Varieties of Rafting Experience: Application of Novel Techniques to Develop Objective Criteria for Distinguishing Cryogenic and Biogenic Transport Processes

Atmospheric CO₂ was very high (~1000-6000 ppm) during the late Triassic (1), a period characterized as one of the few times in Earth's history lacking polar glacial ice. Recently, however, Olsen et al. (2) argued that seasonal freezing occurred in the paleo-arctic (~70°N) during this time based on lacustrine strata of the Junggar Basin, NW China (3). The main observation is the presence of abundant outsized (0.1-12 mm) lithic clasts interpreted to be seasonal lake-ice-rafted debris in otherwise fine-grained, fish and amphibian-bearing mudstone. This discovery provides new context for early dinosaur evolution and the end Triassic extinction (4).

Alternatives to ice rafting in the Mesozoic, when there is little additional evidence of freezing conditions, include biogenic transport mechanisms for dropstones such as algal rafting, root rafting and gastroliths or processes such as mudflows. Distinguishing among transport hypotheses has been problematic because rigorous criteria or quantitative techniques that can discriminate between by cryogenic, biogenic, or mass flow mechanisms are lacking.

We present a novel approach to this problem by using continuous, quantitative, laser diffraction granulometry (0.1 to 2000 microns) on disaggregated Junggar mudstones as well modern sediment samples where depositional processes are known. We show that quantitative granulometry results for ice rafting, algal rafting, and mudflows are significantly different, with uncertainty estimates generated by bootstrap resampling of replicate measurements. We also use image analysis of the CT scanned mudstones to independently calculate distribution, orientation, and size of the coarse grains, thus validating the disaggregated mudstone method, and use SEM images to recognize characteristic grain surface features. Developing and defining these rigorous, quantitative criteria to distinguish specific lithic clast rafting mechanisms are necessary as we continue to explore deep-time environmental archives for evidence of freezing conditions.

1, Schaller+ 2015 GSA Bull 127:661.

2, Olsen+ 2018 Geophys Res. Abstr 20:EGU2018-11440-2.

3, Olsen+ 2018 GSA Abst Prog 50: doi: 10.1130/abs/2018AM-325061

4, Sha+ 2015 PNAS 112:362.