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1.
Based on geological and archaeological proxies from NW Russia and NE Estonia and on GIS‐based modelling, shore displacement during the Stone Age in the Narva‐Luga Klint Bay area in the eastern Gulf of Finland was reconstructed. The reconstructed shore displacement curve displays three regressive phases in the Baltic Sea history, interrupted by the rapid Ancylus Lake and Litorina Sea transgressions c. 10.9–10.2 cal. ka BP and c. 8.5–7.3 cal. ka BP, respectively. During the Ancylus transgression the lake level rose 9 m at an average rate of about 13 mm per year, while during the Litorina transgression the sea level rose 8 m at an average rate of about 7 mm per year. The results show that the highest shoreline of Ancylus Lake at an altitude of 8–17 m a.s.l. was formed c. 10.2 cal. ka BP and that of the Litorina Sea at an altitude of 6–14 m a.s.l., c. 7.3 cal. ka BP. The oldest traces of human activity dated to 8.5–7.9 cal. ka BP are associated with the palaeo‐Narva River in the period of low water level in the Baltic basin at the beginning of the Litorina Sea transgression. The coastal settlement associated with the Litorina Sea lagoon, presently represented by 33 Stone Age sites, developed in the area c. 7.1 cal. ka BP and existed there for more than 2000 years. Transformation from the coastal settlement back to the river settlement indicates a change from a fishing‐and‐hunting economy to farming and animal husbandry c. 4.4 cal. ka BP, coinciding with the time of the overgrowing of the lagoon in the Narva‐Luga Klint Bay area.  相似文献   

2.
Changes in the wind‐energy environment between AD 1359 and 2010 in the Taklimakan Desert (central Asia) are recorded by the evolution of Chinese tamarisk (Tamarix taklamakanensis) nebkhas. The carbonate component and sedimentological properties of the nebkha excavated during the study, together with AMS 14C dating control, indicate that significant regional environmental changes have occurred in the central Taklimakan Desert during the last 700 years. The nebkha data presented show that in the periods of AD c. 1480–1560, c. 1640–1690, c. 1760–1820, c. 1860–1930 and c. 1970–1980 the Taklimakan Desert was a relatively high wind‐energy environment. Although changes in the wind‐energy regime in the desert were mainly in phase with fluctuations of the Siberian High, the wind systems and the variations in wind energy exhibit slight differences when compared with the Tarim Basin. Nebkhas that develop in this region originate from the surfaces of mobile dunes or sand sheets, which differs from the origins of nebkhas found in other arid regions of China.  相似文献   

3.
Trophic resources are an important control governing carbonate production. Though this importance has long been recognized, no calibration exists to quantitatively compare biogenic assemblages within trophic resource fields. This study presents a field calibration of carbonate producers in a range of settings against high‐resolution in situ measurements of nutrients, temperature and salinity. With its latitudinal extent from 30° to 23° N, the Gulf of California, Mexico, spans the warm‐temperate realm and encompasses nutrient regimes from oligo‐mesotrophic in the south to eutrophic in the north. Accordingly, from south to north carbonates are characterized by: (i) coral‐dominated shallow carbonate factories (5–20 m water depth) with average sea‐surface temperatures of 25 °C (min. 18 °C, max. 31 °C), average salinities of 35·06‰ and average chlorophyll a levels, which are a proxy for nutrients, of 0·25 mg Chl a m?3 (max. 0·48, min. 0·1). (ii) Red algal‐dominated subtidal to inner‐shelf carbonate formation (10–25 m) in the central Gulf of California exhibiting average temperatures of 23 °C (min. 18 °C, max. 30 °C), average salinities of 35·25‰, and average Chl a levels of 0·71 Chl a m?3 (max. 5·62, min. 0). (iii) Molluskan bryozoan‐rich inner to outer shelf factories in the northern Gulf of California (20–50 m) with average sea surface temperatures of only 20 °C (min. 13 °C, max 29 °C), average salinities of 35·01‰, and average contents of 2·2 mg Chl a m?3 (max. 8·38, min. 0). By calibrating sedimentological data with in situ measured oceanographic information in different environments, the response of carbonate producers to environmental parameters was established and extrapolated to carbonates on a global scale. The results demonstrate the importance of recognizing and quantifying trophic resources as a dominant control determining the biogenic composition and facies character of both modern and fossil carbonates.  相似文献   

