Henriette Marie Kolling

The Labrador Coastal Current (LCC) is a buoyancy driven current comprised of outflows from Baffin Bay and Hudson Bay that transports freshwater and nutrients to the Scotian Shelf and Gulf of Maine in the NW Atlantic. This study reconstructs Holocene flow speeds over the Labrador Shelf, using the sortable silt index at two core sites off northern and central Labrador. Reconstructions of current speeds at these two sites are similar through time and suggest that the LCC is controlled by Hudson Bay outflow, and experiences a 1,500 cyclicity consistent with the Arctic Oscillation and flow patterns from the NE Atlantic. Surface sediments suggest an increase in LCC velocity in recent decades, consistent with increased freshwater export from the Arctic, and possible recent weakening of the Atlantic meridional overturning circulation. Our new flow speed reconstructions differ from other major North Atlantic Holocene current speed records, but correspond to changes in salinity and bottom water temperature on the Labrador Shelf. This likely reflects the influence of cold and fresh Arctic waters versus warm, saline Atlantic waters. Results indicate recirculating Atlantic waters were likely the main LCC driver during an establishment period from 9 to 6 ka B.P whereas Hudson Bay outflow became the dominant control 6 ka B.P.—present, during a stabilization period once postglacial conditions were established.

The Laurentide Ice Sheet covered vast areas of North America during the Wisconsinan, including the Labrador Shelf, off the coast of eastern Canada. The shelf is characterized by glacial cross‐shelf troughs, which have been considered as predominantly eroded by the last ice advance. Nevertheless, these cross‐shelf troughs contain substantial sedimentary infill that has not yet been thoroughly explored and elucidated. Therefore, we hypothesize that older, potentially non‐glacial, sediments are preserved within these cross‐shelf troughs on the Labrador Shelf. In this study, we investigate the stratigraphic infill of three major trough systems on the Labrador Shelf using 2D seismic reflection data to refine the erosional and depositional processes that produce the geomorphology and stratigraphy of the shelf. The results of the study present an updated Quaternary stratigraphy of the Labrador Shelf, comprising eight seismic units. For the first time, we interpret glaciomarine and open marine deposits between glacial till layers. Additionally, we identified sets of elongated depressions, at three different depth intervals. Some of them are classified as subglacial meltwater channels and some others as ice stream troughs. Finally, we propose a stratigraphic evolution model consisting of 10 phases and spanning two glacial–interglacial cycles. The results support our hypothesis and suggest that the deeper sediment layers on the shelf contain evidence of older glacial–interglacial cycles. They also lend support to the hypothesis of a multiphase origin over multiple glacial cycles for the cross‐shelf troughs. Consequently, new opportunities are provided for the reconstruction of ice‐sheet dynamics and associated oceanographic conditions on the Labrador Shelf, as the sedimentary succession provides an archive to reconstruct dynamics of ice sheets during at least two Quaternary glacial–interglacial cycles.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

The Scotian Shelf on the northwest Atlantic Margin is located at the confluence of two important components of the Atlantic Meridional Overturning Circulation (AMOC). The southward flowing Labrador Current supplies cold, oxygen rich waters and the northward flowing Gulf Stream delivers warm, nutrient rich waters low in O2. Their mixing allows the establishment of a productive marine ecosystem. The relative influence of the current systems is governed by northern hemispheric climate patterns, such as the overall AMOC strength and the North Atlantic Oscillation mode. However, the exact atmospheric and oceanographic mechanisms are still under debate. Due to this knowledge gap regarding the climate-bioproductivity feedback, a deeper insight into the biogeochemical evolution of the region since the Holocene is an important aspect for understanding North Atlantic climate and circulation.On the Scotian Shelf, glacially eroded basins are separated from the open ocean by shallower sills on the outer shelf. Using solid phase and pore water geochemical data from three eight- to twelve-metre-long sediment cores, in combination with reaction-transport modelling, we reconstructed carbon and sulfur cycling at the seafloor along the Scotian Shelf since the last deglaciation. Chloride profiles imply that the basins were filled with freshwater during the earliest phase of the deglaciation. Due to the absence of sulfate reduction in freshwater sediments, reactive Fe oxides escaped pyritization during deposition of the deepest sediment layers. Between 14 and 8 ka BP, a combination of eustatic sea-level rise and isostatic adjustment led to marine transgression and the establishment of fully marine conditions on the shelf, accompanied by increased organic matter deposition and burial. Modelled anaerobic oxidation of methane coupled to reduction of iron oxide minerals in deeper sediment layers in the present day alludes to a geochemical fingerprint of the formerly prevailing freshwater conditions in the shelf basins. Our data and model outcomes allow us to pinpoint the timing of marine transgression for three individual basins along the Scotian Shelf and reconstruct the corresponding evolution of contemporary biogeochemical conditions. We conclude that the diagenetic conditions in Scotia Shelf sediments evolved in a similar manner to those described previously for marginal seas with restricted exchange with the open ocean, such as the Baltic Sea.

