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Professor of Geochemistry and Global Environmental Science Ph.D.
MIT, 1998 |
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Research Interests |
Link to
Caltech Fossil Coral Database |
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Introduction |
Link to ‘Ask a
Climatologist’ as part of GreenWish.org |
I am a chemical oceanographer
interested in using trace metals as tracers of environmental processes.
Most of my current work is centered around the geochemical investigation of
past climates. I am primarily concerned with the last few glacial/interglacial
cycles that span a few hundred thousand years. It is in this time range
that we have both a relatively accurate and precise understanding of age
models (though they are always improving) together with large climatic shifts
that require mechanistic explanation. In particular, we have an amazing
record of the rapidity and magnitude of climate change from polar ice
cores. The figure below shows the record of oxygen isotope variation, a
proxy for air temperature, at the Greenland Summit over the past 110,000
years. The last 10,000 years, the Holocene, is marked by relative
climatic stability when compared to the preceding glacial period where there
are large and very fast transitions between cold and warm times. As an
oceanographer, I try to understand the coupled ocean/atmosphere system during
these shifts by monitoring the deep ocean's behavior. Much of my work
to date has focused on developing a new climate archive, deep-sea corals,
that has the potential to revolutionize the types of information we can
obtain about oceanographic climate change. I describe below five
projects, currently underway in my laboratory, that are related to better
understanding the mechanisms of rapid climate change and climate evolution.
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Deep-Sea Corals and Time Series of Deep-Ocean Change |
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Click on the picture at
left or here for a large detailed image. A
new browser window will open to display the larger image; close it to return
here. Warning:
image is very large. |
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Rate of Deep-Sea Overturning in the Past |
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Sediment Accumulation Rates from Excess 230Th Measurements |
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This problem can be neatly overcome by
normalizing to a measured initial excess of 230Th. Thorium is so insoluble in seawater that
virtually regardless of the total particle rain to the sea floor all of the 230Th produced by 234U
decay is scavenged out of solution. As the Uranium concentration is
conservative (it only varies with salinity), the 230Th rain rate is both constant and known through
time. This feature of Th marine chemistry means that its concentration
in the sediment only varies as the sediment rain rate and can therefore be
used to convert percentage measurements into true accumulation fluxes.
We have been using these measurements to monitor surface water production
variations and atmospheric dust deposition rates at both the Bermuda Rise (in
collaboration with Lloyd Keigwin of WHOI and Ed Boyle of MIT) and off the
coast of Africa (in collaboration with Peter deMenocal and Joe Ortiz at the
Lamont-Doherty Earth Observatory). The figure below shows how
variations in the terrigenous accumulation at our site both induces changes
in the %CaCO3 and masks times of actual CaCO3 accumulation changes. The data also show that
transitions into and out of the African Humid period are abrupt and
fundamentally different than the gradual insolation forcing over this time
period. |
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Pore Water Records of Past Deep-Ocean Salinity and d18O |
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At the last glacial maximum enough
ice was stored on land to increase global ocean salinities by about
3.2%. The combined record of this global change as well as local
variations in salinity due to deep-water mass movements is preserved in
sediment pore waters as broad [Cl] and d18O peaks. Due to
diffusion and compaction induced advection this [Cl] peak typically has an
amplitude of about 1.0-1.5% today. With precise (±0.05%) measurements
made at high depth resolution, this remnant peak can be used to constrain a
1-D pore water model of the history of bottom water salinity over the last
glacial cycle. The result from this model allows us to reconstruct the
Last Glacial Maximum bottom water salinity and d18O. Coupled with benthic
foraminiferal measurements of d18O we can constrain the past
deep temperatures for all of our sites. The figure above shows the
results for a profile taken from the Bermuda Rise in the deep Western
Atlantic. Our data imply that the waters at this site were 4.6±1.0°C
cooler at the LGM than they are today. This places the water mass at or
near the freezing point (click here for our GRLpaper
on this subject). d18O measurements were made by
Prof. Dan Schrag in his lab at Harvard University (click here for a recent paper on the d18O results). Together
with Kate McIntyre (a post-doc here at CIT), we are continuing this project
for sites from around the world’s oceans in an attempt to reconstruct LGM
temperature and salinity for a large part of the water column. |
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Vital Effects and Biomineralization |
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For decades it has been recognized that many
biogenic calcium carbonate minerals do not precipetate at isotopic
equilibrium for oxygen and carbon. It has also been shown for corals
and foraminifera that many species will generate the same slope of d18O vs. d13C for a
growth environment that does not change with time. This so called
"vital effect" is not well understood mechanistically but is
thought to arise from a kinetic fractionation associated with the hydration
of CO2(aq) in the calcifying pool. This effect is
dramatic in deep-sea corals (see figure). Not only is the full range
over 12‰ and 4‰ for carbon and oxygen respectively, but there is a break in slope
at the lightest values. This slope change requires that the "vital
effect" mechanism be something other than kinetic. There is no way
to continue to kinetically fractionate oxygen and stop fractionating carbon
when they are attached to the same molecule. We are exploring a new
thermodynamic model for this fractionation that should be ubiquitous for all
biogenic CaCO3. Implications of this model for the metals we
employ as paleo-tracers are also an active area of research in my lab.
For a pdf version of our recent paper on this subject (submitted to GCA) click
here for text and here for figures. |
Links |
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AT7-35 |