The closing of a seaway: ocean water masses and global climate change     

Caroline H. Lear  Yair Rosenthal  James D. Wright 《Earth and Planetary Science Letters》2003,210(3-4):425-436

The Late Neogene witnessed various major paleoceanographic changes that culminated in intense Northern Hemisphere Glaciation (NHG). The cause and effects of these changes are still debated. We use a multiproxy approach to determine the relative timing of the closure of the Panama gateway, changes in Atlantic circulation, global cooling and ice sheet growth. Benthic foraminiferal Mg/Ca records from a Pacific and an Atlantic Site have been produced and are interpreted in terms of bottom water temperatures. These Mg-temperature records are combined with published benthic δ¹³C, δ¹⁸O and erosion records to reconstruct the flow of proto-North Atlantic Deep Water (proto-NADW) over the past 12 Ma. The results suggest that between 12.5 and 10.5 Ma, and again between about 8.5 and 6 Ma, a nutrient-depleted water mass that was colder (by 1–2°C) and fresher than the intervening deep water mass filled the Atlantic basin. This proto-NADW became warmer (by 1°C) and saltier between 6 and 5 Ma, coincident with the restriction of surface water flow through the Central American Seaway. The Mg-temperature records define a subsequent global cooling trend of 3.5°C between 5 Ma and today. Early NHG in the late Miocene was perhaps related to the formation of the relatively cold, fresh proto-NADW. The formation of the warmer and saltier proto-NADW in the early Pliocene may have initially limited Northern Hemisphere ice growth. However, the increased moisture released at high northern latitudes associated with formation of ‘warm’ proto-NADW, coupled with the global temperature decrease of deep (and hence polar surface) waters, likely helped initiate the intense NHG of the Plio–Pleistocene.  相似文献   

Présence de Paléocène marin dans les Grands Causses (France)     

Bernard Peybernès  Richard Ciszak  Marie-José Fondecave-Wallez  Pierre-Jean Combes  Hubert Camus  Michel Séranne 《Comptes Rendus Geoscience》2003,335(8):681-689

In the Grands Causses, incised valleys, lapies, fissures and sinkholes inherited from successive polyphase karstifications were filled by Palaeocene marine sediments overall assigned to the P1c–P3 interval (Upper Danian–Lower Selandian). These sediments are distributed into three detritic facies, generated by extensional tectonics controlling karstic and erosional processes. Upper Cretaceous marine fossils known within these facies are interpreted as reworked from hypothetically pellicular deposits. The probable palaeogeographic connection with the Pyrenean Palaeocene ‘Breccia trough’ supposes the presence of a SE–NW ‘ria’ running across the continental areas of Lower Languedoc and draining towards the northwest the marine waters of the Palaeocene transgression as far as the Rodez region. To cite this article: B. Peybernès et al., C. R. Geoscience 335 (2003).  相似文献   

Planktic foraminiferal porosity: a water mass proxy for latest Cenomanian paleoceanography, Greenhorn Sea, Western Interior USA and Canada     

C. Fisher   《Cretaceous Research》2003,24(6):633-651

Planktic foraminiferal porosity analyses can be used as a water mass proxy and were conducted on samples from above the latest Cenomanian Neocardioceras or B bentonite from the Western Interior Seaway of North America. This time slice provides a snapshot of water mass characteristics in this vast epicontinental sea during the early phase of Oceanic Anoxic Event 2. Mean sample porosity decreases northward and is interpreted as northward decrease in water temperature at the depth Hedbergella delrioensis (Carsey) calcified. Four water masses are defined by porosity, their boundaries are extremely similar to water mass boundaries previously identified by others using the distributions of macrofossils, microfossils and lithology. The boundary between the Subtropical–Tropical Water Mass and Central Subtropical Water Mass was located in southern Colorado. The boundary between the Central Subtropical Water Mass and the Northern Temperate Water Mass lay at approximately 48°N latitude. This boundary is displaced northward approximately 8° latitude as compared to the Holocene planktic foraminiferal temperate ocean province. Within-sample porosity variation suggests all water masses except the Temperate Water Mass were thermally stratified. Samples from the south indicate that the Subtropical–Tropical Water Mass was the most stratified. The porosity data support a previously published data-based paleoceanographic circulation of the southwestern seaway.  相似文献   