4.
Palaeoseismological and archaeological analysis of a trench enabled us to estimate the Holocene slip rates on the East Helike Fault, flanking the south-western Gulf of Corinth. We recognized two major fault strands within the trench: the ‘north fault’ controls a succession of three colluvial wedges and the deposition of a 2.7 m thick sedimentary sequence. The ‘south fault’ controls the deposition of a 2.9-m thick brownish-red colluvium. Based on colluvial stratigraphy, radiocarbon dating of the sediments suggests that the slip rate was c. 0.3 mm yr−1 from 10 250 to c. 1400 bp , when it increased dramatically to c. 2.0 mm yr−1 after a strong earthquake event near 1400 bp . The faster slip rate evidently increased the sedimentation rate.  相似文献   

5.
Mangerud, J., Gulliksen, S. & Larsen, E. 2009: 14C‐dated fluctuations of the western flank of the Scandinavian Ice Sheet 45–25 kyr BP compared with Bølling–Younger Dryas fluctuations and Dansgaard–Oeschger events in Greenland. Boreas, 10.1111/j.1502‐3885.2009.00127.x. ISSN 0300‐9483. We present 32 accelerator mass spectrometry (AMS) 14C dates obtained on well‐preserved bones from caves in western Norway. The resulting ages of 34–28 14C kyr BP demonstrate that the coast was ice‐free during the so‐called Ålesund Interstadial. New AMS 14C dates on shells aged 41–38 14C kyr BP are evidence of an earlier (Austnes) ice‐free period. The Ålesund Interstadial correlates with Greenland interstadials 8–7 and the Austnes Interstadial with Greenland interstadials 12–11. Between and after the two interstadials, the ice margin reached onto the continental shelf west of Norway. These events can be closely correlated with the Greenland ice core stratigraphy, partly based on identification of the Laschamp and Mono Lake palaeomagnetic excursions. We found that the pattern of the NGRIP δ18O curves for the two periods Greenland Interstadial (GI) 8 to Greenland Stadial (GS) 8 and GI 1–GS 1 (Bølling–Younger Dryas) were strikingly similar, which leads us to suggest that the underlying causes of these climate shifts could have been the same. We therefore discuss some aspects of glacial fluctuations during the Bølling–Younger Dryas in order to elucidate processes during Dansgaard–Oeschger events.  相似文献   

6.
Core SG120 recovered 3.65 m of Quaternary sediment from a northern, shallow-water environment of Spencer Gulf, a marine embayment into the southern continental margin of Australia. Previous investigations had revealed that the upper interval 0 – 148 cm is Holocene marine bioclastic sediment, and that the lower Late Pleistocene interval 250 – 365 cm, with its carbonate palaeosol, had a similar marine origin. However, the age and origin of the interval 148 – 250 cm remained subject to ambiguous interpretation. Re-examination of core SG120, employing detailed foraminiferal analysis, has revealed that this middle unit records the earliest sedimentation associated with the postglacial marine transgression into the northern gulf. These basal Holocene sediments, which incorporated broken, corroded and carbonate-encrusted tests from the underlying palaeosol, together with tests of more pristine appearance, were deposited in a shallow-water, seagrass sandflat environment similar to those in coastal settings of the modern gulf. The lithological change at 148 cm has therefore been reinterpreted as a facies change related to increasing water depth. Radiocarbon analyses of fossil molluscs support this interpretation and reveal that marine transgression, at the site of SG120, was initiated prior to 8600 y cal BP. Selected species of foraminifers (Nubecularia lucifuga, Massilina milletti, Peneroplis planatus, Discorbis dimidiatus, Elphidium crispum and E. macelliforme) together reveal a consistent record of the final stages of the transgression with maximum water depth indicated at a core depth of 90 cm. Subsequent regression, which has been attributed to the combined effects of hydroisostatic uplift and sediment aggradation, is equally recorded by the foraminiferal assemblages.  相似文献   

