IV4 PALAEOENVIRONMENT RECONSTRUCTION
WITH KARST RECORDS
David A.Hubbard , Jr.1 and Janet S.Herman2
1 Virginia Division of Mineral Resources ,
2 Department of Environmental Sciences
Clark Hall , University of Virginia
Karst is a negative relief topography formed by the dissolution of carbonate bedrock. Emergent karstic water, as springs or diffuse stream- bankor -bed seeps , has deposited Quaternary age travertine-marl buildups. Most of the travertine-marl deposits in Virginia are associated with known faults in folded and fractured carbonate rock. Some of these accretionary or positive karst features morphologically resemble spelean formations typical of depositional vadose cave environments .The ubiquitous presence of algae and moss in travertine-marl differentiate it from its spelean counterparts , but whether the role of the biota is only as a passive framework or is also as an active metabolic influence remains to be determined.
Locally valued for their aesthetic waterfalls and as a source of agricultural lime , travertine-marl deposits are an environmental barometer. Land-use practices resulting in increased runoff of precipitation and increased erosion have degraded these features: travertine- marl deposits have been smothered and partially buried by erosional debris; features have been destroyed by the corrasive action of large magnitude floods carrying large volumes of abrasive sediment. Polluted karstic water may inhibit calcite nucleation and the growth of framework flora , both of which influence the rate of travertine-marl deposition.
R.D.1 , Box 159 , Feura Bush , NY 12067
Periods of high glacial meltwater have altered some preglacial cave-passage configurations. Floodwater and fossil karst features , whose formation cannot be explained based on available water from the surrounding watershed , are found superposed on actively forming cave passages. These features may be recognized through correlation of watershed boundaries , peak-runoff observations through a cave system , the presence of anomalous in-cave and surface features , and the geomorphic interpretation of the area in question. Knowledge of minimum rates of karstification may be used to infer climatic conditions , making possible the reconstruction of the hydrology associated with deglaciation.
Clarksville Cave , situated in the hamlet of Clarksville , New York , provides an excellent example of invasion by Wisconsinan meltwater on a preglacial cave system. Vadose development of a major part of the explored cave has occurred preferentially aslant a thrust-fault ramp , often along a calcite bed/limestone contact created by pressure solution. Other fault-related features include slickensides , extension veins , fault- bend folds , stylolites and the repeated basal Onondaga Limestone and impermeable Schoharie Formation thrust below the Onondaga Limestone stratigraphic column.An imbricate thrust east of the cave has upthrown the Esopus Shale against the Onondaga Limestone ,forcing the development of an inefficientresurgence at the base level Mill Pond.
During the Wisconsinan glacial stage , subglacial meltwater formed a series of now abandoned bedrock channels and paleogorges that , due in part to topographic controls , found outlets along and over the flank of the Helderberg Escarpment. Some of this meltwater was pirated into Clarksville Cave where inefficient outlets resulted in the formation of higher in- cave"intermittent phreatic" levels not controlled by the thrust fault. These levels abruptly truncate and grade to lower vadose passages. The characterof these upper levels , the paleogorge and related caves , and elevated paleo-insurgence points correlate with described alpine karst settings.
W.W.S.Yim1 , M.Ivanovich2 , and K.F.Yu1
1 Department of Geography and Geology, University of Hong Kong, Hong Kong
2 Nuclear Physics Division , United Kingdom Atomic Energy Authority ,England
Radiocarbon dates of pre-Holocene marine deposits in Hong Kong ranging from 21,580±1,210 to 45,700±2,000 years B.P. are found to be younger than uranium-series dates of mollusks and other indirect age evidence. Two mollusk samples yielded last interglacial ages of 130,500±5,300 and 142,000±20,000 years B.P., respectively. Palynological and oxygen-isotope evidence shows that the marine deposits containing the mollusks were formed under marginally warmer temperature conditions than in the present day ,which is consistent with a last interglacial age . Since old radio carbonates are likely to be minimum age estimates , similar studies carried out elsewhere would be of value to Pleistocene stratigraphy.
