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Previous Oceans' Seminars:
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Friday, May 31, 2002 3:30pm; Building
370, Rm. 370
Sonya Dyhrman, Ph.D. - Woods Hole Oceanographic Institute
<homepage>
- Title: A molecular demonstration of phosphorus and
iron limitation in the ocean
Recent advances for quantifying phosphorus and iron have led
to hypotheses that marine nitrogen fixation and primary production
are limited by one or both elements in some oceanic regimes. However,
the degree to which the elemental composition of seawater actually
reflects bioavailability to particular organisms is poorly understood.
During a study of the Western North Atlantic we determined that
populations of the significant diazotrophic cyanobacterium Trichodesmium
experienced iron stress in August and phosphorus stress in November
of 2000 using molecular diagnostic tools. Our data suggest that
iron and phosphorus are both important factors controlling Trichodesmium
productivity, and that a dynamic interplay between these two
essential elements may exist in this and many other systems.
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Friday, May 10, 2002 3:30pm
Maureen Raymo, Boston University <homepage>
- Title: Glacial-Interglacial Changes in Ocean Circulation
and the Conveyor Belt Hypothesis
Our framework for interpreting glacial-interglacial deep ocean
circulation changes is heavily biased by reconstructions of the
Last Glacial Maximum. For most of the last two decades, it has been
widely believed that the production of North Atlantic Deep Water
decreased, possibly dramatically, during glacial periods due to
the cessation of Norwegian-Greenland Sea Overflow Water (NSOW). This
"shutting down" of the "conveyor belt" circulation is believed to
be a major factor in the pronounced cooling of the region during
glacial periods. In this talk, I present new d13C data from the
subpolar North Atlantic which give a fresh perspective on NSOW variability
over the Pleistocene.
Raymo, M.E., W.F. Ruddiman, N.J. Shackleton, and D. Oppo (1990)
Evolution of Atlantic-Pacific d13C gradients over the last 2.5
m.y. Earth and Planetary Science Letters, v. 97, p. 353-368.
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Friday, April 19th, 2002 3:30pm
Jonathan Erez, Institute of Earth Sciences at Hebrew University
- Title: Carbon and nutrient cycles in Red Sea Coral reefs and their response to environmental change
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Friday, April 5, 2002 3:30pm
Lynne Talley, Scripps Institute of Oceanography <homepage>
- Title: The contributions of shallow, intermediate
and deep ocean overturn to global heat transport
- Meridional transports of mass, heat and freshwater are
calculated using geostrophic velocity fields from previous studies
[Reid's (1994, 1997) Atlantic and Pacific analyses and Robbins
and Toole's (1997) Indian analysis] and Ekman transports based
on climatological winds. The results are presented as global overturning
streamfunctions. Mass, heat and freshwater transports are also
separated into process-based components: (1) shallowest component
associated with wind-driven subtropical gyres in which warm waters
are advected poleward by western boundary currents, cooled and
returned equatorward in the interior in subducted layers. (2)
The intermediate and deep components are associated with transport
of warm waters which are transformed at subpolar and higher latitudes
and return at depth. The calculated heat transports are too numerous
to recount here - the N. Pacific shows the expected shallow overturn
dominance and the N. Atlantic the Labrador Sea Water / North
Atlantic Deep Water, with about equal contribution from both;
recognition of the importance of LSW formation is critical. In the
southern hemisphere, formation of Antarctic Intermediate Water and
Southeast Indian Subantarctic Mode Water account for as much heat
transport as LSW and NADW formation in the northern hemisphere.
Complications in interpretation arise from the Indonesian throughflow
connection between the Pacific and Indian and the continuity of
the subtropical gyre between the Atlantic and Indian Oceans.
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Friday, March 22, 2002 3:30pm
John Delaney, Professor, School of Oceanography, University
of Washington <homepage>
- Title: NEPTUNE: An Interactive Earth-Ocean Observatory
at the Scale of a Tectonic Plate.
- The NEPTUNE project will deploy a submarine cabled network
on the Juan de Fuca tectonic plate off the U.S. and Canadian
west coasts. Using fiber-optic/power cable and junction boxes,
this plate-scale ocean observatory will establish a linked array
of undersea observatories by providing significant amounts of power
and an Internet communications link to sensors and sensor networks
on, above, and below the sea floor. NEPTUNE's goal is to enable
real-time, long-term, plate-scale studies in the ocean and earth
sciences. The network may also serve as a unique test bed for sensor
and robotic systems designed to explore other oceans in the solar
system.
NEPTUNE will focus on a broad range of scientific inquiries
ranging from research involving microbial productivity from the
oceanic crust related to deformation and volcanic eruptions in
a plate tectonic framework, to a broad array of earthquake and
tsunami-related issues, to tracking fish migration, to observing
sediment transport, to monitoring carbon fluxes related to air-sea
exchange and primary productivity in the upper ocean, including
earthquake generated outgassing of subduction complexes.
Hard-wired to the Internet, NEPTUNE will enable a new generation
of ocean and earth science to be conducted for decades from instrument
arrays immersed in the dynamic environments within and beneath
our oceans. It will also bring real-time data and imagery into
the laboratories, classrooms and living rooms connected to the
Internet. For the first time, the public will be able to electronically
watch over the shoulders of oceanographers and geoscientists as
they explore the time domain within the ocean basins of our planet.
