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1.
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   

2.
On- and off-mound sediment cores from Propeller Mound (Hovland Mound province, Porcupine Seabight) were analysed to understand better the evolution of a carbonate mound. The evaluation of benthic foraminiferal assemblages from the off-mound position helps to determine the changes of the environmental controls on Propeller Mound in glacial and interglacial times. Two different assemblages describe the Holocene and Marine Isotope Stage (MIS) 2 and late MIS 3 (∼31 kyr BP). The different assemblages are related to changes in oceanographic conditions, surface productivity and the waxing and waning of the British Irish Ice Sheet (BIIS) during the last glacial stages. The interglacial assemblage is related to a higher supply of organic material and stronger current intensities in water depth of recent coral growth. During the last glaciation the benthic faunas showed high abundances of cassidulinid species, implying cold bottom waters and a reduced availability of organic matter. High sedimentation rates and the domination of Elphidium excavatum point to shelf erosion related to sea-level lowering (∼50 m) and the progradation of the BIIS onto the shelf. A different assemblage described for the on-mound core is dominated by Discanomalina coronata, Gavelinopsis translucens, Planulina ariminensis, Cibicides lobatulus and to a lower degree by Hyrrokkin sarcophaga. These species are only found or show significantly higher relative abundances in on-mound samples and their maximum contribution in the lower part of the record indicates a higher coral growth density on Propeller Mound in an earlier period. They are less abundant during the Holocene, however. This dataset portrays the boundary conditions of the habitable range for the cold-water coral Lophelia pertusa, which dominates the deep-water reefal ecosystem on the upper flanks of Propeller Mound. The growth of this ecosystem occurs during interglacial and interstadial periods, whereas a retreat of corals is documented in the absence of glacial sediments on-mound. Glacial conditions with cold intermediate waters, a weak current regime and high sedimentation rates provide an unfavourable environmental setting for Lophelia corals to grow. A Late Pleistocene decrease is observed in the mound growth for Propeller Mound, which might face its complete burial in the future, as it already happened to the buried mounds of the Magellan Mound province further north.  相似文献   

3.
《Sedimentology》2018,65(4):1331-1353
The Faxe Quarry in south‐east Denmark offers excellent exposures of Early Palaeocene, Danian deep‐water intercalated coral and bryozoan mounds that form complexes at least 40 m thick and a few kilometres wide along and over submarine highs. The coexisting coral and bryozoan mounds represent two different biogenic carbonate factories with a highly dynamic interplay during growth. The sedimentary facies, mound geometries and the density, diversity and palaeoecology of the associated benthic invertebrates and nannofossils allow recognition of six successive growth units. Unit 1 represents an outer shelf bryozoan mound belt characterized by an oligotrophic cool‐water nannofossil assemblage. Unit 2 comprises a mixed faunal assemblage of bryozoans and octocorals with an initial sparse colonization of hexacorals. The nannofossil assemblage records a decrease in diversity and an increase in warm water forms. Unit 3 marks the onset of dense colonization of the scleractinian coral Dendrophyllia candelabrum with associated low‐diversity macrofauna and nannofossil assemblages. Unit 4 represents the main coral build‐up phase with frame‐building hexacorals of Dendrophyllia and Faxephyllia associated with a high‐diversity invertebrate fauna, and relatively low‐diversity nannofossil assemblages. Unit 5 represents the late coral mound phase showing extensive lateral distribution and finally death and erosion of the coral mounds. This event was contemporaneous with a warming trend in the pelagic environment. The succeeding Unit 6 marks the burial and overgrowth of the coral mound complex by bryozoan‐rich sediments. The coral mound complex in the Faxe Quarry initiated and terminated in global nannofossil zone NP 3 and regional nannofossil zones NNT p2G–3 suggesting a mound growth duration of ca 300 kyr and a mean vertical accretion of the coral mound of 13 cm kyr−1. The mound complex probably serves as the best‐exposed analogue to modern deep and cold‐water coral mounds in the North Atlantic.  相似文献   

