Early Cretaceous giant glendonites: a record of (sub-)millennial-scale cooling?

The Lower Cretaceous succession in Svalbard contains numerous glendonites, pseudomorphs after the cold-water carbonate mineral ikaite, which have been used in conjunction with other evidence to argue for episodic global cooling punctuating the greenhouse climates of the Early Cretaceous. Recent fieldwork in central Spitsbergen has recovered giant bladed glendonites of up to half a metre long, the largest ever recorded in a Lower Cretaceous site, and comparable in size to outlier glendonites found in similar-aged strata of the Sverdrup Basin in Arctic Canada. Unlike the rosette to pineapple-like morphologies seen in some of the largest Canadian Arctic specimens, the new finds in Svalbard appear only as single or crossed blades. These large glendonites, found clo indicate that very local variations in pore water chemistry governed whether numerous small ikaite crystals or few large crystals grew. Taken with evidence from modern ikaite and other large ancient glendonites, we argue that large glendonites such as these (>30 cm long) are pseudomorphs after ikaites that took, at the shortest, decades, but potentially millennia to even tens of millennia to attain their massive size. As growth of the parent ikaite took place in the sediments just below the seafloor of the shallow, epicontinental seas of the High Arctic (then situated at c. 63–66°N), this is consistent with the hypothesis that geologically short-term cooling episodes interrupted the background warmth of the Early Cretaceous greenhouse, although the duration, extent, and cause of such cooling is still debated.