Geomorphic control on the δ¹³C values of organic matter

Goyal, Kanva (2019) Geomorphic control on the δ¹³C values of organic matter. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Many paleovegetation studies unveil the contributions of C₃ and C₄ plants at a given locality by the means of δ¹³C value of soil organic matter. This methodology holds incredible potential in the field of paleoecological reconstruction. However, the intimate connection of vegetation with geomorphic processes and landforms may impart uncertainties that limit the direct application of this method. In fluvial deposits, the response of plants across the landforms may be driven largely by the hydro-geomorphic condition. The variation in the environmental condition influences the plant-adaptive strategies. However, the effect of local geomorphic architecture, along the lateral transects of riverine deposits, on the temporarily stable vegetation and soils with different pedogenic intervals were never quantified from the modern analogs. In this study, sun leaf carbon isotopic composition ( δ¹³CBulk) of dominant tropical deciduous species (Sal- Shorea robusta) and its associated leaf litter was investigated from an undulatory geomorphic surface of north-south trending lateritic alluvium of South Bengal (India). Preliminary study on the δ¹³CBulk values of this deciduous species suggests a variation of ca. 6-7‰ as the water-availability, canopy effect, vegetation density varied along the lateral transects. However, the litter layer δ¹³CBulk values represent a geomorphic and soil texture control in sequestration of the vegetation signals. The badlands or upland areas tend to preserve more positive ¹³C/¹²C ratio (–24‰) compare to the lowland with well-developed soils (–28‰). The use of δ15NBulk values from the same litter soil shows that higher δ¹⁵N value of bulk soil (4‰) is associated with the soils of upland areas suggesting faster decomposition of organic matter (TOC% <0.2) due to higher percolation and soil aeration as coarser grains are present there. Elemental oxide concentration and δ¹³CBulk values measured at 15cms interval in three soil profiles of depth 1.5 m at different locations in the floodplain shows the different rate of pedogenesis in soil profiles developed on same basement material i.e. secondary laterites. There is 7‰ enrichment in δ13CBulk values. If we consider this as paleosol, we would come to a conclusion that there was mixed C₃-C₄ vegetation type but there is no change in the vegetation since Late Pleistocene. Thus, we would come to a wrong conclusion. Therefore, reconstruction of the past vegetation from the preserved organic records is not straightforward and thus requires detail vegetation, geomorphic and physical or chemical characterization before employing it as paleo-record archives.

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