of a deep-water chemocline in this basin, separating an oxic upper water column from a ferruginous deeper water mass. This oxic interval directly post-dates the end-Cryogenian Marinoan Glaciation and is synchronous with an earliest Ediacaran oxygenation event previously described from South China.This evidence from the Adelaide Superbasin provides direct evidence for an earliest Ediacaran oxic water mass penetrating to a substantial paleodepth. The synchronous development of oxic intervals in both Australia and South China supports the notion of a globally developed oceanic oxygenation event and is consistent with the hypothesis that Ediacaran continental margin settings were periodically bathed in oxic water, conducive to the evolution of metazoans.

Oolitic ironstones are iron-rich and chert-poor sedimentary rocks containing concentrically coated grains composed of iron (oxyhydr)oxides and iron phyllosilicates that offer a unique window into iron cycling in ancient coastal environments. These enigmatic deposits are common in the Phanerozoic stratigraphic record yet lack clear modern analogues, and curiously are thought to be absent from Precambrian strata, suggesting a secular control on their deposition. Here we describe a previously unreported ironstone from the middle Tonian (ca. 850 Ma) Katherine Group in the Wernecke Inlier (Yukon, Canada), and show that similar deposits can be found-albeit rarely-throughout the Proterozoic. We investigate the origin of this unit and evaluate its palaeoenvironmental significance, and in light of an extensive literature review, present a holistic model for Precambrian ironstone deposition. The Katherine ironstone occurs in multiple horizons in the McClure and Abraham Plains formations and contains iron ooids and oncoids composed dominantly of authigenic hematite and berthierine, with detrital quartz grains. Textural relationships demonstrate that these coated grains formed on the seafloor with synsedimentary reworking, and the fine interlamination of these phases in grain coatings suggests redox and pH fluctuation during ironstone genesis. Facies associations indicate that the ironstones accumulated in a range of low-energy, shallow marine environments (tidal mudflats and coastal embayments). Geochemical analyses offer insights into genetic processes, and the radiogenic Nd isotope composition and negative Eu anomalies of the Katherine ironstone suggest a continental iron source. We present a model whereby abundant iron, cations, and silica-requisite for the authigenesis of iron phyllosilicates-were supplied from chemical weathering and preferentially enriched in coastal environments due to gradients in pH, Eh and salinity. This continental input would have led to intense iron cycling coupled to organic matter respiration, iron phyllosilicate authigenesis (i.e., reverse weathering). The enrichment of authigenic Fe(III) (oxyhydr)oxides and Fe(II) phyllosilicates took place on a broad coastal plain influenced by both autogenic and allogenic fluctuations in relative sea level, likely in a humid, tropical climate. The lenticular and episodic nature of ironstones in the Proterozoic stratigraphic record suggests that a unique combination of environmental conditions fostered ironstone accumulation. By reviewing the literature on oolitic ironstones, we re-evaluate the temporal distribution of these deposits compared to Archaean-Palaeoproterozoic iron formations, and show that the Great Oxidation Event may have been a prerequisite for ironstone deposition, which may implicate oxidative chemical weathering or suboxic, marine iron cycling. In general, we suggest that the Precambrian record of oolitic ironstones represents an important deep-time archive of iron and nutrient cycling in coastal settings.

The dramatic increase in ocean-atmosphere oxygen levels during the Devonian and Mississippian is increasingly linked to the diversification of land plants, yet the timing and extent of this event remain uncertain. This study uses the redox-sensitive rare earth element cerium (cerium anomaly-Ce/Ce*) to investigate ocean redox conditions during the deposition of globally distributed Paleozoic carbonate strata. Our Paleozoic Ce/Ce* record suggests that Cambrian, Ordovician, and Silurian oceans had relatively low O2 levels (mean Ce/Ce* = 0.86 +/- 0.10, 0.91 +/- 0.14, and 0.91 +/- 0.10 [+/- 1 sigma], respectively). In contrast, shortlived ocean oxygenation events, possibly related to the diversification of small land plants, likely occurred throughout the Early and Middle Devonian (mean Ce/Ce* = 0.80 +/- 0.07 and 0.58 +/- 0.14, respectively). "Modern" Ce/Ce* values (<0.36) first occurred during the Late Devonian, suggesting that the main phase of Devonian and Mississippian oceanic oxygenation was related to the evolution of large vascular plants and the first forests. Despite this, the significant variability of Ce/Ce* values during this time suggests that shallow marine settings were susceptible to redox instability, possibly caused by upwelling of anoxic deep waters. This redox instability potentially provides evidence of a mechanism for contemporaneous mass extinction and metazoan reef collapse events. Development of strongly oxic conditions during the Late Devonian may have resulted in the demise of many Paleozoic-type organisms, facilitated the radiation of the modern evolutionary fauna, and established the modern oxygenated ocean-atmosphere system.

