Inferred spring discharge characteristics of the Saguenay River, Quebec between ca. 1850 and ca. 1900 based on sediment texture proxy data

Abstract

This study explores the potential of sediment textural variation as a proxy for spring freshet magnitude variation of the Saguenay River using fine sand to fine silt fractions that have been preserved in Saguenay River prodelta sediments deposited in the northwestern sector of the Saguenay Fiord's North Arm. High sedimentation rates and particulate organic matter fluxes have created a poorly oxygenated seafloor environment in parts of the North Arm that is virtually free of bioturbating organisms thereby facilitating the preservation of identifiable yearly increments of sediment that accumulate mostly during the River's annual spring freshet. A particle size (2.3 to 6.3 phi i.e., 0.220 mm to 0.013 mm) median diameter (MD) proxy of spring freshet magnitude determined at one cm intervals in a piston core collected in 1982 reflects year-to-year variability of the River's spring freshet magnitude during the 19th and most of the 20th century. This study focuses on the second half of the 19th century during which MD data records an estimated 19 year long interval of relatively small MD's (LMDI) that is estimated to have occurred between ~1871 and ~1888. Compared to the later decades of the 20th century MD record, the second half of the 19th century and the early part of the 20th century to about 1912 shows a relatively higher frequency of larger average MD that appears to imply a more frequent occurrence of stronger spring freshet intervals of comparatively lower temporal variability. Within the 1850 - 1912 interval, the MD proxy suggests a generally decreasing freshet magnitude trend from ~1865 to ~1871 that is followed by the LMDI period of reduced freshet magnitudes featuring MDs that are typically less than 55 um. The LMDI is succeeded by a generally increasing MD sequence suggestive of stronger freshets that persists until about 1912. An explanation for the apparent relatively low freshet magnitudes and low year-to-year variability of spring freshets during the LMDI in relation to previous and following decades is tentatively assigned to lesser amounts of snowfall during January and April and to relatively warmer January and February temperatures acting in concert with relatively lower March and April temperatures. These seasonal conditions show a general correspondence to warm (positive) Atlantic Multi-decadal Oscillation (AMO) phases that occurred between 1860 and 1891. In contrast, during the 20th century, cool (negative) AMO phases seem to be linked to intervals that include some of the highest recorded 20th century Saguenay River freshets witnessed in the 1970's. An analysis of the North Atlantic Oscillation (NAO)/MD relationship yielded mixed results. Not surprisingly, contradicting results also emerged in a comparison of AMO and NAO phases with respect to late 19th century local newspaper weather reports. In general, cooler spring and fall weather and stormy conditions were often associated with negative AMO conditions and with both positive and negative NAO's.