Effects of Precessional Forcing in the Early Stages of the 100-k World

Abstract

Much of what we know about the Andes today comes from studying a heavily human-manufactured landscape. However, before the presence of human modification in this system (~13,000 years ago), past interglacial and glacial cycles, influenced by changes in Earth cycles (eccentricity, obliquity, and precession), were the cause of major landscape re-organization. The variability in intensity and duration of these events and the probable response of the plant community is not well understood. Very few records in South America extend far enough back in time to capture these changes. Those records that do exist, lack an independent chronology and instead rely on wiggle-matching, creating a potentially circular argument in the interpretation of the data. In 2015, a 125 m sediment record was raised from Lake Junín, Peru. Preliminary analysis of the lake data showed that Uranium-Thorium (U-Th) dating was possible, allowing an independent chronology to be developed. The dating indicated that the sediment column spanned the last 700,000 years or seven glacial-interglacial cycles. This dating technique, used in conjunction with radiometric tie points, allowed a completely independent chronology to be developed, and thereby a more accurate record of what was happening so long ago. This study focuses on the interglacials of MIS 13 and 15, the first interglacials of the ‘100-k world,’ and the coolest of the interglacials since then. Fossil pollen abundance, charcoal, sediment chemistry, magnetic susceptibility, and bulk density were all analyzed to investigate whether precessional amplitude and number could predict vegetation change, and whether fire was predictably linked to drought events. It was determined that the Junin area always supported a grassland, albeit one that varied in productivity. Eccentricity and obliquity did not have a marked effect on vegetation although precession did. It was the strength of the precessional cycle, however, rather than the number of cycles that had the strongest effect on productivity. The presence of two drought periods were evident in MIS 15, aligning with the wet season (December to March) insolation minima. These droughts did not cause major vegetation change, however between the dry events, inferred wet events were floristically distinct. Fire was so rare in the landscape during MIS 13 and MIS 15 that it seems unlikely that the native flora was adapted to fire. A finding that highlights how different modern, fire-prone systems, are to those that evolved in the Andes.