The common association between sediment-hosted Zn-Pb deposits and normal faults has led to a view that faults are the main conduits for the transport of mineralizing fluids. This is at odds with the petroleum industry where faults commonly act as seals. Here we present two examples of sediment-hosted Zn-Pb deposits where we can demonstrate that the faults were not active during the time of mineralization. Instead, the main fault formed early in the history of the basin and was responsible for the formation of structural and stratigraphic traps. These traps became the focus of later fluids derived from the basin. The presence of hydrocarbons in the deposits suggests that the mineralizing fluids and the hydrocarbons may have shared the same migration pathway. This does not preclude fault rock and fault derived porosity playing a role at a certain scale. However, the general absence of mineralization or related alteration along faults regionally challenges the notion that they are significant conduits. We must therefore take a more holistic view to the generation and migration of fluids responsible for sediment-hosted Zn-Pb deposits and consider a range of basin related processes.

Phosphorus is a limiting nutrient that is thought to control oceanic oxygen levels to a large extent 1 – 3 . A possible increase in marine phosphorus concentrations during the Ediacaran Period (about 635–539 million years ago) has been proposed as a driver for increasing oxygen levels 4 – 6 . However, little is known about the nature and evolution of phosphorus cycling during this time 4 . Here we use carbonate-associated phosphate (CAP) from six globally distributed sections to reconstruct oceanic phosphorus concentrations during a large negative carbon-isotope excursion—the Shuram excursion (SE)—which co-occurred with global oceanic oxygenation 7 – 9 . Our data suggest pulsed increases in oceanic phosphorus concentrations during the falling and rising limbs of the SE. Using a quantitative biogeochemical model, we propose that this observation could be explained by carbon dioxide and phosphorus release from marine organic-matter oxidation primarily by sulfate, with further phosphorus release from carbon-dioxide-driven weathering on land. Collectively, this may have resulted in elevated organic-pyrite burial and ocean oxygenation. Our CAP data also seem to suggest equivalent oceanic phosphorus concentrations under maximum and minimum extents of ocean anoxia across the SE. This observation may reflect decoupled phosphorus and ocean anoxia cycles, as opposed to their coupled nature in the modern ocean. Our findings point to external stimuli such as sulfate weathering rather than internal oceanic phosphorus–oxygen cycling alone as a possible control on oceanic oxygenation in the Ediacaran. In turn, this may help explain the prolonged rise of atmospheric oxygen levels.

Through new palynological and chronostratigraphic (40Ar/39Ar dating) analyses we constrain the ages of important Australian Eastern Highland macrofloral sites. The Cambalong Creek site is late Paleocene (Selandian-Thanetian), with an age range of ∼ 57.3–59.8 Ma as inferred from palynostratigraphy (Proteacidites angulatus Subzone of the Lower Lygistepollenites balmei Zone). The Brandy Creek, Hotham Heights and Mount Buller sites are middle Eocene (Lutetian-Bartonian), with a minimum age of 39.58 ± 0.30 Ma, as inferred from 40Ar/39Ar analyses of overlying basalts, and a maximum age of ∼ 44 Ma inferred from palynostratigraphy (Lower Nothofagidites asperus Zone). The Kiandra Diggings site is early Miocene (Aquitanian), with a minimum age of 21.67 ± 0.26 Ma as inferred from 40Ar/39Ar dating and a maximum age of 23.03 Ma inferred from palynostratigraphy (Middle Proteacidites tuberculatus Zone). In the context of their revised ages, the recovered macrofloras suggest that subtropical climates prevailed during the late Paleocene (Cambalong Creek) and middle Eocene (Brandy Creek and Hotham Heights), whereas temperate climates existed during the early Miocene (Kiandra Diggings). For the middle Eocene sites, there is no discernible difference in paleotemperature estimates between lowland and highland sites, suggesting relatively low (< 550 m) paleoelevations for the present-day highland sites during the middle Eocene. This suggests that most of the present-day elevation (> 1400 m) was produced by post-middle Eocene uplift.

