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Research Article
Geochemical Characteristics of Spaleotherm Formation in Caves from
Zanzibar Island, Tanzania
Said A. H. Vuai*
School of Mathematical Sciences, College of Natural and Mathematical Sciences, P.O. Box 259, Dodoma, Tanzania
Abstract
This study was conducted to investigate geochemical characteristics of Spaleotherm precipitation from caves
in Zanzibar Island, Tanzania as influenced by hydro geological zones. Nine caves, three from each region of the
Zanzibar Island were selected for investigation. From each cave, water, stalagmite, stalactite, sediments, and
hard rock were collected depending on their availability. Water samples were analyzed for pH, TDS, major cations
and anions while rock samples were analyzed for mineral composition and total chemical composition including
major and trace elements. The results of chemical composition of water showed higher contribution of seawater
as indicated by higher concentration of Na+. Saline water has concentration trend (Na+>Mg2+>Ca2+>K+) similar to
that of sea water while fresh water has the concentration trend of Na+>Ca2+>Mg2+>K+ which can be explained by
mixing of seawater and carbonate water. The influence of hydogeological zone was evident for Mg concentration
and mineral precipitation in cave features probable due to difference in chemical composition of water. The analysis
of spaleotherms showed domination of calcite minerals in all cave features. However quartz and aragonite minerals
exist in some cave sediments and stalactite. Observation of quartz minerals was related to the concentration above
1.6% of SiO2 detected in chemical composition. Inclusion of Mg during stalactite formation showed that it was
hindered by phosphorous concentration. The magnesium partitioning coefficient range agrees well with those
reported in other studies. Moreover the partitioning coefficient found to be low in the cave containing saline water
emphasizing the influence of ionic strength on the Mg partitioning. Therefore the results suggest that chemical
composition of spaleotherm can be used to trace the paleo-hydrological changes associated with salinization of
ground water in cave.
Keywords: Geochemical; Spaleotherm; Zanzibar; Cave water
Introduction
Formation of cave is the common geological features in Zanzibar
islands which is dominated by Quaternary (Q1, Q2, Q3), Pliocene and
Miocene (M1, M2 M3). Both dry and wet cave do exist. Historically,
Zanzibar people have been using caves for various purposes including
cultural such as worship, recreation, such as site seen as well as source of
water for drinking, washing, bathing and other domestic uses. Recently
booming of tourism in the Island turns caves into one among good
attractive sites due to their traceable history to colonization period
especially Portuguese regime.
Zanzibar Island can be divided into three major hydrogeological
zones. These are eastern lime stone/sand stone corridor, the western
sedimentary corridor and coastal limestone deposit called coral rag.
Zanzibar Island has no reliable source of surface water; it depends
entirely on ground water including springs, tube and bore holes as
well as cave water (Figure 1). Cave water is a major source of water
supply in urban and rural area. The quality of cave water is affected
by anthropogenic and natural processes such as weathering rocks. For
instance, McDonald [1] argued, cave drip waters respond to variations
in surface recharge via changes in discharge and chemistry. Such
changes may be encoded in the chemical and physical properties of the
speleothems deposited from these waters, allowing the extraction of
palaeohydrological and other climate-related information. According
to Musgrove and Banner [2] variations in isotopes and trace elements
(Mg/Ca and Sr/Ca ratios) of drip waters and soils from different caves,
as well as phreatic ground waters, provide the potential to distinguish
between local variability and regional processes controlling fluid
geochemistry, and a frame work for understanding the links between
climatic and hydrologic processes. In this regard, chemical composition
of cave water can vary, depending on the nature of the caves, rocks or
soil, as well as weathering process taking place at the particular site.
Stalagmite trace element concentration profiles can reveal evidence
for climatic events that disrupt the local hydrological cycle [3]. The
aim of this study is to investigate the geochemical characteristics of
spaleotherms from cave in Zanzibar. Specifically the study intend to
*Corresponding author: Said A. H. Vuai, School of Mathematical Sciences,
College of Natural and Mathematical Sciences P.O.Box 259, Dodoma, Tanzania
E-mail: [email protected]
Received July 31, 2012; Published November 25, 2012
Citation: Vuai SAH (2012) Geochemical Characteristics of Spaleotherm Formation
in Caves from Zanzibar Island, Tanzania. 1:505. doi:10.4172/scientificreports.505
Figure 1: Triangular diagram showing chemical composition of cave water.
