Australia: The Land Where Time Began

A biography of the Australian continent 

Southwest Labrador Sea off Newfoundland, Oceanographic changes in the Holocene

A regional record of oceanographic changes in the southwest Labrador Sea covering a time period of about 13,000 years was constructed from benthic foraminiferal assemblages together with geochemical data that was recovered from 3 marine sediment cores that had been collected from Placentia Bay, Southeast Newfoundland. The location of the area is boundary zone between the cold Labrador Current (LC) which contains icebergs to the north and the warm Gulf Stream (GS) to the south. The influence of the water from the Gulf Stream increased following the termination of the Younger Dryas and further strengthened at 10.7 cal. kyr BP as a result of the northwards flow of Atlantic water derived from the Slopewater Current. A short-term event that occurred at 8.4 cal,. kyr. BP (8,200 Year Event) increased input of terrestrial water and stratification of the water column is believed to have probably been linked to the distal drainage from Lake Agassiz. A stronger Labrador Current weakened the inflow of warmer subsurface waters from the Gulf Stream after 7.3 cal. kyr BP. Sheldon et al. suggest this may be explained by the release of large volumes of meltwater from ice sheets in Arctic Canada, concurrently with a general shift in oceanic conditions in the region of the Labrador Sea. Conditions became more stable with a slightly increased salinity about 4.0 cal. kyr BP, which indicated there was a decrease in the volume of meltwater carried by the Labrador Current. The Northern Hemisphere neoglacial cooling, that occurred about 2.8 cal. kyr BP, was characterised in southeast Newfoundland by further stabilisation of the current system, which was dominated by the Labrador Current, with some continued influx of Gulf Stream water.

The varying strengths of the cold and warm ocean currents feeding into the subpolar gyre (SPG) strongly influence the climate of the North Atlantic region. The Labrador Sea, the western sector of the gyre, is one of the major sites for the formation of deep water in the North Atlantic region (Marshall & Schott, 1999), which redistributes heat energy and nutrients through deepwater ocean circulation (Delworth & Mann, 2000; Drinkwater et al., 2013; Kuhlbrodt et al., 2007; Lohmann et al., 2008; Rhein et al., 2011; Thornalley et al., 2009). An important element of the Atlantic Meridional Overturning Circulation (AMOC), which influences directly the regional climate (Thornalley et al., 2009), is this deep convection and the associated heat loss to the atmosphere. A crucial role is played by the AMOC in the redistribution of heat from the tropics to the poles and also contributes significantly to the climatic variability of the Northern Hemisphere (Hansen & Østerhus, 2000; Mayewski et al., 2004; Thornalley et al., 2009).

According to Sheldon et al. there have been many different studies carried out on the variability of the AMOC at different time scales (Delworth & Mann, 2000; Wu & Gordon, 2002). Surface density anomalies in the North Atlantic region strongly force interdecadal to centennial scale variability in the AMOC influencing the formation of deep water (Eden & Willebrand, 2001; Zhang & Vallis, 2006). It is shown by coupled atmosphere-climate models that the strength of the Gulf Stream (GS) is linked closely to the strength of the AMOC (Joyce & Zhang, 2010). Since the last Holocene Thermal Maximum there has been a general cooling trend of the surface and subsurface currents of the North Atlantic (Rasmussen at al., 2011; Seidenkrantz et al., 2007, 2008; Sicre et al/. 2014; Ślubowska-Woldengen et al., 2007).

The Labrador Current, cold and relatively fresh, and the Gulf Stream, warmer and more saline, are the 2 main currents that the study area in eastern Newfoundland. In the Labrador Sea (Labrador Sea Water) the formation of deep water to intermediate water is impacted directly by the low salinity Labrador Current and the high salinity Gulf Stream, with saline surface waters increasing deep water formation (Jones & Anderson, 2008; Katsman et al., 2004; Marshall & Schott, 1999; Schmitz & McCartney, 1993; Yashayaev, 2007). Much of the cold outflow water into the North Atlantic and the subpolar gyre (SPG) (Bunker, 1976; Fratantoni & McCartney, 2010) is carried by the cold Labrador Current. As a consequence, an increase in the Labrador Current and a weaker Gulf Stream may cause an increase in stratification in the North Atlantic and, theoretically slow down the AMOC (Jones & Anderson, 2008). According to Sheldon et al. such a scenario has been hypothesised for the last interglacial period (Hillaire-Marcel et al., 2001) and the Younger Dryas (YD) Condron & Winsor, 2012); McManus et al., 2004: Pearce et al., 2013). The Labrador Current appears to have increased in strength over recent decades (Sicre et al., 2014), an increased amount of sea ice being advected southwards (Weckström et al., 2013). An important role in regulating recent and past climate variations as far south as the Gulf of Maine (40^oN) (Wanamaker et al., 2008) is played by the fluctuating strengths of these oceanic currents.

