Chemical dynamics of the “St. Lawrence” riverine system: δD H 2O , δ 18O H 2O , δ 13C DIC, δ 34S sulfate, and dissolved 87Sr/ 86Sr

Abstract

Chemical and stable isotope analyses of the St. Clair, Detroit, Niagara, and St. Lawrence rivers (“St. Lawrence” system) and their tributaries show that the chemical and isotopic compositions of the waters are strongly controlled by the geology of their drainage basins. Tributaries draining the Canadian Shield have very low TDS, HCO 3 −, SO 4 2−, Ca 2+, Mg 2+, NO 3 −, Sr 2+, higher Si and Fe total, and high 87Sr/ 86Sr ratios (0.710–0.713). The Grand and Thames rivers that drain Paleozoic limestones, dolostones, and evaporites are characterized by opposite attributes. The “St. Lawrence” and the tributaries draining the Canadian Appalachians fall between these two endmembers. The St. Clair, Detroit, and Niagara rivers do not show any pronounced seasonal variations in major component chemistry due to buffering by the Great Lakes. In contrast, pronounced seasonal variations characterize the lower St. Lawrence mainly because of significant tributary inputs into the overall water budget. The δD and δ 18O in the “St. Lawrence” range from −60.9 to −44.5‰ and from −8.5 to −6.1‰ SMOW, respectively, much heavier than the comparative values measured for the tributaries (−92.8 to −58.3‰ and −13.1 to −8.5‰). This is a consequence of evaporative loss that, over the residence time of water of 10 2 years, equals about 7% of the water volume in the Great Lakes. The strontium and sulfur isotopic values for the “St. Lawrence” system are relatively uniform, with measured values from 0.70927 to 0.71112 for 87Sr/ 86Sr and from 4.3 to 5.6‰ for sulfate δ 34S. Their seasonal variations are also minor. The strontium and sulfur fluxes of the St. Lawrence river are calculated to be 7.84 × 10 8 and 1.09 × 10 11 mol/a, respectively. The relative contributions of the Great Lakes, tributaries, and other sources to these fluxes are 73:16:11% for strontium and 64:13:23% for sulfur. Isotopic composition of dissolved inorganic carbon ( δ 13C DIC) in the “St. Lawrence” system ranges from −4.7 to +0.7‰, considerably heavier than the values for the tributaries (−16.5 to −6.7‰). The light δ 13C DIC values for the tributaries suggest that CO 2 from bacterial respiration plays an important role in the isotopic composition of riverine DIC. However, in the main stem river(s), this bacterial signal is masked by isotopic equilibration with atmospheric CO 2 due to the long residence time of water in the Great Lakes. Seasonally, the main stem river(s) have heavier δ 13C values in the fall than in the spring, a consequence of preferential 12C consumption by photosynthetic plants in the epilimnion of the Great Lakes during the growth season. In the down-stream portion of the St. Lawrence river, influx of isotopically light tributary waters causes progressive 13C depletion, from −1.3 to −2.0‰ and −1.4 to −3.0‰ in the fall and spring, respectively. The total DIC carbon flux of the St. Lawrence river is calculated to be 3.9 × 10 11 mol/a. Mass balance calculations show that the relative contributions of the Great Lakes, tributaries, decay of organic matter, exchange with the atmosphere, and dissolution of carbonates to this total DIC flux are 81:13:2:-6:10% in the spring, and 83:15:-2:4:0% in the fall, respectively.