4.
A brief review is given of some dynamical processes that influence the benthic dynamics within the carbonate mound provinces located at the Porcupine Bank/Sea Bight margin, NE Atlantic. The depth range of the mounds in this region (600–1,000 m) marks the upper boundary of the Mediterranean outflow water above which Eastern North Atlantic Water dominates. Both water masses are carried northwards by the eastern boundary slope current. In the benthic boundary layer both the action of internal waves, and other tidal period baroclinic waves, may enhance the bottom currents and add to both the residual and maximum flow strength. Both residual and maximum bottom currents vary at different mound locations, with stronger currents found at Belgica (SE Porcupine Sea Bight) mound and Pelagia (NW Porcupine Bank) mound regions, whilst weakest currents are found at the Hovland and Magellan Mounds at the northern Sea Bight margin. The differences may be attributed to the presence of internal waves (Pelagia) or bottom intensified diurnal waves (Belgica). These different dynamical regimes are likely to have implications for the distribution patterns of live coral at the different locations.  相似文献   

5.
Marine sediments from the Integrated Ocean Drilling Project (IODP) Site U1314 (56.36°N, 27.88°W), in the subpolar North Atlantic, were studied for their planktonic foraminifera, calcium carbonate content, and Neogloboqudrina pachyderma sinistral (sin.) δ13C records in order to reconstruct surface and intermediate conditions in this region during the Mid‐Pleistocene Transition (MPT). Variations in the palaeoceanography and regional dynamics of the Arctic Front were estimated by comparing CaCO3 content, planktonic foraminiferal species abundances, carbon isotopes and ice‐rafted debris (IRD) data from Site U1314 with published data from other North Atlantic sites. Site U1314 exhibited high abundances of the polar planktonic foraminifera N. pachyderma sin. and low CaCO3 content until Marine Isotope Stage (MIS) 26, indicating a relatively southeastward position of the Arctic Front (AF) and penetration of colder and low‐salinity surface arctic water‐masses. Changing conditions after MIS 25, with oscillations in the position of the AF, caused an increase in the northward export of the warmer North Atlantic Current (NAC), indicated by greater abundances of non‐polar planktonic foraminifera and higher CaCO3. The N. pachyderma sin. δ13C data indicate good ventilation of the upper part of the intermediate water layer in the eastern North Atlantic during both glacial and interglacial stages, except during Terminations 24/23, 22/21 and 20/1. In addition, for N. pachyderma (sin.) we distinguished two morphotypes: non‐encrusted and heavily encrusted test. Results indicate that increases in the encrusted morphotype and lower planktonic foraminiferal diversity are related to the intensification of glacial conditions (lower sea‐surface temperatures, sea‐ice formation) during MIS 22 and 20.  相似文献   

6.
The Darwin Mounds are small (up to 70 m in diameter), discrete cold‐water coral banks found at c. 950 m water depth in the northern Rockall Trough, north‐east Atlantic. Formerly described in terms of their genesis, the Darwin Mounds are re‐evaluated here in terms of mound growth processes based on 100 and 410 kHz side‐scan sonar data. The side‐scan sonar coverage is divided into a series of acoustic facies representing increasing current speed and sediment transport/erosion from south to north: pockmark facies, ‘mounds within depressions’ facies, Darwin Mound facies, stippled seabed facies and sand wave facies. Mound morphometric changes are quantified and show a south‐to‐north divergence from an inherited morphology, reflecting the outline of coral‐colonized fluid escape structures, to developed, downstream elongated, elevated mound forms. It is postulated that increasing current speeds and bedload sand transport favour mound growth and development by a process of enhanced sand sedimentation within mounds due to current deceleration by frictional drag around coral colonies. Comparisons are made with similar growth processes attributed to comparably sized cold‐water coral mounds in the Porcupine Seabight, offshore Ireland.  相似文献   

