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  • 1
    Publication Date: 2001-01-01
    Description: A series of widespread Maastrichtian and Palaeogene reflectors in the western North Atlantic have been interpreted to record episodes of vigorous bottom-water circulation produced by periodic flooding of the deep North Atlantic basins with southern source waters. In general, the ages of these reflectors have been poorly known with estimated ages spanning several million years. New seismic and core data from Ocean Drilling Program Leg 171B tightly constrain the ages of several of the most prominent reflectors and demonstrate that several of them are associated with geologically short-lived events associated with major palaeoclimatic, palaeoceanographic and evolutionary transitions. On Blake Nose in the western North Atlantic, Reflector Ac formed shortly after the close of the early Eocene warm period between 48 and 49 Ma. The reflector corresponds to an abrupt inception of vigorous deep-water circulation that winnowed foraminiferal sands at 2000-2500 m water depth and caused mass wasting into the deep basins of the Bermuda Rise. Reflector Ac is correlative with a sequence of unconformities present in nearly every part of the global ocean from the shallow shelf to the deep sea, suggesting that this time interval is associated with a global change in ocean circulation, including a major sea-level lowstand. The reflector and unconformities are roughly equivalent in age to glacial tillites on the Antarctic Peninsula, suggesting a link to an early phase of southern hemisphere glaciation. Another widespread reflector, Ab, has a late Palaeocene to earliest Eocene age on the Bermuda Rise. On Blake Nose, the equivalents of Reflector Ab consist of a stack of three closely spaced hiatuses ranging from early late Palaeocene (58.5-60.5 Ma) to latest Palaeocene (c. 55.5 Ma) age. The youngest of these hiatuses is associated with the carbon isotope excursion at the Late Palaeocene Thermal Maximum (LPTM), when there was a major reorganization of deep-water circulation and dramatic, transient warming of high latitudes. Bottom currents appear to have prevented the widespread deposition of sediments at water depths shallower than c. 2200 m from the LPTM until early mid-Eocene time. Erosion on Blake Nose was produced by a strengthened, southward-flowing deep western boundary current at the same time that a southern source watermass produced extensive erosion on the Bermuda Rise. We suggest that the increased flow of the deep western boundary current reflects a stronger outflow of warm intermediate waters shallower than 2000 m from Tethys. The combination of warmer intermediate waters and erosion along the margin may have helped to trigger slope failure of gas hydrate reservoirs around the North Atlantic margin and set the LPTM-greenhouse feedback system in motion. Reflector A* is correlative with highly deformed Maastrichtian sediments on Blake Nose and Maastrichtian chalk interbedded with red claystone on Bermuda Rise. Seismic and coring evidence from Blake Nose shows that the K-P boundary slumping was associated with the magnitude c. 11-13 Richter Scale earthquake generated by the Chicxulub impact event. The chalk sequence on the Bermuda Rise appears to represent the distal turbidites produced by slumping of the margin. Correlation of the chalk beds with Reflector A* shows that the mass wasting deposits are found over nearly the entire western North Atlantic basin. Apparently, much of the eastern seaboard of North American must have catastrophically failed during the K-P impact event, creating one of the largest submarine landslides on the face of the Earth.
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  • 2
    Publication Date: 2001-01-01
    Description: Palaeogene and Cretaceous palaeoceanography has been the focus of intense international interest in the last few years, spurred by deep ocean drilling at Blake Nose in the North Atlantic as well as the need to use past climate change as input for modelling future climate change. This book brings together a number of review papers that describe ancient oceans and unique events in the Earth's climatic history and evolution of biota. The papers show evidence of periods characterized by exceptional global warmth such as the Late Palaeocene Thermal Maximum and Cretaceous anoxic events. Geochemical records and modelling will make the reader aware that these periods were forced by greenhouse gases. This information is essential for understanding the response of the ocean--climate system to the current input of fossil fuels. In this sense, the book contributes to the understanding of fundamental aspects of Earth's climate, the carbon cycle, and marine ecosystems. A number of papers describe massive mass wasting deposits resulting from the energy released by the bolide impact at the Cretaceous--Tertiary boundary as well as the geochemistry of the boundary itself. Additional papers cover aspects of cyclostratigraphy and biostratigraphy of Palaeogene and Cretaceous records. This book will be of interest to a broad audience of Earth Scientists interested in Palaeogene--Cretaceous palaeoceanography, extreme climate modelling, Cretaceous--Tertiary boundary, Late Palaeocene Thermal Maximum, Cretaceous anoxic events, as well as those specifically interested in radiolarian, dinoflagellate and coccolithophorid stratigraphy. ... This 250-word extract was created in the absence of an abstract.
