Thallium isotopes reveal protracted anoxia during the Toarcian (Early Jurassic) associated with volcanism, carbon burial, and mass extinction

For this study, we generated thallium (Tl) isotope records from two anoxic basins to track the earliest changes in global bottom water oxygen contents over the Toarcian Oceanic Anoxic Event (TOAE; ∼183 Ma) of the Early Jurassic. The T-OAE, like other Mesozoic OAEs, has been interpreted as an expansion of marine oxygen depletion based on indirect methods such as organic-rich facies, carbon isotope excursions, and biological turnover. Our Tl isotope data, however, reveal explicit evidence for earlier global marine deoxygenation of ocean water, some 600 ka before the classically defined T-OAE. This antecedent deoxygenation occurs at the Pliensbachian/Toarcian boundary and is coeval with the onset of initial large igneous province (LIP) volcanism and the initiation of a marine mass extinction. Thallium isotopes are also perturbed during the T-OAE interval, as defined by carbon isotopes, reflecting a second deoxygenation event that coincides with the acme of elevated marine mass extinctions and the main phase of LIP volcanism. This suggests that the duration of widespread anoxic bottom waters was at least 1 million years in duration and spanned early to middle Toarcian time. Thus, the Tl data reveal a more nuanced record of marine oxygen depletion and its links to biological change during a period of climatic warming in Earth’s past and highlight the role of oxygen depletion on past biological evolution

Utility of organic carbon isotope data from the Salina Group halite (Michigan Basin): A new tool for stratigraphic correlation and paleoclimate proxy resource

Long-term global carbon isotope records (δ13Ccarb and δ13Corg) for the Silurian have been largely derived from unrestricted openmarine carbonates and shales. Here, we demonstrate how organic carbon harvested from halite-dominated evaporite deposits in a restricted intracratonic basin can be used to produce a carbon isotope record. Inorganic and organic carbon isotope data were generated and compared from four subsurface cores from the Silurian Michigan Basin, representing unrestricted carbonate and restricted evaporite/carbonate deposition. The δ13Ccarb and δ13Corg records exhibit a number of long-term trends and major carbon isotope excursions (CIE) that are correlated with the globally identified Ireviken, Mulde, and Linde events. These data provide temporal and stratigraphic constraints in rocks where paleontological data are sparse or absent. They also potentially highlight the effect of enhanced local evaporation on isotope fractionation. This new technique for generating a long-term organic carbon isotope profile from Silurian halite sequences, which can be correlated to the global curve, is of broad interest to the geoscience and paleoclimate science communities. These data not only provide a valuable tool for understanding the chronostratigraphic framework within an evaporative interior basin, but they also provide a rare temporal link between periods of prolonged evaporite deposition and events of known paleoclimate change

The glass ramp of Wrangellia: Late Triassic to Early Jurassic outer ramp environments of the McCarthy Formation, Alaska, U.S.A.

The marine record of the Triassic–Jurassic boundary interval has been studied extensively in shallow-marine successions deposited along the margins of Pangea, particularly its Tethyan margins. Several of these successions show a facies change from carbonate-rich to carbonate-poor strata attributed to the consequences of igneous activity in the Central Atlantic Magmatic Province (CAMP), which included a biocalcification crisis and the end-Triassic mass extinction. Evidence for a decline in calcareous and an increase in biosiliceous sedimentation across the Triassic–Jurassic boundary interval is currently limited to the continental margins of Pangea with no data from the open Panthalassan Ocean, the largest ocean basin. Here, we present a facies analysis of the McCarthy Formation (Grotto Creek, southcentral Alaska), which represents Norian to Hettangian deepwater sedimentation on Wrangellia, then an isolated oceanic plateau in the tropical eastern Panthalassan Ocean. The facies associations defined in this study represent changes in the composition and rate of biogenic sediment shedding from shallow water to the outer ramp. The uppermost Norian to lowermost Hettangian represent an ~ 8.9-Myr-long interval of sediment starvation dominated by pelagic sedimentation. Sedimentation rates during the Rhaetian were anomalously low compared to sedimentation rates in a similar lowermost Hettangian facies. Thus, we infer the likelihood of several short hiatuses in the Rhaetian, a result of reduced input of biogenic sediment. In the Hettangian, the boundary between the lower and upper members of the McCarthy Formation represents a change in the composition of shallow-water skeletal grains shed to the outer ramp from calcareous to biosiliceous. This change also coincides with an order-of-magnitude increase in sedimentation rates and represents the transition from a siliceous carbonate-ramp to a glass ramp ~ 400 kyr after the Triassic–Jurassic boundary. Sets of large-scale low-angle cross-stratification in the Hettangian are interpreted as a bottom current–induced sediment drift (contouritic sedimentation). The biosiliceous composition of densites (turbidites) and contourites in the Hettangian upper member reflects the Early Jurassic dominance of siliceous sponges over Late Triassic shallow-water carbonate environments. This dominance was brought about by the end-Triassic mass extinction and the collapse of the carbonate factory, as well as increased silica flux to the ocean as a response to the weathering of CAMP basalts. The presence of a glass ramp on Wrangellia supports the hypothesis that global increases in oceanic silica concentrations promoted widespread biosiliceous sedimentation on ramps across the Triassic to Jurassic transition