Department of Geography, Faculty of Sciences ,
Tokyo Metropolitan University , Japan
The atoll lagoons are generally several tens of meters deep .The geographical distribution of atolls seems to be scattered at random on the ocean. Therefore , the geographical variation of atoll reef lagoon depth is also at random. The author proposed a model for coral reef formation on the continental or island shelves ( Hori ,1977) .This model explained systematically the reason of the geographical distribution of present reef types. Atolls are distributed only in lower latitudes , or the Core Zone which is defined as the zone of coral reef formation during the glacial ages .Table reefs are characterized by their wider range of cover than atolls.
The configuration of present coral reefs must be formed on the bases of karst topographies which have been eroded by subaerial solution at several glacial phases of low sea-level during the Quaternary period (MacNeil , 1954; Purdy , 1974; Paulay and McEdward , 1990 etc.). In this short paper ,the author focuses on the geomophological features of atoll lagoon floors ,trying to explain the geographical variation of maximum depth of atoll lagoons.
2. GEOMORPHOLOGICAL FEATURES OF ATOLL LAGOON FLOOR
The "faro" which can be said atolls in an atoll is observed in the Maldive Islands (Gardiner , 1903). The shape of a faro is similar to an atoll with a saucer-like depression , or a reef lagoon , and a rim-frame , The distribution of faros is mainly concentrated on and along the former reef rim of large drowned atolls. The faros are formed by subaerial solution , or karst processes during the low sea-level stands (Purdy , 1974).
Daly's Glacial Control Theory is based on the observation of flat lagoon floors. He explained that flat lagoon floors was the past abrasion platforms which had been formed at the period of low sea-level stand during the last glacial age (Daly , 1934). However , the results of precise echo sounding survey in Bikini Atoll , and its surroundings demonstrated that the topographies of atoll lagoon floors was not so evenly flat as Daly had estimated. Undulating lagoon floor of atolls is explained as the drowned karst topography (Emery et al., 1954 etc.). Purdy (1974) advocated the antecedent karst theory.
3.THE GEOGRAPHICAL DISTRIBUTION OF KNOLLS AND PINNACLES
Knolls and pinnacles in lagoons are generally observed in the lagoon of atolls and barrier reefs. The shapes of knolls and pinnacles are the tower-like reefs which are distributed in wider lagoons. On the other hand , various smaller reefs which are dispersed behind the reef edges and stand on shallower lagoon floors , are termed coral pinnacles , patch reefs , and platform reefs.In this paper , knolls and pinnacles are adopted for short.
The number of knolls and pinnacles distributing in the lagoon of atolls are calculated by using large-scale marine charts ( Hori ,1977) . The following results are summarized. First , the distribution of pinnacles which are smaller topographic order than the knolls , has a relatively wide range from 30°N to 30°S. Second, the knolls are distributed in a narrower range between 17°N and 24°S in the Pacific Ocean , and between 7°N and 14°S in the Indian Ocean (Fig. 1). Most are limited to the lower latitudes. Third ,scattered knolls and pinnacles show a higher concentration in between about 20°N and 20°S in the Pacific Ocean , and between about 8°N and 15 °S in the Indian Ocean (Fig. 1). These facts suggest that knolls and pinnacles in the lagoon may be formed on the bases of mounds and ridges existing among sinkholes and soluted depressions.
Fig.1 Latitudinal variation in the number of knolls and
pinnacles existing in lagoons in the Indian Ocean
(modified from Hori , 1977)
4. THE GEOGRAPHICAL VARIATION OF MAXIMUM DEPTH OF ATOLL LAGOONS
The maximum depths of atoll lagoons are also measured by using large-scale marine charts (Hori, 1977). The latitudinal variation of the maximum depth of atoll and table reef lagoons in the Indian Ocean is shown in Fig. 2. Lagoon depth of atolls in the northern hemisphere in Fig. 2 are obtained from the Maldive Islands group. These data clearly shows the evidence of latitudinal gradient indicating shallower depth in the higher latitude and deeper depth in the lower latitude.
