ABSTRACT: Deciphering dam impacts from the natural background in ultra-high resolution sedimentary records in the NW Gulf of St. Lawrence.

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Abstract

Dam construction is an important factor influencing the sedimentary budget in the proximal offshore catchment area. However, not only the timing of dam construction is decisive on the sedimentation, but also the natural hydro-climatic variations. Therefore, there is a major difficulty in deciphering the timing of the dam construction from the natural variations in the sedimentary records of the catchment area. In this study we illustrate a methodology to measure the background natural sedimentary variations and distinguish the timing of the dam construction in the sedimentary regime using a multi-core and multi-proxy approach combined with wavelet analysis. Onboard the R/V Coriolis II, two box cores were collected between 10 and 20 km offshore in the Sept- Îles area (NW Gulf of St. Lawrence): one within (56BC) and one outside (63BC) the catchment area of the dammed Sainte-Marguerite river. The methods comprised 210Pb-derived age models, CAT-scan imagery, grain size, geochemical and isotopic measurements (C, N, 13C). In addition, ultra-high resolution micro-fluorescence X (X-RF) data were acquired on both cores with a 100 ľm downcore resolution, corresponding to an annual temporal resolution. In order to study the high frequency sedimentological variations, the X-RF data were submitted to a principal component analysis, over which a wavelet analysis was performed. The results reveal a sharp change in the late 1890's in core 56BC (in the dam catchment area) through a flattening of the d50 grain size parameter. This sharp change is not observed in the other core and is associated with the construction of the SM1 dam, the first dam constructed on the Sainte-Marguerite river in 1906. The only other sedimentological events recorded in the grain size data are dated in 1966 AD and 1972 AD or 1976 AD and are linked to the largest historical hydro-meteorological events. Other high frequency sedimentological variations have been investigated by wavelet analysis and compared with historical climatic time series. NAO-like periods (2 to 6 yrs, 6 to 8 yrs, 8 to 12 yrs, 12 to 16 yrs, 18 to 22 yrs, 30 to 35 yrs, 40 to 40 yrs, 60 to 80 yrs and 95 to 100 yrs) and patterns have been uncovered and suggest a relationship between the Gulf of St. Lawrence sedimentary regime and climatic oscillations. Finally, another striking result is the absence of high frequency (periods <30 years) sedimentological variations after AD 1900 in core 56BC (in the catchment area), highlighting once more the possible impact of the dam construction on the sedimentary regime. These results therefore suggest that the proximal offshore sedimentary records are well-suited for measuring the impacts of dam construction on the environment.

Bibliography of Canadian Geomorphology