Part3-IV. Paleoclimatic Reconstruction with Karst Records

ANNUALLY LAMINATED SEQUENCES IN THE INTERNAL STRUCTURE OF SOME BELGIAN STALAGMITE-IMPORTANCE FOR PALEOCLIMATOLOGY

Dominique Genty¹ and Yves Quinif²
1 Universe de paris-Sud. Laboratoire d’Hydrologie et de Geochimie Isotopique,
C.N.R.S.,URA 723,Batiment 504,91405 Orsay Cedex, France
2 Centre d’Etude et de Recherches Appliquees au Karst, Faculte Polytechnique de Mons. 9 rue de Houdain , 7000 Mons Belgium

Fifteen stalagmites from caves and one sealed tunnel in southern Belgium are composed of annually deposited white-porous and dark-compact laminae. This is demonstrated by comparing the number of laminae with the local history of the site for modern stalagnites and with radioisotopic ages for Late Glacial and Holocene stalagmite.Annual cyclicity in the internal structure of these speleothems is explained by the highly seasonal variations of the water excess, which influences underground water flow.

Comparison between climatic data and modern stalagmites of a closed tunnel shows that growth laminae can record climatic variations : (1) there is a good correlation (r=0.84) between lamina thickness in a stalagmite and water excess; (2) during years with a high water excess, dark-compact laminae are more developed, which makes the speleothem darker.

Vertical successions of several laminae represent microsequences that may have recorded climatc variations with a time resolution of 1/2 year . In a Late Glacial stalagmite , successive laminae microsequences from very regular cycles of 11years separated by a thick dark-compact lamina. It is supposed that , as for modern stalagmite , the thick dark-compact lamina corresponds to a period of high water excess. Hence, this 11-year cycle may reflect a climatic cycle.

Ian J. Fairchild
Department of Earth Science , Keele University , Staffs ST5 5BG,

The use of speleothem ( stalagmite and stalactite) geochemistry to record climatic change has been enhanced by the advent of modern precise mass spectrometric U-series age determinations. Stable isotopic compositions can potentially be interpreted in terms of temperature given that the ambient temperature of a cave approximates that of the ground outside . The cationic composition of speleothems reflects the cave water chemistry which in turn reflects that of the overlying soils, variably modified by passage through overlying rock strata. If the variability in water chemistry can be shown to be primarily a function of residence time of water in the soil , then speleothem geochemistry could serve as a proxy for climatic humidity.

This idea is being investigated as part of an EC programme in which the geochemistry of late glacial to Holocene stalagmites from caves in western-Ireland , Belgium , southern France and northern Italy is being studied . The chemistry of cave water is being monitored throughout the year and will be compared with the results of experimental leachates and the chemistry of apparently annually-banded soda straw stalactites.

Macroscopically banded soda straws have been found in three of the caves, and periodic chemical variations in soda straws from a fourth cave, suggesting that the soda straws record changes , probably annual in water chemistry. In the most common type , the bands represent zig-zag growth steps on the inner margin of the straw. An example from Italy exhibits around 130 such bands , around 4 or 5 per mm , but the next 50 bands at least , are significantly more narrow . This change may reflect climatic warming at the end of the little Ice Age in the Alps.

Waters from Grotte Pere-Noel , Belgium display high spatial variability in chemistry which presumably relates to lack of mixing of water parcels within the vertically-bedded strata. Nevertheless , individual soda straw stalactites can exhibit repeated Mg-Sr peaks which could reflect a signature of an annual climatic cycle. In contrast , Grotta d’Ernesto , Italy shows a much closer grouping of the chemistry of the cave waters and recent precipitates in the cave . Leachates of weathered host rocks above the Grotta d’Ernesto indicate varying trace element compositions depending on water-rock contact time , with cave waters more resembling short-term (1 day ) than long-term (1 month ) leachates . Mg chemistry is consistent with greater leaching of the less reactive mineral dolomite when contact times are longer .

Thus , by tempering general principles with a knowledge of the mineralogy and hydrology of individual sites , and the behaviours of the carbonate system , new palaeoclimatic inferences may be obtainable . A test of a crucial part of this system will be a comparison of the characteristics of soil leachates and cave water ; this investigation will be carried out by Anna Tooth ( Keele researcher from 1996 ).

