RECONSTRUCTION OF PALEOENVIRONMENT IN KARST AREA (I)


 

DEPTH OF RILLENKARREN AS A MEASURE OF DEFORESTATION AGE

Ivan Gams (Yugoslavia)

INTRODUCTION

Many karst areas, especially in the Mediterranean where forests were natural vegetation are stony lands at present. Historical evidence for the deforestation age is scarce and it is urgent to develop new methods of dating. Our experiences with  the use of rillenkarren for deforestation dating explained in the  present paper.

Human-made  smooth  surface  of  tombstones,  monuments,  buildings   etc. gradually become more and more rough and dotted with punctual or linear indents. They  are  developed  due  to  euendolithic  and  endolithic  blue-green   algae (cyanobacteria), fungi and green algae. Their activity is mostly  controlled  by cumulative  imbibitions time, which mainly depends on humidity  and temperature. Rain  water solution is also dependent on the exposition of the surface to  rain bringing wind. Among the linear forms of solution rillenkarren (solution flutes) are  the  most known ones. Analysing them, LUNDBERG (1977)  found  no  relation between  their cross-section¡¢ depth¡¢ lenth and slope.  DUNKLEY  (1979)  found little  relationship between their length and slope, and GLOE and  FORD  (1980) established direct relationship between slope up to 60 degrees and length of the flutes, simulated in plaster of Paris (gypsum) in laboratorium.

Stone  face  in  finegrained and homogene limestone is smooth  also  due  to natural  solution processes in the soil cover-bedrock interface. This is  widely known.  But  the smooth stone face originating in subsoil is attributed, as  for present karst literature, only to the cases of minor karst forms, as for example rundkarren.  JENNINGS  described  the subsoil  formation  of  cavernous  subsoil weathering  forms  (kavernose  karren),  solution  pits  (geologische   Orgeln), partially  also solution notches, swamp slots and solution pans  (kamenica).  In the karst monographs of FORD-WILLIAMS (1989, 380-386) the subsoil formations are recognized  as subsoil pit, subsoil clint-and-grike, cutters,  rundkarren,  soil pinnacles and as pinnacle karst and ruiniform karst.

Analyses  of  subsoil  forms in the Dinaric Karst brought  evidence  on  two differently orientated solution processes. The first one, in subsoil karst, with solution of humus and soil moisture makes the bedrock face smooth.  The  second one  ruins the smooth face of the exposed stone due to  mechanical  weathering, algae,  fungi and rain water solution. The subsoil origin of smooth  surface  of the  outcropping  stones is in many cases proved by the  appearence  of  subsoil forms such as solution pipe, scallop, recession, bowl-like solution pan, rounded meanderkarren.  On  the  generally smooth underground  rock  faces  of  compact homogene finegrained limestone and dolomitic limestone places can be found where soil  is stripped off, e.g. behind quarries, new building, roads. Height of  the outcropping  stones with smooth faces can be used to estimate soil erosion.  But there  are some exceptions. Smooth stone face can also be generated  below  snow cover if lasting more than half a year. Dead leaves and litter in dense  forests can  cover  the massive stones rising high above ground:  below  this  material, especially  if  based  on humus sheet, smooth stone face occurs  as  a  natural phenomenon.  In  both  cases the height of outcropping stone  is no  longer  an �indicator  of general soil level lowering but of the removing of the humus  cap from the stone top.

The thesis on subsoil (subhumus) origin of smooth face of outcropping stones has led  to  the  use  of  rillenkarren  for  dating  the  deforestation   age. Nevertheless,  the human or natural origin of smooth stone  face,  density  and depth  of  its indents is a function of passed time after being exposed  to  all sorts  of  weathering in the open air. The idea was first proved  valid  on  the northern side of the Hvar Island.

METHODOLOGY OF ANALYSES

As  for morphometric elements, rillenkarren’s width, depth,  length,  cross-section,  relation  to  slope degree were mostly analysed till  now  .  We  have established their depth as the most suitbable element of dating. But there is  a question,  in which point of their length it has to be measured. The depth of  a channel  is  diminishing downwards until the rillenkarren face  passes  over  to planar  belt  of non-channelled erosion. Locally steeper slope can  modify  this rule.  Table 1 shows the histogramme (series of downward cross-sections) of  one typical example of rillenkarren taken on a 1.3 m high outcropping stone built of gray  Cretaceous  finegrained homogene limestone. This example  of  rillenkarren begins  in  a ripple between two border channels.  Measurements  were  performed along the road from Sezana to Lipica in the Kras Region in Slovenia, Yugoslavia. Depth and width of the channel are increasing fast up to 10 cm distance from the beginning  of the channel, where they achieve their maximal values.  Further  on downward  width  is  still progressing and depth is diminishing.  There  is  an exception  between the distance of 50 and 80 cm on a steeper part of  the  stone surface.  At  the beginning of the steeper part (at 50 cm) the depth  is  still diminishing  but  this  is compensated by the  next  exceptionally  deep cross-section. Presumably, this is the result of the more rapid flow above which  only here  begins  the additional solution. After that, depth  is  diminishing  again despite of still persisting steep slope.

