The rise of pinnacle reefs : a step change in marine evolution triggered by perturbation of the global carbon cycle

The first appearance of pinnacle reef tracts, composed of hundreds to thousands of localized biogenic structures protruding tens to hundreds of meters above the surrounding mid-Silurian seafloor, represents a step change in the evolution of the marine biosphere. This change in seafloor morphology opened a host of new ecological niches that served as "evolutionary cradles" for organism diversification. However, the exact timing and driver's of this event remain poorly understood. These uncertainties remain, in large part, due to a paucity of index fossils in the reef facies, the difficulty of correlating between the offshore pinnacle reefs and more temporally well-constrained shallow marine fades, and cryptic unconformities that separate amalgamated reefs. Here we use delta C-13(carb) stratigraphy within a sequence stratigraphic framework to unravel these complex relationships and constrain the origination of Silurian pinnacle reef tracts in the North American midcontinent to near the Pt. celloni Superzone-Pt. am. amorphognathoides Zonal Group boundary of the mid-Telychian Stage. In addition, we identify a striking relationship between pulses of reef development and changes in global delta C-13(carb) values and sea level. Viewed through this new perspective, we correlate prolific periods of reef development with short-lived carbon isotope (delta C-13(carb)) excursions and eustatic sea level change that, ultimately, reflect perturbations to the global carbon cycle. From changes in the dominance of microbial reefs of the Cambrian to metazoan colonization of reefs in the Middle Ordovician, through the subsequent collapse of metazoan diversity with the Late Ordovician mass extinction, and the first appearance of early Silurian (Llandovery) pinnacle reef tracts and their proliferation during the late Silurian (Wenlock-Pridoli) and Devonian, major reef formation intervals increasingly coincide with delta C-13(carb) excursions. These patterns suggest that Paleozoic reef evolution was the product of environmental forcing by perturbations of the global carbon cycle

Upper Ordovician hardgrounds – from localized surfaces to global biogeochemical events

Upper Ordovician hardgrounds display a spectrum of complexity reflecting a range of local to global-scale processes. Hardgrounds are cemented seafloor surfaces typically marked by the presence of encrusting taxa and borings. Many hardgrounds show evidence for successive episodes of colonization by hard substrate specialists and are associated with localized evidence of seafloor erosion such as overhangs and reworked concretions. They commonly show trace amounts of pyrite and dolomite cements indicating an association with sulfate reduction. The most widespread hardgrounds are highly complex and unravelling their history provides insights into global biogeochemical events. The Curdsville and Kirkfield hardgrounds in the Appalachian Basin (Kentucky and Ontario) represent relatively simple end members of the hardground spectrum. They covered 10s to 100s km2 and formed relatively quickly during the early Katian. They display both planar to subplanar and hummocky to topographically complex surfaces (cm-scale) and contain highly diverse encrusting echinoderm faunas. Study of these surfaces yields important insights into the evolutionary history of encrusting communities. By contrast, the slightly younger hardground at the top of the Galena Group (Ka1) is a surface that is present throughout most of the Midcontinent Basin (>7.5 à 105km2). It is an example of a highly complex surface that was repeatedly modified by erosion and mineralization. Near the eastern margin of the basin in Indiana, the capping Galena hardground is pinnacled with cavity-filling sharpstone clasts, phosphate grains and bored crusts, iron ooids, and pyritic impregnated surfaces. It is onlapped by graptolitic shales of the Kope Formation (Fm) (Ka1) indicating an unconformity of approximately 1 m.y. To the west, in Illinois, the Kope Fm is erosionally truncated and the hardground is directly overlain by graptolitic shales of the Waynesville Fm (Ka3), where the unconformity expands to nearly 4 m.y. Toward Iowa, the hardground is onlapped by meters of phosphorite. Taken together, these observations reveal that the capping Galena Group hardground reflects a complicated history of repeated subaerial exposure, karsting, and marine flooding by a dysoxic to anoxic water mass with fluctuating redox conditions, similar to the age equivalent hardground at the base of the Fjäcka Shale in the Baltic Basin. Thus, hardground studies provide important insights for resolving the temporal continuity of the Upper Ordovician rock record and unravelling processes that controlled carbonate precipitation and dissolution and the evolution of sea floor communities. Some simple hardgrounds may have formed through random exhumation of cemented sediments on the sea floor through the effects of storm scour. However, their clustering into certain portions of the Upper Ordovician suggests that processes that affected sea water chemistry may also be involved. The most complex surfaces reflect major environmental perturbations with large amplitude sea level oscillations and redox changes that in some cases generated rare-earth enriched phosphorites

Can facies act as a chronostratigraphical tool?

