NEWS & EVENTS

The weathering of silicate minerals regulates climate on million-year timescales. The different isotopic proxies that have historically tracked this weathering process provide insight into different aspects of weathering through geologic time. Some silicate bedrock, particularly ophiolites, are more susceptible to enhanced weathering than other lithologies. Their unique characteristics highlight imperfections in the way these proxies track weathering. Lithium isotopes (δ⁷Li) are a stable weathering proxy that can be used to help track weathering processes due to the fractionation of Li during secondary clay mineral formation. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) are a radiogenic weathering proxy that has historically been used to track continental weathering due to the higher abundance of ⁸⁷Sr in continental crust. Here we present data collected from tributaries that source the Nagaland-Manipur Ophiolite Complexes in northeastern India, to compare how the weathering signal is reflected across different weathering proxies. δ⁷Li, ⁸⁷Sr/⁸⁶Sr, and clay mineralogical compositions were measured from river water, bedload, and suspended sediment to gain insight into silicate weathering processes in the region. Our results show disagreement between the two weathering proxies. Sampled river water yielded δ⁷Li_diss values that range from ~21 to 31‰ which is markedly heavier than the global riverine average of ~23‰. In contrast the ⁸⁷Sr/⁸⁶Sr values from the same samples range from 0.7061-0.7114 placing them below the global Sr riverine average of 0.7119. Our δ⁷Li_diss measurements indicate that rivers draining ultramafic lithologies in warm humid climates are likely experiencing higher degrees of chemical weathering intensity and clay mineral formation, thus yielding anomalously high δ⁷Li values compared to other drainages comprised of more felsic lithologies. Additionally, these juvenile mafic lithologies are reflected in the relatively lower ⁸⁷Sr/⁸⁶Sr ratios due to the age of the bedrock and masking the true chemical weathering rates.

Additional information: Mr. Giancarlo DEFRANCESCO, gdefran@connect.hku.hk