7.
A common assumption in the geological analysis of modern reefs is that coral community zonation seen on the surface should also be found in cores from the reef interior. Such assumptions not only underestimate the impact of tropical storms on reef facies development, but have been difficult to test because of restrictions imposed by narrow‐diameter cores and poor recovery. That assumption is tested here using large‐diameter cores recovered from a range of common zones across three Campeche Bank reefs. It is found that cores from the reef‐front, crest, flat and rubble‐cay zones are similar in texture and coral composition, making it impossible to recognize coral assemblages that reflect the surface zonation. Taphonomic signatures imparted by variations in encrustation, bioerosion and cementation, however, produce distinct facies and delineate a clear depth zonation. Cores from the reef‐front zone (2–10 m depth) are characterized by sections of Acropora palmata cobble gravel interspersed with sections of in‐place (but truncated) A. palmata stumps. Upper surfaces of truncated colonies are intensely bioeroded by traces of Entobia isp. and Gastrochaenolites isp. and encrusted by mm‐thick crustose corallines before colony regeneration and, therefore, indicate punctuated growth resulting from a hurricane‐induced cycle of destruction and regeneration. Cores from the reef crest/flat (0–2 m depth) are also characterized by sections of hurricane‐derived A. palmata cobble‐gravels as well as in‐place A. palmata colonies. In contrast to the reef front, however, these cobble gravels are encrusted by cm‐thick crusts of intergrown coralline algae, low‐relief Homotrema and vermetids, bored by traces of Entobia isp. and Trypanites isp. and coated by a dense, peloidal, micrite cement. Cores from the inter‐ to supratidal rubble‐cay zone (+0–5 m) are only composed of A. palmata cobble gravels and, although clasts show evidence of subtidal encrustation and bioerosion, these always represent processes that occurred before deposition on the cay. Instead, these gravels are distinguished on the basis of their limited bioerosion and marine cements, which exhibit fabrics formed in the intertidal zone. These results confirm that hurricanes have a major influence on facies development in Campeche Bank reefs. Instead of reflecting the surface coral zonation, each facies records a distinctive, depth‐related set of taphonomic processes, which reflect colonization, alteration and stabilization following the production of new substrates by hurricanes.  相似文献   

8.
A new geochemical record from the paaleolake Santiaguillo documents the hydrological variability of sub‐tropical northern Mexico over the last ～14 cal. ka. Summer‐season runoff, lake water salinity and deposition of sediments by aeolian activity were reconstructed from concentrations of K, Ca and Zr/K in bulk sediments. More‐than‐average runoff during c. 12.3–9.3 cal. ka BP represented an interval of enhanced summer precipitation. Arid intervals of c. 14–12.3 cal. ka BP and c. 6–4.3 cal. ka BP were characterized by average and more‐than‐average aeolian activity. Comparison with proxy records of summer as well as winter precipitation from tropical and sub‐tropical North America and sea surface temperatures from the Atlantic and Pacific provides insight into the source of moisture and possible forcing. The wet Pleistocene?Holocene transition and early Holocene was contemporary with warmer conditions in the Gulf of California. We suggest that the Atlantic had minimal influence on the summer precipitation of the western part of sub‐tropical northern Mexico and that the source of moisture was dominantly Pacific.  相似文献   

