Abstract
Abstract We show that the Late Triassic–Early Jurassic continental Arctic experienced wintertime freezing conditions, despite the exceptionally high atmospheric CO 2 levels, by quantifying common lake ice-rafted debris (L-IRD) identified in the Junggar Basin of Xinjian, NW China. This L-IRD consists of outsized (0.1–12 mm) lithic clasts ‘floating’ in otherwise fine-grained, profundal lake sediment matrix. Laser-diffraction grain-size analysis demonstrates that the grain-size distribution for lacustrine strata of Junggar Basin is very similar to modern sediments from the seasonally ice-covered Sea of Okhotsk, reflecting a similar depositional mechanism. Three-dimensional computed tomography and two-dimensional thin sections demonstrate that the outsized clasts are dispersed, rather than confined to sand lenses or layers. These results are inconsistent with alternative methods of bimodal sediment deposition such as mud flows, algae rafting or root rafting. The discovery of Triassic–Jurassic continental freezing provides new context for understanding global climate during periods with high-CO 2 conditions and climate and biotic changes in the Mesozoic Era.
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