Results demonstrate that the Appalachian ironstones seem to reflect the same microbially-mediated iron mineralization already documented in the Carnic Alps

Upper Ordovician chronostratigraphic correlation between the Appalachian and Midcontinent basins

Study of a subsurface core (named F688) from northern Indiana provides integrated data sets linking Katian chronostratigraphic records of the Appalachian and Midcontinent basins. The F688 core shows a variety of shallow- and deep-water facies containing numerous, well-preserved and zonally significant fossil species and diagnostic chemostratigraphic patterns. The succession belonging to the Cincinnatian Regional Stage in the F688 core is 210 m thick. Detailed benchtop examination of the succession revealed several phosphatic intervals, rich brachiopod faunas, multiple graptolitic horizons, and at least two tephras. Elemental analysis was conducted at 60 cm spacing quantifying lithofacies composition. Based on these results, the succession was assigned to six previously defined lithostratigraphic units (Kope, Waynesville, Liberty, Whitewater, Elkhorn, and Fort Atkinson formations). This lithostratigraphic succession shares components with both the Appalachian and Midcontinent basins, suggesting deposition near their shared margin. Twenty samples yielded abundant, well-preserved, low-diversity conodont assemblages with long-ranging taxa that clearly demarcate the position of the OrdovicianâSilurian boundary at the top of the succession in the core. More than fifty palynologic samples, targeting graptolite-bearing intervals, were processed for chitinozoans and produced important new insights. The Kope Formation contains the chitinozoan species Belonechitina kjellstromi, Hercochitina downiei, and Clathrochitina sp. nov., co-occurring with a graptolite assemblage suggestive of the Geniculograptus pygmaeus Zone. Samples from the overlying Waynesville Formation produced graptolites indicative of the upper G. pygmaeus to Paraorthograptus manitoulinensis zones co-occurring with the long-ranging chitinozoan species Belonechitina micracantha and Plectochitina spongiosa as well as several new species of the genera Tanuchitina and Hercochitina. Higher in the core, the Liberty, Whitewater, Elkhorn, and Fort Atkinson formations yielded chitinozoan species characteristic of the upper Katian biozones of Anticosti Island and Nevada, such as Tanuchitina anticostiensis, Hercochitina longi, and Eisenackitina ripae. Results of δ13Ccarb analysis reveal partial preservation of the Kope, Waynesville, and Elkhorn excursions. A tephra in the rising limb of the Waynesville Excursion yielded needle-shaped clear zircons that will provide a high-precision U-Pb age. The Fort Atkinson Formation is overlain by the Brassfield Formation containing Silurian conodonts and δ13Ccarb values suggesting an Aeronian age. Chronostratigraphic data from our study of the F688 core resolves longstanding uncertainty about correlations between strata of Katian Age in the Appalachian and Midcontinent basins. Integration of core F688 with our other regional chronostratigraphic data in the Midcontinent Basin demonstrates that the Fort Atkinson Formation of the Indiana and Illinois subsurface is age equivalent to the Fernvale Formation of Tennessee, Arkansas, and Oklahoma. Across this area, the Fernvale is overlain by graptolitic shales of the uppermost P. manitoulinensis to basal Dicellograptus complanatus graptolite zones. By contrast, the type Fort Atkinson Formation of Iowa is interpreted to occur completely within the younger D. complanatus Zone. These regional correlations taken as a whole suggest that the uppermost Katian (all of Ka4) and all but the uppermost Hirnantian are missing throughout much of the Appalachian Basin. By contrast, the Midcontinent Basin contains a much more complete upper Katian and Hirnantian succession. Our comprehensive approach is correcting temporal miscorrelation and providing robust chronostratigraphic context for study of biogeochemical events, which will further enable us to disentangle proxy data and identify the processes that drove the Katian diversity peak and culminated in the Late Ordovician mass extinction