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  • 3
    Publication Date: 2001-01-01
    Description: The Cretaceous-Tertiary (K-T) boundary at Blake Nose, in the NW Atlantic, is recorded by a coarse, poorly graded and poorly cemented layer mostly consisting of green spherules that are mainly composed of smectite. Geochemical patterns across the boundary are governed by the source material of the spherule bed and postdepositional processes. The chemical composition and the nature of this bed indicate that it derived from melted target rocks from the Chicxulub impact structure. Ir and other typical extraterrestrial elements do not present significant enrichments, which suggests that the spherule bed material derived from crustal rocks. Ir instead reaches its highest concentration in the burrow-mottled calcareous ooze above the spherule bed, suggesting that it is associated to the finest fraction deposited after the target-rock-derived material. Only the Ni and Co content show slight enrichments within the upper part of the spherule layer, although most of the trace element profiles resulted from diagenetic alteration. During the alteration of glass to smectite, the concentrations of certain trace elements, such as the rare earth elements, were severely changed. In addition, oxygen-poor conditions also led to the remobilization of redox-sensitive elements, which show enhanced concentration at the top or above the spherule bed. Diagenetic remobilization may have also affected the Ir concentration.
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  • 4
    Publication Date: 2001-01-01
    Description: The Cretaceous-Tertiary (K-T) boundary interval recovered by the ODP Leg 171 at Site 1049 (Blake Nose, NW Atlantic) contains a thick (9-17 cm) spherule bed marking the boundary. The spherules are mainly perfect spheres with a lesser proportion of oval spherules. They usually range from 100 to 1000 {micro}m. This bed represent the diagenetically altered impact ejecta from Chicxulub and further supports this structure as the site of the K-T impact. Mineralogical and geochemical investigations indicate that impact-generated glass was altered to smectite. Transmission electron microscopy observations revealed in some spherules that smectite is forming from a Si-rich or Ca-rich material, which could suggest a precursor similar to Haitian glasses. The variable thickness and the presence of some Cretaceous planktonic foraminifera and clasts of Cretaceous chalk suggest reworking of the ejecta material. However, the spherule bed confirms that a large volume of the Chicxulub ejecta material reached the Blake Nose Plateau.
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  • 5
    Publication Date: 2001-01-01
    Description: Widespread biological, geochemical and sedimentological shifts within the Maastrichtian are well documented, but data are limited for the low-latitude Atlantic. New observations from Ocean Drilling Program (ODP) sites located on Blake Nose in the subtropical western North Atlantic increase information concerning the Maastrichtian history of this critical region. Planktonic {delta}18O results suggest up to 6 {degrees}C of local surface water warming (or 4{per thousand} decrease in salinity) at the same time as most of the globe was cooling. Benthic {delta}13O and {delta}13C values of both planktonic and benthic taxa show little if any directional trend or excursions on long time scales; however, planktonic and benthic taxa exhibit strong {delta}13C and {delta}18O cycles (up to 0.8 and 0.6{per thousand}, respectively) across a short interval of high-resolution sampling. Other portions of the cores have not yet been studied at high resolution. The last occurrence of inoceramid shell fragments on Blake Nose matches previously documented global patterns, i.e. a mid-Maastrichtian extinction event that occurred later in low latitudes than in high southern latitudes. Models for Maastrichtian change seem to be converging on variation in intermediate to deep water ocean circulation as a unifying process. Blake Nose data are consistent with this conclusion, but demonstrate new regional patterns and emphasize the importance of precise and accurate chronostratigraphic correlation in understanding Maastrichtian change.
