What controls submarine channel development and the morphology of deltas entering deep-water fjords?     

Jenny A. Gales  Peter J. Talling  Matthieu J.B. Cartigny  John Hughes Clarke  Gwyn Lintern  Cooper Stacey  Michael A. Clare 《地球表面变化过程与地形》2019,44(2):535-551

River deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet. These systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits. Here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords, and why some locations have well developed submarine channels while others do not. Deltas and associated turbidity current systems are analysed initially in five fjord systems from British Columbia in Canada, and then more widely. This provides the basis for a general classification of delta and turbidity current system types, where rivers enter relatively deep (>200 m) water. Fjord-delta area is found to be strongly bimodal. Avalanching of coarse-grained bedload delivered by steep mountainous rivers produces small Gilbert-type fan deltas, whose steep gradient (11°–25°) approaches the sediment's angle of repose. Bigger fjord-head deltas are associated with much larger and finer-grained rivers. These deltas have much lower gradients (1.5°–10°) that decrease offshore in a near exponential fashion. The lengths of turbidity current channels are highly variable, even in settings fed by rivers with similar discharges. This may be due to resetting of channel systems by delta-top channel avulsions or major offshore landslides, as well as the amount and rate of sediment supplied to the delta front by rivers. © 2018 John Wiley & Sons, Ltd.  相似文献   

Estimating external magnetic field differences at high geomagnetic latitudes from a single station          下载免费PDF全文

Ciarán D. Beggan  Laurence Billingham  Ellen Clarke 《Geophysical Prospecting》2018,66(6):1227-1240

Providing an accurate estimate of the magnetic field on the Earth's surface at a location distant from an observatory has useful scientific and commercial applications, such as in repeat station data reduction, space weather nowcasting or aeromagnetic surveying. While the correlation of measurements between nearby magnetic observatories at low and mid‐latitudes is good, at high geomagnetic latitudes () the external field differences between observatories increase rapidly with distance, even during relatively low magnetic activity. Thus, it is of interest to describe how the differences (or errors) in external magnetic field extrapolation from a single observatory grow with distance from its location. These differences are modulated by local time, seasonal and solar cycle variations, as well as geomagnetic activity, giving a complex temporal and spatial relationship. A straightforward way to describe the differences are via confidence intervals for the extrapolated values with respect to distance. To compute the confidence intervals associated with extrapolation of the external field at varying distances from an observatory, we used 695 station‐years of overlapping minute‐mean data from 37 observatories and variometers at high latitudes from which we removed the main and crustal fields to isolate unmodelled signals. From this data set, the pairwise differences were analysed to quantify the variation during a range of time epochs and separation distances. We estimate the 68.3%, 95.4% and 99.7% confidence levels (equivalent to the 1σ, 2σ and 3σ Gaussian error bounds) from these differences for all components. We find that there is always a small non‐zero bias that we ascribe to instrumentation and local crustal field induction effects. The computed confidence intervals are typically twice as large in the north–south direction compared to the east‐west direction and smaller during the solstice months compared to the equinoxes.  相似文献   

Long‐term sediment yield from a small catchment in southern Brazil affected by land use and soil management changes          下载免费PDF全文

Jean P.G. Minella  Gustavo H. Merten  Claúdia A.P. Barros  Rafael Ramon  Alexandre Schlesner  Robin T. Clarke  Michele Moro  Leandro Dalbianco 《水文研究》2018,32(2):200-211

Sediments produced from eroding cultivated land can cause on‐site and off‐site effects that cause considerable economic and social impacts. Despite the importance of soil conservation practices (SCP) for the control of soil erosion and improvements in soil hydrological functions, limited information is available regarding the effects of SCP on sediment yield (SY) at the catchment scale. This study aimed to investigate the long‐term relationships between SY and land use, soil management, and rainfall in a small catchment. To determine the effects of anthropogenic and climatic factors on SY, rainfall, streamflow, and suspended sediment concentration were monitored at 10‐min intervals for 14 years (2002–2016), and the land use and soil management changes were surveyed annually. Using a statistical procedure to separate the SY effects of climate, land use, and soil management, we observed pronounced temporal effects of land use and soil management changes on SY. During the first 2 years (2002–2004), the land was predominantly cultivated with tobacco under a traditional tillage system (no cover crops and ploughed soil) using animal traction. In that period, the SY reached approximately 400 t·km^?2·year^?1. From 2005 to 2009, a soil conservation programme introduced conservation tillage and winter cover crops in the catchment area, which lowered the SY to 50 t·km^?2·year^?1. In the final period (2010–2016), the SCP were partially abandoned by farmers, and reforested areas increased, resulting in an SY of 150 t·km^?2·year^?1. This study also discusses the factors associated with the failure to continue using SCP, including structural support and farmer attitudes.  相似文献   

