BIOLOGICAL MODULATION OF GLACIAL-INTERGLACIAL VARIATIONS IN SEDIMENT TRANSPORT AND HILLSLOPE EVOLUTION

Given the periodicity of glacial-interglacial climate change and associated ecological shifts, most soil-mantled hillslopes have likely experienced a history of diverse vegetation regimes. Considering the importance of vegetation in erosion mechanics, this suggests that the current morphology of hillslopes does not necessarily reflect a unique suite of processes or process rates. Although the role of vegetation in driving temporal variations in sediment production has been studied for decades, process-based observations relevant to geomorphic timescales are sparse.

Here, we use coupled analyses of soil stratigraphy, tephra concentration and distribution, and topographic data, to reconstruct the glacial-interglacial evolution of a loess-mantled hillslope/unchanneled valley sequence along the Charwell River, South Island, New Zealand. Palynological data suggest that deep-rooted grasses covering the slopes during the last glaciation were replaced by shrublands in the Pleistocene/Holocene transition, and finally by podocarp and beech forest in the early Holocene. Paleo-hillslope profiles constructed from soil stratigraphic data indicate that relatively flat, locally incised forms existed before deposition of the uppermost loess sheet (L1), which is inferred to be a late Marine Isotope Stage (MIS) 3 to MIS 2 deposit. In contrast, the current topography following over 9 ky of forest colonization is broadly convex. This morphological transformation and the pattern of redistributed of glass grains from a 26.5 ka tephra layer within loess sheet L1 are consistent with accelerated transport and mixing of soils within the upper 0.5 m by tree uprooting, root growth, and turnover.

Phytolith analysis of a 7 m continuous core in the adjacent unchanneled valley records colluvial deposition under alternating grassland and forest regimes over more than one glacial-interglacial cycle. Luminescence dating of the core will allow us to quantify how infilling rates have varied through glacial-interglacial cycles and test the notion that forest colonization accelerated rates of transport via bioturbation. Given these findings, anthropogenic and short-term climate controls on forest cover will profoundly affect the redistribution of sediment and soil organic carbon in low-order basins.