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1.
The Gargano Promontory of southern Italy, located on the eastern margin of the Apulia Platform, represents a peculiar Tethyan area where the transition between carbonate platform and adjacent basins is exposed on land. The Aptian stratigraphic record, represented in shallow-water, slope and deep-water deposits, provides a good opportunity to investigate the regional response to the worldwide documented climatic, biotic and palaeoceanographic changes related to Oceanic Anoxic Event 1a (OAE1a). A synthesis of data previously published (Ischitella and Coppitella sections), together with original data (Val Carbonara and Coppa della Guardia sections), from four stratigraphic sections from different depositional settings (proximal to distal) is provided, using an integrated, high-resolution micropalaeontological (planktonic foraminifera and calcareous nannofossils) and, for one section, geochemical (stable carbon and oxygen isotopes) approach. Organic matter preservation is confined to the more distal areas and consists of two thin intervals of black shales in the Aptian portion of the Marne a Fucoidi Formation. Biostratigraphic data assign the older black shale (5 cm thick) to the Selli Level equivalent (OAE1a, Lower Aptian); this carbon-rich interval is immediately followed by another black shale (7–10 cm thick) of early Late Aptian age.OAE1a is generally interpreted as a high-productivity event during a warming interval, followed by a cooling trend. In the Gargano Promontory, although the oxygen isotope curve indicates the above-mentioned climatic evolution, the micropalaeontological data do not support high fertility of the surface water, whereas micropalaeontological and geochemical data for the younger black shale do record high productivity (radiolarian increase in overall abundance, low nannofossil and foraminiferal species richness, increase in abudance of nannofossil fertility indices) associated with a cooling trend. The carbon and oxygen isotope record is in line with evidence from the curves documented elsewhere, whereas, among the biotic events, only the “nannoconid crisis” preceding OAE1a is revealed to be globally correlable.Environmental models for the two episodes of organic matter preservation are proposed, taking into account both global and local controlling factors.  相似文献   
2.
Environmental proxies of soil erosion on Iceland, and oceanographic conditions on the adjacent shelf, were measured on a 50 cm box core taken from the southwest Iceland shelf in 1993 during cruise 93030 of the Canadian ship, CSS Hudson. These data, covering the last several centuries, are compared with the documentary record of sea-ice changes around Iceland since A.D. 1600. The site is under the influence of the Irminger Current, which carries warm, saline, Atlantic water northward along the shelf. Because of the relative warmth of this current, sea ice rarely occurs off southwest Iceland, even during the most severe sea-ice intervals of the historical record. In severe sea-ice years, however, the ice drifts clockwise around Iceland from the northeast and east and, in rare cases, reaches the southern coasts (Ogilvie, 1992). The chronology of the core was established by converting the basal radiocarbon date to calendar years and assuming a linear sedimentation rate from the base of the core to the year of collection, 1993. Organic carbon, stable C and O isotope ratios, planktonic foraminiferal assemblages, and sediment magnetic parameters were measured on samples from the core, plotted against calendar years and compared to the Icelandic sea-ice index. The environmental proxies suggest that increased soil erosion, reduced salinity, and, possibly, decreased marine productivity prevailed during the severe sea-ice interval lasting from the last quarter of the eighteenth century to around 1920. Such a situation could develop with climatic cooling, increased storminess, and loss of vegetation cover to stabilise the soil. Although the core site generally lies outside the sea-ice limits, the evidence clearly shows the influence of sea ice and fresh water, and is sensitive to the overall climatic deterioration manifested by the sea-ice record.  相似文献   
3.
Three provinces, characterized by the presence of carbonate mounds interpreted as cold-water coral banks have been reported in Porcupine Seabight, SW of Ireland and were recently subjected to many detailed studies. This contribution discusses the use of X-ray imagery and physical properties in deciphering palaeoceanographic, sedimentological and biological processes. Physical property core logging and X-ray imagery are used to identify and describe sedimentation regimes and so their respective palaeoceanographic and palaeoclimatological settings in two mound provinces, respectively the Belgica mound province and the Magellan mound province. Both provinces show at present time clear differences in the hydrodynamic environment. This study confirms that also during the past the oceanographic and sedimentological environment of both provinces differ clearly. Impacts of glacial–interglacial variations and locally derived ice rafting events (IRE), comparable with the North Atlantic Heinrich events (HE) have been recognized in both provinces. Moreover, the combination of X-ray imagery, magnetic susceptibility, gamma density and P-wave velocity makes it possible to estimate the coral content and coral distribution in unopened cores localized on top of carbonate mounds. A comparison between on-mound and off-mound cores in both provinces allowed revealing some mechanisms of mound evolution and coral growth versus time.  相似文献   
4.