7.
The structure of the glacial ocean was significantly different to that of the present day with intermediate to mid-depth waters being notably more δ13C enriched than deep waters. This contrast was especially pronounced in the South Atlantic suggesting the development of a sharp chemical divide, or ‘chemocline’, at around 2500 m water depth between upper and lower layers, with implications for deep-ocean carbon storage [Hodell et al., 2003. Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry, Geophysics, Geosystems, 4(1): doi: 1004 10.1029/2002GC000367.]. We evaluate existing benthic foraminiferal δ13C, Cd/Ca and derived carbon isotope air–sea exchange signature (δ13Cas) data sets for the Atlantic during the Last Glacial Maximum (LGM), and Marine Isotope Stages (MIS) 6 and 8 in order to examine the regional extent of the chemocline in the South Atlantic. Benthic δ13C data north of the approximate latitude of the LGM Subantarctic Front (LGM-SAF, 43°S) linearly decrease with water depth, indicative of mixing between upper ‘well’ and lower ‘poorly’ ventilated water masses, with little evidence of the sharp chemical divide. Conversely, benthic δ13C data south of the LGM-SAF below 2500 m water depth are uniformly around ?0.8‰. The apparent δ13C gradient across the LGM-SAF suggests enhanced mid-depth ventilation north of the SAF and reduced ventilation to the south. From this pattern we conclude that the regional chemocline in the South Atlantic constituted a dominantly meridional, rather than a vertical gradient, and was developed during at least the past three glacial periods. Benthic Cd/Ca data indicate that the gradient was not nutrient related, although further data from the South Atlantic are needed for a better assessment of this suggestion. The combined benthic δ13C and Cd/Ca data indicate the source of well-ventilated upper waters in the South Atlantic changed from Northern Component Water (NCW) during early glacial phases to Upper Southern Component Water (USCW) during mid-to-late glacial phases when the Southern Ocean may have become isolated. USCW maintained a positive δ13C and δ13Cas signature simulating a North Atlantic origin that has been implicated in previous studies. The data demonstrate that secular imprints on δ13C must be taken into consideration when assessing the implications of the vertical δ13C gradient. This data also supports a variable water column architecture and modes of water mass formation as primary means to draw down atmospheric CO2 and storage in the abyssal ocean by involving processes occurring on either side of the SAF in the glacial Southern Ocean.  相似文献   

8.
The Belgica cold-water coral banks on the eastern slope of the Porcupine Seabight are closely associated with bottom currents. In order to better understand the local temporal and spatial characteristics, as well as the palaeoclimatologic influences, a 26 m long core, taken on a small contourite drift, was studied. This sediment record of approximately 100 ka BP reveals new insights into the regional glacial and sedimentary processes, which are intrinsically linked to several geological, climatological, biological and hydrodynamic variables. The glacial sequences in the core contain six ice-rafting events (IRE). They are comparable with the North Atlantic Heinrich Events, although their characteristics show dominant influences from the proximal British–Irish Ice Sheet (BIIS). These IRE have a low magnetic susceptibility and are deposited during two or three ice-rafting pulses. The record of ice-rafting suggests a millennial-scaled BIIS destabilisation and confirms the start of a final retreat about 25 ka ago. Additionally, the glacial sequence corresponds to a muddy contourite, influenced by bottom-current strength variations during interstadials, possibly triggered by sporadic reintroductions of Mediterranean Outflow Water in a glacial North Atlantic Ocean. The interglacial sequence features an 11-m thick deep-water massive sand unit, probably deposited under a high-energy bottom-current regime.  相似文献   

