MULTIPLE PATHS TO CLIMATIC STRATHS: HOW LOCAL CONDITIONS AND DATING METHODS OBSCURE TERRACE CORRELATION WITH CLIMATE CYCLES

Strath terraces, an often-used tool in tectonic geomorphology, form in response to climatic, tectonic, volcanic, and anthropogenic forcings. Climate, or more specifically, glacial cycles, cause strath formation most commonly, yet the timing of strath formation among climatically-caused terraces varies. For example, glacial periods as well as interglacials have both been attributed to strath planation in previous studies. Here, we explore this issue further with the goal of better understanding why straths can form in different phases of a glacial cycle. We compiled a database of 421 dated strath terraces, 232 of which are caused by climatic variation, from peer-reviewed sources and compared the timing of strath formation, attributed climate event, and environmental factors. We find that, of the terraces in which the formative glacial cycle is specified, the majority were planed during cold glacial periods (81 of 147), but many were planed during interglacials (30) as well as the transition from glacial to interglacials (31). For strath incision, 73 of 150 terraces incised during interglacials, but 60 incised during the transition from glacial to interglacial periods. Differences in timing of strath formation are due to site specific characteristics, as attributed by the original authors. For example, root cohesion from vegetation causes the timing of increased basin sediment supply at the beginning of a glacial period to lag behind that of a nonvegetated basin. These, and other, characteristics dictate whether a river incises or planes in response to parts of the glacial cycle, and individualizes the fluvial response such that there is no global generalization for when strath terraces form in response to climate. We also found that the method of dating the strath provides different age constraints and thus strath ages from different methods cannot be directly compared. For instance, 14C and OSL dates of terrace alluvium provide maximum ages of strath formation, whereas exposure ages and loess sequences provide minimum ages. Our findings – that timing of terrace formation in response to climate is controlled by site-specific conditions and that age methods provide different constraints – calls for caution when ascribing terraces to climatic events, especially through correlation with other terraces.