Lake sediments as paleoenvironmental records

The Jones 97A core was taken was taken from Jones Lake,
west-central Montana, in order to reconstruct the Holocene
groundwater history and climate, using proxies within the lake
sediments. Previously, Jones core has been sampled at
two-centimeter intervals providing a high resolution of the
stratigraphy. My study examines sediment geochemistry and
grayscale core imagery at the resolution of depositional laminae.
Sampling by depositional units allows examination of the
processes controlling geochemical variability in the core.
Calibrated image analysis may provide a basis for inferring
continuous high-resolution geochemistry throughout the core.

In order to derive sediment geochemistry, carbon coulometry and
stable isotope analyses were performed on a sectioned 25-cm area of
the 97A-5 core.  This involved using the carbon coulometer in the
LRC's core lab and measuring the amount of carbon (organic and
inorganic) in ground, homogenized sediment samples.  The stable
isotope samples were prepared in the core lab as well through a
bleaching process that destroys any organic components that would
alter the isotope data.  The resulting data was plotted and, compared
with an image of the core, the total inorganic carbon (TIC) and total
organic carbon (TOC)curves strongly reflect the visible color
differences shown in the core.  A grayscale image of the core was
used to measure mean grayscale intensity values, with which the
carbon data was compared.  Image analysis reflects a strong
relationship between the TIC and the grayscale intensity; by
contrast, the TOC and grayscale intensity show a relatively weak
relationship.  The heterogeneous nature of the organic components may
be the cause of this weaker relationship.

Oxygen and carbon isotopes indicate changes in lake fluid
balance and biological activity that influence the mode of
lacustrine sedimentation.  A covariant relationship exists
between the oxygen and carbon isotope curves.  Increasing
percentages of TIC broadly correlate with more positive isotopic
values.  I interpret high carbonate concentrations to correlate with
relatively strong evaporative forcing and high biological
productivity.