The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) is commonly associated with the oxygenation of the Earth's ocean-atmosphere system, yet the timing and duration of these redox changes remain uncertain. In particular, it is unclear how shallow marine environments, which were likely crucial habitats for early eukaryotic organisms, responded to Neoproterozoic oxygenation. This study uses trace and rare earth element geochemistry to determine shallow marine redox conditions during the deposition of the late Tonian (ca. 760 Ma) Devede Formation, northern Namibia. In this unit, although carbonate phases (microbialite, former aragonite and high Mg calcite marine cements, and primary dolomite marine cements) generally exhibit no Ce/Ce* anomaly, rare negative values (up to 0.55) are consistent with short-lived periods of oxygenation. In contrast, the dominantly positive Eu/Eu* anomalies of the same phases suggest that basinal conditions were anoxic. These phases also exhibit low concentrations of redox-sensitive (U, V, Mo) and chal-cophile (Co, Cu, Cd, Zn, Pb) elements, suggesting that euxinic conditions were predominant during the depo-sition of the Devede Formation. Integration of this data (Ce/Ce* and Eu/Eu*) with that of analogous late Tonian (ca. 840 to 731 Ma) carbonate strata reveals that shallow marine settings were characterised by persistent anoxia and euxinia, suggesting that any potential increase in atmospheric O2 ca. 800 Ma (e.g. the Bitter Springs Anomaly) was insufficient to facilitate resilient ocean oxygenation. These findings suggest that the late Tonian oceans were likely challenging environments for complex (e.g. eukaryotic) life, and add to a growing body of evidence that the spatially variable redox conditions of late Proterozoic shallow marine settings likely reflect the complex nature of the Neoproterozoic Oxygenation Event.

The Middle-Lower Ordovician Yingshan Formation is an important reservoir unit in the Tazhong oil field (Tarim Basin, northwestern China). This oil field is deeply buried (>5500 m [>18,045 ft]) and has endured a complex diagenetic history. To understand the mechanisms of reservoir formation, we conduct a broad portfolio of geochemical analyses on the calcite cements that fill the pores and fractures in samples retrieved from wells penetrating the Yingshan Formation. Our results identify six diagenetic episodes, each associated with the emplacement of different cements, varying from marine conditions (C1), nearsurface to shallow burial (C2), intermediate-to-deep burial (C3 and C4), and infiltration of the formation with mixing of the underlying basinal brines with meteoric waters and hydrothermal fluids (C5 and C6) along northeast-trending strike-slip faults. The productive wells display evidence of a strong burial diagenetic overprint linked to exotic fluids (C5 and C6) along fractures. We conclude that eogenetic meteoric waters were vital in producing early diagenetic porosity within the Tazhong reservoir, which was subsequently refined by late burial (hyposons for the evolution of other geodynamically active cratonic basins.

The long-term evolution of the biosphere has caused fundamental shifts in environmental conditions and sedimentation at Earth's surface. While effects of the evolution of terrestrial vegetation on river systems have been explored in detail, facies models and possible shifts in sedimentation in pre-vegetation nearshore marine settings have not been sufficiently explored. The circa 800-million-year-old Burra Group, exposed in the Adelaide Fold Belt of South Australia, is a terrestrial to nearshore transgressive sedimentary succession associated with rifting. The Burra Group provides a record of marginal and shallow marine environments before the evolution of land plants. Environments are interpreted to record deposition in low energy fine-grained tidal flat, lagoon, tidal inlet, and beach-barrier environments (a beach-barrier system). The thick mudflats (up to 30 m) of the studied units indicate that vegetation is not essential for mudflat accumulation, contrary to some models. The fluvial-marine transition and the back-barrier-tidal inlet-beach-barrier transition is highly regular and there is little interbedding of units. The Tonian beach-barriers of the Burra Group do not backstep into the back-barrier region and aggrade much like stable modern systems backed by peat. These observations are suggestive of a stable barrier system in the absence of land plants. The high mud percentage and/or early carbonate cementation (including microbialite and stromatolite deposition during transgression) may have contributed to the stability of this beach-barrier system.

