Reduced North Atlantic Central Water formation in response to early Holocene ice‐sheet melting

Abstract

[1] Central waters of the North Atlantic are fundamental for ventilation of the upper ocean and are also linked to the strength of the Atlantic Meridional Overturning Circulation (AMOC). Here, we show based on benthic foraminiferal Mg/Ca ratios, that during times of enhanced melting from the Laurentide Ice Sheet (LIS) between 9.0-8.5 thousand years before present (ka) the production of central waters weakened the upper AMOC resulting in a cooling over the Northern Hemisphere. Centered at 8.54 ± 0.2 ka and 8.24 ± 0.1 ka our dataset records two ∼150-year cooling events in response to the drainage of Lake Agassiz/Ojibway, indicating early slow-down of the upper AMOC in response to the initial freshwater flux into the subpolar gyre (SPG) followed by a more severe weakening of both the upper and lower branches of the AMOC at 8.2 ka. These results highlight the sensitivity of regional North Atlantic climate change to the strength of central-water overturning and exemplify the impact of both gradual and abrupt freshwater fluxes on eastern SPG surface water convection. In light of the possible future increase in Greenland Ice Sheet melting due to global warming these findings may help us to better constrain and possibly predict future North Atlantic climate change. Citation:

Temperature calibration of Mg/Ca ratios in the intermediate water benthic foraminifer Hyalinea balthica

“…On the T ‐ S plot, we show that the reconstructed densities from samples overlain by ENACW, AAIW and MOW water masses yield significantly different values that within the 0.2 σ θ errors are consistent with the densities of the respective water masses. Indeed, this method has already been used to study the interaction between climate change in the northern North Atlantic and changes in meridional ocean circulation [ Bamberg et al , 2010]. It is noteworthy that in down‐core reconstruction, the errors can be further reduced by using either smoothed or binned data, which is often done in high‐resolution records.…”

Detecting Holocene Changes in the Atlantic Meridional Overturning Circulation: Integration of Proxy Data and Climate Simulations

“…Additionally, two 150-year cooling events [(8.54 ± 0.2) ka BP and (8.24 ± 0.1) ka BP] provide evidence for an abrupt central water response to the drainage of Lake Agassiz (8.54 ka BP) in addition to the maximum AMOC slowdown at 8.2 ka BP (Fig. 2) (Bamberg et al 2010). These observations provide a possible analogue for future effects on meridional heat transfer at central water depth from enhanced Greenland Ice Sheet melting and increases in Arctic freshwater export.…”

“…During the winter, NAO modulated wind-stress favors the subduction of Subpolar Mode Water, which in turn comprises a large fraction of Eastern North Atlantic Central Water (ENACW) (Poole and Tomczak 1999). Flowing south along the eastern margin of the North Atlantic basin at densities between σ θ = 27.3 and 27.6 kg/m 3 , ENACW circulation thus contributes to the ventilation of the thermocline and provides an 'oceanic tunnel', transmitting subpolar ocean-atmospheric climate signals to lower latitudes between 300 and 900 m water depth throughout the Holocene (Bamberg et al 2010;Morley et al 2011Morley et al , 2014.…”

Can oceanic paleothermometers reconstruct the Atlantic Multidecadal Oscillation?

“…Determined standard errors for candidate proxies suggest that subsurface temperature variations resulting from the AMO could only be reconstructed with relatively low SNR values. For example, benthic foraminiferal Mg/Ca based temperature estimates have quoted s values of 1.7 K (Lear et al, 2002), 2.4 K (Marchitto et al, 2007) and 0.7 K (Bamberg et al, 2010), suggesting that only the latter case would be capable of producing reconstructions with SNR > 1.…”

“…It is common for the quality of temperature proxy calibration studies to be assessed using the standard error of estimation (for examples see Saenger et al, 2009;Lear et al, 2002;Marchitto et al, 2007;Bamberg et al, 2010). If T is the observed temperature,T is the temperature predicted by the estimated calibration relationship and N is the number of samples employed in the calibration, the standard error of estimation, s, is given by (Taylor, 1997):…”