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Previous Oceans' Seminars:
October 8th, 2002
Mike McPhaden, Senior Research Scientist, NOAA/Pacific Marine Environmental Laboratory
“Evolution of El Nino conditions in the tropical Pacific during 2002”
The classic signatures of El Nino--warming sea surface temperatures, weakening trade winds, and shifts in Pacific rainfall patterns--were detected in the tropical Pacific in early 2002. These conditions developed erratically, being modulated by episodic westerly wind events associated with the Madden-Julian Oscillation. In contrast to the onset of the 1997-98 El Nino, warming in 2002 was less intense and less rapid to develop. However, by mid-year, most ENSO forecast models predicted that warm conditions would be sustained in the tropical Pacific through early 2003. This presentation reviews the current status of evolving El Nino conditions using data from the ENSO Observing System. Development of these conditions will be compared with previous El Niños and interpreted in light of recent theories. Implications for the predictability of ENSO will also be discussed.
October 15th, 2002
Benjamin Bostick, Post-Doc, Dept. of Geological and Environmental Sciences, Stanford University
Title: Molybdenum Scavenging Reactions in the Oceanic Water Column and Seafloor
Molybdenum is an essential element for life in that it is required for nitrogen fixation and assimilation. Molybdenum is usually found in the ocean as molybdate, which has a long residence time in the ocean (>800,000 years). This conservative behavior implies that molybdate is largely unreactive towards ocean-borne particulate matter; however, sulfide minerals from anoxic environments and in manganese-rich hydrothermal crusts scavenge molybdenum efficiently. In fact, the Mo enrichment in sulfidic zones is commonly used as a paleoredox indicator of strongly reducing conditions. Despite the importance of these scavenging mechanisms to the global cycling of molybdenum, little is known about their mechanisms. Here, we present the results of a lab-based study to determine the mechanism of Mo sorption on pyrite, the most important sulfide mineral for Mo enrichment, and a structural investigation of Mo-rich manganese crusts from the Fiji triple junction. Molybdate (MoO42-) and tetrathiomolybdate (MoS42-) adsorb to pyrite surfaces through distinct ligand-exchange mechanisms, with tetrathiomolybdate forming kinetically stable cubane-type surface complexes. Therefore, we suggest that changes in molybdenum speciation, which are driven by changes in the dissolved activity of bisulfide rather than mineralogy, are responsible for Mo enrichment in sulfidic zones. Molybdenum incorporated in hydrothermal Mn crusts was retained through a distinct process, as an inner-sphere heptamolybdate (HxMo7O24x-6) surface complex. This complex is normally stable only under acidic (pH < 4-5) conditions; consequently, its presence suggests that Mo was scavenged from acidic fluids (the hydrothermal fluids) rather than from seawater. Thus, Mn oxides in this system influence the source of oceanic Mo rather than acting as a sink for water-column derived Mo.
October 22nd, 2002
Robert B. Dunbar, Professor of Geological and Environmental Sciences, Stanford University
“Southern Ocean impacts on global climate: clues from the Antarctic margin”
October 29th, 2002
Kristen Averyt, Graduate Student, Dept. of Geological and Environmental Sciences, Stanford University
Title: TBA
November 5th, 2002
Paul D. Quay, Professor of Marine Chemistry, University of Washington
“Changes in the 13C/12C of carbon in the ocean: a tracer of anthropogenic CO2 uptake”
The 13C/12C (d13C) of CO2 in the atmosphere has decreased during the industrial era because the d13C of the CO2 produced during the combustion of fossil fuels (at ~ -28 ‰) is substantially lower than the d13C of atmospheric CO2 (~ -8 ‰). As a result of air-sea CO2 gas exchange, some of this d13C perturbation has been transferred to dissolved inorganic carbon (DIC) in the ocean. Measuring the magnitude of the ocean’s d13C change provides a means to estimate the amount of anthropogenic CO2 taken up by the ocean. Measurements of the d13C of DIC during the WOCE and OACES ocean-wide sampling programs in the 1990s (a total of ~25,000 measurements) have substantially improved estimates of the ocean d13C change and, in turn, the uptake rate of anthropogenic CO2. Using these new d13C data and a box-diffusion ocean model simulation of the oceanic uptake of anthropogenic CO2 and its d13C perturbation indicate that a CO2 uptake rate of 1.9±0.4 Gt C yr-1 (1970-1990) explains both the observed surface ocean and depth-integrated d13C changes. Constraining a box diffusion ocean model to match both the observed anthropogenic d13C and bomb 14C changes yields an oceanic CO2 uptake rate of 1.7±0.2 Gt C yr -1.
