Landscape preservation under post‐European settlement alluvium in the south‐eastern Australian tablelands, inferred from portable OSL reader data

Abstract

Human land-use changes leading to widespread erosion and gully incision have been well studied, but the effects that erosion and sediment mixing, which accompany the deposition of post-(European) settlement alluvium (PSA), have in valley bottoms and wetlands receive considerably less attention. PSA overlying pre-disturbance swampy meadow (SM) wetland sediments is commonly exposed along incised stream channel gully walls throughout the south-eastern Australian Tablelands, providing an ideal setting in which to assess and understand better how PSA deposition affects valley bottoms and the wetland environments that often occupy them. Portable optically stimulated luminescence (pOSL) reader data were measured on bulk sediment samples from SM-PSA stratigraphies at 16 locations throughout the south-eastern Australian Tablelands to assess the effects of erosion and sediment mixing at the SM-PSA boundary. Trends of pOSL data with depth at each profile were used in conjunction with visual profile descriptions to identify the stratigraphic boundary between SM and PSA sediment and to infer the degree of valley bottom erosion and sediment mixing during PSA deposition. At most sites, SM sediments experienced minimal, if any, disturbance during PSA deposition, and we refer to these as non-eroded sites. Many sites, however, experienced a significant degree of erosion and sediment mixing -eroded sites -often corresponding to visually diffuse sedimentary boundaries between the two stratigraphic units. Our findings demonstrate that SM landscapes in the Tablelands can be preserved with minimal disturbance under PSA at non-eroded sites and are preserved beneath a mixing zone at all eroded sites.

Combining bulk sediment OSL and meteoric10Be fingerprinting techniques to identify gully initiation sites and erosion depths

“…[]. The dashed white lines at PSA‐1 and PSA‐2 indicate the bulk OSL transition depth from swampy meadow to post‐European settlement alluvium sediment accumulation [ Portenga et al ., ].…”

“…To counter this possible effect, we smoothed the bulk OSL depth profiles by using a three‐sample moving average (Figure a). The bulk OSL data from sample replicates measured from well‐bleached SM sediments converge on similar values; the bulk OSL data from PSA sediment replicates show variability, however, likely resulting from measuring the luminescence of incompletely bleached samples or from the inclusion of a few bright grains [ Portenga et al ., , Figure 4]. Although PSA exhibits more variable luminescence in replicate sampling, the overall depth trends of bulk OSL data through PSA profiles replicate well, even when measurements are made years apart [ Portenga and Bishop , ].…”

“…In the Tablelands region of southeastern Australia (Figure ), the causes and timing of gully incision and the connection between gullying and PSA deposition have been at the center of research efforts for decades [ Crouch , ; Eyles , ; Mould and Fryirs , ; Muñoz‐Salinas et al ., , ; Neil and Fogarty , ; Olley et al ., ; Olley and Wasson , ; Portenga et al ., , ; Prosser and Slade , ; Rustomji and Pietsch , ; Starr , ; Wasson et al ., ]. Catchment conditions and land use practices leading to initial gully incision are often not considered [ Prosser and Slade , ; Prosser and Winchester , ], although they are generally understood to involve vegetation disturbance along valley sides and bottoms, like those imposed by arid climate conditions or those introduced by European settlers in the 19th and 20th centuries [ Eyles , , ; Portenga et al ., ; Prosser , ; Rustomji and Pietsch , ; Scott , ; Starr , ; Zierholz et al ., ].…”

“…Photograph previously used in Portenga et al . [], taken facing upstream. (c) Expansive modern gully eroding through SM sediments at SM‐1.…”

“…PSA in Birchams Creek is a sandy loam with lenses of gravel exposed stratigraphically above SM in the lower and middle reaches of Birchams Creek (Figure ) and can be >1 m thick (Figure ). Although we were unable to map the lateral extent of PSA, it is seen exposed on both sides of gully walls; as elsewhere in the Tablelands, we assume that PSA was deposited across the valley bottoms [ Portenga et al ., , ; Rustomji and Pietsch , ].…”