Numerical  data  from the histogramme are listed in Table 1.  The  areas  of cross-sections which are the channel volumes at the same time, are also cited in it.  By dividing them with the width value of the profile, we get  thickness  of soluted sheet in mm. Relation between width and depth is augmenting up to 30  cm of distance and after that, it is diminishing (with the exception of the steeper sector).

The transformation of a series of rillenkarren on the 12-13 cm long  profile is  also shown at the end of Table 1. After the beginning the  depth  diminishes also  here. Later, deeper and wider channels nearly supplant the middle  channel and  finally (distance 64 cm) absorb it near the transition to the belt of  non-channelled  erosion. Steeper slope locally modifies the  normal  transformation also here.

If  we take the deepest channel at its beginning (5-10 cm in our cases).  We approach  the maximal thickness of solution sheet. At this distance the  ripples are usually still evenly high and corresponding to the natural stone face.

But  in line with the deepest channels in the middle. We  usually  encounter some minor rillenkarren on the border side and also in between them. An  average depth along  the line can be two times smaller than the  deepest  channels.  We chose  the deepest series for the sake of opinion that the minor  channels  were hindered  in their development by lithological anisotropy, differences in  water quantity, border channels supplanting, later removal of soil or humus cover etc.

The Carpenter’s Profile Gauge was used for measuring. South expositions (SW, SE) were  favoured in the forest of the Kras Region, northern sides  are  often more covered  with  moss. Under the patches of moss the tops  of  ripples  have already been lowered. According to these field observations, moss cover can  not smooth out the stone face. The face under the moss is dotted with micropits  and micropans.

Table 1 Histogramme of one single example of rillenkarren

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RESULTS

With   analyses   of  smooth  stone  faces  we  joined   the   international archaeological  investigation in the northern part of the Hvar Island.  The  low karst plain  Velo polje spreads among the bays of the settlements  Stari  Grad. Vrboska  and Jelsa in the altitude 0-35 m. Stone walls bordering the parcels  or being inside them are dense. According to the regular size of the parcels,  some archaeologists support the idea of this being old Greek parcelling of the fields and erecting of stone walls. The Greek colony in Stari Grad (Pharos) lasted from 385 to 219 B.C. But the smooth faces of the stones in these walls have no  signs of rain water solution that would have originated in open air. The opinion about the  young  age of the walls is also supported by the  history  of  agriculture.Inland convenction of warm air in summer attracts cooler and humid air from  the nearby  sea.  Therefore  on Hvar, as elswhere on the Yugoslav  plains  near  the Adriatic  Sea.,  viniculture had not been possible until the beginning  of  this century,  when  vine  diseases  could be suppressed  by  chemicals. After  that vineyards  began  to be moved from higher slopes to lowlands. At ploughing  for cereals  in the Middle Ages and planting of fruits (olives, fig-trees) later  on stones were removed down to 15-30 cm of soil depth and for vineyards down to 50-80  cm deep. That’s why the stone walls became higher, wider and denser. In  the stone  walls along the 1830 m long profile through Velo polje we found so  many stone that 236 kg of stones in average must have been removed from each  square meter of land.

Rillenkarren are an exeptional phenomenon on the rare outcropping stones  in Velo  polje  and their depth is in span from 0.9 to 2.5 mm. Their  number  being scare,  we are not allowed to calculate the average depth. They are  absent  not only from dolomite and limestone breccia but also from homogene limestone.

On the slope bordering the north side of Velo polje and on the rocky terrace rising  up  to  90  m above the karst plain, the  soil  is  far  shallower  and discontinuous, bushes and forest much denser. We found more rillenkarren  there. Their average depth is 6.13 mm.

The  second  locality  of our measurements is  the  already  mentioned  Kras between Sezana and Lipica. In the mixed forest there, there are several hundreds of outcropping  stones  of  the  height up to  2  m.  There  are  thousands  of rillenkarren on them, 41 measured samples gave their average depth of 6.2 mm.

The  nearly  equal depth of rillenkarren on the plain of Rudine and  on  the Kras  is astonishing when we take into account the differences in  precipitation and  temperature. This can not be explained by facts, observed at simulation  of rillenkarren  in  plastics which consists of unchanged  lowering  of  channelled stone surface after their full development. The level tops of  ripples  between the channels testify in our cases against this thesis.

