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LDEO

Lamont-Doherty Earth Observatory
Country: United States
13 Projects, page 1 of 3
  • Funder: UKRI Project Code: NE/F004966/1
    Funder Contribution: 329,978 GBP
    Partners: University of Cambridge, LDEO

    Large areas of the floor of the oceans are draped with sediment chiefly composed of biogenic calcium carbonate, the remains of calcareous organisms (foraminifera, coccolithophores, pteropods) whose shells are composed of the CaCO3 minerals calcite or aragonite. The CaCO3 contents of marine sediments in many oceanic regions have varied with climate over glacial-interglacial cycles: lower contents of CaCO3 coinciding with the build-up of continental ice sheets. The existence of 'CaCO3 sediments' in the deep ocean has been crucial for moderating the limits of variation in atmospheric CO2. This is because there is an inverse relationship between the concentration of carbonate ion in the deep ocean and the concentration of atmospheric CO2. Carbonate ion concentration, [CO32-], is a major factor controlling the solubility of CaCO3. Because of this inverse relationship, palaeoceanographers have strove for many years to find a proxy for deep sea [CO32-]. The records of deep-ocean CaCO3 content provide important evidence of how ocean chemistry changed with climate but the evidence is indirect because the CaCO3 records represent a response to changes in the carbonate chemistry of ocean waters or of pore waters. Other methods in use are also indirect and rely on the dissolution of the shells of the calcareous organisms. We have developed a new method to estimate deep sea [CO32-] in past oceans using the incorporation of boron (B) in benthic (deep sea) foraminiferal calcite. Benthic B/Ca allows us to define [CO32-] of ocean waters and thus the depth of the water column 'saturation horizon' above which water is oversaturated, and below which is undersaturated, with respect to CaCO3 solubility. We aim to generate records of deep-ocean [CO32-] in critical regions of the oceans that should add significantly to understanding the role of the oceans in atmospheric CO2 cycles.

  • Funder: UKRI Project Code: NE/F019084/1
    Funder Contribution: 261,704 GBP
    Partners: University of Bristol, LDEO

    One of the outstanding mysteries in the Earth sciences is the composition of the core. We know from seismic and cosmochemical constraints that the core is made of a nearly pure iron (~95% Fe + 5% Ni), and that the inner core is solid and the outer core is molten. However, based on our knowledge of the behavior of molten iron at the extreme pressure and temperature conditions of the core, it is apparent that there must be some other light element or elements dissolved in the molten outer core as well. It is thought that the light element is related to convection in the outer core and is therefore important for spawning the Earth's magnetic field. The nature and abundance of the light element will also determine the kinds of reactions that might occur at the boundary between mantle silicate and the molten metal core. For the last half-century the primary candidates for the light elements in the core have included H, O, S, C, and Si. We will never be able to sample the core directly, hollywood movies notwithstanding, so other approaches are required to deduce the identity of the light elements. Basically, the approach has been to try and determine which elements can dissolve into molten iron at core conditions using experiment and theory. A perusal of the vast literature on this subject reveals that individual elements and cocktails of elements have come into, out of, and back into favor with time. Different experimental and theoretical approaches often lead to very different interpretations as to the identity of the light elements. Here we propose a method for deducing the light element in the core that relies on a combination of experiment, thermodynamic modeling, and seismic observations. Seismic data constrain the velocity at which compressional waves can move through molten iron as well as the density. They can also detect whether the core liquid has separated into more than one liquid (immiscibility). In principle, if one knows the same properties for various iron alloy - light element mixtures, one can deduce the composition of the core. The seismic observations are available. An internally consistent model for the properties of molten alloys at core conditions is not. However, thermodynamic relationships allow the physical properties to be determined through the equation of state of molten alloys. The parameters required to develop the thermodynamic model can be deduced through the melting curves of iron - light element alloy compositions. Here, we are proposing to make measurements of the melting curves of two-component (binary) alloys such as FeO, Fe3C, FeS, FeH and FeSi in order to derive the quantities required for the thermodynamic model. We have developed robust techniques in our lab for measuring melting points to very high pressures and temperatures using the laser-heated diamond anvil cell. Further, we have developed an exciting and novel new X-ray imaging technique with which we can measure directly the minimum melting compositions (eutectics) in iron - light element systems. These data further help constrain the thermodynamic models. In summary, we will use an experimental approach to measure how iron - light element alloys melt and from this data we will develop a multi-component thermodynamic model that will allow us to predict the seismic wave velocities and density of a wide range of possible core liquids. We will then compare the model with actual observations to deduce the identity of the elusive light elements in the molten outer core.

  • Funder: UKRI Project Code: NE/H014144/1
    Funder Contribution: 73,969 GBP
    Partners: Imperial College London, NERC British Geological Survey, LDEO