November 12th, 2002
Kelly Kryc, Post-Doc, Dept. of Geological and Environmental Sciences, Stanford University
Title: TBA
November 12th, 2002
David M. Anderson, Research Scientist, NOAA Paleoclimatology Program and INSTAAR, University of Colorado, Boulder
“Century-scale changes in the Asian summer monsoon and their links to the North Atlantic”
Reconstructing the Asian summer monsoon during the past millennium has relevance to process-oriented studies as well as efforts to predict future climate change. We reconstructed the Asian summer monsoon winds for the last 1,000 years using fossil Globigerina bulloides abundance in box cores from the Arabian Sea. The Arabian Sea sediments are nannofossil-rich foraminifer oozes, and the low oxygen content of the Arabian Sea minimizes the bioturbation that would otherwise smooth the record. The composite record based on two cores shares several similarities with the time series of northern-hemisphere warming, namely weaker winds between 1000-1600 AD with a brief increase 1200-1400, a minimum around 1600, and an increase during the past 4 centuries. This is not surprising because both model and observation-based studies reveal a link between cooling/ increased snow cover over Eurasia, and a weaker monsoon the following summer. Alternately, the forcing implicated in the recent warming trend (volcanic aerosols, solar output, greenhouse gases) may directly affect the monsoon. Either interpretation is consistent with the hypothesis that the SW monsoon strength will increase during the coming century as greenhouse gas concentrations continue to rise and northern latitudes continue to warm. Preliminary study of the longer Holocene interval indicates 8 intervals of weaker monsoon winds that are correlated with cooling events in the North Atlantic. We infer that the observed link between Eurasian warmth/ snow cover and the SW monsoon persists on the millennial scale.
November 26th, 2002
Alessandro Tagliabue, Graduate Student, Geophysics Dept., Stanford University
Title: TBA
December 3rd, 2002
James Cloern, Research Scientist, USGS Menlo Park
“Fertilization of Coastal Ecosystems: Another Mode of Global Change”
The National Research Council recently concluded that anthropogenic fertilization of coastal ecosystems "has major impacts, from economic losses associated with reduced fisheries to potential human health impacts, and is likely to increase in severity as nutrient loading from upstream sources increases as a result of continuing urbanization, deforestation, agriculture, and atmospheric deposition". This talk is a review of our contemporary understanding of the mechanisms through which nutrient enrichment causes change in living resources, habitat quality and biogeochemical cycling, using examples from coastal ecosystems around the world.
January 21st, 2003
Dr. Stephen Monismith, Professor of Civil and Environmental Engineering, Stanford University
Title: "Buoyancy driven flows over a coral reef in the Gulf of Aqaba"
I will discuss observations of flows over the coral reef in Eilat Israel, a fringing reef located at the northern end of the Gulf of Aqaba, a branch of the Red Sea. Originally designed to quantify fluxes of phytoplankton to the reef, our measurements of currents, temperatures, and salinities make clear several important aspects of the physical environment of the reef. Firstly, turbulence measurements show that in the absence of surface waves, drag coefficients are roughly 4 time larger over the reef than over a nearby sandy region, rather than as much as 100 times larger as has been suggested in the literature. Moreover, like other canopy flows, near the bed there appears to be an excess of dissipation of the relative to local shear production of turbulence. Secondly, we argue that because barotropic tidal currents are weak, the semi-diurnal currents that produce this turbulence can be attributed to internal tides generated at sill in the Strait of Tiran, 180km to the south of Eilat. Thirdly, we find that the main mechanism for offshore-onshore transport of plankton etc. is thermal convection associated with differential heating and cooling of waters over the reef, i.e. temperature differences that arise because of variations in depth of water over the reef. Remarkably, the strength of flows we observe seem to match well predictions based on scaling and laboratory experiments. An interesting twist is an asymmetry in strength of the flows that arises because evaporation also increases salinity and thus can weaken the density contrast that appears during heating phases.
