Core C-06 from Pumacocha Laguna Pumacocha, Peru
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Holocene Climate of the Central Peruvian Andes
Inferred from High-Resolution Lacustrine Sedimentary, Stable Isotope, and
Geochemical Indicators Project Summary As part of an NSF
funded research initiative, I am investigating Holocene climate variability in the
central Andes of Peru at 5 to 20-year resolution for the last 2,000 to 5,000
years, between 8¡S and 12¡S, based on two to three high-resolution
multi-proxy sediment records collected from undisturbed, high-altitude,
alkaline montane lake systems.
Using stable isotopes (i.e., d18O, dD, d13C), I am reconstructing
precipitation and evaporation ratios (P/E) through time with the goal of
identifying and determining the causes of droughts and pluvials. With the exception of four ice cores,
previous work from Peru has been limited to centennial to millennial-scale
resolution. In order to identify
modes of climate variability on time scales relevant to humans (5 to
20-years), a network of high-resolution proxy climate records are
needed. This project is a first
step toward creating such a network.
This research addresses the following questions: 1) Is there evidence for prolonged (decadal to
centennial-scale) arid events in the central Peruvian Andes? 2)
Given evidence
for arid events, what is the spatial and temporal structure of the events?
How regional are these events?
How do they compare with other high-resolution regional records (i.e.,
ice cores, lake sediments)? 3) Can large-scale ocean/atmosphere meteorological
phenomenon, such as ENSO or the PDO, explain the observed periodicity and
intensity of Holocene drought events in TSA, or must other additional
mechanisms, such as solar forcing, be considered as well? 4) What is the impact of climate variability on
ancient South American societies? How do periods of drought/pluvials compare
with regional archeological records? Pumacocha The
most promising archive collected to date is from Pumacocha, a small alpine
lake located in the Cerro de Pasco region of Peru at 4200 meters above sea
level (mASL; 10¡ 42' 1.02"S, 76¡ 3' 53.26"W). One of three lakes in
the watershed, Pumacocha is a high priority sight for several reasons. First, the entire watershed is underlain
by the Chambar‡ Formation, a Jurassic marine limestone of the Pucar‡
Group. This results in highly
alkaline lake waters throughout the watershed (e.g., Pumacocha ALK =182.2 ppm
HCO3-).
Second, the lake basin itself is small (~ 300 m2), but deep
(~ 23 meters), with anoxic bottom waters starting at about 11 m. This is an
important feature because anoxic bottom waters preclude benthic organisms
from disturbing the sediment stratigraphy, thereby preserving the original
time-resolution of the record.
Another key morphologic feature of the lake basin is that the entire
margin is buffered by at least 20 meters of flat marshland on all sides.
These marshlands act as a filter, which helps to reduce contamination of the
authigenically-precipitated calcite with limestone particles eroded from the
surrounding bedrock during the wet season. Third, the watershed area is small, ~ 8 km2. This means the lake responds to and
records changes in hydrologic conditions specific to that basin. While this is an attractive quality,
it does mean that more than one site in the area is needed in order to
discern local signals from regional climate responses. Fourth, the entire watershed faces
east and is protected by a moderate to steep head wall of ~4500 mASL. These
morphologic characteristics are significant because they physically shelter
the lake from strong easterly winds during the monsoon season, and thus help
maintain the vertical stability and stratification of the water column. Finally, all of the above factors
combine to create an environment that has allowed very finely laminated
carbonate sediments to accumulate in the upper 2 meters of the Pumacocha
cores. X-ray diffraction
analysis confirms the presence of calcite in the sediments and scanning
electron microscope analysis strongly suggests that the calcite is authigenically
precipitated from the water column with essentially no contamination of
limestone from the basin bedrock. Notably, the fine laminations appear to
form couplets resembling varves.
Whether or not these sediments are true varves has yet to be established. 210Pb and 137Cs
dating techniques combined with layer counting will be performed to test this
possibility. Regardless, the
Pumacocha record is clearly unique with regards to other lacustrine records
from South America. Preliminary age control using AMS 14C dating
of selected charcoal fragments indicates that the finely laminated section of
the core encompasses the last 2500 cal yr B.P. and therefore may provide a
record on par with ice cores.
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