Stable isotope clue to episodic sea ice formation in the glacial North Atlantic

Abstract

Sea ice is a determinant parameter of the climate system, which acts as amplifier through positive ice-albedo feedbacks and induces deep-water formation through brine production. It is also one of the most elusive parameters in the paleoclimate record. Most proxies of sea ice provide information on its presence and extent of (e.g., diatoms or dinocysts), but do not permit assessment on its rate of formation. However, seemingly off-equilibrium, low δ18O values in mesopelagic planktic foraminifer species, as observed today in the Arctic Ocean, are thought to relate to the production of isotopically light brines during sea ice formation, and might thus provide a clue on the rate of sea ice growth. With reference to the isotopic properties of modern planktic foraminifers in the Arctic Ocean and using multi-proxy data sets from the last glacial stage in the northwest North Atlantic, we re-examined some of the current interpretations of planktic isotope records in the glacial NW North Atlantic. The large amplitude light isotopic excursions recorded in Neogloboquadrina pachyderma left coiling during Heinrich events 1, for example, correspond to extensive sea ice cover as reconstructed from dinocysts, and do not seem unequivocally linked to low-salinity pulses. Such light isotopic excursion more likely responded to enhanced rates of sea ice formation resulting in the production and sinking of isotopically light brines. On the contrary, the isotopically heavy planktic foraminifers of the last glacial maximum (LGM) interval stricto sensu would rather suggest relatively low rates of brine production, thus low sea ice growth rates in the area. This would imply that the LGM distribution of sea ice in the North Atlantic was primarily linked to spreading and drifting from marginal and Nordic source areas and not from enhanced in situ production. At the scale of glacial–interglacial cycles, isotopic distillation processes relating to sea ice production at high latitudes could be one of the factors controlling the 18O–salinity relationship in deep ocean water masses that might deserve closer examination, as it would be independent of continental ice volume changes.