798 research outputs found
Evolution of sea surface temperatures in the Eocene and Oligocene reconstructed using organic proxies (abstract of paper presented at: 19th Annual V. M. Goldschmidt Conference, Davos, Switzerland, 21-26 June 2009)
No full textSouthern Ocean, Antarctic ice and climate interactions during Neogene cooling
No full textThe partial pressure of atmospheric carbon dioxide (pCO2) has increased from 280 to 420 ppm (parts per million) since the industrial revolution due to anthropogenic emissions. As a result, the Earth's atmosphere, ocean, and cryosphere are undergoing changes due to increased radiative forcing, leading to a warming planet, loss of ice sheets and sea ice, sea level rise, ocean heat redistribution, and fluctuations in ocean circulation, as predicted in scenarios up to the year 2100. Nevertheless, future changes in these components remain highly uncertain—better understanding of these processes is crucial for human communities. To better comprehend the interactions between the Southern Ocean, the Antarctic ice sheet (AIS), pCO2, and climate in the future, I have studied sea surface temperatures, ocean front migrations and deep-sea temperatures during climate transitions in the geological past, specifically the Neogene (2.58–23.04 million years ago, Ma), which experienced pCO2 levels as high as, and sometimes higher than, the current levels.
A distribution model of modern dinocysts was developed based on newly collected surface sediment samples near Antarctica to reconstruct the positions of oceanic fronts in the Southern Ocean in the past (Chapter 2). Subsequently, I demonstrated a significant long-term cooling of ocean surface and northward migrations of ocean fronts during the Neogene near Tasmania. The substantial cooling at mid-latitudes is not caused by solar radiation combined with polar amplification but is attributed to ocean frontal migrations and the northward expansion of the polar sea. Additionally, I identified a substantial deep-ocean cooling, which nearly completely explains the increase in benthic δ18O, leaving little room for an ice volume effect. The relatively stable ice volume during climate cooling seems counterintuitive given the northward migrations of ocean fronts and other geological evidences suggesting the AIS advancing. Hence, I proposed a hypothesis that the AIS gradually decreased in height while expanding seaward, maintaining a relatively stable volume during the mid-Miocene climatic transition (~14.5 Ma) and verified using ice sheet modelling. The migrations of the subtropical front near Tasmania are closely linked to a high-resolution pCO2 dataset for the Pliocene (2.58–5.3 Ma), which shows a delay of ~10,000 years in pCO2 compared to δ18O during the M2 glaciation and, at least, an elevated pCO2 during glaciation. I proposed that carbon emissions from the deep sea, driven by frontal migrations, was the dominant process in regulating pCO2, rather than physical diffusion or the biological carbon pump. This leads to an higher pCO2 during cold phases because less CO2 is stored in the ocean than during deglaciation. In another crucial geographical area, the Agulhas Plateau, I discovered that migrations of the subtropical front, regulating the inflow of warm water into the Atlantic Ocean, play a significant role in the variability of the Atlantic Meridional Overturning Circulation in the Pliocene. I concluded that there was a connection between frontal migrations and the locality of North Atlantic Deep Water formation, significantly altering heat transport to high northern latitudes and providing insights into understanding future AMOC changes
A magneto- and chemostratigraphically calibrated dinoflagellate cyst zonation of the early Palaeogene South Pacific Ocean
No full textInvestigation of the early Palaeogene palaeoecological and palaeoclimatological evolution of the Polar Regions is
hindered by the absence of calcite microfossils in sedimentary archives, which are conventionally the main dating
tool. To overcome this problem, we have generated large datasets of organic dinoflagellate cyst (dinocyst)
assemblages fromSouthern Ocean shelf sediments over the past decade, and we here calibrate these to the Geomagnetic
Polarity TimeScale (GPTS) using magnetostratigraphy and stable isotope stratigraphy. This now for the
first time allows a high-resolution Southern Pacific Ocean dinocyst zonation for the late Palaeocene to late
Eocene (58–36 million years ago; Ma). We compile published dinocyst chronologies from Ocean Drilling Program
(ODP) Hole 1171D on the South Tasman Rise, Hole 1172A/D on the East Tasman Plateau and Integrated
Ocean Drilling Program (IODP) Hole U1356A on the Wilkes Land margin. Correlation to dinocyst zonations
from New Zealand lead to revisions of the magnetostratigraphic age model at Holes 1171D and 1172A/D. Stable
carbon and oxygen isotope records reveal the stratigraphic location of the Palaeocene–Eocene Thermal
Maximum (~56 Ma) and the Middle Eocene Climatic Optimum (~40 Ma), respectively. The resulting zonation
consists of thirteen dinocyst zones, calibrated to the Geomagnetic Polarity Time Scale (GPTS) of Vandenberghe
et al. (2012), which can likely be applied to the entire Southern Ocean. Finally,we apply the revised stratigraphy
to all published TEX86 data, a biomarker-based proxy for sea surface temperature (SST), from ODP Site 1172 to
assess long-term climate evolution. This shows that Southwest Pacific SST trends mimic the global compilation
of benthic foraminiferal oxygen isotopes even better than previously appreciated
Global change across the early Eocene and its warming events
No full textUnder present-day anthropogenic climate change, the extent of future warming and its global distribution have become pressing scientific concerns. Crucial information for addressing these unknowns is captured within the geological record, offering a window into ancient climates under vastly different or fluctuating CO2 concentrations.
