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

Abstract

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.