TYPHOON IMPACTS ON THE CHEMICAL WEATHERING REGIME OF A HIGH MOUNTAINOUS RIVER, TAIWAN

Dr. Berry Lyons has contributed a wealth of valuable information and insight into the geochemistry of small mountainous rivers and High Standing Island (HSI) watersheds, the role they play in global carbon budget, and on their material fluxes from land to sea. HSIs represent a major source of physically weathered particulate matter and chemically weathered solutes to the world’s oceans, and chemical weathering of silicate minerals is a primary control on carbon export from the atmosphere over geological time. Because of the direct exposure of many HSIs to open ocean, they are commonly exposed to the full force of intense storm events that may dramatically affect the weathering regimes of the mountainous watersheds. Taiwan alone experiences an average of four typhoons each summer.

Strontium isotope ratios were analyzed for water samples collected from the Choshui River during a typhoon event in 2004, as well as the days preceding and following the storm. Storm samples were collected at three hour intervals at the upstream Renlun Bridge for the first 72 hours following impact of Typhoon Mindulle. These data are used in conjunction with major ion analyses to assess changes in preferential weathering between different lithologies associated with isotopically distinct tributaries. Following the onset of the storm, more radiogenic ⁸⁷Sr/⁸⁶Sr ratios are seen. These are higher than any previously reported ⁸⁷Sr/⁸⁶Sr ratios from the watershed, suggesting increased fractional input from the furthest upstream tributaries compared with normal flow. This may suggest increased preferential weathering of the more metamorphic high mountain lithologies. Ca/Sr ratios for the storm samples reach peaks during peak discharge periods. Previously analyzed, but unreported dissolved silica concentrations were provided, and used along with storm hydrograph data to determine total Si flux. These are compared to previously reported values from a sampling site located ~21 km downstream. Dissolved Si flux closely mirrors the storm hydrograph, suggesting total CO₂ drawdown scales with storm intensity.