January 28th, 2003
Dr. J. R. Toggweiler, Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, NJ.
Title: “Link between atmospheric CO2 and the physical climate system”.
Ice core records show a strong temporal correlation between the state of the earth's climate and atmospheric CO2 over the last 400,000 years. Cold intervals over Antarctica coincide with the times of lowest atmospheric CO2; cold glacial intervals terminate with rapid increases in temperature and atmospheric CO2. The mechanism that ties atmospheric CO2 to the rest of the climate system has so far eluded any simple description. The most direct link that one can imagine, higher (lower) atmospheric CO2 forced by warmer (colder) ocean temperatures, accounts for no more than 25% of the overall CO2 change between glacial and interglacial states. Most explanations call upon changes in ocean chemistry that make the ocean's biological pump stronger or weaker. Here I suggest that atmospheric CO2 variations occur through a feedback-type interaction between the ocean's physcial circulation, atmospheric CO2, tropospheric temperatures, and the position of the mid-latitude westerlies. Changes in ocean chemistry and biological production are not the main source of atmospheric CO2 variations. A feedback of this sort has major implications for the CO2-induced climate changes underway at the present time.
February 4th, 2003
ROCHELLE G. LABIOSA , Graduate Student, Geophysics, Stanford University
Guest Appearance: DR. JEFF SHRAGER, Research Fellow Carnegie Institution, Plant Biology Branch
Title: "Linking the micro - to the meso-scale: an adventure in molecular biology"
Understanding phytoplankton photosynthesis is critical to several fields including ecology and global biogeochemistry. The efficiency with which phytoplankton fix carbon depends upon the ambient light field, which is in turn dependent upon sun angle and the depth of mixing in the water column. How phytoplankton adjust to short-term changes in light field, such as diurnal changes, and longer term, moreextreme changes, such as those caused by seasonal stratification in open ocean waters, are of particular interest. In this pilot project, Synechocystis PCC 6803 was chosen as a model organism with which to study the molecular and physiological responses of phytoplankton to diurnal changes in light levels. Our goal in this project was to detect the levels of mRNA in the cells at each point in time; given that "DNA codes for RNA, and RNA codes for proteins", this, in turn, would tell us something about the enzymatic processes taking place in the cell at different periods of time in the light cycle. Using the latest molecular biology tools including microarray technology, whereby the whole genome of the organism is used to detect levels of mRNA in the cells, we were able to examine all pathways in the cell, including photosynthesis and respiration. At approximately every two hours in the light cycle for several days, cells were subsampled and RNA was extracted. Initial results from Northern Blot hybridizations (examining RNA levels for individual genes) indicate that, the transcripts encoding photosynthetic proteins (i.e., PsbA2, PsaA and CpcB, in photosystem II, photosystem I, and phycobilisomes, respectively) are highest during the light. Early results from microarrays containing the full genome of the organism show that almost all genes display higher transcript abundances during the day (including photosynthetic genes), with a few notable exceptions, and a few others display higher transcript abundance at night than during the day. This pilot project will be followed by another cyclodyne experiment, with Prochlorococcus spp. in culture. Prochlorococcus is ubiquitous in the world's oceans and comprises the dominant phytoplankton in oligotrophic regions. Prochlorococcus will be grown under similar conditions as the Synechocystis, however the experiment will be taken a step further, and the boundaries of photoacclimation to relatively abrupt, high intensity changes in light field will also be examined. The application of the Prochlorococcus microarray data, in combination with physiological measurements, for paramaterizing oceanographic biological models will be discussed.