9.
《Applied Geochemistry》2005,20(3):519-527
Pleistocene vegetation history on the Chinese Loess Plateau has been traditionally investigated using palynological methods, and questions remain regarding whether an extensive broadleaf deciduous forest ever developed on the loess table under favorable climatic conditions. The authors have employed a C isotope approach to address this question by comparing δ13C values in soil organic matter from different loess ecological domains with known source vegetation to the C isotope values obtained from a paleosol section that can be dated back to 130 ka. The C isotopic compositions of modern soils from the loess table and the loess–desert transition gave δ13C values of −24.5‰ to −18.2‰ and −25.7‰ to −20.7‰, respectively. These C isotopic ratios are consistent with the standing modern vegetation that is dominated by a mixture of C3 and C4 plants and can be distinguished from that in the patchy forest areas where exclusive C3 trees yield a narrow δ13C value range from −26.9‰ to −25‰ (average −26.1‰). Obtained δ13C compositions from paleosols and loess sediments in the Lantian and the Luochuan profiles vary from −24‰ to −16.9‰, indicating a grass-dominated steppe with shifting C3 and C4 contributions controlled mainly by paleoclimatic changes during the late Pleistocene. The present results suggest no extensive forest coverage on the loess table during the past 130 ka even under the most suitable conditions for forest development. This conclusion supports the explanation of natural causes for the development of only patchy forests on the modern loess table and provides critical historical information toward the vegetation restoration project that is currently underway on the Chinese Loess Plateau.  相似文献   

10.
Uranium-series dated stalagmites from Oman indicate that pluvial conditions prevailed from 6.3 to 10.5, 78 to 82, 120 to 130, 180 to 200 and 300 to 330 kyr B.P.; all of these periods coincide with peak interglacials. Oxygen (δ18O) and hydrogen (δD) isotope ratios of speleothem calcite and fluid inclusions reveal the source of moisture and provide information on the amount of precipitation, respectively. δ18O and δD values of stalagmites deposited during peak interglacials vary between ?8 and ?4 ‰ (VPDB) and ?53 and ?20‰ (Vienna Standard Mean Ocean Water [VSMOW]) respectively, whereas modern stalagmites range from ?2.6 to ?1.1‰ in δ18O (VPDB) and ?7.6 and ?3.3‰ in δD (VSMOW), respectively. The growth and isotopic records indicate that during peak interglacial periods, the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coinciding with an interglacial stage of the marine oxygen isotope record.  相似文献   

11.
The recent discoveries of deeply buried Cretaceous reservoir bodies in the Atlantic Ocean revealed that relationships between the distribution of spring carbonate deposits and faults are poorly understood. The well‐exposed Quaternary deposits at Obruktepe (Denizli Basin, Turkey) provide an opportunity to reconstruct the three‐dimensional sedimentary architecture of such a system. Integration of sedimentological, lithofacies and geochemical analyses reveals complexity in the lateral relationships between sedimentary environments, faults and geothermal spring carbonates. Five environmental systems are distinguished based on the lithofacies analysis: (i) vent; (ii) smooth slope; (iii) travertine‐terrace; (iv) tufa‐barrage; and (v) flood systems. Encrusting, baffling and settling sedimentary processes are reflected in data acquired at several scales, from lithofacies observations to the morphology and arrangement of geobodies, together with microfabrics and stable carbon and oxygen isotope data. Mean values of +4·9‰ δ13C and −8·74‰ δ18O Vienna PeeDee Belemnite reflect geothermal circulation of springwaters. The environmental distribution and lithofacies indicate a lateral continuum between travertine and tufa deposits within this hot spring system. This finding supports two depositional models in which water flow variation is the main control on both CaCO3 precipitation and the resulting formation of travertine and tufa. The proposed models address the factors responsible for the development of these complex mound‐shaped carbonate spring deposits, and how they are related to fluid circulation at depth and in association with faults.  相似文献   

