Potassium isotopic signatures of modern offshore detrital sediments from different climatic regimes and the implications

Potassium isotopes are a novel tracer for continental weathering. Previous K isotope studies on chemical weathering generally targeted weathering profiles under a particular climate region, yet the effects of chemical weathering on K isotopes under different climatic backgrounds remain unclear. Moreover, little is known about the K isotope signatures of modern unconsolidated detrital sediments. Here, we report K isotopic data of surficial seafloor sediments from continental shelves along the east coast of China (ECC), as well as those around the tropical Hainan island in the northern South China Sea. The ECC sediments have a relatively narrow distribution of δ⁴¹K (with reference to NIST3141a) values, which range from (−0.40±0.01)‰ to (−0.57±0.04)‰, with an average of (−0.51±0.09)‰. By contrast, δ⁴¹K values of Hainan offshore sediments display a larger variation, ranging from (−0.28±0.07)‰ to (−0.67±0.02)‰. The δ⁴¹K values of Hainan offshore sediments exhibit negative correlations with the chemical index of alteration (CIA), Al/K, Ti/K, and total iron (Fe_T), which underlines the control of chemical weathering on K isotopic signatures of detritus inputs into oceans. We also measured Mg isotope compositions for the same samples; interestingly, the variability in δ²⁶Mg of the samples is small (~0.24‰) for all ECC and Hainan offshore sediments, and δ²⁶Mg values do not show clear correlations with indexes of chemical weathering. Our study demonstrates the link between K isotopic variability of detrital sediments and climatic conditions including rainfall intensity, which indicates that K isotopes of the detrital component of marine sediments could be applied to study Earth’s climate in deep time. The δ⁴¹K values of the offshore detrital sediments are significantly less variable than those of pelagic marine sediments, highlighting the importance of distinguishing the effects of diagenesis and neoformation of clay minerals from continental weathering in attempts to study deep-time climate-weathering link by K isotopes in detrital sedimentary records.