Guy Bilodeau

Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes

The sensitivity of oceanic thermohaline circulation to freshwater perturbations is a critical issue for understanding abrupt climate change. Abrupt climate fluctuations that occurred during both Holocene and Late Pleistocene times have been linked to changes in ocean circulation, but their causes remain uncertain. One of the largest such events in the Holocene occurred between 8,400 and 8,000 calendar years ago,, (7,650–7,200 14C years ago), when the temperature dropped by 4–8 °C in central Greenland and 1.5–3 °C at marine, and terrestrial, sites around the northeastern North Atlantic Ocean. The pattern of cooling implies that heat transfer from the ocean to the atmosphere was reduced in the North Atlantic. Here we argue that this cooling event was forced by a massive outflow of fresh water from the Hudson Strait. This conclusion is based on our estimates of the marine 14C reservoir for Hudson Bay which, in combination with other regional data, indicate that the glacial lakes Agassiz and Ojibway, (originally dammed by a remnant of the Laurentide ice sheet) drained catastrophically ∼8,470 calendar years ago; this would have released >1014 m3 of fresh water into the Labrador Sea. This finding supports the hypothesis,, that a sudden increase in freshwater flux from the waning Laurentide ice sheet reduced sea surface salinity and altered ocean circulation, thereby initiating the most abrupt and widespread cold event to have occurred in the past 10,000 years.

Absence of deep-water formation in the Labrador Sea during the last interglacial period

The two main constituent water masses of the deep North Atlantic Ocean—North Atlantic Deep Water at the bottom and Labrador Sea Water at an intermediate level—are currently formed in the Nordic seas and the Labrador Sea, respectively. The rate of formation of these two water masses tightly governs the strength of the global ocean circulation and the associated heat transport across the North Atlantic Ocean. Numerical simulations have suggested a possible shut-down of Labrador Sea Water formation as a consequence of global warming. Here we use micropalaeontological data and stable isotope measurements in both planktonic and benthic foraminifera from deep Labrador Sea cores to investigate the density structure of the water column during the last interglacial period, which was thought to be about 2 °C warmer than present. Our results indicate that todays stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period. Instead, a buoyant surface layer was present above a single water mass originating from the Nordic seas. Thus the present situation, with an active site of intermediate-water formation in the Labrador Sea, which settled some 7,000 years ago, has no analogue throughout the last climate cycle.

Quantitative assessment of carbonate dissolution in marine sediments from foraminifer linings vs. shell ratios: Davis Strait, northwest North Atlantic

A new method to evaluate quantitatively the degree of calcium carbonate preservation and dissolution in high-latitude marine sediments is proposed on the basis of relative abundance of CaCO 3 shells and organic linings of benthic foraminifers. This method was applied to a late Quaternary sequence from Davis Strait in the northwest North Atlantic and shows that CaCO 3 dissolution in sediments has increased since the last glacial maximum (ca. 18,000 B.P.) and peaked when subpolar interglacial conditions were established in surface waters. The dissolution in Davis Strait sediments appears to be closely related to organic biogenic production in surface waters, with a regional pattern of bottom-water formation and circulation.

Cyclic climatic records during the Olduval subchron (Uppermost Pliocene) on Zakynthos Island (Ionian Sea)

Abstract This contribution is focused on a high-resolution cyclostratigraphic investigation by means of an integrated study on the deposits on Zakynthos island (Ionian sea) within the Olduvai polarity subchron (from 1.95 to 1.77 Ma). Accurate palaeomagnetic study has been carried out to specify the precise position of the Olduvai subchron in the Zakynthos city series, named “Citadel” section. New detailed lithological observations have pointed out the presence of turbiditic and laminated facies within clay deposits. Layers corresponding to the Olduvai subchron have been studied in detail for pollen analysis and CaCO 3 content in order to evidence responses to the glacial/interglacial cycles. Four climatic cycles have been recognized by pollen and CaCO 3 analyses. In order to achieve a correct correlation with the isotopic stages, the δ 18 O measurements of the Zakynthoss foraminifer Globigerinoides ruber have been provided; they reveal the Artemisia expansions (e.g. steppes) to be contemporaneous with interglacial phases. This behaviour does not conform to the previously evidenced responses of plant associations (especially Artemisia ) to the glacial cycles in the western Mediterranean. Hence, the “west Mediterranean model” cannot be upheld for the whole Mediterranean domain.

Distribution of benthic foraminiferal populations in surface sediments of the Saguenay Fjord, before and after the 1996 flood

In the summer of 1996, a ‘flash’ flood occurred in the Saguenay-Lac-St-Jean region (Quebec) leading to the deposition, in less than 2 days, of an estimated 6–15×106 m3 of sediments at the head of the Saguenay Fjord. In order to evaluate the impact of such a sedimentary event on the benthic meiofauna, foraminiferal analyses were performed in surface sediment at eight stations in 1994, 1997, 1998 and 1999 along the main axis of the fjord. A 10-cm sedimentary sequence taken from a box core collected in 1997 from the deepest part of the fjord and spanning the last ca. 35 years provides a reference state for pre-flood foraminiferal assemblages. A significant change in foraminiferal assemblages is observed downcore, with the decrease of Spiroplectammina biformis relative to Adercotryma glomerata. 210Pb measurements suggest that this change occurred during the late 1970s. Surface samples (0–1 cm) collected between 1997 and 1999 in the inner basin also reveal a decrease of S. biformis percentages. Given the relative stability of this environment, such a change in faunal assemblages could be related to reduced industrial waste inputs or to a differential preservation of taxa. The 1996 flood had a major negative impact on the concentration and species diversity of benthic populations near the head of the fjord (Baie des Ha! Ha!), where the flood material is up to 50 cm thick. However, two years after the flood, benthic foraminiferal populations recolonized the sediments in the Saguenay Fjord.

Derivation of δ18O from sediment core log data: Implications for millennial‐scale climate change in the Labrador Sea

Sediment core logs from six sediment cores in the Labrador Sea show millennial-scale climate variability during the last glacial by recording all Heinrich events and several major Dansgaard-Oeschger cycles. The same millennial-scale climate change is documented for surface water δ18O records of Neogloboquadrina pachyderma (left coiled); hence the surface water δ18O record can be derived from sediment core logging by means of multiple linear regression, providing a paleoclimate proxy record at very high temporal resolution (70 years). For the Labrador Sea, sediment core logs contain important information about deepwater current velocities and also reflect the variable input of ice-rafted debris from different sources as inferred from grain-size analysis, the relation of density and P wave velocity, and magnetic susceptibility. For the last glacial, faster deepwater currents, which correspond to highs in sediment physical properties, occurred during iceberg discharge and lasted from several centuries to a few millennia. Those enhanced currents might have contributed to increased production of intermediate waters during times of reduced production of North Atlantic Deep Water. Hudson Strait might have acted as a major supplier of detrital carbonate only during lowered sea level (greater ice extent). During coldest atmospheric temperatures over Greenland, deepwater currents increased during iceberg discharge in the Labrador Sea, then surface water freshened shortly thereafter, while the abrupt atmospheric temperature rise happened after a larger time lag of ≥ 1 kyr. The correlation implies a strong link and common forcing for atmosphere, sea surface, and deep water during the last glacial at millennial timescales but decoupling at orbital timescales.