The glass ramp of Wrangellia: Late Triassic to Early Jurassic outer ramp environments of the McCarthy Formation, Alaska, U.S.A.

The marine record of the Triassic–Jurassic boundary interval has been studied extensively in shallow-marine successions deposited along the margins of Pangea, particularly its Tethyan margins. Several of these successions show a facies change from carbonate-rich to carbonate-poor strata attributed to the consequences of igneous activity in the Central Atlantic Magmatic Province (CAMP), which included a biocalcification crisis and the end-Triassic mass extinction. Evidence for a decline in calcareous and an increase in biosiliceous sedimentation across the Triassic–Jurassic boundary interval is currently limited to the continental margins of Pangea with no data from the open Panthalassan Ocean, the largest ocean basin. Here, we present a facies analysis of the McCarthy Formation (Grotto Creek, southcentral Alaska), which represents Norian to Hettangian deepwater sedimentation on Wrangellia, then an isolated oceanic plateau in the tropical eastern Panthalassan Ocean. The facies associations defined in this study represent changes in the composition and rate of biogenic sediment shedding from shallow water to the outer ramp. The uppermost Norian to lowermost Hettangian represent an ~ 8.9-Myr-long interval of sediment starvation dominated by pelagic sedimentation. Sedimentation rates during the Rhaetian were anomalously low compared to sedimentation rates in a similar lowermost Hettangian facies. Thus, we infer the likelihood of several short hiatuses in the Rhaetian, a result of reduced input of biogenic sediment. In the Hettangian, the boundary between the lower and upper members of the McCarthy Formation represents a change in the composition of shallow-water skeletal grains shed to the outer ramp from calcareous to biosiliceous. This change also coincides with an order-of-magnitude increase in sedimentation rates and represents the transition from a siliceous carbonate-ramp to a glass ramp ~ 400 kyr after the Triassic–Jurassic boundary. Sets of large-scale low-angle cross-stratification in the Hettangian are interpreted as a bottom current–induced sediment drift (contouritic sedimentation). The biosiliceous composition of densites (turbidites) and contourites in the Hettangian upper member reflects the Early Jurassic dominance of siliceous sponges over Late Triassic shallow-water carbonate environments. This dominance was brought about by the end-Triassic mass extinction and the collapse of the carbonate factory, as well as increased silica flux to the ocean as a response to the weathering of CAMP basalts. The presence of a glass ramp on Wrangellia supports the hypothesis that global increases in oceanic silica concentrations promoted widespread biosiliceous sedimentation on ramps across the Triassic to Jurassic transition

New evidence for a long Rhaetian from a Panthalassan succession (Wrangell Mountains, Alaska) and regional differences in carbon cycle perturbations at the Triassic-Jurassic transition

