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Deposits within caves are often used to interpret past landscape evolution and climate conditions. However, cave passage shapes also preserve information about past conditions. Despite the usefulness of passage shape, no previous models simulate cave cross-section evolution in a realistic manner. Here we develop a model for evolving cave passage cross-sections using a shear stress estimation algorithm and a shear stress erosion rule. Our model qualitatively duplicates observed cave passage shapes so long as erosion rates vary with shear stress, as in the case of transport limited dissolution or mechanical erosion. This result provides further evidence that erosion rates within caves are not typically limited by surface reaction rates, even though current speleogenesis models predict surface-rate limitation under most turbulent flow conditions. By adding sediment transport and alluviation to the model we successfully simulate paragenetic channels. Simulations duplicate the hypothesized dynamics of paragenesis, whereby: 1) the cross-section of a phreatic passage grows until shear stress is sufficiently reduced that alluviation occurs, 2) the floor of the passage becomes armored and erosion continues on the ceiling and walls, 3) negative feedback produces an equilibrium cross-sectional area such that shear stress is sufficient to transport incoming sediment. We derive an approximate scaling relationship that indicates that equilibrium paragenetic channel width scales with the square root of discharge, and weakly with the inverse of sediment supply. Simulations confirm this relationship and show that erosion mechanism, sediment size, and roughness are secondary controls. The inverse scaling of width with sediment supply in paragenetic channels contrasts with surface bedrock channels, which respond to larger sediment supplies by widening. Our model provides a first step in simulating cave cross-section evolution and points to the need for a better understanding of the dominant erosion mechanisms in soluble bedrock channels. © 2020 John Wiley & Sons, Ltd. 相似文献
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Surface sediment, speleothems, and bat guano from two adjacent, topographically different cave sites in eastern Spain have been studied palynologically to elucidate the potential of cave sediments for palaeoenvironmental reconstruction. A cave opening with a large entrance and constant width presented far fewer problems of alteration in the pollen assemblages than a sac‐like cave opening with a constricted entrance. Pollen concentration is linked primarily to the amount of pollen input rather than to the result of post‐depositional alteration. Sampling should be undertaken away from parietal and rear areas and avoiding moisture zones. Lateral differences in the pollen spectra indicate that sampling should be on the basis of a multiple‐profile approach and selection of dry rather than wet sediments. If these procedures are followed, within‐cave sediments can realistically reflect not only local but also regional vegetation of the site. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
105.
Genuario Belmonte Gianmarco Ingrosso Marco Poto Gianluca Quarta Marisa D'Elia Raffaele Onorato & Lucio Calcagnile 《Marine Ecology》2009,30(3):376-382
Submarine caves at the Cape of Otranto (SE, Italy) host pseudostalactites. Only recently have they been recognised as originating mainly from the marine tube worm Protula tubularia (Serpulidae, Polychaeta). Their formation depends on the adaptability, life history and behaviour of that species in the cave habitat. The dating of calcareous tubes in the pseudostalactite core represents the first attempt to describe the formation of such bio-construction, and the gregarious behaviour of the species. Calcareous tubes along the core axis of three pseudostalactites removed from the cave lu Lampiùne (Otranto, South East Italy) have been dated using the 14 C method. Measures established ages from 2600 to 5000 years for the three structures. This extremely long period (when compared with the life span of each worm) suggests that each tube aggregation is the result of punctuated growth periods alternated with long standstill phases. At the end of a growth period, each tube aggregation was covered with a brownish concretion, completing the pseudostalactite. 相似文献