12.
Variations in the carbon isotope composition in gases and waters of mud volcanoes in the Taman Peninsula are studied. The δ13C values in CH4 and CO2 vary from ?59.5 to ?44.0‰ (δ13Cav = ?52.4 ± 5.4‰) and from ?17.8 to +22.8‰ (δ13Cav = +6.9 ± 9.3‰), respectively. In waters from most mud volcanoes of the peninsula, this parameter ranges from +3.3 to +33.1‰, although locally lower values are also recorded (up to ?12‰. Fractionation of carbon isotopes in the CO2-HCO3 system corresponds to the isotope equilibrium under Earth’s surface temperatures. The growth of carbon dioxide concentration in the gaseous phase and increase in the HCO3 ion concentration in their water phase is accompanied by the enrichment of the latter with the heavy 13C isotope. The δ13CTDIC value in the water-soluble carbon depends on the occurrence time of water on the Earth’s surface (exchange with atmospheric CO2, methane oxidation, precipitation of carbonates, and other processes), in addition to its primary composition. In this connection, fluctuations in δ13CTDIC values in mud volcanoes with stagnant waters may amount to 10–20‰. In the clayey pulp, concentrations of carbonate matter recalculated to CaCO3 varies from 1–4 to 36–50 wt %. The δ13C value in the latter ranges from ?3.6 to +8.4‰. Carbonate matter of the clayey pulp represents a mixture of sedimentogenic and authigenic carbonates. Therefore, it is usually unbalanced in terms of the carbon isotope composition with the water-soluble CO2 forms.  相似文献   

13.
Continuous surface cores of cold-seep carbonates were recovered offshore Pacific Nicaragua and Costa Rica from 800 to 1,500-m water depths (Meteor 66/3) in order to decipher their evolution and methane enriched fluid emanation in contrasting geological settings. Cores from the mounds Iguana, Perezoso, Baula V and from the Jaco Scarp escarpment were used for a multi-method approach. For both settings aragonite was revealed as dominant authigenic carbonate phase in vein fillings and matrix cementation, followed by Mg-calcite as second most abundant. This common precipitation process of CaCO3 polymorphs could be ascribed as indirectly driven by chemical changes of the advecting pore water due to anaerobic oxidation of methane. A more direct influence of seep-related microbial activity on the authigenic mineral assemblage in both settings is probably reflected by the observed minor amounts of dolomite and a dolomite-like CaMg carbonate (MgCO3 ~ 42 %). δ13C data of Jaco Scarp samples are significantly lower (?43 to ?56 ‰ PDB) than for mound samples (?22 to ?36 ‰ PDB), indicating differences in fluid composition and origin. Noteworthy, δ18O values of Scarp samples correlate most closely with the ocean signature at their time of formation. Documenting the archive potential, a high resolution case study of a mound core implies at least 40 changes in fluid supply within a time interval of approximately 14 ky. As most striking difference, the age data indicate a late-stage downward-progressing cementation front for all three mound cap structures (approx. 2–5 cm/ky), but a significantly faster upward carbonate buildup in the bulging sediments on top of the scarp environment (approx. 120 cm/ky). The latter data set leads to the hypothesis of chemoherm carbonate emplacement in accord with reported sedimentation rates until decompression of the advective fluid system, probably caused by the Jaco Scarp landslide and dating this to approximately 13,000 years ago.  相似文献   

14.
《Quaternary Science Reviews》2003,22(15-17):1717-1728
Sedimentological and geochemical proxy records of a deep-sea sediment core from the southern central Nordic seas were used to reconstruct the development of glacial and interglacial conditions during the Early and Middle Pleistocene, i.e., late Matuyama to middle Brunhes Chron (1.5–0.35 Ma). An enhancement of both glacial and interglacial characteristics is observed during early Brunhes oxygen isotope stages (OIS) 16 and 15, respectively. Any intensification of the climatic conditions prior to this, as was previously described for the eastern part of the Nordic seas, is not recognized at our study site. It is further shown that the glacial–interglacial environmental contrasts increased from the early to the middle Bruhnes Chron. Of all glacial periods investigated OIS 12 is characterized by the most severe conditions, showing both maximum input of iceberg-rafted debris (IRD) as well as planktic foraminiferal δ18O values comparable to those of the Last Glacial Maximum. Among the interglaciations, OIS 11 is by far the longest interval and the first to show fully developed interglacial conditions, i.e., Holocene-like δ18O values and a minimum of IRD deposition. Hence, our comparison supports bottom water δ18O studies that have indicated the existence of a gradual intensification of glacial–interglacial climate contrasts during the Middle Pleistocene.  相似文献   