The sedimentary record of recent (last 500 years) environmental changes in the Seixal Bay marsh,Tagus estuary,Portugal     

Freitas  M.C.  Andrade  C.  Moreno  J.C.  Munhá  J.M.  Cachão  M. 《Geologie en Mijnbouw》1998,77(3-4):283-293

The inner Tagus estuary is essentially a sedimentation basin that receives cohesive sediment from terrestrial, marine, biological and anthropogenic sources. Three short cores from one site in a marsh area of this estuary (Seixal Bay) were analysed for sedimentary, geochemical and micropalaeontological contents (benthic foraminifera and nannoplankton). The length of the cores represents about half a millennium of sedimentation. Textural analysis suggests a highly uniform mud sedimentation for most of the cores but geochemical, mineralogical and micropaleontological results indicate climatic and environmental changes and anthropogenic disturbance. Three Foraminifera zones were identified. The lower part of the lower zone indicates sedimentation in an open channel or a lower domain of an exposed high-energy sandflat. Sediments of the upper part of the lower zone and of the middle zone were deposited in a lower-energy environment, probably associated with a sheltered, vertically aggrading mudflat located within the Seixal Bay. Biological and mineralogical indicators suggest that periods of total or partial closure of this bay occurred. Clay minerals indicate that drier and colder conditions prevailed in the lower half of this zone evolving gradually to a wetter and warmer environment towards the top. The upper zone indicates persistence of low-energy sedimentation and evolution towards the present salt-marsh conditions. Anthropogenic pollution is clear in geochemical proxies at the top of the sedimentary column and was used for dating purposes.  相似文献   

Middle Miocene to Pleistocene magneto-biostratigraphy of ODP Sites 1150 and 1151, northwest Pacific: Sedimentation rate and updated regional geological timescale     

Isao Motoyama  Nobuaki Niitsuma  Toshiaki Maruyama  Hiroki Hayashi  Shin-ichi Kamikuri  Masamichi Shiono  Toshiya Kanamatsu  Kaori Aoki  Chikako Morishita  Kyoko Hagino  Hiroshi Nishi  Motoyoshi Oda 《Island Arc》2004,13(1):289-305

Abstract Shipboard and shore‐based investigation on siliceous and calcareous microfossil biostratigraphy, magneto‐stratigraphy and tephrostratigraphy identified numerous datum events from the sedimentary sequences of Sites 1150 and 1151 drilled on the forearc basin of northern Japan by the Ocean Drilling Program Leg 186. Some 83 datum events were selected to construct new age–depth models for the sites. Based on the reliable magneto‐stratigraphy from the Pleistocene to the Upper Miocene, which were correlated to the standard geomagnetic polarity timescale, and on excellent records of diatom and radiolarian biostratigraphy throughout the sequences, the shipboard age model was revised. Major revisions referred to stratigraphic position of the Miocene–Pliocene boundary that has been shifted more than 200 m downward in each sequence. The age–depth relations of the forearc sites represent drastic changes in the sedimentation rate—extremely high (40 cm/k.y. on average) in the Early Pliocene and low (less than 2 cm/k.y. on average) in the Middle Miocene—and several hiatuses exist throughout the sequence. The drastic changes can be related mostly to changes in diatom sedimentation and the tectonics of the Japanese Island Arc. Local ages for some foraminiferal, calcareous nannofossil and radiolarian bioevents are estimated from the age–depth models at each site. These newly calibrated bioevents and biozones as well as established diatom biostratigraphy are incorporated into the updated magneto‐biochronologic timescale, which will contribute to an improvement in biochronologic accuracy of Neogene sediments in northern Japan and adjacent areas.  相似文献   

