Emplacement Of Central Atlantic Magmatic Province Research Articles

Onset of long-lived silicic and alkaline magmatism in eastern North America preceded Central Atlantic Magmatic Province emplacement

Abstract The White Mountain magma series is the largest Mesozoic felsic igneous province on the eastern North American margin. Previous geochronology suggests that magmatism occurred over 50 m.y., with ages for the oldest units apparently coeval with the ca. 201 Ma Central Atlantic Magmatic Province, the flood basalt province associated with the end-Triassic mass extinction and the opening of the Atlantic Ocean. We use zircon U-Pb geochronology to show that emplacement of White Mountain magma series plutons was already underway at 207.5 Ma. The largest volcanic-plutonic complex, the White Mountain batholith, was emplaced episodically from ca. 198.5 Ma to ca. 180 Ma and is ~25 m.y. older than published ages suggest, and all samples we dated from the Moat Volcanics are between ca. 185 Ma and 180 Ma. The Moat Volcanics and the White Mountain batholith are broadly comagmatic, which constrains the age of a key Jurassic paleomagnetic pole. Our data indicate that a regional mantle thermal anomaly in eastern North America developed at least 5 m.y. prior to the main stage of Central Atlantic Magmatic Province flood basalt volcanism and suggest a geodynamic link between the White Mountain magma series and the Central Atlantic Magmatic Province.

Volcanic temperature changes modulated volatile release and climate fluctuations at the end-Triassic mass extinction

Emplacement of the Central Atlantic Magmatic Province (CAMP) is thought to have triggered global environmental changes and the end-Triassic mass extinction (ETE). However, the mechanisms linking volcanism and environmental change are unclear. Here we provide new insight into these linkages by measuring the abundance of both sedimentary five- to six-ring polycyclic aromatic hydrocarbons (PAHs) and mercury from strata deposited in shallow marine environments across the ETE at the GSSP Kuhjoch section in Austria and St. Audrie's Bay section in the UK. To contextualize these data, we report results from laboratory experiments measuring the production of SO2 and CO2 during heating of limestone and mudstone. ETE sediments record parallel enrichments of mercury and five- to six-ring PAHs, which could have been produced by intrusive magma (mainly sills) and lava flows during the early stage of the CAMP emplacement; these data indicate a direct link between massive gas emission from sill contact metamorphism and the ETE. The fraction of coronene – a highly condensed six-ring PAH that requires greater energy to form compared to smaller PAHs – accumulated in the sediments during these initial volcanic events is low, and it coincides with the terrestrial plant turnover and initial marine extinction. Coronene increases to medium values coinciding with the final marine extinction level. Our heating experiments of typical carbonate and mudstone materials show that relatively low temperature heating (>350°C) by sills releases massive amounts of SO2 on a 100 yr time scale, whereas higher temperature heating (500–600°C) forms more CO2 on the same time scale. The combination of our end-Triassic geochemical data and laboratory results implies that low heating by sills caused SO2-dominated gas emission to the stratosphere and low CO2 emission, inducing global cooling that could have precipitated the mass extinction. The subsequent increase in coronene content indicates higher volcanic temperature that would have volatilized CO2 rich gas; the consequence was a switch to greater CO2 release and long-term (>105 yr) global warming.

Ecosystem change and carbon cycle perturbation preceded the end-Triassic mass extinction

