Sedimentological and ichnological implications of rapid Holocene flooding of a gently sloping mud‐dominated incised valley – an example from the Red River (Gulf of Tonkin)          下载免费PDF全文

Andreas Wetzel  Agata Szczygielski  Daniel Unverricht  Karl Stattegger 《Sedimentology》2017,64(5):1173-1202

The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year^?1 landward during Holocene sea‐level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full‐marine deposits. Thus, the river mouth represented a mud‐dominated open funnel‐shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea‐level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base‐level rise is documented by non‐bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short‐term seawater incursions. Siderite in overlying deposits points to low‐salinity estuarine conditions. The open funnel‐shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short‐term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr^?1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full‐marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated.  相似文献   

Evolution of sedimentary environments of the middle Jiangsu coast, South Yellow Sea since late MIS 3   总被引：3，自引：0，他引：3  

Fei Xia  Yongzhan Zhang  Qiang Wang  Yong Yin  Karl W. Wegmann  J. Paul Liu 《地理学报(英文版)》2013,23(5):883-914

An evolutionary model of sedimentary environments since late Marine Isotope Stage 3 （late MIS 3, i.e., ca. 39 cal ka BP） along the middle Jiangsu coast is presented based upon a reinterpretation of core 07SR01, new correlations between adjacent published cores, and shallow seismic profiles recovered in the Xiyang tidal channel and adjacent northern sea areas. Geomorphology, sedimentology, radiocarbon dating and seismic and sequence stratigraphy are combined to confirm that environmental changes since late MIS 3 in the study area were controlled primarily by sea-level fluctuations, sediment discharge of paleo-rivers into the South Yellow Sea （SYS）, and minor tectonic subsidence, all of which impacted the progression of regional geomorphic and sedimentary environments （Le., coastal barrier island freshwater lacustrine swamp, river floodplain, coastal marsh, tidal sand ridge, and tidal channel）. This resulted in the formation of a fifth-order sequence stratigraphy, comprised of the parasequence of the late stage of the last interstadial （Para-Sq2）, including the highstand and forced regressive wedge system tracts （HST and FRWST）, and the parasequence of the postglacial period （Para-Sql）, including the transgressive and highstand system tracts （TST and HST）. The tidal sand ridges likely began to develop during the postglacial transgression as sea-level rise covered the middle Jiangsu coast at ca. 9.0 cal ka BP. These initially submerged tidal sand ridges were constantly migrating until the southward migration of the Yellow River mouth to the northern Jiangsu coast during AD 1128 to 1855. The paleo-Xiyang tidal channel that was determined by the paleo-tidal current field and significantly different from the modern one, was in existence during the Holocene transgressive maxima and lasted until AD 1128. Following the capture of the Huaihe River in AD 1128 by the Yellow River, the paleo-Xiyang tidal channel was infilled with a large amount of river-derived sediments from AD 1128 to 1855, causing the emergence of some of the previously submerged tidal sand ridges. From AD 1855 to the present, the infilled paleo-Xiyang tidal channel has undergone scouring, resulting in its modern form. The modern Xiyang tidal channel continues to widen and deepen, due both to strong tidal current scouring and anthropogenic activities.  相似文献   

PERSPECTIVE ON CLIMATE CHANGE IN NORTH AMERICA DURING THE TWENTIETH CENTURY     

Thomas R. Karl 《自然地理学》2013,34(3):207-229

The climate record from many parts of North America provides ample evidence of climate change. For specific elements such as temperature or precipitation, a substantial portion of these changes can be shown to be statistically significant; that is, they either reflect an unusual combination of events in a stationary process or they reflect a nonstationary process. The practical significance of a climate change, however, even when it can be shown to be statistically significant, cannot be assumed. Conversely, it is entirely possible to have a practically significant climate change without statistical significance, although this is less likely to occur.