9.
Carbonate sediments in non‐vegetated habitats on the north‐east Adriatic shelf are dominated by shells of molluscs. However, the rate of carbonate molluscan production prior to the 20th century eutrophication and overfishing on this and other shelves remains unknown because: (i) monitoring of ecosystems prior to the 20th century was scarce; and (ii) ecosystem history inferred from cores is masked by condensation and mixing. Here, based on geochronological dating of four bivalve species, carbonate production during the Holocene is assessed in the Gulf of Trieste, where algal and seagrass habitats underwent a major decline during the 20th century. Assemblages of sand‐dwelling Gouldia minima and opportunistic Corbula gibba are time‐averaged to >1000 years and Corbula gibba shells are older by >2000 years than shells of co‐occurring Gouldia minima. This age difference is driven by temporally disjunct production of two species coupled with decimetre‐scale mixing. Stratigraphic unmixing shows that Corbula gibba declined in abundance during the highstand phase and increased again during the 20th century. In contrast, one of the major contributors to carbonate sands – Gouldia minima – increased in abundance during the highstand phase, but declined to almost zero abundance over the past two centuries. Gouldia minima and herbivorous gastropods associated with macroalgae or seagrasses are abundant in the top‐core increments but are rarely alive. Although Gouldia minima is not limited to vegetated habitats, it is abundant in such habitats elsewhere in the Mediterranean Sea. This live–dead mismatch reflects the difference between highstand baseline communities (with soft‐bottom vegetated zones and hard‐bottom Arca beds) and present‐day oligophotic communities with organic‐loving species. Therefore, the decline in light penetration and the loss of vegetated habitats with high molluscan production traces back to the 19th century. More than 50% of the shells on the sea floor in the Gulf of Trieste reflect inactive production that was sourced by heterozoan carbonate factory in algal or seagrass habitats.  相似文献   

10.
The mechanisms that govern porphyroblast crystallization are investigated by comparing quantitative textural data with predictions from different crystallization models. Such numerical models use kinetic formulations of the main crystallization mechanism to predict textural characteristics, such as grain size distributions. In turn, data on porphyroblast textures for natural samples are used to infer which mechanism dominated during their formation. Whereas previous models assume that the rate‐limiting step for a porphyroblast producing reaction is either transport or growth, the model advanced in this study considers the production of nutrients for porphyroblasts as a potentially rate‐limiting factor. This production reflects the breakdown of (metastable) reactants, which at a specific pressure (P) and temperature (T) depends on the bulk composition of the sample. The production of nutrients that potentially contribute to the formation of porphyroblasts is computed based on thermodynamic models. The conceptual model assumes that these nutrients feed into some intergranular medium, and products form by nutrient consumption from that medium, with rates depending on reaction affinity. For any sequence of P–T conditions along a P–T–t path, the numerical model first computes an effective supersaturation (σeff) of the product phase(s), then an effective nucleation rate (J), and finally the amount of (porphyroblast) growth. As a result, the model is useful in investigating how the textural characteristics of a sample (of given bulk composition) depend on the P–T–t path followed during porphyroblast crystallization. The numerical model is tested and validated by comparing simulation results with quantitative textural data for garnet porphyroblasts measured in samples from the Swiss Central Alps.  相似文献   

11.
The Na, K, Mg and Ca contents of certain deep‐origin groundwater discharges have been used by Giggenbach (1988) to define a series of ‘geoindicators’, which may provide hints on the up‐flow depth of origin, on the duration of the fluid ascent to the ground surface and on the associated CO2 flux. On occurrence of a Mw = 6.0 Vrancea earthquake, significant fluctuations of Giggenbach’s geoindicators have been recorded in a saline spring, some 50 km away from the epicentre. A pre‐seismic overall anomaly was monitored for 1 year and a half, the sharpest variations occurring about 3 months before the earthquake. Processes controlling the geoindicator fluctuations assumedly took place at 7–8 km depth, while the earthquake hypocenter depth was about 100 km. This could be an evidence for a mechanical coupling still existing between the seismogenic body in the lithosphere and the overlying crust.  相似文献   