During the Phanerozoic, major global upheavals in life history and the carbon cycle are predominantly linked to the emplacement of large igneous provinces, but the delineation of a cause and effect framework remains unclear. The end-Triassic mass extinction (ETE) is temporally associated with emplacement of the Central Atlantic magmatic province (CAMP). A better understanding of precursor events to the ETE is essential if the mechanisms for this mass extinction are to be fully delineated. Here, we present new high-resolution data integrating petrographic, biotic, mercury, and carbon isotope analyses of the pre-extinction interval at the Ferguson Hill locality, Nevada (USA). We document the “precursor” carbon isotope excursion along with low Hg concentrations and sulphidic sediments prior to the ETE. A combination of proxies reveals disruptions to shallow marine ecosystems and biogeochemical cycles prior to the main phase of CAMP volcanism. We propose that episodic anoxic conditions led to the restructuring of shallow marine benthic ecosystems towards overall lower diversity, including more low oxygen tolerant taxa preceding the ETE. The timing of the initial marine ecosystem restructuring in eastern Panthalassa could be related to the early phase of CAMP emplacement, and implies that an early intrusive event initiated the ecosystem changes. These restructured marine ecosystems reflect the deteriorating environmental conditions leading up to the ETE that ultimately resulted in the ETE.

Thermogenic carbon release from the Central Atlantic magmatic province caused major end-Triassic carbon cycle perturbations

The Central Atlantic magmatic province (CAMP), the end-Triassic mass extinction (ETE), and associated major carbon cycle perturbations occurred synchronously around the Triassic-Jurassic (T-J) boundary (201 Ma). Negative carbon isotope excursions (CIEs) recorded in marine and terrestrial sediments attest to the input of isotopically light carbon, although the carbon sources remain debated. Here, we explore the effects of mantle-derived and thermogenic carbon released from the emplacement of CAMP using the long-term ocean-atmosphere-sediment carbon cycle reservoir (LOSCAR) model. We have tested a detailed emission scenario grounded by numerous complementary boundary conditions, aiming to model the full extent of the carbon cycle perturbations around the T-J boundary. These include three negative CIEs (i.e., Marshi/Precursor, Spelae/Initial, Tilmanni/Main) with sharp positive CIEs in between. We show that a total of ∼24,000 Gt C (including ∼12,000 Gt thermogenic C) replicates the proxy data. These results indicate that thermogenic carbon generated from the contact aureoles around CAMP sills represents a credible source for the negative CIEs. An extremely isotopically depleted carbon source, such as marine methane clathrates, is therefore not required. Furthermore, we also find that significant organic carbon burial, in addition to silicate weathering, is necessary to account for the positive δ13C intervals following the negative CIEs.

Evidence for magma–evaporite interactions during the emplacement of the Central Atlantic Magmatic Province (CAMP) in Brazil

Mafic sills of the Central Atlantic Magmatic Province (CAMP) in Brazil are temporally linked to the end-Triassic extinction (ETE), one of the largest mass extinctions of the Phanerozoic. The sills were emplaced into the volatile-rich sedimentary rocks of the Amazonas and Solimões basins in northern Brazil. Here we present a new geochemical study of 26 dolerite samples from 6 deep boreholes in the Brazilian basins, including whole-rock major and trace elements, whole-rock Sr–Nd isotopes and detailed biotite mineral chemistry. The bulk of the dolerites are characterized by phenocrysts of clinopyroxene and plagioclase in subophitic to intergranular textures, Fe–Ti oxides, and rare olivine and orthopyroxene. A different mineralogical assemblage (microphenocrysts of alkali-feldspar, quartz, biotite and apatite) is found in small independent domains, localized within the framework of coarser plagioclase and clinopyroxene laths. These fine-grained evolved domains crystallized in late-stage, evolved melt pockets in the interstitial spaces between earlier crystallized coarser grained crystals. The majority of the studied dolerites are generally evolved tholeiitic basalts and basaltic andesites with low TiO2 concentrations (<2.0 wt.%). Four samples have high TiO2 concentrations (>2.0 wt.%), and are found in the eastern part of the Amazonas Basin. Whole-rock major and trace element and Sr–Nd isotope geochemistry of both low- and high-Ti sills is similar to that of previously published CAMP rocks from the two magma types. Low-Ti sills show enriched isotopic signatures (143Nd/144Nd201Ma 0.51215 to 0.51244; 87Sr/86Sr201Ma 0.70568 to 0.70756), coupled with crustal-like characteristics in the incompatible element patters (e.g. depletion in Nb and Ta). Unaltered high-Ti samples show more depleted isotopic signatures (143Nd/144Nd201Ma 0.51260 to 0.51262; 87Sr/86Sr201Ma 0.70363 to 0.70398). Low-Ti dolerites from both the Amazonas and Solimões basins contain biotite with extremely high Cl concentrations (up to 4.7 wt.%). We show that there is a strong correlation between host-rock lithology and Cl concentrations in biotite from the dolerites, and interpret this to reflect large-scale crustal contamination of the low-Ti magmas by halite-rich evaporites. Our findings support the hypothesis that sill-evaporite interactions increased volatile release during the emplacement of CAMP. This strengthens the case for active involvement of this LIP in the end-Triassic crisis, and suggests that the sub-volcanic part of a LIP can be of major importance in driving climate change and mass extinctions.

