Surface and subsurface Labrador Shelf water mass conditions during the last 6,000 years

Annalena Lochte,Janne Repschläger,Thomas Blanz,Ralph Schneider,Dieter Garbe-Schönberg,Nils Andersen,Markus Kienast

doi:10.5194/egusphere-egu2020-3160

Abstract

&lt;p&gt;&lt;span&gt;The Labrador Sea is important for the modern global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) that has been hypothesized to stabilize the modern mode of North Atlantic deep-water circulation. The rate of LSW formation is controlled by the amount of winter heat loss to the atmosphere, the expanse of freshwater in the convection region and the inflow of saline waters from the Atlantic. The Labrador Sea, today, receives freshwater through the East and West Greenland Currents (EGC, WGC) and the Labrador Current (LC). Several studies have suggested the WGC to be the main supplier of freshwater to the Labrador Sea, but the role of the southward flowing LC in Labrador Sea convection is still debated. At the same time, many paleoceanographic reconstructions from the Labrador Shelf focussed on late Deglacial to early Holocene meltwater run-off from the Laurentide Ice Sheet (LIS), whereas little information exists about LC variability since the final melting of the LIS about 7,000 years ago. In order to enable better assessment of the role of the LC in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening and sea ice cover on the Labrador Shelf during the last 6,000 years. Our records reveal that the LC underwent three major oceanographic phases from the Mid- to Late Holocene. From 6.2 to 5.6 ka BP, the LC experienced a cold episode that was followed by warmer conditions between 5.6 and 2.1 ka BP, possibly associated with the late Holocene Thermal Maximum. Although surface waters on the Labrador Shelf cooled gradually after 3 ka BP in response to the Neoglaciation, Labrador Shelf subsurface/bottom waters show a shift to warmer temperatures after 2.1 ka BP. Although such an inverse stratification by cooling of surface and warming of subsurface waters on the Labrador Shelf would suggest a diminished convection during the last two millennia compared to the mid-Holocene, it remains difficult &lt;/span&gt;&lt;span&gt;to assess whether hydrographic conditions in the LC &lt;/span&gt;&lt;span&gt;have had a significant impact on Labrador Sea deep-water formation. This study was conducted within the HOSST research school with the aim to improve our understanding of the critical processes involved in the North Altantic thermohaline circulation, which is particularly important in light of current climate change.&amp;#160;&lt;/span&gt;&lt;/p&gt;

Highlights

In order to enable better assessment of the role of the Labrador Current (LC) in deep-water formation and its importance for Holocene climate variability in Atlantic Canada, this study presents high-resolution middle to late Holocene records of sea surface and bottom water temperatures, freshening, and sea ice cover on the Labrador Shelf during the last 6000 years
Since the early Holocene, the Labrador Sea has been an important region of the global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) in the central basin (Clarke and Gascard, 1983; Myers, 2005; Rhein et al, 2011; Yashayaev, 2007), which forms the upper part of North Atlantic Deep Water (NADW) and thereby contributes to the strength of the Atlantic Meridional Overturning Circulation (AMOC)
While changes in the heat loss are predominantly related to atmospheric forcing, Labrador Sea salinity is controlled mainly by two freshwater supply routes (Wang et al, 2018): the eastern route via the East Greenland Current (EGC), which mixes with the Irminger Current (IC) south of Greenland to form the West Greenland Current (WGC), and the western route via the Labrador Current (LC), the extension of the Baffin Current (BC; Fig. 1a)

Summary

Introduction

Since the early Holocene, the Labrador Sea has been an important region of the global thermohaline circulation system through the formation of intermediate Labrador Sea Water (LSW) in the central basin (Clarke and Gascard, 1983; Myers, 2005; Rhein et al, 2011; Yashayaev, 2007), which forms the upper part of North Atlantic Deep Water (NADW) and thereby contributes to the strength of the Atlantic Meridional Overturning Circulation (AMOC). 7 ka (e.g. Jennings et al, 2015; Ullman et al, 2016) and the early Holocene strengthening of the West Greenland Current (WGC) bringing a larger proportion of warm, more saline Atlantic waters into the Labrador Sea (e.g. Lloyd et al, 2005; Lochte et al, 2019a; Seidenkrantz et al, 2013; Sheldon et al, 2016). The rate of Labrador Sea convection is controlled by two main processes: heat loss to the atmosphere and the relative supply of buoyant freshwater to the convection region. While changes in the heat loss are predominantly related to atmospheric forcing, Labrador Sea salinity is controlled mainly by two freshwater supply routes (Wang et al, 2018): the eastern route via the East Greenland Current (EGC), which mixes with the Irminger Current (IC) south of Greenland to form the WGC, and the western route via the Labrador Current (LC), the extension of the Baffin Current (BC; Fig. 1a). While early studies suggested the LC as the main source of central Labrador Sea freshwater (Lazier, 1973, 1988; Khatiwala et al, 1999), more recent work advocated the WGC as the main supplier of freshwater (Cuny et al, 2002; Straneo, 2006; Schmidt and Send, 2007)

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