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Relatively little is known about the long-term sediment accumulation dynamics of Naracoorte Cave Complex (NCC) solution pipe cavities, and many of the megafauna-bearing infill deposits at this globally significant Australian Pleistocene fossil locality remain partially dated or lack any numerical age control. In this study, we assess the suitability of three different luminescence dating signals for improving existing chronologies at six Late and Middle Pleistocene NCC sites (n = 22 samples), and we undertake multi-site examinations of NCC sediment infill dynamics spanning the last 550 thousand years (ka). Modern analogue samples collected from above and beneath two active cave entrances confirm that single-grain OSL, single-grain TT-OSL and multi-grain pIR-IRSL signals can be reset down to insignificant residual levels (<10−1–100 Gy) when compared with the natural dose ranges of interest for most NCC palaeontological applications. Replicate luminescence dating comparisons performed at six NCC fossil sites (n = 15 samples) reveal consistent ages for twenty-eight out of thirty-one paired OSL–TT-OSL, OSL–pIR-IRSL and TT-OSL–pIR-IRSL datasets. Nineteen of the twenty Middle to Late Pleistocene samples analysed from the NCC sites produce homogeneous OSL, TT-OSL and pIR-IRSL De datasets suggesting that the NCC solution pipe deposits considered here are generally not affected by syn-depositional mixing complications that can take place within closed karst cavities (i.e., the remobilisation of unbleached grains from pre-existing cave floor sediments, and their subsequent translocation and incorporation into deposits within deeper parts of the cavity along with externally bleached, allochthonous grain populations). Detailed examination of solution pipe dynamics at Smoke Tortoise Cave (SMT) reveals a complex accumulation history focused on the marine isotope stage (MIS) 9 and MIS 7 interglacial complexes, as well as the MIS 8e interstadial. The SMT case study highlights that NCC solution pipes are not simply associated with short-lived opening and sediment accumulation events, but may involve multiple, discontinuous deposition episodes and reactivation events. An initial multi-site examination of all published NCC infill chronologies (n = 70) appears to suggest statistically significant, preferential solution pipe development during the relatively wet parts of interglacial or interstadial cycles. The non-uniform infill age distribution implies that NCC solution pipe dynamics may have exerted taphonomic biases on fossil accumulation, which should be taken into consideration when reconstructing long-term palaeoecological histories from NCC solution pipe cavities. 相似文献
107.
针对地下溶洞不同枯盈状态下的地震波响应差异问题,选择三种枯盈状态下的溶洞模型进行了一项数值模拟实验。先从弹性波方程理论出发,利用有限差分离散近似,再运用CUDA编程平台进行GPU并行计算,模拟了三种枯盈状态下的溶洞地震波响应。模拟结果表明,P-P波的能量随着溶洞含水量增多而减少,而溶洞尖灭点的绕射现象随着溶洞含水量增多而变得更加明显。 相似文献
108.
济南南部山区是山东喀斯特洞穴发育地区之一,所统计的34个中,长度在50~500 m的较大洞穴有7个.从洞穴发育的基本特征看,较大的洞穴多呈单通道水平廊道状,洞内保留有许多典型的溶蚀小形态(如窝穴、波痕类),但洞内沉积物较为单一,主要是一些崩积物、残积物和渗流水造成的碎屑物,碳酸钙化学沉积物相对较少,溶洞多表现为一种单调的干洞特点.从成因上看:较大洞穴属于具有潜流成因的次生渗流带洞穴;较小洞穴有裂隙状的渗流带洞穴、差异风化成因的壁龛式洞穴等.这些洞穴具有较大的开发潜力. 相似文献
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Evaporitic‐lagoonal marl and dolomite laminar fill sediments are preserved in relict dry caves of the Dead Sea Fault Escarpment (Israel) which has been tectonically active since the Late Neogene. The hosting caves are located within Turonian massive carbonate bedrock and at higher altitudes than previously documented fill sediments of the Dead Sea depression. Based on the relative altitudes of the cave sediments, the ‘reversed stratigraphy’ of the Dead Sea depression fill sediments, possible partial correlation of the cave sediments with other fill sedimentary units of the depression, and sedimentary, geochemical and mineralogical characteristics, it is concluded that: (i) the cave sediments are among the oldest of the depression fill; and (ii) the deposition of the cave sediments took place in hypersaline dolomite‐precipitating water bodies of Late Neogene age, during the initial morphotectonic stages of the depression formation. Variable and relatively low Sr/Ca and δ34S ratios of the cave sediments (assuming precipitation from sea water) suggest variable fresh water input into the depositional brine. The present altitudes of the cave sediments reflect Late Neogene levels of water bodies in the depression, modified by vertical post‐Late Neogene tectonic movements within the still active fault escarpment. According to these altitudes, a 50 to 250 m uplift of the western margins of the depression since the Late Miocene to Early Pliocene is inferred. 相似文献