Duration of and decoupling between carbon isotope excursions during the end-Triassic mass extinction and Central Atlantic Magmatic Province emplacement

Changes in δ13Ccarb and δ13Corg from marine strata occur globally in association with the end-Triassic mass extinction and the emplacement of the Central Atlantic Magmatic Province (CAMP) during the break up of Pangea. As is typical in deep time, the timing and duration of these isotopic excursions has remained elusive, hampering attempts to link carbon cycle perturbations to specific processes. Here, we report δ13Ccarb and δ13Corg from Late Triassic and Early Jurassic strata near Levanto, Peru, where intercalated dated ash beds permit temporal calibration of the carbon isotope record. Both δ13Ccarb and δ13Corg exhibit a broad positive excursion through the latest Triassic into the earliest Jurassic. The first order positive excursion in δ13Corg is interrupted by a negative shift noted in many sections around the world coincident with the extinction horizon. Our data indicate that the negative excursion lasts 85±25 kyrs, longer than inferred by previous studies based on cyclostratigraphy. A 260±80 kyr positive δ13Corg shift follows, during which the first Jurassic ammonites appear. The overall excursion culminates in a return to pre-perturbation carbon isotopic values over the next 1090±70 kyrs. Via chronologic, isotopic, and biostratigraphic correlation to other successions, we find that δ13Ccarb and δ13Corg return to pre-perturbation values as CAMP volcanism ceases and in association with the recovery of pelagic and benthic biota. However, the initiation of the carbon isotope excursion at Levanto predates the well-dated CAMP sills from North America, indicating that CAMP may have started earlier than thought based on these exposures, or that the onset of carbon cycle perturbations was not related to CAMP.

Mercury evidence for pulsed volcanism during the end-Triassic mass extinction

The Central Atlantic Magmatic Province (CAMP) has long been proposed as having a causal relationship with the end-Triassic extinction event (∼201.5 Ma). In North America and northern Africa, CAMP is preserved as multiple basaltic units interbedded with uppermost Triassic to lowermost Jurassic sediments. However, it has been unclear whether this apparent pulsing was a local feature, or if pulses in the intensity of CAMP volcanism characterized the emplacement of the province as a whole. Here, six geographically widespread Triassic-Jurassic records, representing varied paleoenvironments, are analyzed for mercury (Hg) concentrations and Hg/total organic carbon (Hg/TOC) ratios. Volcanism is a major source of mercury to the modern environment. Clear increases in Hg and Hg/TOC are observed at the end-Triassic extinction horizon, confirming that a volcanically induced global Hg cycle perturbation occurred at that time. The established correlation between the extinction horizon and lowest CAMP basalts allows this sedimentary Hg excursion to be stratigraphically tied to a specific flood basalt unit, strengthening the case for volcanic Hg as the driver of sedimentary Hg/TOC spikes. Additional Hg/TOC peaks are also documented between the extinction horizon and the Triassic-Jurassic boundary (separated by ∼200 ky), supporting pulsatory intensity of CAMP volcanism across the entire province and providing direct evidence for episodic volatile release during the initial stages of CAMP emplacement. Pulsatory volcanism, and associated perturbations in the ocean-atmosphere system, likely had profound implications for the rate and magnitude of the end-Triassic mass extinction and subsequent biotic recovery.