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  • 6
    Publication Date: 2001-01-01
    Description: The well-calibrated mid- to late Eocene sediment record of ODP Leg 171B (Site 1053A, Blake Nose) allows a detailed stratigraphic and palaeoenvironmental analysis of the dinoflagellate cyst (dinocyst) content. The recovered assemblages are a mixture of inner neritic, outer neritic and oceanic species. The autochthonous dinoflagellates, principally those of the Impagidinium group, indicate an oceanic milieu, with possibly some shallowing of water depth towards the top of the section. This trend is also indicated by a corresponding increase of inner neritic dinocysts. The close agreement in the abundance peaks of inner neritic dinocysts and terrestrial palynomorphs indicats that both are allochthonous. This is confirmed by the much higher number of neritic species found in JOIDES Holes 1 and 2, on the continental shelf of eastern Florida, immediately to the west of the Blake Nose. Lower-latitude species found in Hole 1053A, but not occurring at higher latitudes during late Eocene time, are Diphyes colligerum and Thalassiphora delicata. The presence of these, and other lower-latitude species, confirms that warmer-water conditions persisted during mid- to early late Eocene time in the vicinity of Site 1053. Eighteen new taxa are described, two of them formally: Charlesdowniea proserpina sp. nov. and Oligosphaeridium anapetum sp. nov.
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  • 7
    Publication Date: 2001-01-01
    Description: Understanding the nature and causes of the variability associated with past warm, high pCO2 climates presents a significant challenge to palaeoclimate research. In this paper we investigate the early Eocene climatic response in the North Atlantic region to forcing from an indirect effect of atmospheric methane (via polar stratospheric clouds (PSCs)), and we investigate the response of the climate system to forcing from a combination of orbital insolation changes and high atmospheric pCO2 concentration. We find that sea surface temperatures (SSTs), sea ice extent, net surface moisture, continental runoff and upwelling in the North Atlantic Ocean are all sensitive to those forcing factors, and that the degree of sensitivity is a function of location and season. Our results suggest that high-latitude SST values can vary by as much as 20 {degrees}C during the winter season in response to precessional and polar cloud forcing, whereas in contrast summer temperature varies by 4 {degrees}C or less. Model predictions of net surface moisture balance also vary substantially with our prescribed forcing. There is a large difference in variability between the localized net surface moisture results and the mean North Atlantic Ocean results, which suggests that large-scale assumptions about past surface ocean salinities and seawater {delta}18O may need to be reassessed. According to model results, the influx of terrigenous material via continental runoff to the North Atlantic Ocean should be highly seasonal, with greatest runoff occurring in spring. Our model results also indicate that changes in wind-driven upwelling and in continental runoff on a precessional time scale should be seen in regions of the central North Atlantic.
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  • 8
    Publication Date: 2001-01-01
    Description: Ocean Drilling Program (ODP) Leg 171B recovered continuous sequences that yield evidence for a suite of critical' events in the Earth's history. The main events include the late Eocene radiolarian extinction, the late Palaeocene benthic foraminiferal extinction associated with the Late Palaeocene Thermal Maximum (LPTM), the Cretaceous-Palaeogene (K-P) extinction, the mid-Maastrichtian event, and several episodes of sapropel deposition documenting the late Cenomanian, late Albian and early Albian warm periods. A compilation of stable isotope results for foraminifera from Leg 171B sites and previously published records shows a series of large-scale cycles in temperature and {delta}13C trends from Albian to late Eocene time. Evolution of {delta}18O gradients between planktic and benthic foraminifera suggests that the North Atlantic evolved from a circulation system similar to the modern Mediterranean during early Albian time to a more open ocean circulation by late Albian-early Cenomanian time. Sea surface temperatures peaked during the mid-Cretaceous climatic optimum from the Albian-Cenomanian boundary to Coniacian time and then show a tendency to fall off toward the cool climates of the mid-Maastrichtian. The Albian-Coniacian period is characterized by light benthic oxygen isotope values showing generally warm deep waters. Lightest benthic oxygen isotopes occurred around the Cenomanian-Turonian boundary, and suggest middle bathyal waters with temperatures up to 20 {degrees}C in the North Atlantic. The disappearance of widespread sapropel deposition in Turonian time suggests that sills separating the North Atlantic from the rest of the global ocean were finally breached to sufficient depth to permit ventilation by deep waters flowing in from elsewhere. The Maastrichtian and Palaeogene records show two intervals of large-scale carbon burial and exhumation in the late Maastrichtian-Danian and late Palaeocene-early Eocene. Carbon burial peaked in early Danian time, perhaps in response to the withdrawal of large epicontinental seas from Europe and North America. Much of the succeeding Danian period was spent unroofing previously deposited carbon and repairing the damage to carbon export systems in the deep ocean caused by the K-P mass extinction. The youngest episode of carbon exhumation coincided with the onset of the early Eocene Warm Period and the LPTM, and has been attributed to the tectonic closure of the eastern Tethys and initiation of the Himalayan Orogeny.
