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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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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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