12.
We compared nekton use of prominent habitat types within a lagoonal system of the northeastern Gulf of Mexico (GoM). These habitat types were defined by combinations of structure (cover type) and location (distance from shore) as: Spartina edge (≤1 m from shore), Spartina (3 m from shore); Juncus edge (≤1 m from shore); seagrass located 3, 5, and 20 m from shore; and shallow non-vegetated bottom at various distances from shore. Although seagrass and Spartina edge sites differed little in environmental characteristics, the density and biomass of most abundant taxa, including pink shrimp (Farfantepenaeus duorarum), were higher in seagrass. Most species within seagrass and Spartina did not differ in abundance or biomass with distance from shore. Our study revealed a shift in peak habitat use in the northeastern GoM to seagrass beds from the pattern observed to the west where nekton is concentrated within shoreline emergent vegetation.  相似文献   

13.
ABSTRACT Field, geochemical, and petrographic data for late Pleistocene dolomites from southeastern Barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters. The dolomite is localized in packstones and wackestones from the algalAmphistegina fore-reef calcarenite facies. Stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization. Carbon isotopes in pure dolomite phases average about -15%0 PDB. This light carbon is attributed to the influence of soil gas CO2, and precludes substantial mixing with seawater. A narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint. Dolomitization likely occurred with as little as a five per cent admixture of seawater. Strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy. The dolomite is characterized by low sodium values. Low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization. A sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix. Limpid, euhedral dolomite cements precipitated in primary intra- and interparticle porosity subsequent to replacement dolomitization. As waters became progressively less saline, dolomite cements alternated with thin bands of syntaxial calcite cement. The final diagenetic phase precipitated was a blocky calcite spar cement, representing diagenesis in a fresh-water lens. This sequence of diagenetic features arose as the result of a single fall in eustatic sea-level following deposition. A stratigraphic-eustatic-diagenetic model constrains both the timing and rate of dolomitization in southeastern Barbados. Dolomitization initiated as sea-level began to fall immediately following the oxygen isotope stage 7–3 high stand, some 216 000 yr bp . Due to the rapidity of late Pleistocene glacio-eustasy, dolomitization (locally complete) is constrained to have occurred within about 5000 yr.  相似文献   

14.
The Permian–Triassic boundary (PTB) is a world‐wide event characterized by the most extensive mass extinction in the history of life. In the Persian Gulf, the rock record of this time interval host one of the most important hydrocarbon reserves in the world: the South Pars Gas Field and its southern extension, the North Dome (or North Field). These carbonate and evaporite successions were sampled in eight wells for petrographic, geochemical and porosity–permeability studies. An important characteristic of the Dalan and Kangan formations is the centimetre‐scale lithological heterogeneities caused by facies changes and diagenetic imprints that led to the compartmentalization of these reservoirs. These Permian–Triassic (P‐T) sediments were deposited in a shallow marine homoclinal ramp. The PTB in this hydrocarbon field is represented by a reworked coarse‐grained intraclastic/bioclastic grainstone facies deposited during a marine transgression. Prolonged subaerial exposure in the P‐T transition caused hypersaline and meteoric diagenesis, including extensive cementation, dolomitization and some dissolution, influencing reservoir characteristics of bordering units. Both δ18O and δ13C values in this succession mirror worldwide excursions typical of other P‐T sections, with some variations due to diagenetic alterations. A pronounced decline in 87Sr/86Sr values, reflective of global seawater geochemistry for most of the Permian is evident in our data. Reservoir quality declines through the late Permian, as a result of facies change and diagenesis. The Late Permian is succeeded by a Triassic transgressive facies and decline in reservoir quality. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