Important roles are played by regional atmospheric circulation and the strength of the wind (Sy et al., 1997). The North Atlantic Oscillation (NAO) significantly impacts the climate of the North Atlantic region (Harrell & Van Loon, 1997), which affects the flow of the Labrador Current as well as sea surface temperatures in the North Atlantic (Flatau et al., 2003; Yashayaev, 2007). Also, it is shown by model simulations that the North Atlantic Oscillation exerts some degree of control over variability of deep convection in the Labrador Sea (Guemas & Salas-Mélia, 2008).

In this study a palaeoceanographic record of the entire Younger Dryas-Holocene time period in Placentia Bay, southeast Newfoundland, Canada is presented. In this study Sheldon et al. investigated the bottom water conditions in the area of the study, situated in the boundary zone between the North Atlantic subpolar gyre and subtropical gyre, which is an important site of water mass mixing (Aagaard & Carmack, 1989; Cuny et al., 2002; Van Aken et al., 2011), while in previous studies the focus was primarily on sea surface and atmospheric conditions (Jessen et al., 2011; Pearce et al., 2013, 2014a; Sicre et al., 2014; Solignac et al., 2011; Weckström & Carmack et al., 2013). Therefore the bay is an excellent site to capture changes occurring in the climate and oceanographic conditions in the North Atlantic (e.g. Pearce et al., 2013). The aim of the study was to reconstruct the variability of the Labrador Current and influence the Gulf Stream on this boundary zone in coastal waters off Newfoundland over the last 13 kry, the primary focus being on subsurface conditions that existed in the Holocene.

Conclusion

In southern Newfoundland, Placentia Bay has been influenced strongly by the Labrador Current and the Gulf Stream over the last 13 cal. kyr BP. In the Early Holocene there was an initial warming, that Sheldon et al. suggest was likely due to the northward shift of the frontal zone between the Gulf Stream and the Labrador Current at the transition from the Younger Dryas (YD), linked to a strengthening AMOC. The benthic foraminiferal record indicates, however, that at about 9.7-9.2 ka BP and 8.4-8.25 ka BP, increased stratification prevailed with the result that there were low levels of oxygen in the bottom water. It is suggested by Sheldon et al. that there were glacial meltwater events that were the cause of these events releasing freshwater to the Labrador Current leading to low density surface layer, thereby reducing convection.

It is revealed by the data obtained by the study that at 7.3 ka BP there was a major oceanographic shift which Sheldon et al. say appears to reflect a fundamental reorganisation of the North Atlantic polewards transport pattern of warm water. During the mid-Holocene a concurrent cooling and freshening of the surface waters off Newfoundland is explained by an increase in the influence of the Labrador Current, and an increase in transport of low-saline, cold meltwater, which is thought to be linked to increased meltwater release from glaciers in the Canadian Arctic during the Northern Hemisphere HTO.

At the end of the Holocene Thermal Optimum, as the overall climate of the Northern Hemisphere cooled, salinity increased again in the bottom waters in Placentia Bay and there was a slight weakening of the Labrador Current which caused a minor northwards movement of the oceanic polar front between the Labrador Current and the Gulf Stream. Therefore, in the Late Holocene there was a minor movement, as well as an increase in the local influence of the Gulf Stream, which also contributed to a more saline and productive environment in Placentia Bay. This shift was not linked to a general strengthening of the AMOC but to the Northern Hemisphere cooling post-HTO, causing reduced meltwater transport. During about the last 3 kyr the Labrador Current strengthened again, which led to cold, low-saline conditions that have continued to the present. A stronger Labrador Current may also have been facilitated by a general shift in the direction of the wind linked to a climate of an overall NAO-type leading to predominantly northerly wind.  

1. Sheldon, C. M., M.-S. Seidenkrantz, C. Pearce, A. Kuijpers, M. J. Hansen and E. Z. Christensen (2016). "Holocene oceanographic changes in SW Labrador Sea, off Newfoundland." The Holocene 26(2): 274-289.