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Jess F. Adkins
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Curriculum Vitae
Publications
GPS Faculty
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Associate
Professor of Geochemistry and Global Environmental Science
Ph.D.
MIT, 1998
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MC
100-23
1200 E. California Blvd.
Pasadena, CA 91125
(626) 395-8550
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Research
Interests
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Link to
Caltech Fossil Coral Database
(It
may not work over the holidays)
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Geochemical investigations of past climates using corals,
sediments and their interstitial waters; Rate of deep ocean circulation and
its relation to mechanisms of rapid climate changes; Metals as tracers of
environmental processes; Radiocarbon and U-series chronology. Chemical
oceanography.
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Introduction
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Link to ‘Ask a
Climatologist’ as part of GreenWish.org
Part2
Part3
Part4
Part5
Part6
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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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 Ice core records of the last
110,000 years, like the one above, reveal that we are living in a unique time
where the climate has been relatively stable. Glacial times, however,
are marked by rapid and large amplitude changes in atmospheric temperature,
snow accumulation at high latitudes, and a variety of other climatic
variables. On orbital time scales, studies from deep-sea sediments have
shown a clear link between deep ocean circulation patterns and global climate
change. As the deep ocean contains nearly all of the mass, heat
capacity and carbon in the ocean/atmosphere system, it is of vital interest
to extend this understanding to times of rapid climate change. Deep-sea
corals offer a unique opportunity to constrain this oceanic behavior.
Individual specimens of the coral D. cristagalli (see figure) live for decades to hundreds
of years and have visible banding patterns that appear to be deposited
annually (though this is still under investigation). We now have a
library of samples that include corals from the past 100,000 years. By
developing precise high-resolution sampling using lasers and Inductively
Coupled Plasma-Mass Spectrometry, we are trying to actualize the potential of
this new archive and generate high-resolution records that match or exceed
the ice cores’ fidelity.
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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 Modern
chemical investigations of ocean circulation rates rely heavily on
measurements of D14C,
the radiocarbon content of a water mass. This radioactive tracer allows
us to calculate the ventilation age, the time since the deep water last
"saw" the atmosphere. However, generally in paleoclimatology, radiocarbon is our chief chronometer
and therefore can not be used as a tracer
itself. Fortunately deep-sea corals are both rich enough in Uranium and
low enough in detrital Thorium that they can be
independently dated to very high precision. Coupled U-Th and 14C dates from the same sample
free radiocarbon from being our sole age control and allow the calculation of
past deep water D14C.
Just as it is used in the modern ocean, this measurement can be directly
related to ventilation age and therefore past circulation rates.
Combined age measurements take us from mapping the volumes and distributions
of past deep-water masses to actually understanding the dynamics associated
with oceans in past climate states. We are beginning to do
paleo-physical oceanography.
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Sediment
Accumulation Rates from Excess 230Th Measurements
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 Measurements of the
concentrations of sediment materials down core is often more usefully
interpreted if they can be converted to actual accumulation fluxes.
Often the percentage of one major core component, which had a constant rain
to the sediment, can vary down core due to changes in another
component. For instance, carbonate production in the surface waters can
be constant through time while the dust flux displays large
variability. Percentage measurements of the two species down core will
both show variation, solely due to the changes in dust flux. Since the
actual carbonate rain was constant, percentage data confuse the
interpretation of water column production through time.
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.
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Links
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CIT Geochemistry
| Global Environmental Science | Scripps
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Ge 154
For Scott McCue
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AT7-35
Stal
U-Series
HoverFiles
CoralPics1
CoralPics2
TasmaniaMaps
Hover
Movie 1, Hover Movie 2
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