Geochemical consequences of intense pulse-like degassing during the onset of the Central Atlantic Magmatic Province

The Triassic–Jurassic boundary (TJB) is marked by one of the five largest mass extinctions of the Phanerozoic and the eruption of a large igneous province: the Central Atlantic Magmatic Province (CAMP). The TJB is characterized by a carbonate production crisis, by negative excursions in the carbon isotopic ratio of buried organic matter, and by successive peaks in atmospheric CO2. Here we use the numerical model GEOCLIM to explore the possible connections between the CAMP emplacement and the observed carbon cycle perturbations. Different degassing scenarios linked to the CAMP eruption are explored. We show that the emission of realistic amounts of CO2 that follow a geologically constrained degassing scenario as short-term peaks (less than 10ka) leads to successive decrease in carbonate production, as observed in the geological record. We also calculate the evolution of carbon isotopes and show that our model reproduces the amplitude of the isotopic excursion with a volcanic degassing of CO2 characterized by a carbon isotopic composition of −20‰. Such low values could be associated to carbon pools of light isotopic composition located at the transition zone [Cartigny, P. 2010, Earth and Planetary Science Letters, v. 296, p. 329–339] and not necessarily to biogenic methane release. Finally, the model predicts a succession of short-term CO2 rises, with an amplitude in close agreement with available proxy-based reconstructions.

The quest for chron E23r at Partridge Island, Bay of Fundy, Canada: CAMP emplacement postdates the end-Triassic extinction event at the North American craton

The Partridge Island stratigraphic section at the Bay of Fundy, Maritime Canada, reveals a continental sedimentary succession with the end-Triassic mass extinction level closely followed by basalts of the Central Atlantic Magmatic Province (CAMP). New Paleomagnetic data show that a short reverse magnetic polarity chron, correlative to E23r of the Newark Geomagnetic Polarity Time Scale (GPTS), is present below the extinction event. Organic carbon isotope data and basalt geochemistry further indicate that the onset of CAMP emplacement in the Bay of Fundy was roughly synchronous with emplacement in the Newark basin, but slightly postdates the oldest CAMP volcanism in Morocco by ∼20 ka. These results confirm the potential for long-distance CAMP correlations based on geochemical trace elements, indicate substantiate provincialism of latest Triassic palynoflora, and suggest a very concise period (&lt;&lt;100 ka) of CAMP emplacement in the northern Atlantic region.

Response of the marine infauna to Triassic–Jurassic environmental change: Ichnological data from southern England

The trace fossil record through the Triassic–Jurassic boundary interval is examined at three sites in central and southern England (St. Audrie's Bay, Somerset; Pinhay Bay, Devon; and Long Itchington, Warwickshire). The lower ‘Pre-Planorbis Beds’ of the Blue Lias Formation record low ichnotaxonomic diversity, low bioturbation intensity, small burrow diameters, and an absence of deep tier bioturbation. The stepwise reappearance of ichnotaxa following this interval is similar at the three sites, suggesting similar rates of recovery in the benthic marine ecosystem and highlighting the potential contribution of these trace fossils to stratigraphic correlation. Mass extinction in the Late Triassic is increasingly linked to the emplacement of the Central Atlantic Magmatic Province (CAMP). Timing of the onset of CAMP volcanism in the UK is currently imperfectly known, but potentially occurred within either the Westbury Formation or the Lilstock Formation. If the onset of CAMP was within the Lilstock Formation, the modestly diverse trace fossil assemblage of the Langport Member of the Lilstock Formation suggests that emplacement of CAMP had little lasting effect on the marine benthos in central and southern England. The decline in benthic activity recorded in the ‘Pre-Planorbis Beds’ appears to be unrelated to the onset of CAMP and any associated environmental change. It is more likely related to an episode of marine anoxia. An episode of marine anoxia also provides a suitable causal mechanism for the reduction of infaunal tiering.