We can estimate the age of 6.13 and 6.2 mm deep channels when comparing them with the solution measurements on human-made vertical walls. KUPPER  established the  average increase of indents depths on the human-made vertical walls of  the tombstones  in  Belgium to 2.5 mm in one century. According to this  thesis  our rillenkarren  could be 24-25 centuries old. According to  JONES_RACHAM WILLIAMS, 1984,  the  13  mm deep indents developed on the  vertical  walls  of dolomitic limestone  after  the Etruscan and the late Etruscan periods (4th  cent.  B.C.), Indents from the late Republican and the Imperial Roman periods (1st cent, B.C.-6th cent.A.D) are 5.5 mm and those from the Middle Ages (5-16th cent.A.D.)  are 2mm deep. If compared with the depth of indents from the Roman period, our  6.13 to 6.2mm deep rillenkarren could be 21 centuries old. At that time  the  Hvar Island was a Roman colony. But the solution on the vertical walls is weaker than the solution on the inclined walls, not even to mention the horizontal  surfaces (solution on the latter measured also with micro erosion meters, is not compared here  as  it  is much faster). DANIN (1983) published a  diagramme  showing  the relation between indent depths mostly produced by cyanobacteria on the  man-made smooth  stones in Israel. According to this diagramme, our 6.13 to 6.2  mm  deep indents  are from the 5th to 6th century B.C. Measurements of solution by  means of  standard limestone tablets hung in the air resulted in a sheet  solution  of 0.29 mm in one century in the Submediterranean climate. Our 6.13 to 6.2 mm  deep rillenkarren  on  Hvar and on the Kras could be according to this  21  centuries old.

According  to  the  measurements  mentioned  above,  our  6.1-6.2  mm   deep rillenkarren  are  21-25  centuries old, most probably from the  middle  of  the first millenium B.C. At that time the Dinaric Karst was inhabited by  Illyrians. They fought  for the land against the Greek colonists. They are  known  in  the history as cattle-breeders who burnt forests to obtain pastures. Illyrians  were first people who densely populated the karst area. In the distance of 3 km  from the place of our measurements, between Sezana and Lipica, there are ruins of the Illyrian sconce on the Tabor hill.

DISCUSSION OF THE RESULTS

After   the  first  Illyrian  deforestation  of  the  Dinaric  Karst,   many deforestation and forestation epochs have succeeded. The question arises as  to which  deforestation epoch we can attribute the beginning  of  our  rillenkarren development.  According to the age ascribed to them above, the first  successive burnings of natural forests for maintaining the pastures were the most fatal for soil and humus (litter) cover of the bedrock. Dead leaves, litter and humus soil horizont  beeing accumulated in the 7 millenia of Holocene also  burnt  togetherwith  trees. After that, the rest of them have weathered in the direct  sunshine or have been washed away by downpowers.

During  the  cattle-breeding  period of agriculture in  the  inland  of  the Dinaric  Karst  which  lasted  till  the recent  century,  the  soil  level  was continuously  lowered.  This can be proved by the measurements  of  rillenkarren depths on the lower outcropping stones. We observed shallower channels there but we measured them less systematically on two localities only: (Tab.2)

On the low stones local conditions can substantially change the velocity  of rillenkarren development.

Although  most  of  the parts of the Dinaric Karst  have  gone  through  the siminar agricultural history, the rillenkarren appear only in some regions and even there, mostly on some stones only. They are nearly absent in the old fields with  deeper  soil which  can be explained  by  progressive  erosion  of  soil, originally  covering nearly all bedrocks. The question about rare  occurence  of the rillenkarren can not be answered and explained only by the factors that  we are familar with at present.

Tab.2

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PRESENT AND FORMER DESERTS: EVIDENCE OF ENVIRONMENTAL
CHANGE FROM CAVE SEDIMENTS IN EAST AFRICA AND THE
AMERICAN SOUTHWEST

George A.Brook, David A. Burney, James B.Cowart,  and Brooks B.Ellwood (USA)

Pollen spectra from speleothems in desert caves indicate that these deposits may  be a valuable new source of paleovegetation data for the arid and  semiarid regions  of the world. In other dryland sediments pollen is  often  absent  or poorly preserved because of the highly oxidizing conditions. Speleothem age  and pollen  data  for Somalia, Zaire, and the U.S.A. indicate that East  Africa  was more  arid  than  today  during glacial maxima of the  last  300,000  years  and relatively  moist during interglacials. By contrast, the American southwest  was much  wetter  during  glacials and relatively arid  during  interglacials.  Only during  interstadials  was water more plentiful in both regions  either  due  to lower temperatures or to increased precipitation.

