Assessing the oxidative weathering of pyrite and its role in controlling atmospheric CO2 release in the eastern Qinghai-Tibet Plateau

Abstract

Carbonate weathering, coupled with sulfuric acid, derived from the oxidative weathering of pyrite (OWP), has been suggested to compensate for the CO2 drawdown by silicate weathering over the Cenozoic. However, the above compensation had not been well documented because of the lack of evidence of OWP and the failure to differentiate between sulfide-derived and gypsum-derived sulfate. In this study, monthly sampling campaigns from July 2012 to June 2014 were conducted at Tangnaihai station in the eastern Qinghai-Tibet Plateau (QTP) to demonstrate the behavior of sulfide-derived sulfate and its effect on the local atmospheric CO2 balance. Sulfur and triple oxygen isotopes of sulfate were employed to discern OWP, and sulfur isotope was used to calculate the precise contributions of sulfide-derived sulfate. The results revealed that fractions of riverine sulfate from OWP ranged from 2% to 99% with a mean value of 61%. The corresponding annual CO2 release flux by carbonate weathering coupled with sulfuric acid (CSW) ranged from 0.005 Tmol /yr to 0.015 Tmol /yr with average values of 0.012 Tmol /yr and 0.007 Tmol /yr in 2012–2013 and 2013–2014, respectively, while the annual CO2 consumption flux by silicate weathering coupled with carbonic acid (SCW) varied from 0.003 Tmol /yr to 0.015 Tmol /yr with average annual values of 0.008 Tmol /yr and 0.010 Tmol /yr in 2012–2013 and 2013–2014, respectively. These results indicated that CO2 release by CSW exceeded CO2 consumption by SCW in 2012–2013, but not in 2013–2014. In the uplifting mountain areas, the pyrite oxidation and produced CO2 release sometimes could not balance the CO2 consumption by SCW, and its roles in long term climate change are uncertain. In future research, the role of sulfide oxidation coupled to carbonate weathering should be reestimated to clarify for present and past global climate change.