Geoscience Reference
In-Depth Information
pollen and other macrofossil evidence to decipher past changes in climate
around Lake Moran. Together with Eric Edlund, then a graduate student,
Byrne collected several cores from the lake.
Radiocarbon dating of these cores showed that Lake Moran formed
15,000 years ago, at er glacial ice began retreating in the Sierra Nevada. h e
sediment cores were also dated by matching layers of volcanic ash that settled
into the lake with eruptions from nearby Mono Craters. An ash layer pro-
vides a basal date of the lake as 14,500 to 15,000 years old. Over the millen-
nia, the lake has been slowly i lling with sediments from its small watershed.
h ese sediments contain a record of environmental change that spans the
critical transition from the late Pleistocene into the Holocene.
Lake Moran contains seasonal layers: dark, organic rich layers deposited
during the winter, and lighter, denser layers during the summer. In total,
about ten feet of sediment have accumulated in the lake during its 15,000-
year history, each foot of sediment containing about 1,500 years of environ-
mental and climatic history.
h e record of environmental change that Byrne and Edlund reconstructed
from these sediments contains insights about how the region experienced the
climatic transition at the end of the relatively moist and cool Pleistocene
and the subsequent climate l uctuations of the Holocene. h ey found some
surprises in this record. h e notion so commonly held by ecologists of “plant
communities,” with specii c species living in close association in specii c
habitats, appears to be a function of particular climatic conditions. Edlund
and Byrne found some unlikely combinations of plants growing in close
proximity to each other around Lake Moran at times in the past that would
never be found today. For example, about 12,000 years ago, a dense conifer-
ous forest surrounded the lake for a period lasting between one thousand and
two thousand years. Sugar pine, which prefers warm summers, grew with
mountain hemlock, which prefers wetter conditions. Today, the sugar pine is
generally restricted to the lower elevation forests in the Sierra, and the moun-
tain hemlock is found at higher, moister locations. h e fact that these were
found together suggests a very dif erent climate regime in the Sierra in latest
Pleistocene, one with warm summers but abundant moisture. h
ese plant
assemblages have no analog in modern vegetation communities.
We can infer certain things about regional climate conditions from these
unusual combinations of plants. Although the region was generally warming
rapidly, the spring season was apparently cooler in the Sierra Nevada than
today, with more snowstorms than rain, and the snowpack persisted later into