INTRODUCTION

Evidence of past environmental conditions in the world’s dry areas is  often limited by the poor preservation of pollen grains and other organic matter. This severely restricts efforts to reconstruct past vegetation characteristics and to provide a 14C chronology for the data. As a result, desert paleoenvironments arein general poorly known and where there is information it rarely extends  beyond about  30,000 B.P., the range of the 14C dating method.  Lake-level  studies are,for  example, largely limited to this time range. These have shown that in  East Africa  the  last  glacial maximum was substantially drier than  today  and  the Holocene,  particularly the early Holocene, substantially wetter. The  situation in the American southwest was apparently very different; lakes were highest here during the last glacial maximum and low or dry during the Holocene.

The  recent discovery by Bastin and Brook et al. that cave  speleothems  may contain  pollen raises the question as to whether these deposits–which  can  be 230Th/234U   dated  to  ca   350,000 B.P. –could be  a  source  of  desert paleoenvironmental data to well beyond the range of 14C dating. In arid  and semiarid  areas  speleothem ages alone can provide information  on  past  wetter phases of climate as deposition is modest or absent today in extremely dry caves.

This  paper reports on research conducted in Somalia, Zaire, and the  U.S.A. between  1982  and  1987.  The research attempts  to  determine  1)  under  what conditions desert speleothems (and cave speleothems in general) contain  pollen, 2) to what extent the fossil pollen provides reliable data on past vegetation in deserts,  and 3)  whether  pollen is  preserved  in  very  old  formation–thus providing  a window on the more distant past. In addition, speleothem  ages  are examined  to  establish  if  the pattern of  glacial  aridity  and  interglacial humidity in East Africa and the reverse in the southwestern U.S.A. (as  revealed by  lake-level studies) was typical of the last 300,000 years not just the  last 30,000 years.

STUDY AREAS AND METHODS

There are numerous caves in and close to the North American Mojave, Sonoran, and  Chihuahuan deserts. Carlsbad Cavern, in the northern Chihuahuan  Desert  at 1,325 m  in  the  Guadalupe Mountains, was  selected  for  study.  Mean  annual precipitation  at the cave is 362 mm and the mean annual temperature is  17.2° C. Using an electric drilling rig three 4.5 cm diameter cores up to 3.2 m long were recovered  from  the Georgia Giant and Texas Toothpick stalagmites in the  Green Lake Room and in the Lower Cave respectively.

There  are extensive areas underlain by soluble rocks in  the  Somali-Chalbi Desert but the caves of the region are poorly known. Sediments from two caves in northern Somalia  were examined in this study. Galweda Cave  in  the  hyperarid coastal desert zone south of the Gulf of Aden is at 20 m elevation and presently receives  less than 50 mm of precipitation annually, mean annual temperature  is 30° C.  Hayla Cave in the Golis Mountains immediately to the south is at 1,800  m with annual precipitation and temperature being 435 mm and 17¡æ respectively.

Work in the Ituri rainforest of northeastern Zaire was conducted principally at Matupi Cave in the Mount Hoyo block southwest of Lake Mobutu Sese Seko.  The cave  is  at 1,100 m elevation on the western edge of the Western  Rift  Valley. Today, vegetation near the cave is species-rich equatorial rainforest. The  mean annual temperature is 23° C and annual precipitation is about 1,687 mm.

The  core drilled from the Georgia Giant stalagmite in Carlsbad  Cavern  was dated at regular intervals providing 46 ages. 230Th/234U ages were  also obtained for  the  outside layers of the Texas Toothpick formation that were  within  the range  of the dating method. 230Th/234U ages were also obtained for  six speleothems from Galweda Cave, nine from Hayla Cave and five from Matupi Cave.

Speleothems  from  Carlsbad, Hayla, and Matupi Cave were  also  examined  to determine if they contained pollen. Ten samples of more than 100 g were cut from selected locations along the Georgia Giant core and five samples from the  Texas Toothpick  core.  Eight  samples  of 22-112 g were cut  from  seven  Hayla  Cave speleothems,  and  sixteen  samples of 9-33 g were cut from  seven  Matupi  Cave formations. Any pollen present was extracted using methods described in Brook et al. (1990).