15.
国际全球变化研究发展态势文献计量评价   总被引：1，自引：0，他引：1  
-全球变化研究是国际上地球系统综合研究的重大跨学科研究领域。随着全球环境问题的日益突出，国际上先后组织发起了全球环境变化研究的四大科学计划WCRP、IGBP、IHDP、DIVERSITAS及其组成的地球系统科学联盟ESSP，开展对全球变化和地球系统科学的研究。当前，全球变化问题不仅是科学界关注和研究的问题，也是政治界、经济界都关注的重大问题。 科技文献能够反映科学前沿的发展动态。对SCIE和SSCI数据库收录的全球变化研究文献进行统计，从文献计量学的角度，分析国际全球变化研究领域的发展态势，了解中国全球变化研究的国际影响力。可以看出：国际上全球变化研究的论文数量一直呈增长趋势，以地球科学多学科、生态学、环境科学、自然地理学、植物学等学科领域为主。中国自2000年以来在该领域的发文数量增长较快，特别是中国科学院的发文量已进入国际前列，但中国在该研究领域尚缺少高影响力的论文。  相似文献   

16.
The Cenomanian–Turonian boundary was characterized by distinctive positive carbon isotope excursions that were related to the formation of widespread oceanic anoxia. High-resolution geochemical proxies (TOC, CaCO3, δ13Corg, and δ13Ccarb) obtained from bulk rock, planktic foraminifers, and inoceramids from four marine marlstone-dominated stratigraphic sections in the Western Canada Sedimentary Basin (WCSB) were used to establish a regional carbon isotope stratigraphic framework and to investigate paleoenvironmental variability in four different depositional settings. Compared to background δ13Corg, (<−27‰) and δ13Ccarb (<2‰) values which were correlative to stable isotope excursions during Oceanic Anoxic Event (OAE) II worldwide, the δ13Corg (>24‰), and δ13Ccarb (>4‰) derived from inoceramid prisms in the studied sections within WCSB, were elevated during the Late Cenomanian–Early Turonian. During this interval, TOC and CaCO3 values which increased sporadically to >40% and 7%, respectively, were not consistent enough to be used for stratigraphic correlations. Based on the δ13Corg excursions, two bentonite beds were regionally correlated across this portion of the Western Interior Seaway (WIS). The eruption associated with the “Red” bentonite occurred approximately coeval with the maximum δ13Corg-excursion during OAE II in the Neocardioceras juddii Zone, whereas the “Blue” bentonite coincides with the termination of OAE II in the latest Watinoceras devonense zone. During the Late Cenomanian–Early Turonian in the WCSB, benthic foraminifers were sparse or totally absent, indicating the existence of fully anoxic bottom-water conditions. Planktic foraminifera were common in the well-oxygenated surface waters. A benthic oxic zone characterized by several agglutinated species occurs in the eastern part of the WSCB at the beginning of OAE II in the Sciponoceras gracile zone. The termination of the OAE II in the WCSB coincides with the first occurrence of small ammonites (Subprionocyclus sp.) in the western part of the basin.  相似文献   

