Interpreting Early Triassic (Smithian) sea-level change and climate using sequence stratigraphy and oxygen isotopes of conodont apatite
The Early Triassic is conventionally interpreted to have a warm and ice-free climate. During this time, three globally recognized depositional sequences developed in response to ~My-scale sea-level changes. The Lower Triassic Lower Thaynes Formation records the Smithian (2nd sea-level cycle in the Early Triassic) in the western United States (Confusion Range and Weber Canyon, Utah). The Smithian portion of the Mikin Formation records an approximately time-equivalent sea-level cycle in northern India (Guling, Himachal Pradesh). The maximum flooding zone combined with existing age-diagnostic biostratigraphy allows for correlation between two sections in the western United States and one section in northern India suggesting the Smithian My-scale sea-level change was likely eustatic. Samples were collected for oxygen isotopic analyses of conodont apatite from the two field locations (in Utah) in a sequence stratigraphic framework to better understand the Smithian paleoclimate. Due to an up to ~1.5 ° disparity of values between the two locations, additional conodont elements were analyzed from the Guling, Himachal Pradesh, northern India, Bear Lake, southern Idaho, Wapiti Lake area in eastern British Columbia, Canada, and Sverdrup Basin in the Canadian Arctic. Conodont Alteration Index or CAI (a measure of post-burial thermal alteration based on color) was estimated for each of the locations listed above and range from 1.5 for Wapiti Lake and the Confusion Range to 5 for Guling. In addition, SEM images were taken to identify potential physical alteration of the conodonts for the Confusion Range (smooth surface with no signs of alteration), Weber Canyon (pitted surface with signs of potential alteration), and Guling (visibly the most pitted surface with signs of potential dissolution). The δ18O values for Weber Canyon range from ~14.4 to 15.8 °, the Confusion Range from ~16 to 16.9 °, northern India from ~15.8 to 16.5 °, Wapiti Lake from ~ 17.2 and 17.6 °, Sverdrup Basin range from ~14.5 and 14.8 °, and the Bear Lake value was ~ 16.5 °. Conodonts with a CAI of ï‚£ 3 or lower produced δ18O values that most likely reflect the primary Smithian ocean isotopic values. Assuming an ice-free ocean value of -1 °, sea-surface temperatures were calculated as ~35 to 38 ï‚°C for the paleotropical and ~32 to 34 ï‚°C for the paleosubtropical regions, which make sense given their latitudinal position. Warm ocean currents in the neo-Tethys Sea can potentially explain this discrepancy. All six locations indicate that the Smithian ocean was significantly warmer than the present ocean, and instead, most resemble the extreme greenhouse sea-surface temperatures calculated for mid- to late Cretaceous.
paleoclimate, carbonate sedimentology and stratigraphy, Triassic climate, paleoceanography, oxygen isotopes, conodonts
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