The fine particles created by glacial comminution of carbonate minerals are susceptible to interaction with meltwater , and the isotopic composition of fine carbonate in glacial deposits has the potential of recording this interaction . Here , we focus on the carbonate fraction of glacial sediment and debris within basal ice at the Tsanfleuron Glacier , Switzerland . Size separates of basal ice debris and melt-out tills show a collinear isotopic trend interpreted as a mixing line between detritus

(d ¹⁸O_PDB= -5‰ and d ¹³C_PDB=0.6‰) and authigenic calcite , which in one case had

d ¹⁸O values as light as -18.7‰ and d ¹³C values as light as -5.7‰ . In most cases , the finer fractions (with calcite crystals as small as 0.1 m m ) are enriched in the isotopically light component , which is interpreted to form largely by freezing in basal ice. The isotopic compositions of ice and included gases could be influenced by this process , Melt-out tills inherit the authigenic calcite; thus, carbonate-bearing till should no longer be regarded simply as clastic sediment. ( GEOLOGY , 1993 , V.21 , P.901-904 ).

PALEOALPINE KARSTIFICATION- THE LONGEST PALEOKARST PERIOD IN THE WESTERN CARPATHIANS (SLOVAKIA)

Juraj Cincura and Eduard Kohler
Geological Institute , Slovak Academy of Science , Dubravska 9,842 26 Bratislava ,
Slovak Republic

The considerable areal extent and great thickness of Middle/Upper Triassic carbonate complexes influenced favourably the formation of karst during subaerial periods. The lower boundary of the Paleoalpine karst period is age-determined by the gradual emergence of the basement-during the Upper Cretaceous in the Central Western Carpathians and even earlier in the Inner Carpathians. The upper boundary can be dated by marine transgression . The start of the transgression is not synchronous and it varies in a broad range from Upper Cretaceous to Upper Eocene and maybe even up to Oligocene/Miocene . The typical products of the period include typical karst bauxites filling karst cavities, ferricrusts, red clays , collapse and crackle breccias with speleothems , freshwater limestones or polymict conglomerates.

Combining with the predecessors studies^[1~3] and based on the comprehensive investigation of the 60 caves such as Xiaoyan Cave, Taiping Cave and Heiyan Cave and their sedi-ments, and comparing the dating of stalagmites and tufas,the stalagmite in Paniong Cave is rich in paleo-environmental informations, and its vertical profile is a typical and systematical one of Holocene.

The stalagmite, 122 cm high and 25~45 cm in diameter is situated on the calcareous plate flanked by rimstone dams and cave pearl pools. It is characterized by clear lamina and structure on its vertical profile. To obtain enough evidences for age division, the layers with depositional cycles, rhythms and dark lamina resulting from the special deposition were se-lected for systematic dating.

Pictures: No.1 in 40KB Size(JPEG) and No.2 in 9KB Size(B/W GIF),
Sketch and comprehensive results of Ams C¹⁴, LSC C¹⁴, U-series dating and stable isotopic analysis for a 1.2m high stalagmite taken from Panlong cave, 37km south of Guilin.

Nine depositional cycles were identified by the staged changes in dropping water from cave ceiling. The stalagmite is mainly composed of yellowish white, grayish white and pure white calcite from bottom to top, and intercalated with some pink calcite lamina in the mid-upper section. It intercalates many dark layers, which are getting darker in colour and more in quantity upwards. The calcites change their size from fine to moderately coarse and then to moderately fine, and take the shapes of needles pillar and gigantical crystals with ring-bedded structure and radiated structure. No.1 and No.2 cycles comprise the core of the sta-lagmite base with slow deposition; No.3 ~ No.8 cycles build up its main body with rapid de-position intercalated with some short-term slow deposition and some breaks; No.9 cycle con-structs the top of the stalagmite with slow deposition, which is intermittently growing now.

A batch of depositional laminas building up the stalagmite body of a certain stage is defined as lamina complex, and its surfacial pattern showing the surfacial characteristics of the lamina as surfacial structure. The depositional ( growing ) surface structure indicates the regime of dropping water. In the course of diagenesis, it is easily going on in water that the calcites were adjusted, moved and recrystallized to form coarser crystals which grow needle- shapedly along C axis and cut across the lamina to produce the radiated structure. Although argillaceous materials move or accumulate locally on the surface of calcite, the structure of the lamina remains reflecting the original deposition. The reason why the surfacial structure and lamina complex are very complicated is because the flowing regime of the dropping water is changeable. The surfacial structure of the lamina is of universal importance. For instance, their gentle surface, dip surface, and uneven surface manifest the dropping water scattered, flowing on one side and of several fixed position, respectively. The lamina complex and its patterns can show the regime of the drips of a certain stage.

