Browsing by Author "Meadows, Michael"

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    Measuring Compound Soil Erosion by Wind and Water in the Eastern Agro–Pastoral Ecotone of Northern China
    (2022-05-21) Lin, Degen; Shi, Peijun; Meadows, Michael; Yang, Huiming; Wang, Jing’ai; Zhang, Gangfeng; Hu, Zhenhua
    Land degradation induced by soil erosion is widespread in semiarid regions globally and is common in the agro–pastoral ecotone of northern China. Most researchers identify soil erosion by wind and water as independent processes, and there is a lack of research regarding the relative contributions of wind and water erosion and the interactions between them in what is referred to here as compound soil erosion (CSE). CSE may occur in situations where wind more effectively erodes a surface already subject to water erosion, where rainfall impacts a surface previously exposed by wind erosion, or where material already deposited by wind is subject to water erosion. In this paper, we use the Chinese Soil Loss Equation (CSLE) and the Revised Wind Erosion Equation (RWEQ) to calculate the rate of soil erosion and map the distribution of three types of soil erosion classified as (i) wind (wind-erod), (ii) water (water-erod), and (iii) CSE (CSE-erod) for the study area that spans more than 400,000 km2 of sand- and loess-covered northern China. According to minimum threshold values for mild erosion, we identify water-erod, wind-erod, and CSE-erod land as occurring across 41.41%, 13.39%, and 27.69% of the total area, while mean soil erosion rates for water-erod, wind-erod, and CSE-erod land were calculated as 6877.65 t km−2 yr−1, 1481.47 t km−2 yr−1, and 5989.49 t km−2 yr−1, respectively. Land subject to CSE-erod is predominantly distributed around the margins of those areas that experience wind erosion and water erosion independently. The CSLE and RWEQ do not facilitate a direct assessment of the interactions between wind and water erosion, so we use these equations here only to derive estimates of the relative contributions of wind erosion and water erosion to total soil erosion and the actual mechanisms controlling the interactions between wind and water erosion require further field investigation. It is concluded that CSE is an important but underappreciated process in semiarid regions and needs to be accounted for in land degradation assessments as it has substantial impacts on agricultural productivity and sustainable development in regions with sandy and/or loess-covered surfaces.
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    Palaeoenvironmental reconstruction in South Africa's year-round rainfall zone using multiproxy geochemical analyses on lake sediments from Swartvlei
    (2019) Maboya, Matjie Lillian; Meadows, Michael; Haberzettl, Torsten
    Coastal lakes in the south coast of South Africa contain sediments with good records of palaeoenvironmental changes. Swartvlei is the largest of the lakes in the Wilderness Embayment and is connected to the Indian Ocean via an estuary. The lake is believed to have been formed during sea-level regressions in the quaternary, and separated from Groenvlei lake between 4000 and 2000 cal BP. There are questions about dominant precipitation regimes as well as the onset, cessation and altitude of marine transgressions in the area. In this study, Holocene sediments from Swartvlei Lake were extracted and investigated using multiple methods. These include organic and inorganic geochemical proxies and multidating approach through radiocarbon and OSL dating. A composite profile was made using marker layers with lithostratigraphic distinctions combining three cores into one continuous, 7 m long core spanning 8600 cal BP. The core was subdivided into two distinct zones namely, Unit A (8600 to 3500 cal BP) and Unit B (3500 cal BP to present), identified using cluster analysis on particle size data. The results reveal low sea-levels with limited precipitation and aquatic productivity during the early Holocene, followed by a marine incursion from 4500 to 3500 cal BP and moister conditions thereafter. This marine incursion, marked by increased Ca and TIC concentrations, occurred when the physical barriers were breached, and the estuarine channel widened due to a landward strandline migration. Pronounced riverine input due to increased precipitation was observed after 3500 cal BP, with strong minerogenic input and lowered sealevel. An influx of silt and clay material that dominate the top half of the core marks Swartvlei’s evolution into more lacustrine conditions and its separation from Groenvlei during the same period. A more humid climate is further inferred from organic proxies that indicate a greater in-wash of vascular vegetation during this period, as well as higher productivity from 3500 to 1400 cal BP. In addition, high biogenic silica concentrations indicate increased bio-productivity during the Little Ice Age (LIA) while increased sedimentation rates suggest that anthropogenic activity impacted the lake from 150 cal BP. This study adds insight to the geomorphic evolution of Swartvlei and highlights the usefulness of geochemical analyses in the elucidation of regional quaternary environmental and climatic changes.
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    Phytolith Analysis as a Palaeoecological Tool for Reconstructing Mid-to Late-Pleistocene Environments in the Olorgesailie Basin, Kenya
    (2012) Kinyanjui, Rahab; Meadows, Michael; Gillson, Lindsey
    Olorgesailie Basin is an important prehistoric locus and holds a prominent place in African Quaternary research. It is located in the southern Kenya rift system (1˚ 35´S and 36˚ 27´E) and has preserved numerous archaeological findings of the Acheulean, Sangoan and Middle Stone Age occupations and most importantly, hominin cranium associated with the Acheulean hand axes. Evidence of past vegetation has hitherto been scarce, because the arid / semi-arid conditions are not conducive for preservation of organic plant microfossils. Phytolith analysis is used to reconstruct the vegetation history and understand hominin habitat preference during mid-late Pleistocene (~746-64 ka). A localised modern phytolith analogue is used to interpret the fossil assemblage. Descriptive analysis identified fifty nine phytolith morphotypes which were categorised into three major groups; grass short-cell phytolith, epidermal silicified appendages and sedge phytoliths. The identification and classification was based on the International Code for Phytolith Nomenclature and other existing literature. In order to determine the significance of the identified morphotypes in interpreting fossil data, two multivariate statistical analyses were performed on the database; Correspondence analysis distinguished vegetation components according to a moisture gradient while Cluster analysis identified unique morphotypes that were taxonomically affiliated to their parent plant species. However, the analyses were not able to distinguish plants along the altitudinal gradient. Fossil phytoliths derived from sixty palaeosol samples extracted from eight geological sections in three localities of the Olorgesailie Basin, were identified and tallied to determine vegetation cover and how this changed both spatially and temporarily during the mid-late Pleistocene period. Fossil phytolith frequencies were plotted on TILIA diagrams against the available 40Ar/39Ar dates. Correspondence analysis identified three vegetation components; grasslands, woody & herbaceous and aquatic and identified moisture availability as the major underlying factor influencing the morphotype clusters on the ordination space. An additional cluster analysis of forty morphotypes derived from grasses confirmed the known taxonomic affiliation of major grass short-cell phytoliths to four grass subfamilies (saddle ovate- C3 Arundinoideae, bilobate concave- and convex- outer margin short shaft and quadralobate-C4 Panicoideae, saddles-C4 Chloridoideae and bilobates convex outer margin long shaft-C4 Aristidoideae). From the non-grass category, achene and papillae phytolith morphotypes were identified as belonging to the Cyperaceae.