Copyright: © 2012 Vuai SAH. This is an open-access article distributed under the
terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
Volume 1 • Issue 11 • 2012
Citation: Vuai SAH (2012) Geochemical Characteristics of Spaleotherm Formation in Caves from Zanzibar Island, Tanzania. 1:505. doi:10.4172/
scientificreports.505
Page 2 of 5
investigate of source of chemical species of cave water, determination of
mineralogical and chemical composition of spaleotherms (stalagmite,
stalactites, sediments and hard rock) and investigation of precipitation
and dissolution of characteristics of minerals during spaleotherms
formation.
Materials and Methods
Study site
Location and climate: This study focused on whole of Unguja
Island in Zanzibar. Zanzibar is a union of two islands namely Unguja
and Pemba, plus a number of small islets. Unguja Island is located
between latitudes 6° south of the equator and 39.5’ east. It lies off the
Tanzania Mainland 40 km (29miles) from Bagamoyo. Unguja Island is
flat, and low lying covering an area of 1,554 sq km (598 sq miles) being
about 85 km (53 miles) long and 20 km (12 miles) wide. Rainfall is
strongly seasonal, related to change of monsoon and movement of the
tropical convergence zone. The seasons of heavy rains (Masika) comes
during March - May. Relative cool and dry season (Kusi) occurs during
June- September while the light rains (Vuli) starts from October and
December. The north east monsoon (Kaskazi) blows during January
and February when the weather is dry and hot. The average annual
rainfall varies between 1000-2,250mm and average annual temperature
is 26°C.
Geology: Zanzibar is a part of the ancient Miocene Rufiji / Ruvu
river delta. Due to periods of isostatic movement and block faulting over
the coastal Tanzania and off shore deltaic zone, only Zanzibar, Mafia
and Latham Island areas remained above the sea level as land blocks
of the original delta. It is a complex junction of four blocks evidenced
by past artesian leakage with ferruginous and siliceous cements, and
anhydrite deposits over the most of eastern Zanzibar.
Geologically, Zanzibar is composed of lower Miocene M1, M2, and
M3 rocks overlain by Quaternary Q1, Q2, and Q3. Q1 is complex of red,
brown and black soils, thin tropical laterites, reworked Miocene sands,
gravels, unsorted colluvial sands, clays and limestone fragments. The
soil cover maintains a water table and acts as confining layer to the Q2
aquifer. It has a maximum of 25m thickness. Q2 is a white cream and
coralline reef limestone with a thickness of 30 to 35m. It forms a thin
veneer over the coastal platform where it is frequently cavernous and
with many solution channels. In coastal areas, the Q2 is open to the sea.
Q3 is marine fluviatile sand, cemented sandstone, and shell fragments,
shark’s teeth with garnets, kyanites and tourmalines. It has a maximum
thickness of 25m in the northeast Upenja-Kibokwa region.
Miocene are rhythmic fluviatile sediments of the dissected Rufiji
river delta. M1 is hard, dense, pearly white crystalline limestone in
strata and lenses, detrital limestone of colluvial origin and recemented.
It has a thickness of 2560m. M2 are grey, white course clean, angular
siliceous sands often sugary in texture and lightly cemented. Closed
and ponded drainage show the interesting mode of formation of local
and isolated limestone lenses. M3 includes marls, sandy clays, clay
sands and frequent gravel stringers (bluish grey to bluish green) which
weather to a red, yellow or brown color. M3 constitute the main base
rock of Unguja Island.
Hydrology: Hydrological Zanzibar Island can be grouped into
three zones (Figure 2). Zone one is Miocene type water, typically of
a low TDS/CaCO3 type. It is a mixture of two simple components of
local recharge (90%) and cannate (10%) water. Flow component in
most cases is zero, which suggests that the steep Miocene water table
gradients are the results of step and stairs type aquifer. Sulphate is
Figure 2: Hydrological Zanzibar Island divided into three zones.
principal contaminant at low season water levels when connate water
percentage rises. Zone two is chloride type water due to either past
ages of higher sea level, island movement or more simply windblown
sea spray from an active coast. Island tilting, as evidenced by raised
abandoned sea cliffs is considered the main cause in Zanzibar. Zone
three is areas where sea water invasion is a natural phenomenon at
depth, and where good quality waters are due to the development of
perched aquifers. In daytime the perched aquifer becomes depleted
and serves it contacts with well and the deeper contaminated take over.
