Fig. 2: Plot of the Δ’¹⁷O versus δ¹⁸O of the I-type cosmic spherules (ICS).

The linear regression (gray solid line) is defined by modern I-type cosmic spherules¹¹. The majority of fossil spherules are diagenetically altered (black ellipse). The gray-blue dashed curves represent equilibration of oxygen from specimens with modern oxygen isotope composition and oxygen from different meteoric waters⁵⁴. The brown-red dotted curves represent equilibration of oxygen from spherules with a presumed unaltered oxygen isotope composition (brown-red hexagon) with oxygen from meteoric waters⁵⁴, indicating a lower Δ’¹⁷O of the Middle Triassic atmosphere (cyan star). The vertical lines of both equilibration line types indicate 10% intervals. Error bars and envelopes are 1SD.