Purdy (1974) explained that the maximum depth of the lagoon floor appears to vary geographically in accordance with the amount of precipitation. Under the humid tropical climate ,solution processes will occur strongly and rapidly than the processes under the semihumid and semiarid tropical climate.
The similar evidence showing latitudinal gradient of maximum depth of atoll lagoon floor in the Pacific Ocean may be indicated (Fig. 3).The most of data existing between 5°N and 10°N in Fig. 3 are corresponding with atolls located in the area of precipitation from 1,500 mm to 3,000 mm in a year. The latitudinal gradient of maximum lagoon depth shows deeper depth in the lower latitude and shallower depth in the higher latitude.
The above-mentioned geographical evidences suggest that atolls with deep lagoon floor had been located in the tropical humid area during the glacial ages ,and had been strongly eroded by subaerial solution processes on the islands protruded from the low-stand sea level.
The present geomorphology of coral reef must be based on karst topographies which have been eroded by subaerial solution at several periods of low sea-level stands during the Quaternary period .In particular , karstic relief is often found on large lagoon floors in the low latitudes.
Therefore , maximum depth of lagoon floor must be carefully evaluated. The deepest values of the maximum depth of lagoon floors are almost equivalent to depth of reef edges , or "Daly point " (Hori , 1983) from the viewpoint of geographical variations .However ,the shallowest values of the maximum depth of lagoon floors seem to indicate a geographical gradient with the amount of precipitation. This may have a close connection with the magnitude of karst topographies in the Indian Ocean and Pacific Ocean.
Fig.2 Latitudinal variation of the maximum depth of atolls and table
reef lagoons on the Indian Ocean (modified from Hori ,1977)
Fig.3 Latitudinal variation of the maximum depth of atolls and table
reef lagoons in the Pacific Ocean (modified from Hori ,1977)
THE GROWTH RINGS OF SPELEOTHEMS, LATE QUATERNARY
CLIMATIC CHANGES AND THE ESTIMATE OF ITS RELATED
RELATIVE SOLUTION RATE OF LIMESTONE
ARAKAWA , Tatsuhiko1 , HORI , Nobuyuki1 and MIURA Hajime2
1 Faculty of Integrated Arts & Sciences , Hiroshima University , Japan
2 Dep.of Geography faculty of Education , Yamaguchi University , Japan
Some studies about carbonate speleothem dating have been developed using paleomagnetism , U-series , ESR etc. However , they lack a geophysical point of view: the speleothem consists of numerous layers (Growth rings) . There are few reports about the causes of growth of speleothems .Many speleothem rings(growth rings) have been analyzed by the ESR dating method. Late Quaternary climatic changes in the Ryukyu Islands , Japan , are estimated from this analysis. Carbonate speleothems named stalagmites,stalactites and columns generally have many concentric layers comparable to tree rings. The formation of growth rings and the volume (or thickness )of each layer of growth rings are indicative of an influence of climatic changes. Similarly , the relationship between the growth rates of each layer in each period show the relative denudation rate of karst terrain. For example , in YoronIsland , consisting of raised coral reef terraces , many speleothems were collected from 5 limestone caves which sit at 5 different levels , and ESRages and the volumes of their rings were determined.
The result of this study demonstrates the new approach to the relationships between the evolution of karst geomorphology and the climatic changes during Quaternary period.
ARAKAWA ,Tatsuhiko1 ; HORI , Nobuyuki2 ; MIURA , Hajime3
1 Hertford College , University of Oxford , Oxford , OX1 3BW England
2 Faculty of Integrated Arts and Sciences , Hiroshima University ,
Hiroshima , 730 Japan
3 Department of Geography , Faculty of Education , Yamaguchi
University ,Yamaguchi , 753 Japan
The Ryukyu Islands Arc is where the Filipino Sea plate strikes the Eurasian plate, hence it is an active area for such movements. Most of the Islands consist of Quaternary raised coral reef limestone. Therefore ,it seems to be a suitable field to research cave developments.
Many speleothems were collected from five different horizontal caves , each from a different terrace , in Yoron Island. They were dated by the ESR dating method. Although , generally , the elevation of a cave floor indicates the underground water level at the time of its formation , all ages of speleothems displayed less than 25,000 yr. B.P. except one.
