Reduced North Atlantic Deep Water Coeval with the Glacial Lake Agassiz Freshwater Outburst

Abstract

An outstanding climate anomaly 8200 years before the present (B.P.) in the North Atlantic is commonly postulated to be the result of weakened overturning circulation triggered by a freshwater outburst. New stable isotopic and sedimentological records from a northwest Atlantic sediment core reveal that the most prominent Holocene anomaly in bottom-water chemistry and flow speed in the deep limb of the Atlantic overturning circulation begins at approximately 8.38 thousand years B.P., coeval with the catastrophic drainage of Lake Agassiz. The influence of Lower North Atlantic Deep Water was strongly reduced at our site for approximately 100 years after the outburst, confirming the ocean's sensitivity to freshwater forcing. The similarities between the timing and duration of the pronounced deep circulation changes and regional climate anomalies support a causal link.

Relative sea-level data from southwest Scotland constrain meltwater-driven sea-level jumps prior to the 8.2 kyr BP event

“…Delta RSL record (Hijma and Cohen, 2010) North Atlantic proxy records Kleiven et al, 2008). It is therefore unlikely 757 that the majority of freshwater was routed south into the sub-tropical gyre (Condron and 758 Windsor, 2011;Hill and Condron, 2014), although it remains possible that this region 759 received a portion of the total discharge volume.…”

“…Timing of events are expressed as age probability density functions (pdfs) (3σ) and horizontal bars (2σ probability ranges). Probability density functions of RSLr 1 , RSLr 2 and RSLr 3 (black curves) in the Cree estuary from this study are plotted alongside DCP6a (blue bar) and DCP6b (purple bar) from Jennings et al (2015), the James Bay drainage deposit (red bar; Roy et al, 2005), the onset of sudden sea-level rise in the Rhine-Meuse Delta (purple curve; Hijma and Cohen, 2010) and Mississippi Delta (brown curve; Li et al, 2012) and the two North Atlantic surface cooling and freshening events (FE 1 ; orange curve, FE 2 light blue curve) inferred from revised chronology of Kleiven et al (2008). Peak meltwater discharge (modelled) associated with separation of the Keewatin and Labrador ice-dome 'saddle' (Gregoire et al, 2012) is shown as a grey bar, but note has no strict statistical definition.…”

“…Of the nine dates between 9.0 to 8.0 kyr BP in the Kleiven et al (2008) Kleiven et al (2008). However, we need to determine the precise properties that is coeval with the onset of the δ 13 C excursion.…”

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

“…The abrupt temperature and salinity decreases in our record suggest that melt-waters from the lake drainage rapidly reached ENACW formation sites and thereby further weakened the upper limb of the AMOC. Separated from the first event by a partial recovery of ∼200-300 years, the second event at 8.17 to 8.31 ka is concurrent with the maximum cooling observed in the d 18 O GISP2 ice core record as well as with the maximum slowdown of LNADW formation associated with the 8.2 ka event [Ellison et al, 2006;Kleiven et al, 2008] (Figure 3). Further, the combined temperature and salinity values suggest that ENACW density tended towards lighter density surfaces during both 150-year-long events, with lightest values centered at 8.24 and 8.52 ka ( Figure S4).…”

“…The final drainage of Lake Agassiz/Ojibway at 8.47 ± 0.3 ka was the largest (∼1.63 × 10 14 m 3 of freshwater [Teller et al, 2002], equivalent to a 0.4-1.2 m sea level rise [Cronin et al, 2007]) and has thus often served as an example to investigate the abrupt response of the AMOC to future meltwater inputs even though the volume and lake discharge rates were much greater than high-end estimates for future global sea level rise of 0.55 to 1.25 cm yr −1 [Rahmstorf, 2007]. Indeed, recent high resolution studies focusing on the abrupt cooling episode of the 8.2 ka event, successfully demonstrated the sensitivity of lower North Atlantic Deep Water (LNADW) formation to the lake drainage [Ellison et al, 2006;Kleiven et al, 2008] and thereby support climate models that predict a decrease in the lower limb of the AMOC associated with a climate cooling [LeGrande et al, 2006;Renssen et al, 2001] in response to the lake drainage. However, numerous climate archives across the Northern Hemisphere also record a broad interval of climate deterioration starting at ∼9.0 ka, well before the drainage of Lake Agassiz/Ojibway [Alley and Ágústsdóttir, 2005;Rohling and Pälike, 2005].…”