DOI: 10.25921/v0w6-b132
title: NOAA/WDS Paleoclimatology - Laguna Pallcacocha, Ecuador Holocene Sediment Color Intensity Data

description: The variability of El Niño/Southern Oscillation (ENSO) during the Holocene epoch, in particular on millennial timescales, is poorly understood. Palaeoclimate studies have documented ENSO variability for selected intervals in the Holocene, but most records are either too short or insufficiently resolved to investigate variability on millennial scales. Here we present a record of sedimentation in Laguna Pallcacocha, southern Ecuador, which is strongly influenced by ENSO variability, and covers the past 12,000 years continuously. We find that changes on a timescale of 2–8 years, which we attribute to warm ENSO events, become more frequent over the Holocene until about 1,200 years ago, and then decline towards the present. Periods of relatively high and low ENSO activity, alternating at a timescale of about 2,000 years, are superimposed on this long-term trend. We attribute the long-term trend to orbitally induced changes in insolation, and suggest internal ENSO dynamics as a possible cause of the millennial variability. However, the millennial oscillation will need to be confirmed in other ENSO proxy records.
description: A continuous lacustrine sedimentary record of Holocene ENSO variability from 
Laguna Pallcacocha, Southern Ecuadorian Andes, Ecuador. LATITUDE/LONGITUDE: 02 46' S, 79 14' W. BATHYMETRY/ELEVATION: 12 meter water depth (07/1999), 4200 m.a.s.l.

Methods: We split the cores and scanned the surface with a Geotek linescan camera, which digitally scans the surface of the sediment and generates a continuous red, green, and blue digital record of the sediment core surface.  A single composite record of color intensity was compiled by sampling the continuous downcore profiles of red, green, and blue color intensity from the core drives at an increment of 0.5 mm.  The three color channels covary (r2 = 0.60-0.99) and we chose to use the red color channel to document the downcore concentration of the laminae in the record because it exhibits slightly higher variance than either the blue or green channels.

The age-model for the new record is based on the same radiocarbon chronology used by Rodbell et al.(1999). The laminae dated by AMS 14C of terrestrial microfossils in the Rodbell et al. (1999) record are distinctive and could be confidently identified in the new cores.  By creating a composite section from overlapping drives, we were able to improve on the original age model.  We adopted the constant carbon accumulation model (CCAM) and an event model (EM) to allocate time between dated intervals (Rodbell et al., 1999; Moy, 2000). The CCAM assumes that the rate of organic carbon deposition has remained nearly constant between radiocarbon-dated intervals through the Holocene, and this continuous sedimentation was punctuated by nearly instantaneous clastic depositional events. The EM is based on the assumption that each of the clastic laminae was deposited during a single ENSO event with a temporal duration of 6 months, and we based this hypothesis on discharge data from the Rio Chira, which display high flow for a period of 6 months during an ENSO event (Martin et al., 1993).  Clastic events were identified in the color intensity profile as peaks in color intensity units that exceed the surrounding background laminae by 15 color intensity units or more (Moy, 2000).  After time was assigned to the clastic laminae, the remaining time was distributed linearly between radiocarbon dates.  One advantage of the EM over the CCAM is that it assigns equal magnitude to each of the events, thereby removing any effects that sediment compaction may have on the color of the light colored laminae at the base of the record.