Early Pliocene onset of modern Nordic Seas circulation related to ocean gateway changes

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.

Highly variable Pliocene sea surface conditions in the Norwegian Sea

“…The gradual SST cooling from 5.3 to 5.0 Ma culminates with the 4.9 Ma cold event (Fig. 2), which is also indicated by dinoflagellate assemblage changes (De Schepper et al, 2015). The gradual cooling may therefore have influenced the regional climate and ice growth leading up to the 4.9 Ma global glaciation event.…”

“…5), suggesting a potential role for increased seasonality in causing local warming and/or the retreat of small-scale marine-based glaciers existing in the Nordic Seas realm. The change in SSTs from the stable warm conditions between 4.64 and 4.45 Ma to the stable cold conditions between 4.3 and 4.0 Ma can also be seen in relation to the onset of northward through-flow in the Bering Strait (Marincovich and Gladenkov, 1999), the establishment of the EGC and the onset of modern circulation in the Nordic Seas, the latter suggested to have occurred around 4.5 Ma (De Schepper et al, 2015). The modern oceanographic pattern of the Nordic Seas is characterized by a strong zonal gradient of SST and salinity between the cold and fresh EGC to the west and the warm and saline NwAC to the east (Fig.…”

“…In previously published U K 37 SST records from ODP Site 907 in the Iceland Sea, no summer bias is assumed (Herbert et al, 2016;De Schepper et al, 2015; Schreck et al, Table 1. AMS 14 C dates from GS15-198-62MUC-A, and calibrated calendar ages.…”

“…The main hypotheses are long-term changes in the atmospheric CO 2 concentration, changes in oceanic gateways and circulation, and changes in orbital forcing. Recently Schreck et al (2013) and De Schepper et al (2015) promoted the importance of the Nordic Seas in priming the Northern Hemisphere for glaciation through the establishment of the East Greenland Current (EGC) and the modern Nordic Seas oceanographic pattern ( Fig. 1) around 4.5 Ma.…”

“…1) around 4.5 Ma. The establishment of the cold EGC, as a potential response to changes in the ocean circulation through the Bering Strait, has been considered important for large-scale glaciation of Greenland because the EGC thermally isolates Greenland from the warm waters transported northward in the Norwegian Atlantic Current (NwAC) (Bohrmann et al, 1990;Sarnthein et al, 2009;De Schepper et al, 2015).…”