Holocene oscillations in temperature and salinity of the surface subpolar North Atlantic

Abstract

The Atlantic meridional overturning circulation (AMOC) transports warm salty surface waters to high latitudes, where they cool, sink and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm northwestern European climate, and acts as a control on the global climate. Past climate fluctuations during the Holocene epoch ( approximately 11,700 years ago to the present) have been linked with changes in North Atlantic Ocean circulation. The behaviour of the surface flowing salty water that helped drive overturning during past climatic changes is, however, not well known. Here we investigate the temperature and salinity changes of a substantial surface inflow to a region of deep-water formation throughout the Holocene. We find that the inflow has undergone millennial-scale variations in temperature and salinity ( approximately 3.5 degrees C and approximately 1.5 practical salinity units, respectively) most probably controlled by subpolar gyre dynamics. The temperature and salinity variations correlate with previously reported periods of rapid climate change. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic. Model studies predict a weakening of AMOC in response to enhanced Arctic freshwater fluxes, although the inflow can compensate on decadal timescales by becoming more saline. Our data suggest that such a negative feedback mechanism may have operated during past intervals of climate change.

Holocene evolution of the North Atlantic subsurface transport

“…The position of core GEOFAR KF16 (38 • N, 31.13 • W) in the vicinity of the AF is ideal to trace changes in STG position and associated varying influence of subtropical and temperate waters (Repschläger et al, 2015). Results are compared to published data from the western subtropical (Bahr et al, 2013;Cléroux et al, 2012) and subpolar North Atlantic (Thornalley et al, 2009) along the warm water route.…”

“…The two studies that investigated the interaction between the STG and SPG (Solignac et al, 2004;Thornalley et al, 2009) mainly address surface water pathways, while changes in the tropical origin of the water masses are not considered. Thus, a systematic investigation of the subsurface transport from the tropical into the subpolar North Atlantic and its potential impact on the stabilization of the Holocene AMOC mode is apparently missing.…”

“…In this scenario, the main stabilization occurred via the formation of the Labrador Sea Water (LSW), which was probably well established at about 7 ka BP (Hoogakker et al, 2011;Kissel et al, 2013;Solignac et al, 2004;Thornalley et al, 2013). A stable LSW formation relies on the interaction between warm water transport in the clockwisecirculating subtropical gyre (STG) and the subpolar gyre (SPG), circulating counterclockwise in the Irminger Basin and the Labrador Sea (Thornalley et al, 2009). Antiphase heat and salt storage within the STG during times when SPG freshens (Cléroux et al, 2012;Thornalley et al, 2009) and the subsequent advection of the warm saline water into the SPG leads to a strengthening of the deepwater formation by convection within the SPG (e.g., Born and Stocker, 2014).…”

The Holocene occurrence of cold water corals in the NE Atlantic: Implications for coral carbonate mound evolution

“…Changes of the density structure at upper intermediate depth through time may cause reduced coral reproduction and thus coral reefs may decline. Thornalley et al (2009) just revealed first evidence of changes of South Iceland density structures for the sub-surface ocean during the Holocene, suspected to be primarily related to changes in the exchange of subpolar and subtropical gyre waters. Times of reduced coral activity at 1.8-2.0 ka and between 6 and 8 ka do correspond to periods of similar surface and sub-surface densities contrasting periods of significantly different salinity values (Thornalley et al, 2009).…”

Model sensitivity to North Atlantic freshwater forcing at 8.2 ka

“…(Condron and Winsor, 2011). However, several proxy records from the Irminger and Labrador Seas combine δ 18 O and Mg/Ca of planktic foraminera to infer decreases in δ 18 O of seawater at 8.2 ka of up to 1 ‰ (Came et al, 2007;Thornalley et al, 2009;Ellison et al, 2006;Winsor et al, 2012;Hoffman et al, 2012), which would be equivalent to a freshening of ∼0.7 psu assuming the Laurentide Ice Sheet meltwater was about −25 ‰ (Hillaire- Marcel et al, 2007). Also, the location of detrital carbonate layers deposited around 8.2 ka indicate greater freshwater transport in the outer branch of the Labrador Current, which typically mixes with the North Atlantic Current and travels to the Nordic Seas (Lewis et al, 2012).…”