February 11th, 2003
Dr. C. Storlazzi. USGS, Santa Cruz
Title: "Sediment Dynamics on Hawai’ian Reefs: Insights from Southern Moloka’i"
We have employed numerous techniques to observe temporal changes in waves, currents, tides and sediment suspension across the heavily impacted reef off Kamiloloa, Moloka’i. These tools have ranged from digital cameras on the hillslopes to instrument packages deployed at depths greater than 10 m on the fore reef. The time-series measurements suggest the following conceptual model of water and fine-grained sediment transport across the reef: Relatively cool, clear water flows up onto the reef flat during flooding tides. At high tide, more deep-water wave energy is able to propagate onto the reef flat and larger trade wind-driven waves can develop on the water depth-limited reef flat, thereby increasing sediment suspension. Wind-driven surface currents and wave breaking at the reef crest cause setup of water on the reef flat, further increasing the water depth and enhancing the development of depth-limited waves and sediment suspension. As the tide ebbs, the water and associated suspended sediment on the reef flat drains off of the reef flat and is advected offshore and to the west by alongshore tidal currents. Observations on the fore reef show higher turbidity levels throughout the water column during the ebb tide. It therefore appears that high suspended sediment concentrations on the deeper fore reef, where active coral growth is at a maximum, are dynamically linked to processes on the muddy, shallow reef flat.
February 18th, 2003
Constanze Weyhenmeyer, Lawrence Livermore National Labs
“Origin and Evolution of Late Pleistocene to Holocene Groundwater in an Arid Coastal Region of Oman Inferred from Chemistry, Isotopes and Noble Gases”
In arid and semi-arid regions, conventional hydrodynamic mass-balance models often fail to adequately describe groundwater systems due to the large spatial and temporal variability of hydrological input parameters (e.g. precipitation, evaporation, runoff) and the overall scarcity of data from the more remote areas. Alternatively, this study of a coastal alluvial aquifer in the Sultanate of Oman utilizes a wide suite of geochemical and isotopic tracers (O, H, C, Sr) for the identification of aquifer units, recharge areas and groundwater flow paths. Groundwater residence times were estimated by a combination of radionuclides with different half-lives and input functions (3H, 85Kr, 39Ar, 14C). Noble gas measurements (He, Ne, Kr, Ar, Xe) were carried out in Holocene and Late Pleistocene groundwater samples to infer changes in recharge temperatures through time. Stable isotopes of groundwater (delta18O, delta2H) provided additional information about changes in moisture sources for precipitation on a Glacial-Interglacial time scale.
February 25th, 2003
Dr. Doug Capone, Wrigley Institute for Environmental Studies & Department of Biological Sciences, University of Southern California.
Title: Reinventing the Marine Nitrogen Cycle: N2 Fixation in the World's Oceans
Recent observations in biological oceanography, geochemistry and molecular ecology have prompted us to review and radically revise our view of the quantitative importance of N2 fixation in the marine N cycle, particularly in the oligotrophic tropics and sub-tropics. It has been generally held that the main source of nitrogen supporting new production by phytoplankton in surface waters through much of the world's upper ocean is the reserves of nitrate in the deep ocean, and that microbial N2 fixation is of relatively minor importance in the global marine N balance. However, geochemical indices point to substantial inputs of recently fixed nitrogen in several major ocean basins. Direct estimates of depth integrated N2 fixation by the most conspicuous planktonic N2 fixer, the cyanobacteria Trichodesmium spp., similarly suggest that oceanic N2 fixation is far greater than earlier projected. Intense inputs of fixed nitrogen can occur during episodic surface aggregations of Trichodesmium although this is only poorly quantified at present; remote sensing may provide the means to resolve the spatial and temporal dimensions of such phenomenon. The simple view that there are only a few key N2 fixers in the sea is also being reevaluated as new studies unveil a much broader suite of potential diazotrophs resident in the oceans. Quantifying their input is a current challenge. Because it can affect the oceanic inventories of fixed nitrogen and thereby the ocean^Ys capacity to sequester carbon, N2 fixation has been proposed as a key components in a suite of interactions and a possible feedback system among the oceans, atmospheric CO2 and climate. Nitrogen fixers have a greater demand for iron than do non-diazotrophs and the delivery of iron to the upper ocean through aeolian dust deposition is hypothesized to be a critical control on this process. Phosphate availability may also place a constraint on the extent of N2 fixation in particular ecosystems. Current investigations are focusing on the interplay of these limiting nutrients in different ocean regimes and the role of nitrogen fixers in ocean carbon sequestration.