Conventional (one-dimensional) chemostratigraphy of marine carbonates assesses the chemical archive of individual stratigraphic sections and their correlation in space and time. Whereas this approach has shown to be of value when linking isobathymetric domains, usually characterised by similar facies, more caution is needed when correlations are extended across different physiographic settings and hence different facies belts. Here, the spatial geochemical record of Pennsylvanian platform-margin-to-basin transects across a bathymetric range of about 800 m is documented and discussed in a process-oriented context. Particularly, the presence of layered palaeo-water masses and their potential control on slope facies distribution and geochemical properties requires attention. Whereas Carboniferous thermo- and/or chemo-clines most likely affected depth-related slope facies zonation, it was facies change and hence, variances in porosity–permeability properties, that controlled differential early burial diagenetic alteration. Specifically, the lower-slope related breccia facies is characterised by higher volumes of early burial carbonate cements. This implies that these sediments entered the shallow-burial domain with a considerable open pore space and gave way to an increased rock:fluid ratio. Whereas the δ13C record is invariant with bathymetry, the more diagenesis-sensitive δ18O proxy, records pronounced shifts observed across major facies boundaries. From this it is concluded that although the primary controlling factor of slope facies distribution with depth is probably palaeoceanographic in nature, it is differential rock:fluid ratios that control the first-order, spatial shifts in δ18O composition. These findings show that one-dimensional chemostratigraphy will severely underestimate the complexity of three-dimensional (bathymetric) data sets across platform margins. This is of relevance for the interpretation of the geochemical archive of fossil carbonate platforms in general.  相似文献   
5.
The Cenomanian/Turonian Boundary Event (CTBE) at Wunstorf, north-west Germany, has been analysed palynologically by high resolution sampling to reconstruct changes in relative sea-level and water mass character within photic zone waters. Based on changes in the ratio of terrigenous sporomorphs to marine palynomorphs (t/m index), the distribution of the organic-walled algal taxa as well as of selected dinocyst taxa and groups the section can largely be subdivided into pre-“plenus-bed” and post-“plenus-bed” intervals, reflecting different stages of third-order relative sea-level cycles and/or changes in water mass influence in the photic zone. Accordingly, the pre-“plenus-bed” interval is placed in a transgressive systems tract starting at the “facies change” event (C. guerangeri/M. geslinianum ammonite Zone boundary) with the maximum flooding surface at the top of the “Chondrites II” bed (top of R. cushmani Biozone). A highstand systems tract is suggested from the base of the “plenus-bed” up the base of the “fish-shale” event. Within the “fish-shale” event interval, a transgressive systems tract is suggested to start at the base of the thin, grey-green marly interbed. The Cenomanian/Turonian boundary proper, as defined by the first occurrence of Mytiloides spp., as well as the lowermost Turonian are located within the initial phase of a transgressive systems tract. With respect to water mass characteristics within photic-zone waters, the pre-“plenus-bed” interval is predominantly characterized by warm water masses that changed gradually towards the deposition of the “Chondrites II” bed, where a strong influence of cool and/or salinity-reduced waters is indicated by various palynological proxies. Within the post-“plenus-bed” interval a mixture and/or alternation of warmer and cooler waters is indicated, with the warmer water influence increasing gradually towards and within the Lower Turonian stage. The increased proportions of prasinophytes within the “Chondrites II” bed and parts of the “fish-shale” interval may indicate availability of reduced nitrogen chemospecies, especially ammonium, within photic-zone waters as a function of a vertical expansion of the oceanic O2-minimum zone.  相似文献   
6.
 Despite the increasing interest in the South Atlantic Ocean as a key area of the heat exchange between the southern and the northern hemisphere, information about its palaeoceanographic conditions during transitions from glacial to interglacial stages, the so-called Terminations, are not well understood. Herein we attempt to increase this information by studying the calcareous dinoflagellate cysts and the shells of Thoracosphaera heimii (calcareous cysts) of five Late Quaternary South Atlantic Ocean cores. Extremely high accumulation rates of calcareous cysts at the Terminations might be due to a combined effect of increased cyst production and better preservation as result of calm, oligotrophic conditions in the upper water layers. Low relative abundance of Sphaerodinella albatrosiana compared with Sphaerodinella tuberosa in the Cape Basin may be the result of the relatively colder environmental conditions in this region compared with the equatorial Atlantic Ocean with high relative abundance of S. albatrosiana. Furthermore, the predominance of S. tuberosa during glacials and interglacials at the observed site of the western Atlantic Ocean reflects decreased salinity in the upper water layer. Received: 9 November 1998 / Accepted: 26 October 1999  相似文献   
7.
The Bonarelli Level (BL) from the upper Cenomanian portion of the reference Bottaccione section (central Italy) is characterized by the presence of black shales containing high TOC concentrations (up to 17%) and amounts of CaCO3 near to zero. In the absence of carbonate and, consequently, of relative carbon- and oxygen-isotopic data, the elemental geochemistry revealed to be a very useful tool to obtain information about the palaeoclimatic and palaeoceanographic evolution of the Tethys Ocean during the OAE2. Based on several geochemical proxies (Rb, V, Ni, Cr, Si, Ba), the BL is interpreted as a high-productivity event driven by increasingly warm and humid climatic conditions promoting an accelerated hydrological cycle. The enrichment factors of peculiar trace metals (Zn, Cd, Pb, Sb, Mo, U) provide further insight about the H2S activity at the seafloor during the organic-rich sediment deposition and permitted us to evaluate the use of Ba as palaeoproductivity tracer in conditions of high rate of sulphate reduction.By comparing geochemical records from the reference Bottaccione section (central Italy) with those previously obtained for the coeval Calabianca section (northwestern Sicily), different degrees of oceanic anoxia were delineated and ascribed to different abundance and type (degradable or refractory) of organic matter, which are limiting factors in the bacterial sulphate reduction reactions and in subsequent euxinic conditions at seafloor in the Tethys realm. Based on a ciclostratigraphic approach, consistent fluctuations at 100 ky scale in the chemostratigraphic signals from the two sections are inferred to be expression of a strong orbital-climatic forcing driving changes in the oceanic environment during the BL deposition.  相似文献   
8.
