Long-term analysis of Hubbard Brook stable oxygen isotope ratios of streamwater and precipitation sulfate

In response to decreasing atmospheric emissions of sulfur (S) since the 1970s there has been a concomitant decrease in S deposition to watersheds in the Northeastern U.S. Previous study at the Hubbard Brook Experimental Forest, NH (USA) using chemical and isotopic analyzes ( $ delta^{34} {text{S}}_{{{text{SO}}_{4} }} $ ) combined with modeling has suggested that there is an internal source of S within these watersheds that results in a net loss of S via sulfate in drainage waters. The current study expands these previous investigations by the utilization of δ¹⁸O analyzes of precipitation sulfate and streamwater sulfate. Archived stream and bulk precipitation samples at the Hubbard Brook Experimental Forest from 1968–2004 were analyzed for stable oxygen isotope ratios of sulfate ( $ delta^{18} {text{O}}_{{{text{SO}}_{4} }} $ ). Overall decreasing temporal trends and seasonally low winter values of $ delta^{18} {text{O}}_{{{text{SO}}_{4} }} $ in bulk precipitation are most likely attributed to similar trends in precipitation $ delta^{18} {text{O}}_{{{text{H}}_{2} {text{O}}}} $ values. Regional climate trends and changes in temperature control precipitation $ delta^{18} {text{O}}_{{{text{H}}_{2} {text{O}}}} $ values that are reflected in the $ delta^{18} {text{O}}_{{{text{SO}}_{4} }} $ values of precipitation. The significant relationship between ambient temperature and the $ delta^{18} {text{O}}_{{{text{H}}_{2} {text{O}}}} $ values of precipitation is shown from a nearby site in Ottawa, Ontario (Canada). Although streamwater $ delta^{18} {text{O}}_{{{text{SO}}_{4} }} $ values did not reveal temporal trends, a large difference between precipitation and streamwater $ delta^{18} {text{O}}_{{{text{SO}}_{4} }} $ values suggest the importance of internal cycling of S especially through the large organic S pool and the concomitant effect on the $ delta^{18} {text{O}}_{{{text{SO}}_{4} }} $ values in drainage waters.