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.