High‐resolution gravity cores and box cores from the North Icelandic shelf have been studied for palaeoceanographic history based on lithological and biostratigraphical foraminiferal data. Results from two outer shelf cores covering the last 13.6 k 14C yr BP are presented in this paper. The sediments accumulated in north–south trending basins on each side of the Kolbeinsey Ridge at water depths of ca. 400 m. Sedimentation rates up to 1.5 m kyr−1 are observed during the Late‐glacial and Holocene. The Vedde and Saksunarvatn tephras are present in the cores as well as the Hekla 1104. A new tephra, KOL‐GS‐2, has been identified and dated to 13.4 k 14C yr BP, and another tephra, geochemically identical to the Borrobol Tephra, has been found at the same level. At present, the oceanographic Polar Front is located on the North Icelandic shelf, which experiences sharp oceanographic surface boundaries between the cold East Icelandic Current and the warmer Irminger Current. Past changes in sedimentological and biological processes in the study area are assumed to be related to fluctuations of the Polar Front. The area was deglaciated before ca. 14 kyr BP, but there is evidence of ice rafting up to the end of the GS‐1 (Greenland Stadial 1, Younger Dryas) period, increasing again towards the end of the Holocene. Foraminiferal studies show a relatively strong GS‐2 (pre‐13 kyr BP) palaeo‐Irminger Current, followed by severe cooling and then by unstable conditions during the remainder of the GI‐1 (Greenland Interstadial 1, Bølling–Allerød) and GS‐1 (Younger Dryas). Another cooling event occurred during the Preboreal before the Holocene current system was established at about 9 kyr BP. After a climatic optimum between 9 and 6 kyr BP the climate began to deteriorate and fluctuate. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   
9.
Pollen and organic-walled dinoflagellate cyst assemblages from core GeoB 9503-5 retrieved from the mud-belt ( 50 m water depth) off the Senegal River mouth have been analyzed to reconstruct short-term palaeoceanographic and palaeoenvironmental changes in subtropical NW Africa during the time interval from ca. 4200 to 1200 cal yr BP. Our study emphasizes significant coeval changes in continental and oceanic environments in and off Senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca. 2900 and 2500 cal yr BP. After ca. 2500 cal yr BP, the environment slowly became drier again as indicated by slight increases in Sahelian savannah and desert elements in the pollen record. Around ca. 2200 cal yr BP, this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen, dinoflagellate cysts, fresh-water algae and plant cuticles, suggesting “episodic flash flood” events of the Senegal River. The driest phase developed after about 2100 cal yr BP.  相似文献   
10.
Contourite deposits in the central sector of the middle slope of the Gulf of Cadiz have been studied using a comprehensive acoustic, seismic and core database. Buried, mounded, elongated and separated drifts developed under the influence of the lower core of the Mediterranean Outflow Water are preserved in the sedimentary record. These are characterised by depositional features in an area where strong tectonic and erosive processes are now dominant. The general stacking pattern of the depositional system is mainly influenced by climatic changes through the Quaternary, whereas changes in the depositional style observed in two, buried, mounded drifts, the Guadalquivir and Huelva Drifts, are evidence of a tectonic control. In the western Guadalquivir Drift, the onset of the sheeted drift construction (aggrading QII unit) above a mounded drift (prograding QI unit) resulted from a new Lower Mediterranean Core Water hydrodynamic regime. This change is correlated with a tectonic event coeval with the Mid Pleistocene Revolution (MPR) discontinuity that produced new irregularities of the seafloor during the Mid- to Late-Pleistocene. Changes in the Huelva Drift from a mounded to a sheeted drift geometry during the Late-Pleistocene, and from a prograding drift (QI and most part of QII) to an aggrading one (upper seismic unit of QII), highlight a new change in oceanographic conditions. This depositional and then oceanographic change is associated with a tectonic event, coeval with the Marine Isotope Stage (MIS) 6 discontinuity, in which a redistribution of the diapiric ridges led to the development of new local gateways, three principal branches of the Mediterranean Lower Core Water, and associated contourite channels. As a result, these buried contourite drifts hold a key palaeoceanographic record of the evolution of Mediterranean Lower Core Water, influenced by both neotectonic activity and climatic changes during the Quaternary. This study is an example of how contourite deposits and erosive elements in the marine environment can provide evidence for the reconstruction of palaeoceanographic and recent tectonic changes.  相似文献   
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