Zone one sources mainly consists of Q1/M1, zone two consist of Q2/
M1 while zone III mainly consists of Q2/M1/M3.
Sample collection and analysis: Nine caves, three from each region
of the Zanzibar Island of Tanzania were selected for investigation
(Figure 2). The sampling covers two hydrological zones. Four samples
(Diman 1-3 and Kizimkazi 3) were collected from hydrological zone
II while the rest were collected from hydrological zone III. From each
cave, water and rock samples were collected. Rock samples collected
include stalagmite, stalactite, sediments, and hard rock depending on
their availability.
Water samples were analyzed for temperature, pH, TDS, electrical
conductivity (EC) and major cations and anions while rock samples
were analyzed for mineral composition and total chemical composition
including major and trace elements. Electrical conductivity and pH
were measured in situ using HEC-100 EC meter and HM-21P pH
meter, respectively. Samples were collected in polyethylene bottles and
brought to the laboratory in a cool container. They were then filtered
through a 0.45 μm Millipore cellulose filter. Water samples for cations
analysis (Na+, K+, Mg2+ and Ca2+) were preserved in 1% v/v of 1.42 sp.
density HNO3 and analyzed using atomic absoption spectrometry.
Total chemical compostion for rock samples was measure using XRF
using powdered sample while mineral compostion was measured using
X-ray differection technique in random oriented powdered sample. The
Volume 1 • Issue 11 • 2012
Citation: Vuai SAH (2012) Geochemical Characteristics of Spaleotherm Formation in Caves from Zanzibar Island, Tanzania. 1:505. doi:10.4172/
scientificreports.505
Page 3 of 5
geochemical computer program PHREEQC [4] was used to calculate
saturation indices and activities of dissolved species in cave water at
formation temperature as measured during the time of sampling.
Results and Discussion
Water quality of cave
Temperature variation of the water was very minimum; the value
changes from 26.8 to 27.8oC. Similarly the pH also did vary slightly with
values 7.29 to 7.74. The salinity of the water as indicated by Electrical
Conductivity (EC) and total dissolved solids (TDS) range from 730
to 9500 µScm-1 and 359 to 4980 ppm, respectively. The chemical
compositions differ according to hydrological zones with two different
trends according to salinity. Zone III showed saline water strongly
influenced by seawater as indicated by higher concentration of Na+.
It has concentration trend (Na+>Mg2+>Ca2+>K+) similar to that of sea
water. This was evident for samples collected from Fukuchani (1, 2 and
3) and Kizimkazi 1 (Table 1). Fresh water showed concentration trend
of Na+>Ca2+>Mg2+>K+. The samples from Dimani (1, 2 and 3) and two
from Kizimkazi (2 and 3) contained fresh water. Although Kizimakazi 1
is located in hydrological zone III, it showed relatively fresh water with
chemical composition similar to those located in zone II. This suggest
that the water from Kizimkazi 2 is from perched aquifer that is not
affected by seawater intrusion and it chemical composition is mainly
due to dissolution of Q2/M1/M3 rock by infiltration water enriched
with chloride due to dissolution of sea salt aerosols. The amount of TDS
varied proportional to the chloride concentration (R=0.999) suggesting
salinization of cave water was due to seawater intrusion (Figure 3). The
increase of chloride concentration in groundwater is most indicative
of groundwater salinization caused by seawater intrusion due to its
conservative nature whereas the concentration variations of other
elements are frequently ignored because of their transformation
and modification due to interactions with aquifer materials [5]. It is
widely known that the hydrogeochemical composition of coastal
groundwater affected by seawater intrusion is mainly controlled by
cation exchange reaction rather than simple mixing process [6]. The
chemical composition of fresh groundwater unaffected by seawater
intrusion is dominated by Ca2+ and HCO3- ions where as in sea water;
Na+ and Cl- are the dominant ions. The cation exchange in the aquifer
is therefore expected to increase Ca2+ in the water. If this takes place
stochiometrically, the slope of the graph of Ca2+ verses Na+ in molar
concentration is expected to be close to 2. The exchange process is
always not linear and follows ligumier curve. The plot of Ca2+ against
Na+ in this study showed value clearly deviating from 2. In addition
to that, chemical composition of cave water (Figure 4) plots along the
straight line of Na+ K and Ca line. The results suggest that mixing of
seawater and carbonate water can explain the chemical composition of
the cave water in Zanzibar.
