Insights into glendonite formation from the Oligocene Sagavanirktok Formation, North Slope, Alaska

The type locality for the upper Oligocene Nuwok Member of the Sagavanirktok Formation (Carter Creek, North Slope, Alaska, USA) contains abundant occurrences of glendonite, a pseudomorph after the calcium carbonate mineral ikaite, which typically forms in the shallow subsurface of cold marine sediments. At the time of deposition, the Nuwok Member was located at a high latitude similar to today, and is characterized by sands and silty muds interpreted here to have been deposited in coastal and shelfal marine environments. Isotopic (Sr) and biostratigraphic (foraminifera) evidence from this study refine the age of the outcrop to approximately 24 Ma. Glendonite crystals occur in two basic forms: radial clusters, commonly centered around a single larger primary crystal (~10 cm; Type A) and larger single blades generally without accessory crystals (~15–25 cm; Type B). Microscopic examination reveals a sequence of multiple types of replacive calcite that formed as a direct result of ikaite transformation: Type 1 rhombohedral crystals characterized by microporous cores and concentric zonation; Type 2A, composed of clear calcite that overgrew and augmented Type 1 crystals; and microcrystalline Type 2B, which formed a matrix with abundant organic matter inclusions surrounding the rhombs and commonly dominates the outer rims of glendonite specimens. Type 3 calcite precipitated as fibrous, botryoidal epitaxial cement atop previous phases and is not ikaite-derived. These phases are similarly distributed in all examined specimens and are consistent with several previously described glendonite occurrences around the world, despite differing diagenetic and geologic histories. Very low stable carbon isotope compositions in glendonite calcite (δ13C values of -15 to -44‰ VPDB) suggests carbon was incorporated from organic sources, including biogenic methane. Oligocene glendonites are uncommon globally; the well-preserved Nuwok Member specimens described here facilitate the development of a more comprehensive ikaite transformation model, and will allow future studies to better reconstruct Arctic environmental conditions and paleoclimates during this time.