The late Katian Elkhorn event: precursor to the Late Ordovician mass extinction

The late Katian Elkhorn event is a biogeochemical perturbation preceding the Late Ordovician mass extinction (LOME) with an exceptional record in the United States (U.S.). Results of our recent studies in this interval allow revised temporal ordering to strata across multiple basins providing insights into the magnitude of environmental disturbance and associated processes and feedbacks. The record of the Elkhorn event spans portions of the Appalachian and Midcontinent basins in the eastern U.S. and the Williston Basin and Cordilleran margin in the west. Our work focuses heavily on the Midcontinent Basin in particular, as it shares many characteristics of size, tectonic setting, and lithofacies with the Baltic Basin, providing the potential for resolving global signatures of the event. In its type-area, the Cincinnatian Series ends with the Elkhorn event. The succession is marked by shallowing from subtidal to marginal marine facies, capped by a karstic sequence boundary. Our new conodont data demonstrate that an overlying white to pink crinoidal grainstone package, previously assigned to the basal Silurian âwhiteâ Brassfield Formation near the Ohio-Indiana state line, is in fact Upper Ordovician. Further, δ13Ccarb values in this unit are elevated, in line with later phases of the Elkhorn event (2â° more positive than reported Rhuddanian values). These findings support a correlation of the grainstone interval with the Fernvale Formation of central Tennessee. To the east, much of the northern Appalachian Basin was overfilled with widespread marginal marine to terrestrial red beds by the onset of the Elkhorn event, while the Midcontinent Basin to the west remained relatively sediment starved. In the southern Midcontinent, the mid-Elkhorn event sequence boundary was onlapped by ironstone deposition (lower Fernvale Formation). The ironstones are overlain by sparry and hematitic grainstones with localized bioherms. In Arkansas, where the Fernvale is thickest (>30 m), the sparry phase gives way upward to manganese carbonates and bioherms. Across the region, the Fernvale is, in turn, cut by a sequence boundary, suggesting a yet higher Katian sequence, and is perforated by paleokarst pockets that are filled and overlain by upper Katian (Ka4) sediments. This sequence boundary is onlapped by black shales and the thickest (>10 m) phosphorite of the Ordovician at the end of the Elkhorn event. Previous studies have suggested age equivalence of the Elkhorn and Paroveja δ13Ccarb excursions in Laurentia and Baltica. Despite the attraction of aligning the latest Richmondian and Pirgu regional stages, our data sets demonstrate that this is a miscorrelation. Critical to this revision are new integrated biostratigraphic and chemostratigraphic data sets in a transect from the margin of the Appalachian Basin into the Midcontinent Basin. The new data reveal that the Elkhorn Shale and Fernvale Formation are overlain by the Brainard and laterally equivalent Sylvan, and Mannie shales. These shale successions contain graptolites of the complanatusand pacificus zones. Thus, the Elkhorn event occurred in the latest manitoulinensis Zone, suggesting correlation with the Baltic Moe δ13Ccarb excursion. Our extensive new data sets provide regional chronostratigraphic correlation of strata deposited during the Elkhorn event. When temporally ordered, these records provide evidence for high amplitude sea level oscillations, major redox fluctuations, and reef pulses that demonstrate the waxing and waning of continental ice sheets on Gondwana and the spread of oceanic anoxia only a few million years before the LOME. These findings further call into question traditional models of rapid glaciation during a long-lived greenhouse state as the sole driver of the LOME and emphasize the need for new integrated Upper Ordovician research initiatives to better characterize Katian events

Composition and source of salinity of ore-bearing fluids in Cu-Au systems of the Carajás Mineral Province, Brazil

The composition and Cl/Br – Na/Cl ratios of highly saline aqueous fluid inclusions from large tonnage (> 100 t) IOCG deposits (Sossego, Alvo 118, and Igarapé Bahia) and a Paleoproterozoic intrusion-related Cu-Au- (Mo-W-Bi-Sn) deposit (Breves; < 50 Mt)) in the Carajás Mineral Province have been analysed by LA-ICP-MS and ion chromatography. In both Cu-Au systems, brine inclusions are Ca-dominated (5 to 10 times more than in porphyry Cu-Au fluids), and contain percent level concentrations of Na and K. IOCG inclusion fluids, however, contain higher Sr, Ba, Pb, and Zn\ud concentrations, but significantly less Bi, than inclusions\ud from the intrusion-related Breves deposit. Cu is consistently below detection limits in brine inclusions from\ud the IOCG and intrusion-related systems, with Fe not detected in the latter. Cl/Br and Na/Cl ratios of the IOCG\ud inclusion fluids range from entirely evaporative brines\ud (bittern fluids; e.g. Igarapé Bahia and Alvo 118) to values\ud that indicate mixing with magma-derived brines. Cl/Br and Na/Cl ratios of the Breves inclusion fluids strongly suggest the involvement of magmatic brines, but possibly\ud also incorporated bittern fluids. Collectively, these data\ud demonstrate that residual evaporative and magmatic brines were important components of the fluid regime involved in the formation of Cu-Au systems in the Carajás Mineral Province

In situ quantification of Br and Cl in minerals and fluid inclusions by LA-ICP-MS: a powerful tool to identify fluid sources

Bromine and chlorine are important halogens for fluid source identification in the Earth's crust, but until recently we lacked routine analytical techniques to determine the concentration of these elements in situ on a micrometer scale in minerals and fluid inclusions. In this study, we evaluate the potential of in situ Cl and Br measurements by LA-ICP-MS through analysis of a range of scapolite grains with known Cl and Br concentrations. We assess the effects of varying spot sizes, variable plasma energy and resolve the contribution of polyatomic interferences on Br measurements. Using well-characterised natural scapolite standards, we show that LA-ICP-MS analysis allows measurement of Br and Cl concentrations in scapolite, and fluid inclusions as small as 16 µm in diameter and potentially in sodalite and a variety of other minerals, such as apatite, biotite, and amphibole. As a demonstration of the accuracy and potential of Cl and Br analyses by LA-ICP-MS, we analysed natural fluid inclusions hosted in sphalerite and compared them to crush and leach ion chromatography Cl/Br analyses. Limit of detection for Br is ~8 µg g−1, whereas relatively high Cl concentrations (> 500 µg g−1) are required for quantification by LA-ICP-MS. In general, our LA-ICP-MS fluid inclusion results agree well with ion chromatography (IC) data. Additionally, combined cathodoluminescence and LA-ICP-MS analyses on natural scapolites within a well-studied regional metamorphic suite in South Australia demonstrate that Cl and Br can be quantified with a ~25 µm resolution in natural minerals. This technique can be applied to resolve a range of hydrothermal geology problems, including determining the origins of ore forming brines and ore deposition processes, mapping metamorphic and hydrothermal fluid provinces and pathways, and constraining the effects of fluid–rock reactions and fluid mixing