This work delves into the 'hothouse' conditions of the early Eocene (56–48 million years ago). This time was characterized by surface temperatures 10–14 °C higher than today and CO2 concentrations approximately three to five times higher than present, and ice sheets were presumably absent. During this period, many repetitive warming events (hyperthermals) occurred, driven by variations in Earth's orbit. Our knowledge of these hyperthermals was primarily limited to a few records of changing ocean bottom water conditions. Yet, these phases offer a great opportunity to study the effects of global warming.
In this work, early Eocene climate was reconstructed using sediment cores retrieved at three locations from tropics to poles: the equatorial Atlantic, the northern Negev, and the Arctic Ocean. The variability of surface ocean temperatures was reconstructed at these sites using temperature-sensitive molecular fossils, derived from marine archaea. Surface ocean conditions were further evaluated by analyzing assemblages of fossil organic walled dinoflagellate cysts.
A main result of this work is that a surface warming signal was observed during multiple hyperthermals and smaller orbital variations at all sites, confirming their global nature. During these variations, the polar regions experienced approximately twice the warming of the tropics, due to the effect of 'polar amplification'. In the absence of polar (sea)ice, this gives an interesting insight in the non-ice-related drivers of present-day polar amplification.
Furthermore, the impact of the hyperthermals on tropical ocean surface ecosystems was investigated. Interestingly, no systematic biotic response was recorded during the smaller hyperthermals (below ~1.5 ºC surface warming). These finding contrasts previous work which documented significant biotic disruption during the larger hyperthermals and suggests a warming threshold for ocean surface biotic response.
Finally, the effects of the warming intervals on the hydrological cycle were explored as well. These findings, amongst others, indicate that subtropical regions became drier during hyperthermals, while polar areas experienced wetter conditions.
In summary, this work presents new insights in the distribution and effects of warming on orbital scale during the early Eocene hothouse world
Enhanced terrestrial carbon export from East Antarctica during the early Eocene
No full textTerrestrial organic carbon (TerrOC) acts as an important CO2 sink when transported via rivers to the ocean and sequestered in coastal marine sediments. This mechanism might help to modulate atmospheric CO2 levels over short- and long timescales (103 to 106 years), but its importance during past warm climates remains unknown. Here we use terrestrial biomarkers preserved in coastal marine sediment samples from Wilkes Land, East Antarctica (~67°S) to quantify TerrOC burial during the early Eocene (~54.4 to 51.5 Ma). Terrestrial biomarker distributions indicate the delivery of plant-, soil- and peat-derived organic carbon (OC) into the marine realm. Mass accumulation rates of plant- (long-chain n-alkane) and soil-derived (hopane) biomarkers dramatically increase between the earliest Eocene (~54 Ma) and the early Eocene Climatic Optimum (EECO; ~53 Ma). This coincides with increased OC mass accumulation rates and indicates enhanced TerrOC burial during the EECO. Leaf wax δ 2H values indicate that the EECO was characterised by wetter conditions relative to the earliest Eocene, suggesting that hydroclimate exerts a first-order control on TerrOC export. Our results indicate that TerrOC burial in coastal marine sediments UOB Open could have acted as an important negative feedback mechanism during the early Eocene, but also during other warm climate intervals
Ships Observing Marine Climate: a catalogue of the VOS participating in the VSOP-NA
No full textOur present knowledge of the marine climate, as represented by data sets such as COADS (Woodruff et al., 1987), is based on meteorological observations from the Voluntary Observing Ships (VOS). Because the VOS are merchant ships, rather than specially designed meteorological platforms, errors and biases exist in the data. However there is little information readily available to the climatologist either on the nature of the VOS fleet or on the observing practises which are used. This report, describing the forty-six ships that participated in the Voluntary Observing Ships' Special Observing Project - North Atlantic (VSOP-NA), therefore serves two purposes:(i) it provides a reference document to aid analysis of the VSOP-NA data set,(ii) it gives a detailed description of a subset of