This result seems to show that many speleothems which had formed by that time had collapsed and disappeared with cave development. Because in a sub-tropical area like Yoron Island where limestone is very porous and soft and crustal movements have been active , limestone solution rate is more rapid ,thus cave development is also quicker. Therefore , it is probable that the general limitation of speleothem existence , one cycle of cave development , is less than 30,000 yr.B.P. in the Quaternary raised coral reef limestone areas.
Yong-liang Yang and Henry Elderfield1 , Miro Ivanovich2
1 Department of Earth Sciences , University of Cambridge ,
2 Isotope Geochemistry Section , Harwell Laboratory ,
Uranium series nuclide concentrations have been measured on sediments from five box cores from an equatorial Pacific transect.230Th excess activities show discontinuities at the Holocene-glacial boundary as dated by14C. The glacial sedimentation rates determined by 230Th and 14C are 2.5-3.0cm/kyr. The Holocene rates from 230Th are much lower than those dated by 14C (1.9-2.3 cm/kyr) because of carbonate dissolution. 230Th sedimentation fluxes exceed water column supply by factors of 1.2-1.8 in the Holocene and 1.8-3.0 in the glacial sections. A number of models have been applied to calculate carbonate dissolution rates. The results show that carbonate dissolution rates in the Holocene (in g/cm2kyr) equal 1.5 ×10-3 exp(1.4D)where D is water depth in kilometers. A point- by- point estimation of sediment fluxes through time show that clay accumulation rates in the area have been near constant at 0.1-0.2 g/cm2kyr over the past 20kyr whereas carbonate accumulation rates have decreased dramatically from 0.6- 1.0g/cm2kyr in the glacial sections of the cores to 0.2-0.6 g/cm2kyr in the Holocene. The errors caused by the uncertainties in the age of the termination of the last glacial period have been investigated and results show that a range of 11-14 kyr leads to an error upper limit of about 30%in the estimation of CaCO3 dissolution rates. The response time of CaCO3 and 230Thex concentrations in the mixed layer of sediments due to an impulse of change in CaCO3 dissolution rate has also been discussed , showing that the observed changes in carbonate dissolution may be explained in terms of a single or a continuous change , depending upon the thickness of the mixed layer.
Jeffrey A. Dorale , Luis A. Gonzalez , Mark K. Reagan
David A. Pickett , Michael T. Murrell , Richard G. Baker
High-precision uranium-thorium mass spectrometric chronology and 18O-- 13Cisotopic analysis of speleothem calcite from Cold Water Cave in northeast Iowa have been used to chart mid-Holocene climate change. Significant shifts in d 18O and d13C isotopic values coincide with well- documented Holocene vegetation changes. Temperature estimates based on 18O/16O ratios suggest that the climate warmed rapidly by about 3°C at 5900 years before present and then cooled by 4°C at 3600 years before present. Initiation of a gradual increase in d 13C at 5900 years before present suggests that turnover of the forest soil biomass was slow and that equilibrium with prairie vegetation was not attained by 3600 years before present.
Wang Fuxing & Cao Jianhua
Institute of Karst Geology , Guilin , China
Stromatolite is an organosedimentary structure commonly found in the rocks formed since Archean to recent. They morphologically occur as columns(branched or unbranched) or laminated layers , which consist of dark and light laminae, composed of carbonate. As a matter of fact , they are one kind of secondary carbonate deposits which formed under the controlling of bioactivities of algae and bacteria as well as lichens etc. Stromatolites have been studied for a long time and described in many thousands literatures. Many Groups ( Genera) and Forms ( Species)of fossil stromatolites have been established. As one kind of fossil , the structures played an important role in biogeological ,especially in Precambrian geology , and paleoenvironmental studies.
Stromatolites as products of secondary carbonate deposits are surely an important object of karstology and should be included into the field of karst study.