March 4th, 2003
Dr Ken Johnson, Monterey Bay Aquarium Research Institute
“The role of the marine biogeochemical cycle of iron in regulating glacial/interglacial cycles. ”
Iron is a key nutrient element for phytoplankton in the ocean. Rates ofphotosynthesis and biomass accumulation are limited by the availability of iron over at least 20% of the ocean's surface. The late John Martin hypothesized that an increase in the flux of iron to surface waters of the ocean could stimulate sufficient primary production and carbon export toreduce the concentration of carbon dioxide in the atmosphere and produce a glacial climate. We have conducted a variety of open ocean iron fertilization experiments to test this hypothesis, including the recent Southern Ocean Iron Experiment (SOFEX). I'll review the evidence that has accumulated to support the Iron Hypothesis, including the SOFEX results, and consider the major challenges that remain in assessing the role of iron in regulating climate. Perhaps the most significant of these challenges is understanding the process at glacial terminations that breaks the positive feedback that iron can exert on global cooling.
March 11th, 2003
Greg Ravizza, University of Hawaii
Title: "Impacts, glaciation & weathering: the marine Os isotope record of the Eocene-Oligocene transition."
Osmium (Os) isotope analyses of bulk sediments from the South Atlantic, Equatorial Pacific, and the Italian Apennines yield a well-dated and coherent pattern of 187Os/188Os variation from the late Eocene to the early Oligocene. The resulting composite record demonstrates the global character of two prominent features of the previously reported low resolution LL44-GPC3 Os isotope record. These are (1) a pronounced minimum in 187Os/188Os (0.22 to 0.27) in the late Eocene, between 34 and 34.5 Ma, and (2) a subsequent rapid increase in 187Os/188Os, to approximately 0.6 by 32 Ma. An ultramafic weathering event, and an increased influx of extraterrestrial particles to the Earth are discussed as alternative explanations for the late Eocene 187Os/188Os minimum. Comparison of the 187Os/188Os to benthic foraminiferal oxygen isotope records demonstrates that the nearly 3-fold increase in 187Os/188Os from the late Eocene minimum coincides with the growth and decay of the first large ice sheet of the Oligocene (commonly refereed to as the Oi1 glacial event). The fine structure of the Os isotope record indicates that enhanced release of radiogenic Os, unrelated to the recovery from late Eocene minimum, lagged the initiation of the Oi1 event by roughly 0.5 Myr. This record, in conjunction with results from weathering studies in modern glacial soils, suggests that exposure of freshly eroded material during deglaciation following Oi1 enhanced chemical weathering rates, and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide. The improved temporal resolution and age control of the refined Eocene-Oligocene Os isotope record also makes it possible to illustrate the late Eocene Os isotope excursion as a tool for global correlation of marine sediments.
April 15th, 2003
Jim McManus, Oregon State University
Title: "Tracing the global silica cycle through time: The proxy potential of germanium"
Records of oceanic Ge:Si preserved in siliceous deposits show temporal variations that reflect changes in the marine Si cycle through time. These variations might be influenced by changes in the ratio of weathering to hydrothermal inputs, changes in the relative rates of Ge removal in opal versus non-opal phases, or possibly by changes attributable to the internal cycling of Si in the ocean and fractionation of Ge:Si by diatoms. One of the limitations in interpreting the documented secular Ge:Si variations has been that the sink term for nearly half of the oceanic Ge is poorly quantified (i.e., the modern geochemical budget is poorly understood). Our recent work along the North and South American margins demonstrates that Ge burial is decoupled from that of silica with nearly half of the oceanic Ge being buried in iron-rich marine sediments. With this finding and the possibility that diatoms discriminate between Ge and Si, further chemical constraints are needed to distinguish among the various influences on the germanium silica cycles. One proposal currently being explored is that germanium and silica isotopes offer the potential for fully exploiting the coupled Ge-Si system.