The preponderance of evidence over recent years suggests that any notion regarding static climate regimes must be dismissed in favor of a dynamic system. The differential heating of our planet which results in a restless atmosphere can also yield a restless climate. Whether this dynamic climate system is merely a manifestation of natural variations or at least partly caused by changes in external influences, we cannot yet be certain. The fact that the climate has changed in North America during the 20th century, and in all likelihood will probably continue to do so in the future, should be considered in any decision-making process related to climate. [Key words: climatic change.]  相似文献   

The Holocene Lake Plain of North Rudolph,East Africa     

Karl W. Butzer 《自然地理学》2013,34(1):42-58

During most of the early and middle Holocene, Lake Rudolf was 75m deeper than today, flooding the Kibish Lake Plain as much as 60–100km north and northwest of the modern shore, with periodic overflow across the White Nile watershed. The landforms and sediment sequences of this lake plain are mapped or profiled, described, and integrated into a regional geomorphologic framework, controlled by a suite of ¹⁴C dates. High lake stands are dated 10,000–7,000, 6500–4000 and about 3250 years ago (uncalibrated), with levels fluctuating around that of the present 7000–6500 and since 2500 years. Environmental changes of such magnitude reflect long-term changes of East African climate, so that the Rudolf Basin provides a well-documented index of regional climatic trends for the last 10 millenia. Since shortly after 10,000 years, prehistoric fishing and hunting settlements dotted the rivers and shorelines, favored by optimal aquatic resources but inhibited by a higher incidence of malaria. Aquatic subsistence patterns survived severe but temporary lake shrinkage 7000 years ago; during the last 5 millennia they have gradually given way to pastoral cultures. The Kibish Lake Plain serves to illustrate that geographic landscape analysis can rarely afford to be ahistorical.  相似文献   

Cyclic and Seasonal Beach Response: A Comparison of Oceanside and Bayside Beaches     

Karl F. Nordstrom 《自然地理学》2013,34(2):177-196

This study compares the cyclic and seasonal geomorphic responses of oceanside and bayside beaches on Sandy Hook Spit, New Jersey. It was hypothesized that the different nature of onshore wind regimes at bayside and oceanside beaches would cause different types of beach change. On the east-facing ocean shoreline, rates of beach change are related to weather patterns associated with the passage of mid-latitude cyclonic storms. Bayside beach change is related to the influence of the prevailing westerlies. Storm erosion and post-storm deposition is more rapid on oceanside beaches. Swell waves occurring between storms rapidly reinstate pre-storm equilibrium conditions. Lower bay-side wave energies occurring between storms have little effect on profile development, and foreshore slopes inherited from previous storms undergo little change. Bayside beaches therefore appear to be more in equilibrium with storm conditions than oceanside beaches.

Changes in foreshore slope, beach volume, and beach position associated with individual storm events are compared to seasonal trends to test the applicability of the Hayes cyclic model of beach development to both oceanside and bayside beaches. There is evidence of a cyclic trend of develop-ment on the more exposed ocean sites and seasonal development on the more sheltered bayside sites, which suggests that cyclic development may be causally related to the difference among the energy levels of the storm and nonstorm wave regimes.  相似文献   

Chemical characteristics (REE,etc.) of Paleozoic and Mesozoic graywackes and sandstones from Central Europe     

Karl?Hans?Wedepohl  Klaus?SimonEmail author 《Contributions to Mineralogy and Petrology》2012,164(4):693-696

During the Variscan orogeny in Central Europe, partial melting in the lower continental crust formed granitic magmas, which intruded into the upper crust and left compounds of Ca (plus Eu²⁺), Mg, etc. in the lower crust. From the late Paleozoic decomposition of the tonalitic upper crust, sedimentary graywackes were produced reflecting the composition of this crust. The repeated reworking of the sedimentary cover caused the formation of sands. Sandstones as their products of consolidation contain increasing fractions of quartz and decreasing feldspar from Carboniferous and Triassic to Cretaceous age. A distinct negative Eu anomaly characterizes the majority of these rocks. The latter is imprinted by the Variscan magmatism. Quartz as used for numerous Medieval wood ash glasses is marked for its Central European origin by a distinct negative Eu anomaly in contrast to many soda glasses produced outside Germany mostly with a small or none Eu anomaly.  相似文献   

Response of single benthic metrics and multi-metric methods to anthropogenic pressure gradients, in five distinct European coastal and transitional ecosystems   总被引：4，自引：0，他引：4  

Borja A  Barbone E  Basset A  Borgersen G  Brkljacic M  Elliott M  Garmendia JM  Marques JC  Mazik K  Muxika I  Magalhães Neto J  Norling K  Rodríguez JG  Rosati I  Rygg B  Teixeira H  Trayanova A 《Marine pollution bulletin》2011,62(3):499-513