Revisiting the death of geography in the era of Big Data: the friction of distance in cyberspace and real space   总被引：1，自引：0，他引：1  

Su Yeon Han  Keith C. Clarke 《International Journal of Digital Earth》2018,11(5):451-469

Many scholars have argued that the importance of geographic proximity in human interactions has been diminished by the use of the Internet, while others disagree with this argument. Studies have noted the distance decay effect in both cyberspace and real space, showing that interactions occur with an inverse relationship between the number of interactions and the distance between the locations of the interactors. However, these studies rarely provide strong evidence to show the influence of distance on interactions in cyberspace, nor do they quantify the differences in the amount of friction of distance between cyberspace and real space. To fill this gap, this study used massive amounts of social media data (Twitter) to compare the influence of distance decay on human interactions between cyberspace and real space in a quantitative manner. To estimate the distance decay effect in both cyberspace and real space, the distance decay function of interactions in each space was modeled. Estimating the distance decay in cyberspace in this study can help predict the degree of information flow across space through social media. Measuring how far ideas can be diffused through social media is useful for users of location-based services, policy advocates, public health officials, and political campaigners.  相似文献   

Waldo R. Tobler (1930–2018)     

Keith C. Clarke 《制图学和地理信息科学》2018,45(4):287-288

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The evolution of zircon during low‐P partial melting of metapelitic rocks: theoretical predictions and a case study from Mt Stafford,central Australia     

WEI‐ Wang  E. Dunkley  G. L. Clarke  N. R. Daczko 《Journal of Metamorphic Geology》2014,32(8):791-808

Palaeoproterozoic metasedimentary migmatite reflects the highest temperature parts of a regional aureole at Mt Stafford, central Australia, comprising rocks that experienced 500–800 °C at ≈3 kbar. Whole‐rock major element concentrations are correlated with Zr content, psammitic compositions having nearly twice the Zr content of pelitic compositions. Zirconium is concentrated in mesosome compared with leucosome. Zircon is largely detrital, mostly lacking any overgrowth contemporary with migmatite formation. Comparatively small proportions of micro‐zircon (<10 μm) in sub‐solidus rocks are mostly hosted by quartz and plagioclase. Much higher proportions (three to five times) of micro‐zircon in migmatite are hosted by prograde K‐feldspar, cordierite and biotite. T–X and P–T NCKFMASHTZr pseudosections constructed using thermocalc model the distribution of Zr between solid and silicate liquid phases. Half of the detrital zircon (~100 ppm Zr) is predicted to be dissolved into silicate liquid at ≈800 °C and all dissolved by 850 °C, if all zircon is involved in the equilibration volume. Melt segregation at relatively low temperature is predicted to enrich the residuum in Zr, consistent with the observed distribution of Zr between mesosome and leucosome. The limited development of metamorphic zircon rims or overgrowths at Mt Stafford is explained by three concurrent processes: (i) Zr liberated during prograde metamorphism formed micro‐zircon, rather than following the prediction that Zr will partition into silicate liquid; (ii) some detrital zircon was probably armoured by other rock‐forming minerals, reducing Zr content in the effective bulk rock composition; and (iii) small proportions of melt loss during migmatization removed Zr that otherwise would have been available to form metamorphic rims.  相似文献   

Clitellate worms (Annelida) in lateglacial and Holocene sedimentary DNA records from the Polar Urals and northern Norway     

Youri Lammers  Charlotte L. Clarke  Christer Ersus  Antony G. Brown  Mary E. Edwards  Ludovic Gielly  Haflidi Haflidason  Jan Mangerud  Emilia Rota  John Inge Svendsen  Inger Greve Alsos 《Boreas: An International Journal of Quaternary Research》2019,48(2):317-329

While there are extensive macro‐ and microfossil records of a range of plants and animals from the Quaternary, earthworms and their close relatives amongst annelids are not preserved as fossils and therefore the knowledge of their past distributions is limited. This lack of fossils means that clitellate worms (Annelida) are currently underused in palaeoecological research, even though they can provide valuable information about terrestrial and aquatic environmental conditions. Their DNA might be preserved in sediments, which offers an alternative method for detection. Here we analyse lacustrine sediments from lakes in the Polar Urals, Arctic Russia, covering the period 24 000–1300 cal. a BP, and NE Norway, covering 10 700–3300 cal. a BP, using a universal mammal 16S rDNA marker. While mammals were recorded using the marker (reindeer was detected twice in the Polar Urals core at 23 000 and 14 000 cal. a BP, and four times in the Norwegian core at 11 000 cal. a BP and between 3600–3300 cal. a BP), worm extracellular DNA ‘bycatch’ was rather high. In this paper we present the first reported worm detection from ancient DNA. Our results demonstrate that both aquatic and terrestrial clitellates can be identified in late‐Quaternary lacustrine sediments, and the ecological information retrievable from this group warrants further research with a more targeted approach.  相似文献   