Planktic foraminiferal and 13C isotopic changes across the Paleocene/Eocene boundary at Possagno (Italy)     

I. Arenillas  E. Molina  B. Schmitz 《International Journal of Earth Sciences》1999,88(2):352-364

 The interval spanning the Paleocene–Eocene (P/E) transition in the Possagno section consists of 1 m of red marls, including a 4-cm-thick, dark-red "dissolution" clay, which represents the Paleocene/Eocene boundary event. The Possagno section is much more condensed than other Tethyan and North Atlantic sections previously studied; however, in this section the most significant biotic, isotopic and sedimentological events across the P/E boundary can be recognized. The Possagno section spans the following planktic foraminiferal subzones: upper part of M. gracilis Subzone, A. berggreni Subzone, A. sibaiyaensis Subzone and probably lowermost part of P. wilcoxensis Subzone. The quantitative analysis indicates a major increase of low-latitude acarininids, including compressed tropical acarininids just above the boundary clay. This acarininid incursion begins just below the boundary clay but reaches its maximum just above the clay. The planktic foraminiferal faunal turnover is gradual except for the acarininid incursion. The isotopic results show a negative excursion in ∂¹³C values at the small benthic foraminifera mass extinction event. The acarininid maximum diversity coincides with this isotopic excursion, and reflects an increase in surface seawater temperature. Despite being very condensed, the Possagno section allows us to further confirm that the different biotic, isotopic and sedimentological events recognized in the Spanish sections (Alamedilla, Campo, Caravaca, Zumaya) are not local in nature and allows the establishment of a detailed chronostratigraphic framework to define the P/E boundary stratotype. Received: 8 April 1998 / Accepted: 12 April 1999  相似文献   

Benthic Foraminifera in Siliceous Black Ore from the Ezuri Kuroko Deposit, Hokuroku District, Japan     

Kosei KOMURO  Naoaki AOKI  Yoshimichi KAJIWARA 《Resource Geology》2002,52(2):111-121

Abstract: Abundant benthic foraminifera have been identified in thin sections of the siliceous black ore in the Ezuri Kuroko deposit, Hokuroku, Japan. By treating samples with conventional hydrofluoric acid digestion techniques, sponge spicules and radiolaria have also been recognized in the residue. Under microscopic observations, 94 individual foraminiferal specimens have been detected. However, as it is difficult to identify species or genera by means of microscope observations alone, only a small number of genera have been identified based on morphology. The foraminiferal assemblage is composed predominantly of agglutinated species (83%) with subordinate calcareous species (17%), and is assigned to the Cyclammina Assemblage based on the preponderance of Cyclammina (57%). The foraminifera are generally well preserved within micro‐crystalline to cryptocrystalline quartz, and exhibit no obvious features related to compaction or secondary deformation. Textural observations suggest that the siliceous component of this rock was not derived from an allochthonous block but instead constitutes autochthonous proto‐Kuroko sediment. The Cyclammina Assemblage in the ore is different from recently described foraminiferal assemblages in the vicinity of present deep‐sea hydrothermal vents, but is identical to those found in black shales of the Onnagawa to Funakawa stages in the Green Tuff region. The proto‐Kuroko sediment is assumed to have been deposited in an oxygen‐deficient environment within a closed, deep‐seated basin. The existence of siliceous microfossils suggests that the silica in the siliceous ore did not originate from silica sinter deposits produced by submarine hydrothermal activities, but from a biogenic siliceous ooze, probably composed of diatoms. Sulfide mineralization in the interstices of some of the microfossils is inferred to relate primarily to bacterial sulfate reduction associated with the decomposition of organic matter. The later sulfide mineralization associated with larger crystals (which contain fluid inclusions with homogenization temperatures of approximately 250C) cuts across the siliceous masses and foraminiferal septa, and may have been formed after consolidation of the siliceous ooze, accompanying the formation of acidic intrusive rocks during the late Onnagawa stage.  相似文献   