New types of stromatolitic structure ( speleothem ) have been found near the entrances of some caves in Xinping , Yangshuo , Guilin , China , called previously the "scale-like speleothem" by the author in some publications. It is considered karst cave stromatolitic structure based on its apparent laminated construction consisted of light organic-poor and dark organic- rich layers produced by algae and mosses etc. . Three kinds of karst cave stromatolites (KCS) were described an analysed morphologically and biologically from the Jiaotiandayan Cave in Xinping. The first one calls the Incline Type KCS , and the second and the third are the Erect Type KCS and the Dam type KCS , respectively.
1. GROWTH ENVIRONMENT OF THE KCS
The studied stromatolites developed on the heliotropic surface of somestalagmite , about 50-80 m from the entrance of the cave , where the cave is about 50 m wide and 20-30 m high , with a temperature of 24¡æ- 25¡æ for the cave air (1991.8.31 , midday , the air temperature outside the cave is 33¡æ) , and 23¡æ for the temperature of an underground river (faces NW 310¡ã) . The cave developed in limestone of the Donggangling Fm., the middle Devonian. Near the entrance there are many kinds of shrubs , mosses ,algae etc. flourishing on every part of the cave , which gradually decreased with the increasing of the distance toward inside. In the studied point many kinds of speleothems grew on the bottom , wall and ceiling of the cave , in which some are the so-called heliotropic stalactite ( Phytokarst) . Also , on the heliotropic side of many stalactites and curtain draperies etc.there shows some colored (green, purple and blue) organosediments. The environment of the studied area is well for organism growing and also for forming the organosedimentary structure.
2. BIOSYNTHESIS OF THE KCS
On the basis of studies with binocular body microscope ,SEM , optical biomicroscope etc.,the morphologies and biological features justify the studied structures are one kind of stromatolite.
(a). The forms grow only on the heliotropic side of stalagmites , and only in the "twilight-zone" near the entrance of a cave.
(b). The structures consist of dark and light layers which both are composed of two (dark and light)intercalated laminae.The dark laminae produced mainly by organic , carbon , ferrous matter , micritic calcite and microfossils, and the dark laminae of the dark layers are commonly thicker(1-5 times) than the light one. The dark layer is often thicker than the light one. Obviously, the dark layer are biological layer and throughout facing the cave entrance (light).
(c). The dark layers are porous and relative loose ,and with rough surface containing microorganisms and microfossils (algae , mosses etc.) indicating a biosedimentary nature.
(d). The structures show a distinct stromatolitic structure both information and microstructure. The studied samples show coagulum- striate stromatolitic microstructure.
(e). In the dark laminae , especially those in the dark layers ,there are a lot of calcified algal and moss microfossils. Some of the dark layers were almost formed of densely piled- up calcified bundles of algal colums or filaments.
3. FORMATION MECHANISM OF THE KCS
Marine or lake stromatolites are formed under the sea or fresh water. The known desert stromatolites "are never covered by standing water , and running water (heavy rainfall) covers them for only a few hours during the year"(Krumbein & Guile , 1979). The so-called lichen-stromatolite were formed on fresh rock surface in coastal regions (Klappa ,1979). The studied karst cave stromatolites formed on the stalagmite covered with thin water dropped from the cave ceilings , and keep the structure in water- saturated ( sheet-flood) condition.
Marine and lake stromatolites were formed under the mechanism of inorganic deposition in night and organosedimentation in day time by the binding , trapping and assimilation of algae or bacteria. "As a result of biophysical disintegration ,biochemical decomposition and biosynthesis of mineral components , protosoils are formed. Induration of these superficial biologically weathered rinds , followed by further lichen colonization , leadsto the formation of lichen stromatolites"(Klappa , 1979) , and "the formationof desert stromatolites with extremely short wetting periods is in itself astonishing similar to supratidal stromatolites crusts discussed by Monty , 1973 (Krumbein & Guile , 1979)".
And what happen with the karst cave stromatolites? The analyses clearly demonstrate that the laminated structures are produced by activities of algae and mosses , namely calcification of organisms and trapping and binding of sheet-flood transported particles. The dark layer of the KCS were formed due to the strongest activities of micro-organisms , on the other hand , the light layer under the relative weaker activities which is less in light.