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Friday, March 8, 2002 3:30pm
Karen Cascioti, Princeton University
- Title: Genetic and Stable Isotopic Characterization of N2O Production byNitrifying Bacteria
- Nitrous oxide (N2O) is a potent greenhouse gas whose concentration is
increasing in the atmosphere. N2O distributions in the ocean suggest
that nitrifying bacteria are the primary source of N2O in the ocean.
However, moderately enriched isotopic signatures of N2O (N-15 and O-18)
have been difficult to rectify with our previous understanding of N2O
production in a nitrifier culture and its extreme isotopic
fractionation. Here I will present my work on N2O production by marine
nitrifying bacteria. The genetic pathway for N2O production has been
elucidated as well as isotope effects for key steps in the N2O
production pathway. The genetic information suggests that nitrifying and
denitrifying bacteria use the same pathway for N2O production from
nitrite and highlights the important steps needed to constrain the
isotopic signature of N2O from nitrifiers. Using a simple steady-state
model incorporating the isotope effects we measured for ammonia
oxidation and nitrite reduction, we show that the isotopic signature of
N2O production in many regions of the ocean is consistent with a
nitrification source.
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Friday, January 18, 2002 3:30pm
Dr. Alan Mix,Oregon State U. <homepage>
- Title: Ice age cooling of the equatorial pacific - is it driven from the tropics or the poles?
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Friday, December 7, 2001 3:30pm
Porf. Peter Barret, Victoria University of Wellington, New Zealand (visiting faculty in GES)
- Title: Climate Change - An Antarctic Perspective
- The Antarctic continent is currently covered by 30 million
cubic km of ice - 66 m of sea level equivalent. The continent
and its ice sheet influence the earth's climate and ocean circulation
by pinning the south polar vortex, annually doubling the ice cover
with sea ice growth, and ventilating the deep ocean basins with
shelf-generated Antarctic bottom water. How long has it been doing
this, and how stable is this land-ice-atmosphere-ocean system,
especially in the face of the projected 1.4 to 5.8\272C increase
by the end of this century? In this talk I will outline some features
of Antarctic ice cover today, some patterns of ice sheet behaviour,
and what we have learned over the last 3 decades about the history
of ice on Antarctica over the last 40 million years.
The current view of this history is that continental ice sheets
first developed at around the Eocene-Oligocene boundary, growing
and shrinking cyclically in response to orbital forcing. A permanent
core of ice formed around 15 million years ago, limiting the
extent of the response, but some have argued for quite extensive
collapse in Pliocene and late Quaternary times. The reasons for this
decline in climate, which tracks the global decline of around
5\272C over this period, has been in the past attributed to the
opening of the Southern Ocean, but declining atmospheric CO2 may
well have played a major role.
Although Antarctic glacial history is most widely known from
interpretations of the deep-sea oxygen isotope record, a mixed
temperature-ice volume proxy, I will focus on the sedimentary record
from drilling on the Antarctic continental shelf, and especially the
results from the Cape Roberts Project off the Ross Sea coast. This
multinational project cored 1500 m of cyclic shallow marine strata
from 17 to 34 Ma old at the edge of the present East Antarctic ice
sheet. The palynomorph record indicates a progressive shift in climate
from cool temperate to sub-polar over this period, at variance with
the recently revised isotope record.
We will conclude by speculating about what might be found
in a lake core from the middle of the Antarctic ice sheet.
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Friday, November 9, 2001 3:30pm
Benjamin Bostick, GES, Stanford University
- Title: Mineral and Dissolved Sulfide Controls on the
Fate and Transport of Arsenic
- Arsenic is a common contaminant of soils and waters that
jeopardizes environmental quality, the consequence of which have
recently been manifested in Bangladesh. Under anaerobic conditions,
it is generally perceived that arsenic has a greater mobility
than in well-aerated environments owing to the formation of arsenite.
The means by which arsenic is reduced and secondarily partitioned
in anaerobic environments, however, is not well resolved. Here
we described the reaction of arsenic species with dissolved and
mineral sulfides and assess the reaction kinetics. Comparison
with reduction rates measured for dissimilatory arsenate reduction
by bacteria are used to evaluate the probable reaction pathway.
Arsenate undergoes a complex reaction with sulfide generating
numerous dissolved arsenic-sulfur complexes prior to the formation
of arsenite; precipitation of orpiment also results at higher dissolved
concentrations. Arsenate is also reduced by mineral sulfides such
as pyrite and results in a thio-arsenite surface complex. Thus,
sulfide components of reduced soils and waters will have a dramatic
impact on the behavior of arsenic and needs to be considered when
evaluating the fate of this hazardous element.
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Friday, November 2, 2001 3:30pm
Francisco Chavez, Biological
Ocean Group, Monterey Bay Aquarium Research Institute (MBARI)
- Title: Climate, fish populations, ocean biogeochemical
cycles and atmospheric carbon dioxide
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Friday, October 5, 2001 2:00pm
(note time change)
John Hayes, MIT/Woods Hole Oceanographic Institution <homepage>
- Molecular-isotopic investigations of geomicrobial
systems, or "How molecular structures and isotopic abundances
tell us about the bugs in the mud"
- Analyses of the abundance of carbon-13 in specific organic
molecules that are produced by microorganisms make it possible
to map pathways of carbon flow in sedimentary systems. A particularly
interesting example is provided by the anaerobic oxidation of
methane, a process now recognized as catalyzed by a consortium
involving a still-unknown archaeon (possibly a methanogen operating
in reverse) and a sulfate-reducing bacterium. Evidence bearing
on this system now includes ribosomal-RNA gene libraries, structures
and isotopic compositions of microbial lipids, and ion-microprobe
analyses of microbial colonies.
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