Bias in sediment chemical weathering intensity evaluation: A numerical simulation study

Abstract

Silicate weathering is critical to global carbon cycle and climate change, and has attracted considerable attention in Earth science studies. Chemical weathering intensity evaluation and paleoclimatic reconstruction based on siliciclastic sediment composition analysis are popular topics. However, chemical weathering signals are difficult to be accurately extracted from sediment compositions due to multiple, complex processes and factors in sediment source-to-sink systems. To better understand the bias in evaluation of chemical weathering intensity, we conduct Monte Carlo simulations to explore the relationships between sediment compositions and widely-used major elements-based weathering indices, i.e., Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), Weathering Index of Parker (WIP), the modified CIA (CIX) and representative element ratios (i.e., K/Al, Na/Al, Na/K, SiO2/Al2O3). Most chemical weathering indices are dominated by the total clay mineral content in sediments and exhibit remarkable grain size bias from hydrodynamic sorting, manifesting by variations of ca. 20 in CIA, CIW, PIA, WIP and CIX values between simulated sandy (15 wt% clay minerals) and muddy (47 wt% clay minerals) sediments. Clay mineral species, which might be shaped by source lithology, climate and even diagenesis, display noticeable effects to weathering indices in clay mineral-rich sediments. Although the plagioclase/K-feldspar ratios almost have no influence on CIA, the effects of detrital feldspar types and abundances are prominent on most weathering indices. Thus, source lithological bias cannot be overlooked in sediment weathering intensity evaluation under various climatic conditions because of the significant lithology controls on feldspar fertility and clay mineral species. The WIP and SiO2/Al2O3 indices are highly sensitive to quartz contents (e.g., quartz/feldspar ratios), which are closely related to sediment recycling and source lithology. This study, from the numerical simulation perspective, provides quantitative insight into biases in sediment chemical weathering intensity evaluation. Our findings highlight the importance of a comprehensive understanding how the sediment source-to-sink processes and factors influence siliciclastic sediment compositions, textures and chemical weathering indices. We advocate that a systematic analysis (e.g., mineralogy, grain size, provenance, etc.) on targeted sediments or sedimentary rocks is necessary for interpreting sedimentary weathering records.