The Effect of River-Ice Break-Up on Suspended Sediment and Select Trace-Element Fluxes

Ice break-up on northern rivers presents a unique research challenge because of its dynamic nature and complexity of physical, chemical and biological processes. Rapidly moving ice, ice jamming, flood levels, and enhanced flow velocities can produce significant sediment transport and trace-element fluxes. Systematic sampling for these parameters, however, is rarely conducted because of logistical difficulties. This paper discusses the magnitude and relative significance of sediment and trace-element fluxes during break-up of the Liard River in northern Canada. Historical data for the open-water and stable ice-covered periods are compared to that measured during the 1987 and 1993 break-up events. Analysis reveals that a break-up pulse occurs during this period that if not accounted, can lead to significant underestimation of suspended sediment and trace-element fluxes. More generally, any estimates of annual sediment or trace-elements for northern rivers that do not include data for the critical break-up period must be regarded as being conservative.

Grenville metagabbro complexes of the Otter Lake area, Quebec

Metagabbro complexes in a portion of the Grenville Province lying northwest of Ottawa occur as sheets, cylinders, and irregular bodies within a medium- to high-grade marble–gneiss–amphibolite terrane. The largest bodies (0.5–10 km in greatest dimension) consist principally of felsic metagabbro, mafic metagabbro, and minor metapyroxenite. Major-element and select trace-element analyses show that different complexes contain distinctly different amounts of K and other elements resulting in different interelement trends.Microstructure and microprobe mineral analyses provide evidence that the following metamorphic changes have occurred: (i) recrystallization of Ca pyroxene, orthopyroxene, and plagioclase; (ii) reaction of Mg-rich olivine with plagioclase to produce reaction zones consisting of orthopyroxene and a hornblende–spinel intergrowth; (iii) reaction of Mg–Fe olivine with plagioclase to produce garnet and hornblende; (iv) production of anthophyllite and hornblende from orthopyroxene and plagioclase; (v) production of hornblende (locally as rims about Ca pyroxene) from Ca pyroxene and plagioclase; (vi) crystallization of biotite, possibly by reaction between orthopyroxene and K-feldspar; and (vii) crystallization of small inclusions of spinel and ilmenite in Ca pyroxene and of spinel and biotite in plagioclase.With regard to the reaction olivine + plagioclase = orthopyroxene + hornblende + spinel, the anorthite and locally the forsterite components were extracted preferentially from plagioclase and olivine; K and Ti (for hornblende) and Zn (for spinel) were evidently obtained from the surrounding minerals; and H, F, and Cl (for hornblende) were obtained from beyond the gabbro bobies. Locally the reaction occurred within large crystals of Ca pyroxene where embedded olivine and plagioclase crystals were in contact.The production of hornblende rims about Ca pyroxene evidently involved plagioclase as a reactant, but the rims formed regardless of the contacting minerals. For example, rims were locally produced where Ca-pyroxene crystals were embedded in large crystals of orthopyroxene.Application of five geothermometers to crystals of both igneous and metamorphic origin yield temperatures of about 700 °C, similar to temperatures recorded for the enclosing marble and gneiss.