THE SPELEOTHEM AGE DATA

Under  the present climatic conditions there is no speleothem deposition  in Galweda  Cave (rainfall < 50 mm) and minimal growth in  Carlsbad  (precipitation 363  mm)  and Hayla Cave (precipitation 435 mm). With a precipitation  close  to 1,700  mm  per year there is extensive deposition at Matupi Cave.  The  ages  of speleothems from the northern Somali and southwestern U.S.A. caves can therefore provide   information  on  past  wetter  climatic  conditions  when   speleothem deposition was more rapid and more extensive than today. The ages of Matupi Cave speleothems indicate only that at the time of their deposition conditions at the cave were still relatively wet–as they are today (precipitation is 1,687  mm). Age  data  for the  Georgia Giant and  Texas  Toothpick  cores,  together  with speleothem  ages for Ogle Cave and Carlsbad Cavern from Hill (1987)  and  Harmon and Curl (1978) are shown in Fig.1. Periods of speleothem deposition in northern Somalia are shown in the same figure for comparison.

It is clear from Fig.1 that maximum rates of speleothem accumulation were at very different times in the American southwest and East Africa. In North America deposition  was  most extensive during glacial and interstadial periods  of  the last 300,000 years, particularly during isotope stage 6 (the Illinoisian or Riss Glaciation). The warmer phases of interglacials were apparently much drier. This was  particularly so during isotope stages 1 and 5e when there were hiatuses  in the  growth  of  the Georgia  Giant. Isotope stage  5e  appears  to  have  been particularly  dry as two other speleothems from Carlsbad and one from Ogle  Cave also stopped growing at this time.

In  contrast to the pattern in the southwestern U.S.A. there is no  evidence of extensive speleothem growth in the Somali-Chalbi Desert during full  glacial periods. In   this  region  deposition  was  apparently  most   rapid   during interglacials  and  to a  lesser  extent  during  interstadials.   Particularly noticeable is that when deposition of the Georgia Giant speleothem ceased at the beginning of isotope stages 5 and 1, there was increased speleothem formation in northern Somalia.

DISCUSSION

Speleothem age and pollen data for the Chihuahuan and Somali-Chalbi deserts, and for the Ituri rainforest suggest that East Africa was much more  arid  than today during the last glacial maximum and probably also during  other  glacial maxima  of the last 300,000 years. Interstadials appear to have been cooler  and wetter  than today, the increased availability of moisture being due  either  to reduced evapotranspiration or to a slight increase in precipitation. The wettest periods  of  the  past were the times of greatest global warmth  (peaks  in  the marine d 18O curve) when there is strong evidence of substantially increased monsoonal  precipitation  in  the  Horn of Africa. By  contrast,  the  American southwest  was  wetter than today during glacial maxima and at least as  dry  as today during  interglacial  maxima. Only during  interstadial times  were  the deserts in both North America and East Africa wetter than they are today and  it is  unresolved whether  this was because of  lower  temperatures  or increased precipitation.

Fig.1. Phases of speleothem deposition in East Africa and the American southwest  during the last 300,000 years and arid phases indicated by pollen data.
The marine record is from Martinson et al. (1987)

Pollen  spectra from actively growing speleothems in Hayla and  Matupi  Cave were found to reflect the present vegetation near these two caves implying  that fossil pollen from  speleothems can be used  to  reconstruct  past  vegetation characteristics. The recoverry of pollen from one Somali  speleothem  deposited 176,000  B.P. suggests that speleothems may be able to provide  vegetation  data for  a considerable part of the Quaternary period. The absence of pollen in  the two  Carlsbad  speleothems  can probably be attributed  to  the  great  distance between them and the cave entrance and to their great depth beneath the surface. The Georgia Giant is 215 m below the cave entrance and the Texas Toothpick 245 m below  it.  The  Hayla and Matupi Cave speleothems that  contained  pollen  were growing fairly close to natural entrances and in passages close to the  surface. In  our  view  most of the pollen deposited on speleothems  is  airborne  pollen and/or  pollen  introduced into the cave by animals, particularly  bats.  Modest numbers  of  pollen  grains may be introduced by groundwaters  under  favorable conditions. Being so far from the natural cave entrance and at so great a depth, neither  airbone nor waterborne pollen was apparently able to reach the  Georgia Giant and Texas Toothpick stalagmites in Carlsbad Cavern.

In conclusion, cave speleothems in arid and semiarid regions appear to be  a largely untapped yet extremely viable source of paleoenvironmental  data.  They may be able to provide important new information about the impact of man on  his environment. In Africa the “Cradle of Mankind” they may also provide information on  possible relationships  between natural  environmental  changes  and  man’s evolution. In the last 200,000-300,000 years, a period within the range of 230Th/234U  dating, man has evolved from Australopithecus  through  Homo erectus to Homo sapiens –modern humans.

(TO BE CONTINUED)

SOURCE  www.karst.edu.cn/

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