PRE-QUATERNARY PALYNOMORPHS AS TRACERS OF FRESHWATER AND SEDIMENT INFLUX IN THE MACKENZIE TROUGH, CANADIAN BEAUFORT SEA

The Mackenzie River is the most important Canadian river in terms of drainage area and sediment discharge. It drains an area of more than 1.8 million km² and a total of 127 M tons of sediments are transported annually and discharged in the Beaufort Sea. The transported sediments are of Mesozoic and Cenozoic age and contain abundant microfossils, among which pre-Quaternary organic-walled palynomorphs (pollen and spores, dinoflagellate cysts and acritarchs). These palynomorphs constitute some of the main micropaleontological tracers in sub-Arctic and Arctic environments: they are abundant, present in coastal and oceanic environments, highly resistant to diagenetic processes, and they do not suffer dissolution like biogenic silica or calcium carbonate microfossils.

A 36 cm-long sediment core spanning the last ~150 years was recovered from the Mackenzie Trough area in 2009 to quantitatively reconstruct the evolution of sea surface parameters using dinoflagellate cysts as proxy indicators, and to assess the freshwater influx from the Mackenzie River to the Beaufort Sea area using pre-Quaternary palynomorphs. Their concentrations varied between 100 and 3600 palynomorphs/cm³ and were correlated with instrumental data of the Mackenzie River discharge since 1935 AD. Therefore, they were used as indicators of freshwater discharge by the Mackenzie River for the time period covered by the core.

The pre-Quaternary palynomorphs displayed 4 zones over the last 150 years: Zone 1 (1860-1890 AD) characterized by low concentrations (av. 1100 specimens/cm³) associated with reduced freshwater discharge and widespread drought throughout central Canada; Zone 2 (1890-1980 AD) characterized by relatively high palynomorph concentrations (av. 1400 specimens/cm³) during a period favourable for large scale agriculture in central Canada; Zone 3 (1980-1995 AD) characterized by maximum concentrations (av. 2240 specimens/cm³) and associated with the maximum freshwater discharge on record; Zone 4 (1995-2003 AD) characterized by a steady decrease of palynomorph concentrations and associated with the decrease in river discharge since the 1989 AD maximum. This highlights the potential of these palynomorphs as freshwater indicators. Work is underway to assess the sedimentological origin of the palynomoprhs based on their age.