Geologic Evidence of Rapid, Multiple, and High-Magnitude Climate Change During the Last Glacial (Wisconsinan) of North America

Abstract

Over the last few decades data derived from ice cores and dendrochronologic sequences indicate that former climate change during the Quaternary has been rapid and of high magnitude. The Estancia Valley, central New Mexico, U.S.A. contains a high-resolution sedimentologic and palaeontologic pluvial-lake record that supports these data. The known Estancia Valley record, in outcrop and subcrop, spans the Wisconsinan (last glacial episode of North America) and represents relatively continuous deposition through that time. The period from early to middle Wisconsinan (ca. 60,000 BP to 30,000 BP) is marked by a relatively stable climate, represented by dry playa and saline lake sediment and fossils. At the beginning of the late Wisconsinan, however, climatic instability becomes the norm. The first fresh-water pluvial lake, as evidenced by fossils, appears at ca. 24,000 BP, followed by additional discrete fresh-water lakes at ca. 21,000 BP, ca. 20,500, ca. 20,000 BP, ca. 17,000 BP, and ca. 14,000 BP. Each of these seemingly deeper-water stands was separated by desiccation events (the last dating after ca. 12,400 BP) indicating that each lake phase resulted from high magnitude climate change (full pluvial to full inter-pluvial climate). It also appears that each lake phase developed rapidly, stabilized for only a short period, and desiccated rapidly. Late Lake Estancia, the highest lake stand of about 52 m deep and dated at ca. 20,000 BP (coinciding with the late Wisconsinan glacial maximum) offers the most conclusive evidence of rapid lake development from dry-playa climatic conditions. The Late Lake Estancia sedimentary sequence consists of deposition on an unconformable surface, an interval of saline-lake adapted organisms, brackish-water-adapted organisms, and finally infrahaline-adapted organisms, all within a 10 cm stratigraphic interval. Radiocarbon ages and sedimentation rates suggest that change from dry-playa conditions to full pluvial-lake conditions (maximum lake depth) back to a dry playa occurred within a few decades to a few centuries. It is likely that the other fresh-water stands developed and disappeared in a similar time span. The rise and fall of major pluvial systems in the southwest U.S.A. demonstrate that the geologic response to rapid climate change can also be rapid.