ALBERT

Description: 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.

Description: 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.

Description: The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO2. In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206Pb/238U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.

Description: The emplacement of the Karoo Large Igneous Province (LIP) occurred synchronously with the Toarcian crisis (ca. 183 Ma), which is characterized by major carbon cycle perturbations. A marked increase in the atmospheric concentration of CO2 (pCO2) attests to significant input of carbon, while negative carbon isotope excursions (CIEs) in marine and terrestrial records suggest the involvement of a 12C-enriched source. Here we explore the effects of pulsed carbon release from the Karoo LIP on atmospheric pCO2 and δ13C of marine sediments, using the GEOCLIM carbon cycle model. We show that a total of 20,500 Gt C replicates the Toarcian pCO2 and δ13C proxy data, and that thermogenic carbon (δ13C of −36 ‰) represents a plausible source for the observed negative CIEs. Importantly, an extremely isotopically depleted carbon source, such as methane clathrates, is not required in order to replicate the negative CIEs. Although exact values of individual degassing pulses represent estimates, we consider our emission scenario realistic as it incorporates the available geological knowledge of the Karoo LIP and a representative framework for Earth system processes during the Toarcian.