The composition, colour, lamina complex and structure of the stalagmite indicate the geological environment inside and outside the cave. There exist 8 dark layers, the synthetical reflection of the environment out of the cave (Tab.1).
There are differences in age, compo-sition, structure and depositional properties between them. In them are found depositional breaks being the upper and lower limits of depositional cycles, i.e. the indicator to determine the geological age, especially in No.1, No.5 and No. 7 layers, they are very clear.

Number of layer	Location from the top (mm)	Thickness of the layer (mm)	Structure	Age(a) Top	Age(a) Bottom	Calculated lower and upper limit ages and their difference (a) Top	Calculated lower and upper limit ages and their difference (a) Bottom	Depositional properties
8	90~114	2~19	Branched semi-ring-bedded	2265+80	2288+80	(23)	(23)	Depositional break at the bottom, intermittent deposition upwards.
7	120~170	4~70	Conical semi-ring-bedded	2295+140	2354+140	2290+140 ---------- 2295+140 (5)	2344+80 ---------- 2354+140 (10)	Depositional break at the bottom, intermittent deposition upwards.
6	270~300	2~18	Biconical semi-ring-bedded	3608+180	3905+150	(297)	(297)	Depositional break at the bottom, slow deposition upwards.
5	344~400	4~56	Conical semi-ring-bedded	4219+190	4296+150	4172+150 ---------- 4219+190 (47)	4162+150 ---------- 4296+170 (134)	Depositional break at the bottom, slow and intermittent deposition upwards.
4	468	1	Conical ring-bedded	5151+190	5481+190	(330)	(330)	Depositional break
3	520~540	1~18	Conical ring-bedded	5502+220	5510+220	5502+220 ---------- 5517+220 (17)	5510+220 ---------- 5581+220 (12)	Depositional break at the bottom, slow and then intermittent deposition.
2	739~750	4~11	Conical ring-bedded	5578+110	5581+110	5578+220 ---------- 5457+110 (12)	5490+110 ---------- 5581+220 (91)	Depositional break at the bottom, slow deposition upwards.
1	956~1030	4~74	Conical ring-bedded	6461+250	32375+380	6383+110 ---------- 6539+290 (156)	29437+650 ---------- 32375+380 (2938)	Depositional break at both top and bottom, slow and intermittent deposition in the middle section.

With few or no fossils in the stalagmite, U-series, ¹⁴C, ESR, and heat luminescence are available for dating. To make as accurate dating as possible, the samples were taken at the tops of the lamina comprising the core of the stalagmite. The dating data about 40 lamina in-dicate that the ages of the core of its base is 36000+1800 a B.P. , 10 mm from the top of the stalagmite 1060+80 a B. P. , and the other 30 data between them all are in normal sequence. Calculating with these data, the lower limits of No.1, No.5 and No. 7 dark layers are 32375+380 a B.P., 4296+150 a B.P. and 2354+140 a B.P., respectively, and their upper limits, and the lower and upper limits of other dark layers are listed on Tab. 1. These data can be taken as the basis of division of the geological ages because they are in line with the values measured.