15.
The lacustrine deposits of lakes in arid central Asia (ACA) potentially record palaeoclimatic changes on orbital and suborbital time scales, but such changes are still poorly understood due to the lack of reliable chronologies. Bosten Lake, the largest freshwater inland lake in China, is located in the southern Tianshan Mountains in central ACA. A 51.6‐m‐deep lacustrine succession was retrieved from the lake and 30 samples from the succession were used for luminescence dating to establish a chronology based on multi‐grain quartz OSL and K‐feldspar post‐IR IRSL (pIRIR290) dating. Quartz OSL ages were only used for samples from the upper part of the core. The K‐feldspar luminescence characteristics (dose recovery test, anomalous fading test, first IR stimulation temperature plateau test) are satisfactory and from the relationship amongst the quartz OSL, IR50 and pIRIR290 doses we infer that the feldspar signals are likely to be well bleached at deposition. Bacon age‐depth modelling was used to derive a chronology spanning the last c. 220 ka. The chronology, lithology and grain‐size proxy record indicate that Bosten Lake formed at least c. 220 ka ago and that lake levels fluctuated frequently thereafter. A stable deep lake occurred at c. 220, 210–180, c. 165, 70–60, 40–30 and 20–5 ka, while shallow levels occurred at c. 215, 180–165, 100–70, 60–40 and 30–20 ka. Bosten Lake levels decreased by at least ~29 m and possibly the lake even dried up between c. 160 and c. 100 ka. We suggest that the water‐level fluctuations in the lakes of ACA may not respond directly to climatic changes and may be affected by a number of complex factors.  相似文献   

16.
We present elemental concentrations and magnetic susceptibility data from a new 270‐cm‐long sediment core collected from the western part of palaeolake Babicora and infer millennial‐scale hydrological variations over the last 27 cal. ka in the western Chihuahua Desert. Variations in the available water content at the sediment–air interface of the watershed, lake salinity and lake productivity are inferred from values of the chemical index of alteration (CIA), CaCO 3 and Corg, respectively. An abrupt increase in runoff at c. 24 cal. ka BP appears correlative with the Heinrich 2 (H2) event. Except for this event, diminished runoff between c. 27 and 19 cal. ka BP indicates lower annual precipitation (weak summer rainfall) during the Last Glacial Maximum. The deposition of chemically altered sediments between c. 25 and 22 cal. ka BP results from the higher sediment–water interaction in the watershed owing to lower evaporation, cooler conditions and higher precipitation during the H2 event. Since 19 cal. ka BP the runoff has been characterized by high‐amplitude fluctuations with intervals of reduced precipitation identified at c. 19, 18, 17.5, 13–14, 11.5, 10, 7.5 and 3 cal. ka BP.  相似文献   

17.
《Sedimentology》2018,65(6):1973-2002
Dolomitization is commonly associated with crustal‐scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass‐imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1·43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface‐breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0·7 m year−1 into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal‐scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a mechanism for dolomitization from seawater that has none of the inherent mass balance problems of classical, conceptual models of hydrothermal dolomitization.  相似文献   

18.

