FIRST REPORT OF GLENDONITE IN THE UPPERMOST CAMBRIAN

“Glendonite”, a pseudomorph of mineral ikaite (CaCO₃·6H₂O), occurs in the uppermost Hwajeol Formation (Furongian; Stage 10) in the Taebaeksan Basin, Korea, eastern margin of Sino-Korean Block (North China) which was located in equatorial region near Siberia and Tarim during Late Cambrian. Owing to its idiosyncratic stability in water below 10°C, ikaite has long been regarded as indicator of ancient cold-water conditions.

The glendonites up to 10 mm long and 3 mm wide include pyramid, bipyramid and radial crystals now filled with calcite, and occur scattered within intervals of outer platform deposits in the uppermost part of the Hwajeol Formation. Available conodont biostratigraphy from the studied section shows that the glendonite-bearing interval occurs 15 m above FAD of Eocodonontus notchpeakensis, indicating the base of Cambrian Stage 10, which is coincident with global HERB negative carbon isotope excursion. Recurring mass extinctions of invertebrates combined with sharp fluctuations in global carbon cycles in the latest Cambrian are apparent, but the causes of these end-Cambrian events have not been fully unfolded yet.

Though preservation of local record of cold water by upwelling cannot be completely ruled out, the Hwajeol “glendonites” do provide fresh and ample evidence demonstrating the presence of “transient” cold water in the low latitude “greenhouse ocean” during the latest Cambrian for the first time. Future research on coeval late Cambrian intervals of other regions, which might possess similar minute crystals of ikaite pseudomorphs coupled with interdisciplinary approach by carbon isotope study would hold some vital clues to the understanding of enigmatic global changes during the latest Cambrian to Ordovician transition.