    Warming of the climate system is unequivocal, and evident from observations of increases in global average air and ocean temperatures. Based on projections published in the Nobel Peace Prize winning report of the Intergovernmental Panel on Climate Change in 2007, the Earth's surface is estimated to warm between 1.8 to 4.0 degree Celsius by the end of the century. Already now, we are observing the retreat of mountain glaciers, limited extent of sea ice in the Arctic Ocean, and melting around the edges of the polar ice caps in Greenland and parts of Antarctica. Projected sea level rise by 2100 ranges between 0.2 and 0.6 m. This range however does not include potential contributions from the ice caps, which today tie up a water mass equivalent to about 60-70 m of sea level change. From studying the climate in the past, we know that the polar ice caps can wax and wane with changing climatic conditions. The last time Earth climate was significantly warmer than today, for an extended amount of time, was 4.5 to 3 million years ago, during the time period of the Pliocene. Global temperatures in the warm Pliocene were about 3 degree Celsius higher than today. Sea level estimates for this time range from 5 to 40 m higher than today, with a most commonly used value around 25 m. Such an elevated sea level would not only mean the disappearance of the Greenland and West Antarctic ice sheets, but also instability in parts of the massive East Antarctic ice sheet. Data are however sparse, and models give conflicting results on whether melting only acted on the more vulnerable West Antarctic ice sheet, or also on parts of the large East Antarctic ice sheet. With our project we will try to address exactly this dilemma. We are going to analyse the chemical composition of ice-rafted sediments from the Southern Ocean off East Antarctica. This material was carried to its place of deposition by icebergs, and its geochemical fingerprint contained in mineral grains, allows us to track back the exact area of the continent where the iceberg was coming from. This in turn tells us which part of the ice margin became instable in the past. It is such evidence from the geological record that is vital to test and refine existing climate models. We will investigate which adjustments have to be made to climate models to reproduce our observational data. Once we succeed to produce a better database for understanding ice extent during the Pliocene warmth, and once we have a climate model that matches the observations of the past, we should have higher confidence to use the same type of model to foresee what may happen in the future.

  • Funder: UKRI Project Code: NE/F002734/1
    Funder Contribution: 422,259 GBP
    Partners: UAB, Cardiff University, LDEO

    The last glacial period was punctuated by repeated, high-amplitude millennial-scale shifts in northern hemisphere climate known as Dansgaard-Oeschger (D-O) oscillations. These events were characterised by the extremely rapid alternation between cold and warmer conditions with temperature increases over Greenland and NW Europe sometimes exceeding 10 degrees Celsius within a few decades. The discovery of D-O oscillations has provided a major stimulus for climate research and fuelled debate over the possible nature of climate change in the future, yet very little is known about the origin of these events. Ocean circulation is a fundamental component of Earth's climate system. Changes in circulation are known to be associated with major shifts in global climate, including glacial-interglacial transitions as well as more rapid events such as D-O oscillations. The transition from the last interglacial period (similar to today) to full glacial conditions, around 75,000 years ago, saw a large build-up of continental ice sheets and a decrease in atmospheric CO2. This period also saw the first appearance of D-O climate variability, suggesting that a critical threshold, between the relative stability of the last interglacial and the instability of glacial times, had been crossed. However, fundamental uncertainties currently exist concerning the role of ocean circulation within the evolving climate system and its apparent threshold behaviour at this time. This project will investigate changes in Atlantic Ocean circulation and their role in global climate change during the MIS 5a/4 transition. The project will improve our understanding of the links between ocean circulation and climate change and will therefore inform scientists working in related fields and potentially policy makers. Palaeoclimate reconstructions will be made on each of five sediment cores taken from the North Atlantic, covering both east and west basins in intermediate and deep waters. Proxies for nutrient distribution and carbon chemistry will be used to reconstruct the evolving deep ocean chemical structure during MIS 5a/4 while dynamical palaeocirculation proxies will enable assessment of physical changes in bottom current speed and mass volume transport. Direct temporal correlation of the reconstructions with records from ice cores and absolutely dated cave deposits will enable the development of extremely robust age models, vital for investigation of potential leads and lags between the reconstructed changes and other climate relevant parameters such as ice volume and atmospheric CO2. A further circulation proxy record will be produced from a core from the Atlantic/Indian sector of the Southern Ocean. This will provide unambiguous evidence for the relative timing between changes in ocean circulation and the global carbon cycle. The data produced as part of this project will provide critical constraints for differentiating between key physical and chemical changes in Earth's climate system during the transition to full glacial conditions and the onset of glacial climate instability.

  • Funder: UKRI Project Code: NE/K013181/1
    Funder Contribution: 470,958 GBP
    Partners: University of Southampton, Nat Oceanic and Atmos Admin NOAA, LDEO, Woods Hole Oceanographic Inst

    During the last three decades, the Antarctic Botttom Water (AABW) filling the bulk of the global ocean abyss has exhibited a striking warming and contraction in volume over much of the world ocean, particularly in the Atlantic basin. While the causes of these changes are unknown, available evidence suggests that, in the Atlantic Ocean, the warming and contraction of AABW may be caused by changes in winds over the northern Weddell Sea, where much AABW is produced. This hypothesis asserts that those winds regulate the volume and temperature of the AABW exported northward via the Orkney Passage (a major AABW exit route from the Weddell Sea) by altering the intensity of the turbulent mixing between AABW and overlying warmer waters in the passage. In this proposal, we set out to test and, if necessary, redefine this hypothesis by: (1) carrying out the first systematic measurements of how AABW flows through the Orkney Passage, how its properties change along the way, and what processes are important in determining the AABW flow and transformation in the passage; (2) determining how and why the flux and properties of AABW in the Orkney Passage respond to wind forcing on time scales of up to several years. To address task (1) above, we will measure the velocity and properties of AABW and the intensity of turbulent mixing at several key locations in the passage. The observations will be obtained both with instruments lowered to the seabed from a ship and with a novel autonomous underwater vehicle, which is particularly effective at measuring a range of potentially crucial processes occurring near the ocean floor. To address task (2), we will enhance an array of moorings recently deployed in the Orkney Passage by the British Antarctic Survey to monitor the flux and properties of AABW. We will equip the moorings with sufficient oceanographic instrumentation to identify the processes determining the AABW's response to wind forcing, which are not resolved by the present array. We will use our findings from tasks (1) and (2) to define how and why the volume and properties of the AABW escaping the Weddell Sea through the Orkney Passage react to changes in winds. Armed with this new understanding, we will revisit the widespread AABW warming and contraction observed over recent decades, and inform the international strategy to monitor future changes in AABW circulation.