17.
To reconstruct oceanographic variations in the subtropical South Pacific, 271-year long subseasonal time series of Sr/Ca and δ18O were generated from a coral growing at Rarotonga (21.5°S, 159.5°W). In this case, coral Sr/Ca appears to be an excellent proxy for sea surface temperature (SST) and coral δ18O is a function of both SST and seawater δ18O composition (δ18Osw). Here, we focus on extracting the δ18Osw signal from these proxy records. A method is presented assuming that coral Sr/Ca is solely a function of SST and that coral δ18O is a function of both SST and δ18Osw. This method separates the effects of δ18Osw from SST by breaking the instantaneous changes of coral δ18O into separate contributions by instantaneous SST and δ18Osw changes, respectively. The results show that on average δ18Osw at Rarotonga explains ∼39% of the variance in δ18O and that variations in SST explains the remaining ∼61% of δ18O variance. Reconstructed δ18Osw shows systematic increases in summer months (December-February) consistent with the regional pattern of variations in precipitation and evaporation. The δ18Osw also shows a positive linear correlation with satellite-derived estimated salinity for the period 1980 to 1997 (r = 0.72). This linear correlation between reconstructed δ18Osw and salinity makes it possible to use the reconstructed δ18Osw to estimate the past interannual and decadal salinity changes in this region. Comparisons of coral δ18O and δ18Osw at Rarotonga with the Pacific decadal oscillation index suggest that the decadal and interdecadal salinity and SST variability at Rarotonga appears to be related to basin-scale decadal variability in the Pacific.  相似文献   

18.
记述了新生代深海冷水碳酸盐泥丘近期的9个重要研究事件;总结了冷水泥丘具有全球海洋(大陆斜坡为主)分布、形态各异、冷水枝状珊瑚构筑泥丘的特点;介绍了冷水泥丘形成的(地质流体渗流和微生物作用)内因及(海底牵引底流作用)外因两种主要观点.对2005年IODP 307航次实施的北大西洋Porcupine Seabight冷水泥丘大洋钻探工作初步成果进行了编译,公布了中国科学家在碳氧同位素方面的初步实验结果.实验结果显示上新世中期以来的2 Ma里冷水碳酸盐泥丘启动和发育过程中存在2次碳氧同位素偏移事件(Ⅰ和Ⅱ),碳氧同位素偏移事件Ⅰ与泥丘的启动相呼应,暗示北大西洋古海洋气候发生巨大变化,可能与北极冰盖极盛有关.  相似文献   

19.
Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of 24Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of 26Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ26Mgsw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ26Mgsw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ26Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in 24Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ26Mgsw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ26Mgsw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ26Mg of the Huangjin limestone makes it difficult to reconstruct δ26Mgsw using bulk carbonate/calcareous sediments. Finally, δ26Mgsw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.  相似文献   

20.
Palaeotemperature reconstruction for the period of 20?18 ka BP in Siberia is here based on δ18O analysis and 14C dating of large syngenetic ice wedges. Dozens of yedoma exposures, from Yamal Peninsula to Chukotka, have been studied. Snow meltwater is considered to be the main source of ice‐wedge ice. The modern relationship between δ18O composition of ice‐wedge ice and winter temperature is used as a base for reconstruction. In modern ice wedges (elementary veins that have accumulated during the last 60–100 years) δ18O fluctuates between ?14 and ?20‰ in western Siberia and between ?23 and ?28‰ in northern Yakutia. The trend in δ18O distribution in ice wedges dated at 20?18 ka BP is similar to the modern one. For example, the δ18O values in Late Pleistocene wedges are more negative going from west to east by 8–10‰, i.e. from ?19 to ?25‰ in western Siberian ice wedges to ?30 to ?35‰ in northern Yakutia. However, values are as high as ?28 to ?33‰ in north Chukotka and the central areas of the Magadan Region and even as high as ?23 to ?29‰ in the east of Chukotka. The same difference between the oxygen isotope composition of ice wedges in the eastern and western regions of Siberian permafrost (about 8–10‰) is also preserved from 20?18 ka BP to the present: δ18O values obtained from large ice wedges from the Late Pleistocene vary from ?19 to ?25‰ in western Siberia to ?30 to ?35‰ in northern Yakutia. We conclude that, at 20?18 ka BP, mean January temperatures were about 8–12°C lower (in Chukotka up to 17–18°C) than at present.  相似文献   

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