LATE JURASSIC GLACIERS DURING THE MESOZOIC GREENHOUSE: EVIDENCE FROM THE LOWER NAKNEK FORMATION, SOUTHERN ALASKA

Fluctuations in global climate represent one of the major secular trends in earth history. The Mesozoic era is widely regarded as the longest episode of sustained greenhouse conditions in the Phanerozoic, driven in part by the elevated concentration of atmospheric carbon dioxide. However, a number of paleoclimate proxies suggest that the Jurassic period may not have been uniformly warm, but may instead have included brief episodes of anomalous cold. Lines of evidence supporting episodic cooling include eustatic sea level changes, oxygen isotope paleothermometry, and paleobiogeographic changes. However, direct physical evidence for periodic cooling (e.g., glacial scouring, ice-rafted debris, glendonite crystals, etc.) has been largely lacking, leading to considerable debate regarding the continuity of greenhouse climate conditions in the Jurassic. Here, we report new stratigraphic observations from southern Alaska that are interpreted as evidence for glaciomarine deposition, strongly supporting the episodic departure from a greenhouse state and the local development of glacial conditions in the early Late Jurassic.

The Oxfordian-age Chisik Conglomerate of the Naknek Formation is well exposed in sea cliffs at McNeil Head in southwestern Cook Inlet. Stratigraphic relations are complex and include abundant soft-sediment deformation and irregular contacts exhibiting many meters of incised relief. The facies include thick packages of poorly sorted sandstone and conglomerate, both of which locally contain abundant outsized clasts interpreted as dropstones from melting ice. Intervals of finer grained strata include thin bedded, laminated, and rhythmically graded siltstone and very fine grained sandstone, also locally hosting dropstones. At one location, the sandstone included prominent fine grooves on bedding planes, interpreted as ice-contact striae.

The totality of observations strongly suggest that these rocks represent the most persuasive physical evidence yet found for the development of at least local alpine glaciers during the Late Jurassic. If confirmed, notions of uninterrupted Mesozoic greenhouse conditions will require re-evaluation, and new paleoclimate models will need to account for the possibility of brief, but significant, episodes of cooling.