EPSc 484/584 Final Problem Set! (50 points)
Name:
Due 4/29 5 pm
Corals.
You have been provided with two coral data sets (on
separate worksheets within the same Excel file) from the
places indicated on the map below. Both records have
seasonal or better resolution, and cover ~1960 to ~1980.
You may wish to use the Southern Oscillation Index graph
below for comparison with your data sets; a negative (red)
SOI indicates El Nino conditions, positive (blue) equals La
Nina; peaks or troughs indicate significant “Events”
(3) Based on the Punta Pitt data, is coral d18O a reliable indicator of SST? Why/why not? Give
the transfer function that relates the two.
(2) Could you use this same transfer function to calculate SST from Tarawa Atoll d18O data?
Why/why not?
(2) Based on what you know about El Nino, what relationship would you expect between SST at
Punta Pitt and Tarawa Atoll?
(4) Does that data you have (you only have d18O for Tarawa Atoll) suggest such a relationship?
Suggest an interpretation (or interpretations) for the similarities/differences between the two
records.
(2) Which (if either) the Punta Pitt or Tarawa d18O/SST records appears to be the best match to
the SOI as graphed above?
(8) Do d13C, Ba/Ca, Cd/Ca, and Mn/Ca of Punta Pitt corals seem to be reliable indicators of El
Nino conditions? (Ba is thought to be influenced by upwelling and/or temperature, while Mn is
thought to reflect incorporation of lagoonal mud into corals, triggered by strong winds remixing
the ocean bottom). For those which are, explain the basic mechanism suggested by the nature of
the correlation (e.g., Ba/Ca is higher/lower when SST is higher/lower because…)
(2) What about Mn/Ca at Tarawa? Reliable or not, and if reliable, what is the nature of its
relationship to ENSO?
(4) Even though you’ve examined only 2 coral datasets, compare coral geochemistry as a
paleoclimate indicator to other indicators we’ve discussed over the semester- what are its
particular advantages or disadvantages, where would you rank it on the reliability scale, and how
would you feel extending coral analyses back into the pre-Quaternary geologic record?
Lake balance.
These next few questions will be about the (closed; i.e. no outflow) Lahontan Basin, Nevada.
During the last glacial, Lake Lahontan filled a large portion of the basin, with a lakeshore at 1335
m above sea level, a surface area of 22,800 km2 and a volume of 2130 km3. Currently there are
two small perennial lakes occupying the basin (Pyramid and Walker), with surface areas of 563
and 326 sq. km respectively and volumes of 36 and 14 km3. For the purposes of this exercise,
we’re going to ignore the modern existence of Walker Lake. An abrupt decrease from highstand
(fullest) conditions to those similar to today seems to have occurred around 13,000 bp.
Modern Data:
Groundwater input to Pyramid lake: negligible
Outflow: none
River input: Pyramid Lake: Truckee River, 0.725 km3/yr
On lake precipitation: Pyramid= 0.2 m/yr
Lake balance equation:  S = Is + Iu + Pl – Qs – Qu - El
 S = change in water storage, Is = Surface inflow, Iu = Underground inflow, Pl = Lake
precipitation, Qs = Surface outflow, Qu = Underground outflow and El = Lake evaporation.
Bathymetry of Lahontan basin indicates: Lake surface area = 5449.8 ln (lake volume) - 18966
St = Si + S (storage at any given time = initial plus change)
El (total evaporative flux) = Evaporation rate X lake surface area
Pl (total precipitation input to lake) = On-lake precipitation X lake surface area
Understanding even something as relatively simple as a change in lake level in terms of climate
can actually be quite difficult….
(3) First, presuming that Pyramid Lake is currently in balance (that is, not changing its amount of
storage), calculate, from the information given, the annual volume loss of water from the lake due
to evaporation. Given the current surface area of Pyramid Lake, what evaporation rate does this
correspond to (in m/yr)?
(5) The problem with examining change over time is that the volume of water lost or input to the
system from on-lake precipitation and evaporation changes with surface area, so even if rates
(ppt/evaporation) stay the same, the volume of the lake may change. Build a simple mathematical
model of Lake Lahontan (use Excel!) in which you evaluate the water balance of the lake system
at yearly increments (be careful with units!!). Start with an initial storage (volume) and surface
area equal to that of modern day Pyramid Lake. Keep evaporation rate constant at what you just
calculated above, but increase river input to 1 km3/yr. (You will need to calculate a new lake
volume (and therefore surface area) each year.) Let the total flux of water due to evaporation and
on-lake precipitation vary with changing lake surface area. Run your model out for ~25 yrs.
Describe what happens to lake volume and why. (provide a graph of lake volume with time)
(2) What happens to lake volume if evaporation increased to 2 m/yr, but all else was the same as
initial conditions (river input = .725 km3/yr). (provide a graph of lake volume with time)
(3) If evaporation rates stayed constant at what you calculated for today, approximately how
many years would it take to fill Lahontan Basin up to its highest recorded level if we increased
on-lake precipitation by .1 m/yr every year, and river input correspondingly by .3625 km3/yr
Explain your process/show work (provide a graph of lake volume with time).
(3) It is unlikely that evaporation rates would stay constant with rainfall increasing so
dramatically; if evaporation decreased by .01 m/ yr in addition to the changes described above,
how long would it take to fill the basin to highstand levels? Graph lake volume with time for this
scenario and the last one you calculated on the same graph to compare.
(1) How sensitive are lake balance models, then, to the particular assumptions you make
regarding values of hydrologically important rates (and/or changes in those values)?
(2) What potentially important data/sources of change in lake level has our model ignored?
(2) Where/what kind of evidence might you look for to help you determine what piece of the lake
system (river input, rainfall, evaporation) was controlling lake level change?
(2) Finally, then, what would you consider “reasonable” use of lake level data and basic water
balance modelling- despite all the assumptions, unknowns, etc., what does lake level model
reliably tell you (if anything)?