April 22nd, 2003
Paul Falkowski, Rutgers University
Title: “The evolution of marine phytoplankton in the Phanaerozoic oceans: From dinosaurs to diatoms”
Unlike terrestrial plants, phytoplankton are represented by relatively few species that are taxonomically diverse. Since the end-Permian extinction, 251 Ma before present, the major taxa of eucaryotic phytoplankton have all been dominated by organisms containing plastids derived from the “red” line of the ancentral cyanobacterium. In the Paleozoic oceans however, the fossil evidence suggest that the “green” line was dominant. The evolutionary trajectory that led to the evolutionary success of the red line appears to have been set by oceanic conditions in the Triassic, out of which emerged thecate dinoflagellates and the first coccolithophorids. The origin of diatoms remains unclear, however this group rose to ecological prominence in the Cenozoic. I will trace the evolutionary processes and ecological traits that have led to the success of the red line, and the role of the Wilson cycle and the ensuing climatic shifts that have led to the long term successions in phytoplankton community structure.
Wednesday April 30th in GeoCorner Room 109
Greg Cutter, Old Dominion University
Title: "The marine biogeochemical cycle of carbonyl sulfide and its role in the Earth's radiation budget"
Abstract
May 6th, 2003
Christina Ravelo, UC Santa Cruz
Title: "Do tropical conditions determine climate sensitivity?: Lessons from the warm Pliocene"
The last 5 Ma of Earth's history includes a shift from global warmth of the early Pliocene to globally cold conditions of the Pleistocene, and therefore provides an opportunity to understand the causes of major climate transitions. It also provides an opportunity to compare how climate behavior such as its sensitivity to solar forcing is different in a warm versus cold global state. Although high latitude records of climate change exist, the lack of records from tropical and subtropical regions has prevented scientists from testing theories about the causes of the warm to cold transition and of the changes in climate sensitivity. Using tropical and subtropical records from the Pacific we document that major reorganization of low latitude conditions, specifically the development of Walker circulation in the tropical Pacific and cooling of upwelling regions in the subtropics, occurred just after 2.0 Ma. This is well after major tectonic events in the tropical regions and after the onset of significant Northern Hemisphere Glaciation, indicating that glaciation was not driven by tropical processes. However, an analysis of how ice sheet respond to solar forcing demonstrates that the sensitivity of the response is greatest just after about 2.0 Ma. This indicates that mean tropical conditions potentially have a large influence on the sensitivity of global climate to perturbations.
May 13th, 2003
Stuart Fallon , Lawrence Livermore National Laboratory
Title: "Surface water processes in the Indonesian Seas as documented by high-resolution coral D14C record"
Radiocarbon has been used as a tracer of ocean circulation and marine geochemistry since the discovery of carbon-14 dating. D14C has been used to study global ocean circulation through the distribution of natural and bomb-derived 14C. Two expeditions, GEOSECS (1970s) and WOCE (1990s) identified spatial gradients in both the surface and deep-water D14C, which define large-scale ocean circulation. The main limitation of these studies is that they provide only a “snap shot” of 14C variability. In order to get a complete picture of the temporal variation of 14C, proxy records, such as those provided by corals are needed. Coral can provide time series of seasonal and interannual 14C variations of the surface ocean over time scales of hundreds of years. Bi-monthly radiocarbon and stable isotope (d13C, d18O) have been recovered from a coral in the Indonesian Seaway in order to better understand seasonal, interannual and decadal variability in the surface water masses that contribute to the Indonesian Throughflow. The d18O record displays strong regular seasonality but is clearly dominated by fresh-water flux as this area receives ~2400mm rainfall annually. During drought (El Niño) years the d18O begins to reflect water temperature. The d 13C record also has a clear and regular seasonality with depleted values that occur during the cloud cover and rainfall peak in January. The radiocarbon time series is relatively stable (little to no seasonality) from 1890 to 1955 at which point the record increases rapidly, a response to the increased atmospheric 14C caused by nuclear weapons testing. From 1954 to 1986 the record displays clear seasonal variability from 15 to 60‰. The quick response to the increased atmospheric 14C and the high post-bomb peak (+160‰) indicate a significant amount of the waters entering the Indonesian Throughflow originate in the North Pacific. The coupled model/data approach will be used to assess the model parameterization of ocean dynamics and improve the models.