Basin-scale processes such as regional fluid flow and organic maturation are commonly associated with the pre-cipitation of Pb-Zn sulphide minerals. Despite extensive study, the origin of carbonate-hosted Pb-Zn mineralisa-tion (Mississippi Valley-type and Irish-type) remains controversial, with deposits viewed as exceptional features derived from unusual basin conditions (e.g. regional hydrothermal fluid flux). In order to explore the links be-tween mineralisation and broader-scale basinal processes, we have examined the relationship of base metal sul-phides to regional diagenetic phases in the Waulsortian Limestone (Feltrim Formation) of the world-class Irish Midlands Zn-Pb province. Using combined sedimentology, petrography, and trace/rare earth element chemistry of regional calcite cements, we have reconstructed the basinal fluid history before, during and after sulphide min-eralisation. The non-bright-dull cathodoluminescence and trace metal composition of calcite cements indicates that the Waulsortian Limestone was subjected to three major chemical environments during progressive burial diagenesis: 1. Near-surface/shallow-burial oxic conditions; 2. subsurface euxinic conditions; and 3. subsurface ferruginous (ferrosulphidic) conditions. Zn-Pb mineralisation occurred under stage 2 euxinic conditions. We suggest that euxinic conditions were initiated when hydrocarbons entered the Waulsortian succession re-gionally, associated with the introduction of sour gas (H2S) accumulations. Mixing between more oxic base metal-bearing and H2S-bearing fluids produced ideal conditions for voluminous Zn-Pb sulphide precipitation. In-creasing fluid anoxia then led to iron oxide dissolution with consequently increased iron sulphide precipitation, causing eventual depletion of H2S, and marked the onset of ferruginous conditions that terminated metal sulphide precipitation. Base metals were potentially derived from a range of sources that include Carboniferous seawater (and the dissolution of associated Mn/Fe oxyhydroxides), hydrocarbons, and basement-interacted ba-sinal brines. The chemical model proposed by this study is compatible with existing data on the Irish Zn-Pb Orefield and indicates that sulphide mineralisation is linked to normal basin-scale processes such as subsidence, aquifer redox evolution and hydrocarbon generation. These results may explain many other global occurrences of carbonate-hosted Zn-Pb mineralisation and provide new insights into the chemical and redox processes that occur during burial diagenesis.(c) 2022 Elsevier B.V. All rights reserved.

The Latrobe Group in the Gippsland Basin consists of well-developed, stacked shoreface and coastal plain deposits, cut by large, incised channel systems up to 500 m deep. These large channels are conspicuous on seismic intersections and have previously been interpreted as the result of fluvial incision associated with basin inversion/compressional tectonics. Here we have used 3D seismic, well log, and core to re-evaluate the origin of these channel systems. We interpret three major phases of channel incision-infilling; Tuna Channel 1, Tuna Channel 2 and the Marlin Channel. Biostratigraphic data shows that the channels range in age from the Palaeocene-Eocene transition to the Late Eocene. These ages indicate channel formation occurred prior to the onset of compressional tectonics in the Gippsland Basin, which began at the Eocene-Oligocene transition. Correlation of these channels with coeval palaeoshoreface systems reveals that the channels are located seaward of, and in close proximity to these shorelines. Well data indicates that the channel fill sediments are of marine origin, consisting of glauconite, dinoflagellates, foraminifera, and marine macro fossils. We do not observe evidence for a regional unconformity which would correlate with over 500 m of subaerial channel incision. These channels are therefore interpreted as submarine canyons, rather than channels of fluvial origin. They are interpreted as shelf-incising, with canyon heads located close to coeval palaeoshorelines where they received sediment via longshore/littoral processes. The erroneous identification of submarine canyons as incised valleys in sedimentary basins has significant implications for tectonic and eustatic histories globally. (C) 2022 Elsevier B.V. All rights reserved.