The end-Triassic mass extinction is one of the big five extinction events in Phanerozoic Earth history. It is linked with the emplacement of the Central Atlantic Magmatic Province and a host of interconnected environmental and climatic responses that caused profound deterioration of terrestrial and marine biospheres. Current understanding, however, is hampered by (i) a geographically limited set of localities and data; (ii) incomplete stratigraphic records caused by low relative sea-level in European sections during the Late Triassic and earliest Jurassic; and (iii) major discrepancies in the estimated duration of the latest Triassic Rhaetian that limit spatiotemporal evaluation of climatic and biotic responses locally and globally. Here, we investigate the Late Triassic–Early Jurassic time interval from a stratigraphically well-preserved sedimentary succession deposited in tropical oceanic Panthalassa. We present diverse new data from the lower McCarthy Formation exposed at Grotto Creek (Wrangell Mountains, southern Alaska), including ammonoid, bivalve, hydrozoan, and conodont biostratigraphy; organic carbon isotope (δ13Corg) stratigraphy; and CA-ID TIMS zircon U–Pb dates. These data are consistent with a Norian-Rhaetian Boundary (NRB) of ∼209 Ma, providing new evidence to support a long duration of the Rhaetian. They also constrain the Triassic-Jurassic boundary (TJB) to a ∼6 m interval in the section. Our TJB δ13Corg record from Grotto Creek, in conjunction with previous data, demonstrates consistent features that not only appear correlative on a global scale but also shows local heterogeneities compared to some Tethyan records. Notably, smaller excursions within a large negative carbon isotope excursion [NCIE] known from Tethyan localities are absent in Panthalassan records. This new comparative isotopic record becomes useful for (i) distinguishing regional overprinting of the global signal; (ii) raising questions about the ubiquity of smaller-scale NCIEs across the TJB; and (iii) highlighting the largely unresolved regional vs. global scale of some presumed carbon cycle perturbations. These paleontological and geochemical data establish the Grotto Creek section as an important Upper Triassic to Lower Jurassic succession due to its paleogeographic position and complete marine record. Our record represents the best documentation of the NRB and TJB intervals from Wrangellia, and likely the entire North American Cordillera

New evidence for a long Rhaetian from a Panthalassan succession (Wrangell Mountains, Alaska) and regional differences in carbon cycle perturbations at the Triassic-Jurassic transition

The end-Triassic mass extinction is one of the big five extinction events in Phanerozoic Earth history. It is linked with the emplacement of the Central Atlantic Magmatic Province and a host of interconnected environmental and climatic responses that caused profound deterioration of terrestrial and marine biospheres. Current understanding, however, is hampered by (i) a geographically limited set of localities and data; (ii) incomplete stratigraphic records caused by low relative sea-level in European sections during the Late Triassic and earliest Jurassic; and (iii) major discrepancies in the estimated duration of the latest Triassic Rhaetian that limit spatiotemporal evaluation of climatic and biotic responses locally and globally. Here, we investigate the Late Triassic–Early Jurassic time interval from a stratigraphically well-preserved sedimentary succession deposited in tropical oceanic Panthalassa. We present diverse new data from the lower McCarthy Formation exposed at Grotto Creek (Wrangell Mountains, southern Alaska), including ammonoid, bivalve, hydrozoan, and conodont biostratigraphy; organic carbon isotope (δ13Corg) stratigraphy; and CA-ID TIMS zircon U–Pb dates. These data are consistent with a Norian-Rhaetian Boundary (NRB) of ∼209 Ma, providing new evidence to support a long duration of the Rhaetian. They also constrain the Triassic-Jurassic boundary (TJB) to a ∼6 m interval in the section. Our TJB δ13Corg record from Grotto Creek, in conjunction with previous data, demonstrates consistent features that not only appear correlative on a global scale but also shows local heterogeneities compared to some Tethyan records. Notably, smaller excursions within a large negative carbon isotope excursion [NCIE] known from Tethyan localities are absent in Panthalassan records. This new comparative isotopic record becomes useful for (i) distinguishing regional overprinting of the global signal; (ii) raising questions about the ubiquity of smaller-scale NCIEs across the TJB; and (iii) highlighting the largely unresolved regional vs. global scale of some presumed carbon cycle perturbations. These paleontological and geochemical data establish the Grotto Creek section as an important Upper Triassic to Lower Jurassic succession due to its paleogeographic position and complete marine record. Our record represents the best documentation of the NRB and TJB intervals from Wrangellia, and likely the entire North American Cordillera