An ~15,000-Year Record of El Niño-Driven Alluviation in Southwestern Ecuador

Abstract

Debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the Pacific Ocean, in Ecuador. These storm-induced events were dated by radiocarbon, and the age of laminae that are less than 200 years old matches the historic record of El Nino events. From about 15,000 to about 7000 calendar years before the present, the periodicity of clastic deposition is greater than or equal to 15 years; thereafter, there is a progressive increase in frequency to periodicities of 2 to 8.5 years. This is the modern El Nino periodicity, which was established about 5000 calendar years before the present. This may reflect the onset of a steeper zonal sea surface temperature gradient, which was driven by enhanced trade winds.

A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador

“…Inasmuch as the Holocene section of most cores is a dark gyttja with 15-20 wt.% organic C, the light-gray tephras are distinct. However, some cores contain a complex sequence of thin laminae of gyttja alternating with light-colored inorganic clastic sediment, such as the core from Laguna Pallcacocha (Rodbell et al,1999). In these cases, tephras were identified through the inspection of smear slides and confirmation of the presence of angular and vesicular glass shards and phenocrysts.…”

“…Because Laguna Pallcacocha is well dated (Rodbell et al, 1999), most tephra age assignments are based on correlation to this core. In addition, basal dates were also obtained from each core and all well-preserved terrestrial macrofossils found in the other cores were also dated (Bagnato, 2000); these dates were also used in developing the tephrochronology.…”

“…Two radiocarbon dates from this material yielded nearly identical ages of ∼15,620 (+230; −350) and 15,490 (+200; −560) cal yr B.P., which are more than 1000 yr older than radiocarbon dates from the gyttja either above or below this interval. Because of the apparent problem arising from the recycling of organic matter on the landscape, we have based much of our chronology on the laminated core from Laguna Pallcacocha (Rodbell et al, 1999). This lake is located in the floor of a cirque that is surrounded by steep slopes with little, if any, peat deposits.…”

Biogeochemical processes controlling oxygen and carbon isotopes of diatom silica in Late Glacial to Holocene lacustrine rhythmites

“…The onset of the Holocene is characterised by a weakening and reduction of amplitude of the ENSO (Moy et al, 2002;Rodbell et al, 1999). The establishment of a long-term La Niña pattern in the tropical Pacific during this period seems to be a consequence of the ENSO activity reduction (Hernández et al, 2010;Koutavas et al, 2002).…”

Inferences about winter temperatures and summer rains from the late Quaternary record of C4 perennial grasses and C3 desert shrubs in the northern Chihuahuan Desert

“…and a more La-Niña-like average state, a condition correlated with drier winter conditions in the Southwest (e.g. Rodbell et al, 1999;Clement et al, 2000). With the onset of greater ENSO frequency and magnitude in the later part of the midHolocene, the southwestern United States would have experienced more regular winter precipitation with greater extremes (Cane, 2005) and possibly higher freezing level heights, i.e., the elevation above sea level at which the air temperature is close to 0˚C (Diaz et al, 2003).…”