Given the recent historical disastrous tsunamis and the knowledge that the Arabian Gulf (AG) is tectonically active, this study aimed to evaluate tsunami hazards in Kuwait from both submarine earthquakes and subaerial landslides. Despite the low or unknown tsunami risks that impose potential threats to the coastal area’s infrastructures and population of Kuwait, such an investigation is important to sustain the economy and safety of life. This study focused on tsunamis generated by submarine earthquakes with earthquake magnitudes (M w ) of 8.3–9.0 along the Makran Subduction Zone (MSZ) and subaerial landslides with volumes of 0.75–2.0 km3 from six sources along the Iranian coast inside the AG and one source at the Gulf entrance in Oman. The level of tsunami hazards associated with these tsunamigenic sources was evaluated using numerical modeling. Tsunami model was applied to conduct a numerical tsunami simulation and predict tsunami propagation. For landslide sources, a two-layer model was proposed to solve nonlinear longwave equations within two interfacing layers with appropriate kinematic and dynamic boundary conditions. Threat level maps along the coasts of the AG and Kuwait were developed to illustrate the impacts of potential tsunamis triggered by submarine earthquakes of different scales and subaerial landslides at different sources. GEBCO 30 arc-second grid data and others were used as bathymetry and topography data for numerical modeling. Earthquakes of M w 8.3 and M w 8.6 along the MSZ had low and considerable impacts, respectively, at the Gulf entrance, but negligible impacts on Kuwait. An earthquake of M w 9.0 had a remarkable impact for the entire Gulf region and generated a maximum tsunami amplitude of up to 0.5 m along the Kuwaiti coastline 12 h after the earthquake. In the case of landslides inside the AG, the majority impact occurred locally near the sources. The landslide source opposite to Kuwait Bay generated the maximum tsunami amplitudes reaching 0.3 m inside Kuwait Bay and 1.8 m along the southern coasts of Kuwait.

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19.
After the Marinoan glaciation, macroscopic organisms thrived in the Yangtze Sea, South China, during the Ediacaran period. The Wenghui biota, which is found from the upper Doushantuo Formation black shales (>551 Ma) in northeastern Guizhou, South China, includes macroscopic algae, metazoans and ichnofossils. Most macroalgae in the Wenghui biota bear a holdfast to secure them onto seafloor and have a thallus of various lengths extending into the water column. This biota can be divided into Globusphyton, Sectoralga–Longifuniculum, Cucullus, Beltanelliformis and Baculiphyca–Gesinella assemblages. A fossil-barren interval containing a thin layer of feldspathic sandstone separates the macroscopic organisms into two distinct parts. From the Globusphyton assemblage through the Sectoralga–Longifuniculum assemblage to the Cucullus assemblage, metazoans show a positive correlation with the abundance and diversity of branching macroalgae at both metre and millimetre scales. Nevertheless, both Beltanelliformis and Baculiphyca–Gesinella assemblages, in which the number and diversity of macroscopic algae and metazoans, especially the shorter branching macroalgae, are obviously decreasing or even lacking, might be related to a special environment and a fragile ecosystem. In addition, the ratios of Ni/Co, U/Th and V/(V + Ni) display zigzagged profiles at millimetre scales indicating frequent redox fluctuations. Variations in macrofossils and trace elements at both millimetre and metre scales indicate that the oxygen content in the northeast Guizhou Sea fluctuated frequently during the middle–late Ediacaran period and the Wenghui biota possibly lived in the redox buffering zone. Moreover, the abundance and diversity of macroalgae, especially the branching macroalgae, could have significantly influenced the redox conditions in water column. The increase in oxygen may have improved the environment for the growth and reproduction of macroalgae and metazoans.  相似文献   

20.
A high‐resolution study of coccolithophores in a sediment core from the East China Sea (ECS) region of the Northwest Pacific revealed that fluctuations in coccolithophore assemblages responded well to the Pacific Decadal Oscillation (PDO) during 1901–2013. Emiliania huxleyi and Gephyrocapsa oceanica were the most dominant species, accounting for more than 97% of all the ten detected species. The variation of the two dominant species showed a significant positive correlation, and they both increased during peak values of the PDO. Decreased stratification and increased nutrient supplementation from the bottom water induced by lower sea surface temperatures in the ECS, which were associated with high PDO values, were responsible for the dominant coccolithophore increase. Similarly, the increase of Calcidiscus leptoporus also corresponded to low‐temperature and high‐nutrient conditions associated with the cold phase in the ECS (positive phase of the PDO). The responses of decadal C. leptoporus variations to changes in cold and warm phases of the PDO in the Northwest and Northeast Pacific indicate that C. leptoporus preserved in sediment cores represents a potential indicator for reconstructing phase changes of the PDO during ancient times.  相似文献   

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