the VOS, which will be of value in the interpretation of marine climate data sets.This report is in two parts, Part 1 is an overall summary of the ship characteristics, Part 2 is a ship by ship description. The next section will briefly describe the VSOP-NA project, followed by a summary of the characteristics of the VSOP-NA ships (Section 3). Since these ships were specially selected (Section 2.2), the degree to which they are representative of the whole VOS fleet will be carefully considered. The meteorological instrumentation used by the VOS varies depending on which meteorological agency recruited the ships. That used on the chosen VSOP-NA ships is typical of VOS recruited by the countries bordering the North Atlantic, and will be described in Section 4. Section 5 is a summary of Part 1 of the report.Part 2 presents the VSOP-NA ship catalogue. This includes, for each ship, diagrams of the layout (indicating in particular the exposure of the sensors), a summary of the geographical positions at which observations were obtained, and details of the instrumentation used.<br/
Transient Middle Eocene atmospheric CO2 and temperature variations
No full textThe long-term warmth of the Eocene (~56 to 34 million years ago) is commonly associated with elevated partial pressure of atmospheric carbon dioxide (pCO2). However, a direct relationship between the two has not been established for short-term climate perturbations. We reconstructed changes in both pCO2 and temperature over an episode of transient global warming called the Middle Eocene Climatic Optimum (MECO; ~40 million years ago). Organic molecular paleothermometry indicates a warming of southwest Pacific sea surface temperatures (SSTs) by 3° to 6°C. Reconstructions of pCO2 indicate a concomitant increase by a factor of 2 to 3. The marked consistency between SST and pCO2 trends during the MECO suggests that elevated pCO2 played a major role in global warming during the MECO
Menjawab Serangan Pemikiran Liberalisme Terhadap Islam menerusi Novel Daun Karya Malim Ghozali P.K.
No full textThis study aims to analyse the liberal critiques of Islamic ideology as depicted in the novel Daun by Malim Ghozali P.K. The main issues highlighted in this article are: [i] the historical context of the development of liberalism in Malaysia as presented in both within and outside the creative work; [ii] explaining how liberal groups attempt to shape an interpretation of Islam to fit their respective agendas. This study is conducted based on a literature review that finds the attitudes of postmodernist scholars, who are evidently unsatisfied and continually deconstructing new theories that ohlm.se the true values of Islam. Therefore, this study employs the concept of New Historicism proposed by Stephen Greenblatt (1988) as an analytical framework, utilizing the concepts of constraint and mobility to view texts as constructed from sociocultural references, specifically the social values that permeate the author's identity through the societal crises the author experienced. The findings of the study indicate that Malim Ghozali P.K. strives to provide a new meaning in order to reject the liberalism ideology prevailing in society, while also critising the government's attitude at the time the novel was written, which use religious issues as a political agenda to gain support.
Full text: PDFKajian ini bertujuan untuk menganalisis serangan pemikiran liberalisme terhadap pemikiran Islam tradisional menerusi novel Daun karya Malim Ghozali P.K. Isu utama yang diketengahkan dalam artikel ini ialah: [i] perhubungan konteks sejarah perkembangan liberalisme di Malaysia yang wujud dalam karya dan luar karya; [ii] menjelaskan bagaimana golongan liberalis cuba membentuk acuan Islam yang ditakrifkan sesuai berdasarkan agenda masing-masing. Kajian ini dijalankan berdasarkan sorotan literatur yang mendapati sikap ahli pascamodenis yang ternyata tidak menyenangi dan sentiasa mendekonstruksi teori-teori baharu yang menentang nilai Islam sebenar. Kajian ini memanfaatkan gagasan teori Historisisme Baru yang dikemukakan oleh Stephen Greenblatt (1988) sebagai kerangka analisis dengan memanfaatkan konsep kekangan dan mobiliti yang melihat teks dibina daripada rujukan sosiobudaya, iaitu nilai sosial yang meresap dalam diri pengarang melalui kemelut yang melanda masyarakat dalam situasi pengarang. Dapatan kajian menunjukkan bahawa Malim Ghozali P.K. berusaha memberikan pemaknaan baharu demi menolak gagasan pemikiran liberalisme yang sedang berlaku dalam masyarakat seterusnya mengkritik sikap pemerintah ketika novel ini ditulis yang menjadikan isu agama sebagai agenda politik untuk meraih sokongan.
