CHEMICAL-DYNAMICS OF THE ST-LAWRENCE RIVERINE SYSTEM - DELTA-D-H2O, DELTA-O-18(H2O), DELTA-C-13(DIC), DELTA-S-34(SULFATE), AND DISSOLVED SR-87 SR-86/

Chemical and stable isotope analyses of the St. Clair, Detroit, Niagar a, and St. Lawrence rivers (''St. Lawrence'' system) and their tributa ries show that the chemical and isotopic compositions of the waters ar e strongly controlled by the geology of their drainage basins. Tributa ries draining the Canadian Shield have very low TDS, HCO3-, SO42-, Ca2 +, Mg2+, NO3-, Sr2+, higher Si and Fe-total, and high Sr-87/Sr-86 rati os (0.710-0.713). The Grand and Thames rivers that drain Paleozoic lim estones, dolostones, and evaporites are characterized by opposite attr ibutes. The ''St. Lawrence'' and the tributaries draining the Canadian Appalachians fall between these two endmembers. The St. Clair, Detroi t, and Niagara rivers do not show any pronounced seasonal variations i n major component chemistry due to buffering by the Great Lakes. In co ntrast, pronounced seasonal variations characterize the lower St. Lawr ence mainly because of significant tributary inputs into the overall w ater budget. The delta D and delta(18)O in the ''St. Lawrence'' range from -60.9 to -44.5 parts per thousand and from -8.5 to -6.1 parts per thousand SMOW, respectively, much heavier than the comparative values measured for the tributaries (-92.8 to -58.3 parts per thousand and - 13.1 to -8.5 parts per thousand). This is a consequence of evaporative loss that, over the residence time of water of similar to 10(2) years , equals about 7% of the water volume in the Great Lakes. The strontiu m and sulfur isotopic values for the ''St. Lawrence'' system are relat ively uniform, with measured values from 0.70927 to 0.71112 for Sr-87/ Sr-86 and from 4.3 to 5.6 parts per thousand for sulfate delta(34)S. T heir seasonal variations are also minor. The strontium and sulfur flux es of the St. Lawrence river are calculated to be 7.84 x 10(8) and 1.0 9 x 10(11) mol/a, respectively. The relative contributions of the Grea t Lakes, tributaries, and other sources to these fluxes are 73:16:11% for strontium and 64:13:23% for sulfur. Isotopic composition of dissol ved inorganic carbon (delta(13)C(DIC)) in the ''St. Lawrence'' system ranges from -4.7 to +0.7 parts per thousand, considerably heavier than the values for the tributaries (-16.5 to -6.7 parts per thousand). Th e light delta(13)C(DIC) values for the tributaries suggest that CO2 fr om bacterial respiration plays an important role in the isotopic compo sition of riverine DIC. However, in the main stem river(s), this bacte rial signal is masked by isotopic equilibration with atmospheric CO2 d ue to the long residence time of water in the Great Lakes. Seasonally, the main stem river(s) have heavier delta(13)C values in the fall tha n in the spring, a consequence of preferential C-12 consumption by pho tosynthetic plants in the epilimnion of the Great Lakes during the gro wth season. In the down-stream portion of the St. Lawrence river, infl ux of isotopically light tributary waters causes progressive C-13 depl etion, from -1.3 to -2.0 parts per thousand and -1.4 to -3.0 parts per thousand in the fall and spring, respectively. The total DIC carbon f lux of the St. Lawrence river is calculated to be 3.9 x 10(11) mol/a. Mass balance calculations show that the relative contributions of the Great Lakes, tributaries, decay of organic matter, exchange with the a tmosphere, 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, respectivel y.