Aviv Bachan

Modelling the impact of pulsed CAMP volcanism on pCO2 and δ13C across the Triassic–Jurassic transition

A sharp negative δ 13 C excursion coincides with the end-Triassic mass extinction. This is followed by a protracted interval of 13 C enrichment. These isotopic events occurred simultaneously with the emplacement of the Central Atlantic Magmatic Province (CAMP). Here we use a carbon cycle box model to explore the effects of episodic carbon release – constrained by recently developed high-resolution chronology – on atmospheric p CO 2 , ocean chemistry and the δ 13 C of the ocean–atmosphere carbon pool. Our results are consistent with previous modelling efforts in suggesting that the sharp negative δ 13 C excursion and acidification event associated with the extinction are best explained by the rapid release (<20 ka) of highly 13 C-depleted carbon (−70‰). However, our model also indicates that the likely short duration of the excursion requires organic carbon burial to have closely followed carbon injection. The age within the Hettangian of the large positive δ 13 C excursion which follows is currently uncertain. If early Hettangian in age, then our modelling indicates that the interval of 13 C enrichment was closely associated with the volcanic CO 2 pulses and p CO 2 peaks. If late Hettangian in age, then the 13 C enrichment must have lagged the carbon input substantially (by hundreds of thousands of years) and was associated with CO 2 drawdown and over-cooling. Our modelling highlights the need for improved age constraints on Hettangian stratigraphic sections in order to test between two distinct and contrasting possibilities: continuing carbon cycle instability due to recurrent perturbations from CAMP activity or a delayed recovery arising from internal biosphere dynamics.

Additive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundary limestones

The end-Triassic mass extinction coincided with a negative δ13C excursion, consistent with release of 13C-depleted CO2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high-resolution calcium isotope (δ44/40Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ13C, δ44/40Ca decreases by ∼0.8‰ in 20 m of section and then recovers to preexcursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ44/40Ca excursion is too large to be explained by changes to seawater δ44/40Ca alone, regardless of CO2 injection volume and duration. Less than 20% of the δ44/40Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ44/40Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end-Triassic mass extinction, as well as for interpreting other environmental events in the geologic past.

Uranium isotope evidence for an expansion of marine anoxia during the end‐Triassic extinction

The end-Triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia, suggesting that anoxia played a role in suppressing marine biodiversity. However, global changes in ocean anoxia are difficult to quantify using proxies for local anoxia. Uranium isotopes (δ238U) in CaCO3 sediments deposited under locally well-oxygenated bottom waters can passively track seawater δ238U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238U(VI) relative to 235U(VI) in anoxic marine sediments. We measured δ238U in shallow-marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m. We observe a ∼0.7‰ negative excursion in δ238U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ13C excursion and persisting for the duration of subsequent high δ13C values in the lower-middle Hettangian stage. The δ238U excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium. Based on output from a forward model of the uranium cycle, the excursion is consistent with a 40–100-fold increase in the extent of anoxic deposition occurring worldwide. Additionally, relatively constant uranium concentrations point toward increased uranium delivery to the oceans from continental weathering, which is consistent with weathering-induced eutrophication following the rapid increase in pCO2 during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ238U implies that anoxia could have delayed biotic recovery well into the Hettangian stage.

A model for the decrease in amplitude of carbon isotope excursions across the Phanerozoic

The geological cycling of carbon ties together the ocean-atmosphere carbon pool, Earths biosphere, and Earths sedimentary reservoirs. Perturbations to this coupled system are recorded in the carbon-isotopic (δ13C) composition of marine carbonates. Large amplitude δ13C excursions are typically treated as individual events and interpreted accordingly. However, a recent compilation of Phanerozoic carbon isotopic data reveals that δ13C excursions are a ubiquitous feature of the geologic record, and thus should be considered in concert. Analysis indicates that Phanerozoic carbon isotope excursions, as a whole, have characteristic durations of 0.5 to 10 M.yr. and exhibit declining amplitude over time. These commonalities suggest a shared underlying control. Here we demonstrate that sinusoidal modulation of the sensitivity of organic carbon and phosphate burial in a simple numerical model of the geologic carbon cycle results in large, asymmetric δ13C oscillations that exhibit their largest amplitudes in the 0.5 to 10 M.yr. period range. As anoxia is known to strongly modulate the C:P burial ratio of organic matter in sediments, we propose that sea-level oscillations were the primary source of sinusoidal modulation for the geologic carbon cycle, and that their degree of influence on the carbon cycle was determined by the state of oxygenation of bottom waters overlying the continental shelves. When oxygen minimum zones (OMZs) were large, shallow, and prone to expansion, sea-level changes would have had the capacity to drive large changes in the areal extent of OMZs in contact with the sea-floor, resulting in strong leverage on the burial sensitivity of organic carbon and phosphate, and thus on δ13C. Progressive oxygenation of the oceans, which was facilitated by biological innovations, resulted in a decline in the amplitude of δ13C excursions over the Phanerozoic, and the biogeochemical stabilization of the Earth System.