Abstract
The globally warm climate of the early Pliocene gradually cooled from 4 million years ago, synchronous with decreasing atmospheric CO2 concentrations. In contrast, palaeoceanographic records indicate that the Nordic Seas cooled during the earliest Pliocene, before global cooling. However, a lack of knowledge regarding the precise timing of Nordic Seas cooling has limited our understanding of the governing mechanisms. Here, using marine palynology, we show that cooling in the Nordic Seas was coincident with the first trans-Arctic migration of cool-water Pacific mollusks around 4.5 million years ago, and followed by the development of a modern-like Nordic Seas surface circulation. Nordic Seas cooling precedes global cooling by 500,000 years; as such, we propose that reconfiguration of the Bering Strait and Central American Seaway triggered the development of a modern circulation in the Nordic Seas, which is essential for North Atlantic Deep Water formation and a precursor for more widespread Greenland glaciation in the late Pliocene.
Highlights
The globally warm climate of the early Pliocene gradually cooled from 4 million years ago, synchronous with decreasing atmospheric CO2 concentrations
Deep-water exchange across the Central American Seaway likely halted in the late Miocene[12,13,14], a further shoaling phase in the early Pliocene has been interpreted between B4.8 and 4.2 Ma
This Nordic Seas circulation was hypothesized to be initiated in the early Pliocene as a consequence of Central American Seaway closure[17], but a precise timing, encompassing mechanism and conclusive data from the Nordic Seas remained elusive
Summary
The globally warm climate of the early Pliocene gradually cooled from 4 million years ago, synchronous with decreasing atmospheric CO2 concentrations. The strong east–west sea surface temperature and salinity gradient is caused by inflow of relatively warm saline Atlantic water in the southeast, continuing as the Norwegian Atlantic Current towards the Arctic Ocean, and by export of cool, fresher Arctic water via the East Greenland Current in the west[22] This Nordic Seas circulation was hypothesized to be initiated in the early Pliocene as a consequence of Central American Seaway closure[17], but a precise timing, encompassing mechanism and conclusive data from the Nordic Seas remained elusive. 23), we demonstrate that the Nordic Seas cooled and a zonal sea surface temperature gradient developed around 4.5 Ma, preceding the global climate cooling and atmospheric carbon dioxide decline from 4.0 Ma. Here, using new marine palynological data (palynomorphs including dinoflagellate cysts and acritarchs) from Norwegian Sea Ocean Drilling Program (ODP) Hole 642B and an existing record from Iceland Sea ODP Hole 907A We link this change in surface ocean conditions to inflow of fresher, cooler Pacific water via the Bering Strait into the Nordic Seas along the East Greenland Current pathway around 4.5 Ma
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