CONCEPTUAL FRAMEWORKS FOR INTERPRETING PALEOSOLS IN LOESS SEQUENCES

Paleosols in loess sequences, formed in landscapes of the past and then preserved by burial, are a rich but complex source of information on past environments. Here we review key insights from the extensive literature on this topic and note issues that need more critical attention. Interpretation of paleosols in loess necessarily involves analogies with surface soils. This modern analogue approach can range from using surface soils to develop transfer functions relating individual magnetic parameters and climate, to comprehensive comparisons of morphology, micromorphology, and mineral weathering between paleosols and surface soils sampled across gradients of climate and vegetation. Complicating this approach, the surface soils used as analogues have often formed under climate, vegetation, and erosion rates that varied over time. In fact, it can be difficult to estimate the age of these analogue soils and how much they have been affected by human land use, especially in rapidly eroding loess landscapes.

At the same time, it is important to recognize the geomorphic setting in which the paleosols formed, a land surface aggrading through loess accumulation at various rates that can be faster, slower, or similar to rates of pedogenic processes. Researchers in multiple loess regions have concluded that paleosols and/or surface soils have undergone upbuilding through ongoing loess accumulation. In this setting, soils have varied residence times in the zone of pedogenesis, A horizons can become B horizons, and a stack of weakly developed soils in one locality can be equivalent to a single well-developed soil where loess accumulation is slower. Despite clear evidence for upbuilding in some cases, it can be difficult to confirm in other settings. Similar profiles of radiocarbon or luminescence ages in soils can result from upbuilding or other processes.

Does it matter if the surface soils we use as analogues formed under a range of climates and vegetation types? Does it matter if they did not experience upbuilding, or were affected by a different rate of aggradation than the paleosols we are interpreting? These are questions that can be answered—and in some cases have been at least partially answered—through well-established methods of pedology. We consider examples such as soil evidence of past vegetation change and clay-rich B horizon development in aggrading and non-aggrading soils.