ENSO-Induced Floods Recorded in Lake Sediment Cores from Las Cajas National Park, Ecuador

Although the El Niño Southern Oscillation (ENSO) is the greatest source of global interannual climate variability, our ability to predict the response of ENSO to changes in the mean state of climate are limited by a paucity of long-term records of ENSO. The sediment record from Laguna Pallcacocha in El Cajas National Park, southern Ecuador (4060 masl; 2°46’S; 79°14’W) records El Niño floods spanning the Holocene (Rodbell et al., 1999; Moy et al., 2002). The sediment record is unusual for the nearly continuous dark- and light-colored laminations (0.1-2.0 cm thick) that comprise the Holocene. Light laminae typically have erosive basal contacts and fine-upwards, whereas dark laminae possess abrupt or gradational lower contacts and reveal no grain size trends. Light laminae contain <2% organic carbon, <2.5% biogenic silica and are well sorted whereas dark laminae are poorly sorted, organic rich (>7%), and contain 3-10% biogenic silica. Light laminae represent deposition during periods of increased precipitation, mobilization of unvegetated sediment above the lake, and increased stream discharge, all of which generate density-driven undercurrents. Conversely, dark laminae are deposited relatively slowly by sedimentation of organic matter, some derived from surface soil horizons. Time series analysis of light laminae reveal the ENSO spectral signature (2-8 yr) that evolves through the Holocene. In order to better constrain the timing and magnitude of shifts in ENSO we developed a series of predictive metrics designed to identify catchments with geomorphic properties conducive to recording and preserving clastic flux. We recovered Livingstone cores from four lakes, which ranked highly according to our system. Each of the selected lakes reveals finely resolved clastic laminations similar to Laguna Pallcacocha, and multiple regional tephra enable precise correlation among lake cores. These results suggest that we were successfully able to identify the geomorphic controls that influence the quality and fidelity of the alluviation signal associated with flooding during El Niño events. The additional flood records from the region will better elucidate the severity and variability of ENSO throughout the Holocene