Pleistocene oceanographie changes indicated by deep sea benthic foraminifera in the northern Indian Ocean     

Ajai K. Rai  M. S. Srinivasan 《Journal of Earth System Science》1994,103(4):499-517

An attempt has been made to understand the Pleistocene bottom water history in response to the paleoclimatic changes in the northern Indian Ocean employing quantitative analyses of deep sea benthic foraminifera at the DSDP sites 219 and 238. Among the 150 benthic foraminifera recorded a few species show dominance with changing percent frequencies during most of the sequence. The dominant benthic foraminiferal assemblages suggest that most of the Pleistocene bottom waters at site 219 and Early Pleistocene bottom waters at site 238 are of North Indian Deep Water (NIDW) origin. However, Late Pleistocene assemblage at site 238 appears to be closely associated with a water mass intermediate between North Indian Deep Water (NIDW) and Antarctic Bottom Water (AABW). Uvigerina proboscidea is the most dominant benthic foraminiferal species present during the Pleistocene at both the sites. A marked increase in the relative abundance ofU. proboscidea along with less diverse and equitable fauna during Early Pleistocene suggests a relative cooling, an intensified oceanic circulation and upwelling of nutrient rich bottom waters resulting in high surface productivity. At the same time, low sediment accumulation rate during Early Pleistocene reveals increased winnowing of the sediments possibly due to more corrosive and cold bottom waters. The Late Pleistocene in general, is marked by relatively warm and stable bottom waters as reflected by low abundance ofU. proboscidea and more diverse and equitable benthic fauna. The lower depth range for the occurrence ofBulimina aculeate in the Indian Ocean is around 2300 m, similar to that of many other areas.B. aculeata also shows marked increase in its abundance near the Pliocene/Pleistocene boundary while a sudden decrease in the relative abundance ofStilostomella lepidula occurs close to the Early/Late Pleistocene boundary.  相似文献   

Calcareous phytoplankton from the Barremian/Aptian boundary interval in NW Germany     

Helmut Keupp  Jrg Mutterlose 《Cretaceous Research》1994,15(6)

Two cored boreholes in the central part of the North West German Basin recovered a unique section of Upper Barremian to Lower Aptian strata. Calcareous nannofossils show a distinctive shift from boreal endemic assemblages in the Barremian to cosmopolitan ones in the Aptian. This onset of new cosmopolitan species (e.g., Chiastozygus litterarius, Flabellites oblongus, Rhagodiscus angustus, Braarudosphaera sp., Eprolithus sp.) is spread over an interval of 25m, starting well below the early Aptian "Fischschiefer", a dark laminated shale rich in organic matter. These changes in the composition of calcareous nannofossils indicate that major palaeoceanographic changes occurred before the deposition of the Fischschiefer.The distribution pattern of calcispheres allows the differentiation of two sedimentary successions, separated by the Fischschiefer. The lower succession, which includes the sediments below the base of the Fischschiefer, indicates a boreal-pelagic environment. The lower part of the upper successions which includes the Fischschiefer, represents warmer inner shelf conditions. This corresponds to the presence of rich Tethyan-derived nannoconid assemblages and the presence of the planktonic foraminifera Hedbergella in the Fischschiefer. The topmost part of the upper succession (middle Aptian, Hedbergella marl) suggests a change to a pelagic warmer water environment. Two new taxa are introduced: the calcareous dinoflagellate cyst Obliquipithonella laqueata n. sp. and the foraminiferan Choanaella fortunate n. gen. n. sp.  相似文献   

琉球岛弧两侧底质中有孔虫的特征     

郝治纯  曾学鲁 《地球科学》1990,15(3):243-250

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