Sponge meadows and glass ramps: State shifts and regime change
Abstract Glass ramps are coastal marine sedimentation regimes of siliceous biomineralization, and present alternatives to carbonate or siliciclastic deposition. Prominent chert deposits around the world record glass ramps from the Eocene, Jurassic, Permian, and Mississippian. These events ranged from regional to continental scale, and lasted from a couple million years to over ten million years. This review compares established cases of glass ramp deposition to clarify the bio-depositional features that distinguish glass ramps from other bedded or mounded biosiliceous deposits, and from carbonate ramp regimes. It also compares environmental contexts of glass ramp initiation and persistence. The primary sediment on these glass ramps is disaggregated spicules of siliceous sponges. The sponges formed meadows on level-bottom inner- and mid-ramp settings, and loose spicules were redistributed along the shelf. Carbonate bioclasts were generally present but diluted by the great abundance of spicules. Carbonate factories were either limited to small, dense patches; moved offshore; or re-established at a later time. Glass ramps initiated in a wide variety of climates and oceanographic settings but appeared to initially require partial protection from storm energy. The importance of marine silicic acid for such widespread biosiliceous factories is also estimated, showing that glass ramps required long-term adjustments in silica supply. Glass ramps lack modern analogues, but their ecology can be interpreted in light of stable state dynamics in other modern invertebrate communities. It is suggested that siliceous sponge meadows were an alternate stable state locally on carbonate ramps, occurring with greater frequency and carrying greater ecological significance than have been previously expected. This model warrants additional scrutiny on sponge meadows' possible stabilizing feedbacks and on the thresholds of state switching.