In the existing Quaternary researches in Guilin, the data of hundreds of boreholes were collected, meanwhile the studies on some profile were done, also much data about fossil, ar-chaeology and isotopic dating were recently obtained through the special studies on the sedi-ments in the caves and glacial vestiges. However, there is not a division with enough evidences for Quaternary, Holocene in particular. Combining the stratigraphical, paleontological and archaeological methods with the studies on the depositional cycles, the dating of the lamina and the depositional breaks at the tops and bottoms of the cycles, the stalagmite in Paniong Cave is believed to be formed in Late Pleistocene-Holocene. A detail division of age is made (Tab.2). The comprehensive researches showed that the No.2 cycle, i.e. No.1 dark layer, is an indicator showing an vio-lent climatic changes from cold to warm, which is a weathering crust-typed depositional cycle formed in a long-term depositional break. No.2 cycle has an unconformable contact with un-derlying No. 1 cycle. In the transitional period from cold to warm after long-term deposition-al break, the dark lamina building up the low-mid section were formed. In Younger Dryas glacial period (11080+280 a B.P.), the parallel disconformity and unconformity were pro-duced because of depositional break and intermittently slow deposition. With the intermit-tent hot deposition after the glaciation, the grayish white (intercalated with grayish black) calcite lamina were deposited to form the top cone of the core of the stalagmite base. When the depositional break came again, it contacted unconformably with overlying No. 3 cycle. Therefore, the intermittent depositions around Younger Dryas glacial period is classified into the dry depositional cycle after Dali glacial period. The lower limit of Holocene(Q₄) is 11080+280 a B.P., i.e. the time when Younger Dryas glacial period began[4]. Before Younger Dryas glacial period, the cold deposition building up the core of the stalagmite base may be included in the last stage of Late Pleistocene(Q₃³); after Younger Dryas glacial period, the hot deposition building up the top cone of core of the stalagmite base in the early stage of Holocene (Q₄¹). It is difficult to further divide No. 2 cycle because it is only 5-74 mm thick, but its upper and lower boundaries are clear unconformable. Accordingly, it can be taken as the upper or lower boundary of other cycles. In a word, Holocene is subdivided into 4 stages by different depositional cycles, regime of dropping water and characteristic indicators. The stages are delimited by the dark layers and contact unconformably with each other. Their lower and upper limit ages are available. The division is nearly consistent with the ones of Pearl River Delta^[5], loess of north China, the plain of north China^[6], the glacial periods of China and Europe (Tab.2).

(1) Like the ages determined by fossils and archaeology, the isotopic dating of speleothems is reliable and correct, sometimes, however, of multiple solutions. With the more systematical and definite properties, the comprehensive studies and systematical dating of the vertical profile of a stalagmite is one of the available and effective methods for the geological age division of Quaternary. To ensure the ages being correct and in normal sequence, the samples should be taken at the top of the lamina along the central axis of the stalagmite.
(2) The stalagmite has 9 depositional cycles showing its whole growth and 8 main dark layers indicating the rapid changes of deposition. No.1 and No.2 cycles build up the core of the stalagmite base with a very slow growth, No.3~No.8 cycles comprise the main body of the stalagmite with a quick growth, and No.9 cycle makes up of the two cones of the stalag-mite top. On the basis of the observation of the seasonal dropping water on the stalagmite, we found the stalagmite is still slowly growing as a result of the intermittent deposition of calcite.
(3) The stalagmite was formed in the period from the last stage of Late Pleistocene to Holocene, its main body in Holocene. It is suggested that the boundary between Late Pleis-tocene (Q₃) and Holocene (Q₄) would be Younger Dryas glacial period with the lower limit of 11080+280 B.P.. The time before Younger Dryas glacial period belongs to the last stage of Late Pleistocene (Q₃³), and the one after that incorperate into Early Holocene (Q₄¹). The lower limits of Middle Holocene (Q₄²), Late Holocene (Q43) and the last stage of Holocene (Q₄⁴) are respectively 6461+250 a B.P., 4296+170 a B.P. and 2354+140 a B.P. , which can further be divided into early and late sub-periods by the unconformity and parallel uncon-formity or disconformity between the deposional cycles. Seeing that the predecessors often took the time about 10000 a B.P. as the boundary be-tween Late Pleistocene and Holocene, grounded on the specific properties of the stalagmite profile in Panlong Cave, the lower limit age of Holocene is def ined as 11080+280 B.P.. It is difficult to further divide No.2 cycle being too thin. However, it is demarcated by both up-per and lower unconformities. The lower limit of Holocene would be defined by 20000 a earlier, i.e. 32375+380 a B.P., providing using the lower limit of No.2 cycle as the one of Holocene. It must further be verified whether the viewpoint is available.

This is funded by the project of the Ministry of Geology and Mineral Resources (8502218) and the project of National Natural Science Fundation (49070155), and instructed by Prof. Yuan Daoxian, a academician of the Chinese Academy of Sciences. Also Wang Fuxing, Liu Zhaihua et al. of the Institute of Karst Geology, Yuan Sixun, Li Kun of Beijing University, and Liu Yuyan of Geological University of China participated in the work.