Carbonate and elemental accumulation rates in arid soils of mid-to-late Pleistocene outwash terraces, southeastern Wind River Range, Wyoming, USA

Abstract

An important result of the past few decades of soil–geomorphic research is the awareness of the major role of dust and related carbonate precipitates in arid soils. The temporal evolution of soils developed on outwash terraces is still a matter of debate particularly regarding the type of evolution: progressive evolution and constant rates vs complex and/or interval-accumulation rates (progressive or regressive evolution possible). Soils on a chronosequence of outwash terraces just outside and downstream of canyon mouths of the Wind River Range (USA) were analysed for carbonate accumulation, organic carbon and poorly-crystalline pedogenetic oxyhydroxides stocks and pathways of chemical weathering. The soils and their carbonate stocks in the fine earth fraction (<2mm) can be used as a relative dating tool and appear to correlate with regional glacial chronostratigraphy [Pinedale (c. 20ky), Bull Lake (with two different units around 160 and 260ky) and Sacagawea Ridge (c. 660ky)]. Carbonate stocks of soils on these terraces increase with increasing soil age, but at generally decreasing rates. A quasi-steady state situation, however, was not reached before 1My. The time-split approach showed that CaCO3 accumulation rates were the highest during marine isotope (MIS) cold stages (2.13gCaCO3/m2/y), presumably due to a sparse vegetation cover during MIS2 (Pinedale) and 8 (lower Bull lake). Compared to organic carbon (c. 8–10kgCorg/m2), the inorganic carbon stocks of the fine earth fraction were much higher (c. 60kgCinorg/m2). Although the dominant pedogenic process here appears to be carbonate accumulation, chemical weathering and elemental leaching also are significant. Weathering data were obtained using immobile elements and rare earth elements (REE) as tracers. Chemical weathering and related leaching were particularly notable for Na originating from plagioclase. In addition, pedogenetic Al and Mn oxyhydroxides accumulate (220 and 32g/m2, respectively, after 660ky). These form at relatively low rates, however, when compared to moister and cooler conditions at higher altitudes in the Wind River Range. Due to the dry climate under which the terrace soils formed (annual precipitation 32cm/yr−1), weakly crystalline and amorphous Fe-oxyhydroxide is rapidly transformed into crystalline forms (hematite) over time. Consequently, the stocks of weakly crystalline and amorphous Fe-oxyhydroxide was significantly lower in the ~660ky soils (50g/m2) than in the Holocene soils (1000g/m2).The development of soils on Quaternary outwash terraces in this region is controlled through the rates of aeolian influx over time as modified by glacial/interglacial climate variations rather than by moisture availability. Consequently, carbonate accumulation rates in these soils are ‘influx limited’ rather than ‘moisture limited’.