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  • 9
    Publication Date: 2001-01-01
    Description: Outstandingly well-preserved benthic foraminiferal successions from upper Aptian-lower Albian sediments at Site 1049 (Leg 171, Blake Nose escarpment, western North Atlantic) provide a detailed record of the faunal turnover across Oceanic Anoxic Event 1b (OAE 1b). Changes in abundance, diversity and species composition reflect strong fluctuations in carbon flux and bottom-water oxygenation. Before the onset of black shale sedimentation, the originally diverse assemblages are replaced by low-diversity associations, dominated by species inferred to be opportunistic phytodetritus feeders and thriving on an enhanced carbon flux to the sea floor. The 46 cm thick laminated black shale horizon corresponding to OAE 1b is virtually devoid of benthic formainifers or contains highly impoverished assemblages, suggesting that intense eutrophication and/or strong stratification triggered near anoxia at the sea floor during black shale deposition. Above the black shale, reoccurrence of the pre-black shale fauna points to relatively rapid bottom-water reoxygenation. The benthic formainiferal record of Leg 171 provides clear evidence that no major extinctions occurred across OAE 1b, as most of the species occurring below the black shale reappear above it. In contrast to other Cretaceous anoxic events, OAE 1b may have been more limited in duration or in geographical and water-depth extent, allowing recolonization from adjacent, more hospitable areas, once local conditions improved at the sea floor. Prolific radiation within the suborder Rotaliina and diversification of Textulariina with calcareous cement appear to have started in the Aptian time before OAE 1b, and continued into early Albian time to give rise to many of the modern lineages.
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  • 10
    Publication Date: 2001-01-01
    Description: Clay mineral analyses were performed on Eocene sediments from drill sites in the western Central Atlantic. The investigated sites cover the full range of early Palaeogene deep waters above and below the calcite compensation depth (CCD), but otherwise represent different depositional and hydrographic regimes. Palygorskite clays with authigenic microstructures were discovered in Lower Eocene hemipelagic sediments from the distal end of the Blake Nose depth transect and in pelagic clays of the same age from the distal Nares Abyssal Plain, where terrigenous input was reduced. Palygorskite clays were not detected in coeval sediments from a distal near-CCD setting on Bermuda Rise that received major terrigenous input. The distribution of palygorskite clays at these sites, the microstructures of the constituent minerals, their absence from contemporaneous deposits on the American margin, and the position of the northerly sites outside the range of a potential African aeolian supply strongly suggest an authigenic origin of these clays at the early Eocene sea floor. Palygorskite clays are widely distributed in lower Eocene sediments from about 50{degrees} N to 50{degrees} S palaeolatitude. The most widespread distributions and peak abundances in Atlantic oceanic sediments are reported from shelf to deep-water sites of the palaeo-tropical and -subtropical belt and correlate with the Early Eocene period of extreme warmth. Marine authigenic palygorskite clay may provide an indication of the localities and the time periods that were characterized by high bottom-water temperatures, by elevated alkalinity, silica and magnesium concentrations, and by reduced sediment accumulation rates.
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