Ph.D. MIT, 1998
 |
MC 100-23 1200 E. California Blvd. Pasadena, CA 91125 (626) 395-8550 |
(It may not work over the holidays)
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.
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.
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.
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.
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.
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.
Stal U-Series
HoverFiles
CoralPics1
CoralPics2