In recent times many benthic indices have been proposed to assess the ecological quality of marine waters worldwide. In this study we compared single metrics and multi-metric methods to assess coastal and transitional benthic status along human pressure gradients in five distinct environments across Europe: Varna bay and lake (Bulgaria), Lesina lagoon (Italy), Mondego estuary (Portugal), Basque coast (Spain) and Oslofjord (Norway). Hence, 13 single metrics (abundance, number of taxa, and several diversity and sensitivity indices) and eight of the most common indices used within the European Water Framework Directive (WFD) for benthic assessment were selected: index of size spectra (ISS), Benthic assessment tool (BAT), Norwegian quality index (NQI), Multivariate AMBI (M-AMBI), Benthic quality index (BQI), (Benthic ecosystem quality index (BEQI), Benthic index based on taxonomic sufficiency (BITS), and infaunal quality index (IQI). Within each system, sampling sites were ordered in an increasing pressure gradient according to a preliminary classification based on professional judgement. The different indices are largely consistent in their response to pressure gradient, except in some particular cases (i.e. BITS, in all cases, or ISS when a low number of individuals is present). Inconsistencies between indicator responses were most pronounced in transitional waters (i.e. IQI, BEQI), highlighting the difficulties of the generic application of indicators to all marine, estuarine and lagoonal environments. However, some of the single (i.e. ecological groups approach, diversity, richness) and multi-metric methods (i.e. BAT, M-AMBI, NQI) were able to detect such gradients both in transitional and coastal environments, being these multi-metric methods more consistent in the detection than single indices. This study highlights the importance of survey design and good reference conditions for some indicators. The agreement observed between different methodologies and their ability to detect quality trends across distinct environments constitutes a promising result for the implementation of the WFD’s monitoring plans. Moreover, these results have management implications, regarding the dangers of misclassification, uncertainty in the assessment, use of conflicting indices, and testing and validation of indices.  相似文献   

Rapid macrofaunal colonization of water-based drill cuttings on different sediments     

Trannum HC  Setvik S  Norling K  Nilsson HC 《Marine pollution bulletin》2011,62(10):2145-2156

A field experiment was conducted to investigate how water-based drill cuttings and sediment type influence colonization of soft bottom communities. Bottom frames with trays containing defaunated sediments were placed at the seabed for 6 months to study colonization of macrofauna. Two different sediments (coarse and fine) were used, and 6 or 24 mm layer of water-based drill cuttings were added on top of these sediments. Some of the sediments were controls with no additions. In the end of the experiment, the oxygen availability in sediment porewater and macrofaunal abundance were reduced in treatments with 24 mm drill cuttings compared to controls. Tube-building annelids were particularly sensitive to drill cuttings. However, these responses were only minor, and notably, the drill cuttings initiated a weaker faunal response than sediment type and site of the bottom frame. Sediments capped with water-based drill cuttings thus showed a rapid colonization of macrofaunal communities.  相似文献   

The chemical composition including the Rare Earth Elements of the three major glass types of Europe and the Orient used in late antiquity and the Middle Ages     

Karl Hans Wedepohl  Klaus Simon  Andreas Kronz 《Chemie der Erde / Geochemistry》2011,71(3):289-296

Sets of 20 soda ash glasses, 16 soda lime glasses and 23 wood ash glasses mainly from excavations in Europe (additional soda ash glasses from Egypt) were analysed on 61 chemical elements. Average SiO₂ is about 62% in soda glasses and 50% in wood ash glasses. The three groups of glasses contain on average 13% Na₂O, 18% Na₂O and 13% K₂O as fluxes to lower the melting temperature of quartz at their production. The starting materials beside quartz were halophytic plant ash for soda ash glass, trona (Na₃H(CO₃)₂·2H₂O) and lime (clamshells) for soda lime glass and beech ash for wood ash glass. Each of the three major glass types contains specific Rare Earth Element (REE) concentrations mainly contained in quartz and its intergrown minerals. 50 Paleozoic and Mesozoic sandstones from Central Europe represent the quartz composition. The REE pattern of these glasses apparently indicates major compositional stages of the Continental Earth's Crust. The boron to lithium and sodium to potassium ratios as in seawater suggest reactions of materials for soda glass with seawater. Negative Ce anomalies in the three glasses are caused by reactions of quartz with seawater.  相似文献   

Using stable isotopes to understand hydrochemical processes in and around a Prairie Pothole wetland in the Northern Great Plains,USA     

Christopher T. Mills  Martin B. Goldhaber  Craig A. Stricker  JoAnn M. Holloway  Jean M. Morrison  Karl J. Ellefsen  Donald O. Rosenberry  Roland S. Thurston 《Applied Geochemistry》2011

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