Particle‐size evidence of barrier estuary regime as a new proxy for ENSO climate variability          下载免费PDF全文

David W. Clarke  John F. Boyle  Andrew J. Plater 《地球表面变化过程与地形》2017,42(10):1520-1534

We investigate a new proxy for ENSO climate variability based on particle‐size data from long‐term, coastal sediment records preserved in a barrier estuary setting. Corresponding ~4–8 year periodicities identified from Wavelet analysis of particle‐size data from Pescadero Marsh in Central Coast California and rainfall data from San Francisco reflect established ENSO periodicity, as further evidenced in the Multivariate ENSO Index (MEI), and thus confirms an important ENSO control on both precipitation and barrier regime variability. Despite the fact that barrier estuary mean particle size is influenced by coastal erosion, precipitation and streamflow, balanced against barrier morphology and volume, it is encouraging that considerable correspondence can also be observed in the time series of MEI, regional rainfall and site‐based mean particle size over the period 1871–2008. This correspondence is, however, weakened after c.1970 by temporal variation in sedimentation rate and event‐based deposition. These confounding effects are more likely when: (i) accommodation space may be a limiting factor; and (ii) particularly strong El Niños, e.g. 1982/1983 and 1997/1998, deposit discrete >cm‐thick units during winter storms. The efficacy of the sediment record of climate variability appears not to be compromised by location within the back‐barrier setting, but it is limited to those El Niños that lead to barrier breakdown. For wider application of this particle size index of ENSO variability, it is important to establish a well‐resolved chronology and to sample the record at the appropriate interval to characterize deposition at a sub‐annual scale. Further, the sample site must be selected to limit the influence of decreasing accommodation space through time (infilling) and event‐based deposition. It is concluded that particle‐size data from back‐barrier sediment records have proven potential for preserving evidence of sub‐decadal climate variability, allowing researchers to explore temporal and spatial patterns in phenomena such as ENSO. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Geology and regional significance of the Sarnoo Hills,eastern rift margin of the Barmer Basin,NW India          下载免费PDF全文

Andrew J. Bladon  Stuart D. Burley  Stuart M. Clarke  Hazel Beaumont 《Basin Research》2015,27(5):636-655

The Barmer Basin is a poorly understood rift basin in Rajasthan, northwest India. Exposures in the Sarnoo Hills, situated along the central eastern rift margin of the Barmer Basin, reveal a sedimentary succession that accumulated prior to the main Barmer Basin rift event, and a rift‐oblique fault network that displays unusual geometries and characteristics. Here, we present a comprehensive study of Lower Cretaceous sedimentology on the basin margin, along with a detailed investigation of rift‐oblique faults that are exposed nowhere else in the region and provide critical insights into Barmer Basin evolution. Lower Cretaceous sediments were deposited within a rapidly subsiding alluvial plain fluvial system. Subsequent to deposition, the evolving Sarnoo Hills fault network was affected by structural inheritance during an early, previously unrecognised, rift‐oblique extensional event attributed to transtension between India and Madagascar, and formed a juvenile fault network within the immediate rift‐margin footwall. Ghaggar‐Hakra Formation deposition may have been triggered by early rifting which tectonically destabilised the Marwar Craton prior to the main northeast–southwest Barmer Basin rift event. The identification of early rifting in the Barmer Basin demonstrates that regional extension and the associated rift systems were established throughout northwest India prior to the main phase of Deccan eruptions. Inheritance of early oblique fault systems within the evolving Barmer Basin provides a robust explanation for poorly understood structural complications interpreted in the subsurface throughout the rift. Critically, the presence of syn‐rift sedimentary successions within older oblique rift systems obscured beneath the present‐day Barmer Basin has significant implications for hydrocarbon exploration.  相似文献   

Implications of simultaneously mitigating and adapting to climate change: initial experiments using GCAM     

Katherine Calvin  Marshall Wise  Leon Clarke  Jae Edmonds  Page Kyle  Patrick Luckow  Allison Thomson 《Climatic change》2013,117(3):545-560

Most research on future climate change discusses mitigation and impacts/adaptation separately. However, mitigation will have implications for impacts and adaptation. Similarly, impacts and adaptation will affect mitigation. This paper begins to explore these two veins of research simultaneously using an integrated assessment model. We begin by discussing the types of interactions one might expect by impact sector. Then, we develop a numerical experiment in the agriculture sector to illustrate the importance of considering mitigation, impacts, and adaptation at the same time. In our experiment, we find that climate change can reduce crop yields, resulting in an expansion of cropland to feed a growing population and a reduction in bioenergy production. These two effects, in combination, result in an increase in the cost of mitigation.  相似文献