The Scotian Shelf lies at the confluence of warm Gulf Stream (GS) waters and the cold Labrador Current (LC), making it highly sensitive to large- and small-scale climate variability. Modern observations show rapid regional warming accelerated by episodic GS-derived intrusions, yet Holocene paleoceanographic reconstructions from this margin are sparse and often conflicting with respect to the frequency and extent of intrusion events. Here, we present high-resolution Mg/Ca-derived sea-surface temperature (SST) and planktonic δ¹⁸O records from St. Anns Basin on the north-eastern Scotian Shelf that provide new insights into the hydrographic surface-ocean variability of the past 8.5 ka calibrated Before Present (cal BP). While the SST record does not capture the 8.2 ka event, this event is evident in the δ¹⁸O and Ca/Sr records, indicating that its freshwater signal reached the Scotian Shelf. Reconstructed SSTs are generally cold from ~8.5 to ~6.2 cal ka BP, followed by a gradual increase in mean SSTs punctuated by multiple short-lived warm and saline events beginning around 6 cal ka BP, at 6.0–5.8, 5.5–5.4, 5.1–4.9, 3.2–3.1, 2.5–2.2 and 1.05–0.8 cal ka BP, which we interpret as intrusions of GS-sourced slope waters. We attribute these events to basin-scale reorganizations of the GS-LC system, consistent with the minimum/maximum modal state framework of Pickart et al. (1999). Minimum modal state circulation, characterized by a strong onshore LC and an intensified Deep Western Boundary Current (DWBC), which is dominated by Denmark Strait Overflow water, creates a sharp front which restricts intrusions of warm water onto the Scotian Shelf. Maximum modal state conditions feature a weakened LC and increased Labrador Sea Water (LSW) contribution to the DWBC, and reduce cross-slope temperature and salinity gradients that permit GS-derived waters to penetrate the shelf. Overall, our results indicate that warm-water intrusions occurred regularly throughout the past 6.5 ka BP with magnitudes of 6.7 °C and 1.5 psu comparable to those observed today.

New hydroacoustic measurements combined with old data reveal the widespread occurrence of contourite drift deposits - indicative of persistent strong bottom currents - at rather great water depths in the northern Baltic Sea and Eastern Canadian coastal waters (Foxe Basin, Hudson Bay, Gulf of St. Lawrence). In addition, lag deposits suggest that strong bottom currents temporary eroded sediments most likely during the cold Little Ice Age. For example, the Little Ice Age lag deposits are found to a water depth of ~ 300 m in Foxe Basin and to ~ 150 m in the Baltic Sea. In all ecosystems the depositional environment changed drastically with the onset of climate warming after the Little Ice Age: calm conditions prevailed leading to the accumulation of fine-grained sediments. A possible mechanism to explain the strong bottom currents during the Little Ice Age is an enhanced deep-water formation caused by accelerated convection and/or brine formation (Eastern Canadian waters) during colder winter conditions. Attempts to model the enhanced winter-time deep-water formation / convection remain inconclusive and do not match the hydroacoustic and sedimentological evidence. However, solving this issue is critical as it could allow to, e.g., reconstruct past winter temperatures based on sedimentological grain-size studies. Yet, most proxies used in paleo-oceanographic temperature reconstructions only relate to spring and summer (growing season) conditions. Our results indicate that winter temperature changes (strength and length of sea-ice season) are of critical importance for the depositional environment and bottom water ventilation in the Eastern Canadian and Baltic Sea ecosystems.