March 20th, 2003
Peggy Delaney, UC Santa Cruz
Title: "Southeast Pacific Paleoceanographic Transects, (ODP Leg 202), Interstitial Water Geochemistry"
I present a biogeochemical overview of sites drilled during the Southeast Pacific Paleoceanographic Transects expedition (Ocean Drilling Program Leg 202) from the perspective of their interstitial water geochemistry. The sites cover a wide latitudinal and water depth range in this productive eastern boundary current region. The oceanographic and geographic settings of Leg 202 sites influence their biogeochemical environments, as reflected in the volatile hydrocarbon, interstitial water, and sediment geochemistry. Oxidation of organic matter is the major influence on interstitial water geochemistry, with resulting effects on volatile hydrocarbon geochemistry and on authigenic carbonate mineralization reactions. The dissolution of opaline silica influences the dissolved silicate profiles, and in some sites, temperature appears to be a primary control on the dissolved concentrations. In addition to the prevailing influence of organic matter degradation, two Chile Margin sites are affected by methane hydrates as shown in chlorinity profiles, and a Panama Basin site shows the signature of fluid flow of relatively unaltered seawater in the underlying basement.
May 27th, 2003
Mike Gagan, Australian National University
Title: "Holocene Ocean-Atmosphere Interactions in the Pacific Warm Pool and the Tropical General Circulation"
Calcium carbonate paleoenvironmental archives are important for reconstructing dynamic changes in tropical climate systems and for understanding the processes controlling climate change. We are in the process of developing a multi-proxy geochemical approach (d18O, d13C, Sr/Ca, Mg/Ca, D14C) to tropical paleoceanography using surface-dwelling foraminifera and corals. The primary aim of this presentation will be to demonstrate the utility of both archives for reconstructing ocean-atmosphere climate change on millennial to seasonal time-scales in the Pacific Warm Pool region. The picture emerging from Sr/Ca, Mg/Ca and 18O/16O data is that the southern sector of the Pacific Warm Pool was anomalously warm and a strong source of evaporated water vapor, between 11,000 and 4,000 years ago. Analysis of the seasonal cycle of sea-surface temperature and 18O/16O reveals a positive feedback between sea-surface temperature gradients across the Pacific, trade wind velocity and enhanced evaporation in the southern Warm Pool region. The resulting intensification of the tropical general circulation during the early Holocene may have triggered prolonged amplification of postglacial global warming. Coupled measurements of Sr/Ca and 18O/16O in corals indicate that the fresh, buoyant lens of Warm Pool seawater may be a relatively recent feature. High-resolution coral records of freshwater fluxes in the Great Barrier Reef reveal that 20th century deforestation for agriculture has lead to higher land-sea hydraulic gradients and groundwater discharge to the nearshore reefs. Hence nearshore corals may now be exposed to lower aragonite saturation states, which could reduce coral calcification and contribute to recent reef degradation. In 1999, we drilled a sequence of well-preserved Porites corals within a rapidly uplifted paleo-reef in Alor, Indonesia, with 230Th ages spanning 8,400 to 7,600 calendar years before present. Measurements of coral Sr/Ca and d18O have yielded a semi-continuous record (310 years) showing that sea-surface temperatures were essentially the same as today from 8,400 to 7,600 years ago. However, both tracers show that this period of climatic stability is interrupted by an abrupt ~3C cooling over a period of ~100 years, reaching a minimum 8,000 years ago. The rapid cooling of ~0.3C per decade in the Warm Pool is nearly synchronous with abrupt cooling in the North Atlantic region, as indicated by the sudden decrease in d18O values of ice from the GISP2 ice core. This finding supports the hypothesis that the tropical ocean-atmosphere serves to propagate abrupt climate change between the northern and southern hemispheres without a significant time lag.
June 3rd, 2003
Organizational Meeting for 2003-2004 Oceans Seminar