Text penuh: PD
Rapid expansion of meso-megathermal rain forests into the southern high latitudes at the onset of the Paleocene-Eocene Thermal Maximum
No full textCurrent knowledge of terrestrial ecosystem response to the Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma) is largely based on the midlatitudes of the Northern Hemisphere. To more fully reconstruct global terrestrial ecosystem response to the PETM, we generated vegetation and biomarker proxy records from an outcrop section on the southern coast of Australia (~60°S paleolatitude). We documented a rapid, massive, and sustained vegetation turnover as a response to regional PETM warming of ~1–4 °C, abruptly transitioning from a warm temperate to a meso-megathermal rain forest similar to that of present-day northeastern Queensland, Australia. The onset of this vegetation change preceded the characteristic PETM carbon-isotope excursion (CIE) by several thousand years. The reconstructed ecosystem change is much stronger than in other Southern Hemisphere records, highlighting the need for consideration of regional paleoceanographic, paleogeographic, and biogeographic characteristics to fully understand the global terrestrial ecosystem response to PETM climate forcing
Hydraulic Fracture Containment in Sand
No full textThe mechanism of hydraulic fracturing in soft, high permeability material is considered fundamentally different from that in hard, low permeability rock, where a tensile fracture is created and conventional linear elastic fracture mechanics (LEFM) applies. The fracturing and associated modeling work is then a relatively new area. Particularly, the fracture containment in layered formations remains unknown. This research is aiming to capture the basic physics of the process of hydraulic fracture initiation and propagation in such materials, and further the fracture containment in layered samples. It consists of experimental and simulation studies with application in the petroleum industry. Laboratory tests are performed on lightly cohesive/cohesionless sands. They are pure sand, sand with silt and sand with cement. The mechanical behavior is independently determined in triaxial tests at different confining stress (up to 60 MPa) and porosities. The material is described within the framework of elasto-plasticity. Material parameters are then derived from the simulations, which are performed to match stress-strain behavior of uniform deformation in triaxial tests. In addition, particle breakage and boundary induced deformation localization in large strain tests are also investigated. Hydraulic fracturing injection tests are first performed using different fluids to select an appropriate fracturing fluid. They are viscous Newtonian fluid, bentonite slurry, linear gel, crosslinked gel, and BXLG. Fluid rheology and leak-off have a strong influence on the tendency to fracturing. The test observation shows that BXLG is a fairly efficient fracturing fluid at high stress, so that the following injection tests are performed with BXLG. Injection tests are carried out on sand at different confining stress (up to 20 MPa). Utilizing an X-ray CT scanner provides real-time visualization of the fracture geometry during injection. This technique helps reveal the mechanism of the fracturing. Based of injection tests and associated simulations, the considered fracturing behavior involves leak-off, initiation pressure, propagation behavior, effect of material parameters, and fracture closure. Leak-off in high permeability material is characterized by two-dimensional whole gel leak-off. Both external and internal filtercakes are observed. This results in the fracture tip lagging behind the fluid leak-off front; also, a significant part of the pressure drop occurs across the internal filtercake. Pressurization of the borehole is intrinsically related to the fracture initiation. The onset of shear bands of a pressurized borehole can be considered as the upper bound of fracture initiation. The observed high pressure is then determined by the borehole instability due to shearing. The propagation behavior is related to the leak-off and the associated change in pore pressure. Shear failure occurs at the fracture tip within the internal filtercake. BXLG that builds a relatively efficient filtercake results in smooth closure of the fracture tip. The fracture initiates and propagates at an oblique angle. This is consistent with the mechanism that it propagates in shear. Simulation of fracture propagation shows that besides the confining stress, all the considered constitutive parameters have influence on the predicted pressure and geometry. A higher pressure is required for material of larger Young’s modulus, smaller Poisson ratio, smaller friction angle and larger dilation angle. Furthermore, dilatancy plays a more significant role for fracturing in soft, high permeability material than in hard rock. Fracture recession is an important phenomenon during closure, which can be explained by more intensive leak-off at the tip. Also, CT scans show that the fracture closes mechanically far below the confining stress. The most important deviation from elastic prediction is a larger injection pressure, larger width, and closure pressure much lower than the confining stress. In addition, injection tests on cemented sands of different strength show that the threshold value of soft material and hard rock is about 0.5 MPa in tensile strength, below which the dominant fracturing mechanism appears to be failure in shear. The fracture initiation and propagation across layers are tested on layered samples, which have a difference in permeability or strength. A uniform confining stress is applied over the entire sample so that any containment would be determined by material properties. In tests with a permeability contrast, the sand layer has permeability some 3-4 times larger than the sand+silt layer; and, they both have no tensile strength. The tests show that fractures may be strongly contained by the high permeability layer. The simulation of a two-dimensional layered model qualitatively explains the mechanism. When the tip penetrates the high permeability layer, the pressure must increase to open the fracture. Also, the fluid needs to leak-off to build enough effective stress. This requires a relatively long time. In a three-dimensional situation it may explain the larger propagation rate in the other direction, within the low permeability layer. That means that the fracture will propagate much further into low permeability layers. In another kind of layered test, the two layers are sand with cement and sand with silt. They differ in tensile strength and shear resistance but they have the same permeability. The fracture develops better in the cemented sand. The simulation shows that this can be explained by the combined influence of the constitutive parameters. Comparing qualitatively strength and permeability contrast, most fracture containment is observed in the low permeability layer. Under the conditions of the present study, permeability is more important than strength for containment.GeotechnologyCivil Engineering and Geoscience
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