From rock to soil: Saprock genesis and its legacy for subsoil structure and micro-aggregate formation during pedogenesis

• Role of weathering and saprock genesis for (micro-)aggregate formation is unraveled. • Microaggregate formation is not limited to pedogenesis. • Primary particles and dissolution residues are nuclei for aggregate formation. • Translocated topsoil-material involved in aggregation in subsoil and saprock. • Rock alteration and soil formation already since in the Pleistocene. The development of soil structure and the formation of (micro-)aggregates starts with the alteration of the fluid-exposed surfaces of the parent rock during weathering, with the bedrock type as an essential determinant. We shed light on the intricate processes that gradually convert the pristine bedrock lithology into subsoil during weathering and incipient pedogenesis, thereby already commencing aggregation. Applying a profile-for-time approach, we explore the bedrock-soil continuum, from the pristine bedrock to regolith and subsoil of different bedrock types, including sedimentary and igneous rocks in temperate climate conditions exemplary in central Germany. Based on textural, petrological, mineralogical, and chemical information obtained by complementary analytical methods, including stereo and polarization microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, infrared-spectroscopy, and X-ray diffraction, we show that bedrock type determines the material inventory of the (micro-)aggregates formed during pedogenesis. We demonstrate that the formation of aggregates is not limited to processes in soil but must be considered an intrinsic process in saprocks due to the translocation of soil-born matter from proximate environments. We found that the structure and the material inventory of (micro-)aggregates found to date are determined to a large extent by the bedrock type and are governed by processes operative in past geologic times, already during petrogenesis and finally during saprock genesis.