Figure 3: Map of Zanzibar Island showing sampling points.
Figure 4: Evolution of Chemical Species in Cave water.
S/N
Location
Temp.
o
C
pH
EC
µScm-1
TDS
Na+
K+
Mg2+
Ca2+
Cl-
HCO3231.8
mgL-1
1
Dimani 1
28.2
7.3
861.0
431.0
75.5
1.7
10.7
39.0
129.6
2
Dimani 2
28.1
7.3
769.0
383.0
57.2
1.4
8.7
39.3
106.5
256.2
3
Dimani 3
28.6
7.4
737.0
359.0
55.5
1.2
7.9
40.1
92.3
207.4
4
Kizimkazi 1
28.0
7.7
7020.0
3530.0
1180.8
25.5
127.0
43.1
2236.5
170.8
5
Kizimkazi 2
27.8
7.6
898.0
442.0
105.8
2.3
11.8
33.3
188.2
158.6
6
Kizimkazi 3
26.8
7.3
730.0
364.0
57.9
1.8
8.0
36.7
103.0
219.6
7
Fukuchani 1
27.7
7.4
9080.0
4980.0
1771.8
34.6
181.8
49.4
2983.8
207.4
8
Fukuchani 2
28.1
7.3
9500.0
4800.0
1930.0
36.7
194.8
49.8
3079.6
268.4
9
Fukuchani 3
27.9
7.4
9310.0
4640.0
1771.0
35.6
187.0
49.3
2946.5
231.0
Table 1: Chemical Characteristics of Cave Water.
Volume 1 • Issue 11 • 2012
Citation: Vuai SAH (2012) Geochemical Characteristics of Spaleotherm Formation in Caves from Zanzibar Island, Tanzania. 1:505. doi:10.4172/
scientificreports.505
Page 4 of 5
Parameter (%)
Hard Rock
Sediment
Stalagmite
Stalactite
LOI
42.2
38.7
42.9
43.9
Na2O
0.0
0.3
0.0
0.0
MgO
0.4
0.4
0.3
0.3
Al2O3
0.4
1.5
0.2
0.5
0.8
SiO2
0.8
6.8
0.7
P2O5
0.2
0.5
0.1
0.2
K 2O
0.0
0.1
0.0
0.0
CaO
53.4
47.2
52.5
50.7
SrO
0.1
0.2
0.1
0.1
Calcite
Calcite, Quartz,
Aragonite
Calcite,
Calcite,
Quartz
Minerals
Table 2: Mineralogical and chemical composition of Cave features.
SN
Mg/Ca
P/Ca
SG
ST
1
0.0056
0.0057 0.0017
SG
Sr/Ca
ST
SG
KMg
ST
2
0.0078
0.0081 0.0016
0.0013
3
0.0024
0.0044
0.00080
0.0023
SG
ST
0.020
0.0018
0.0011
0.035
0.0030
0.0069 0.0028
4
0.0055
0.0045 0.0018
0.0043
0.00091 0.00072 0.025
5
0.0016
0.0026 0.0014
0.0042
0.0043
0.0031
Ave 0.0046
0.0051 0.0015
0.002639 0.0023
0.0024
0.026
0.0125
0.0068 0.021
Table 3: Calcium ratios and Magnesium portioning coefficients of Spaleotherm.
Minerals and chemical characteristics of cave features
In each cave the study intended to collect stalagmite, stalactite,
hard rock and sediments from cave flow. However, in most caves these
features could not be collected. Only four caves provided stalactite
and stalagmite samples. The cave features showed uniform chemical
composition (Table 2) reflecting carbonate minerals of the bed rock.
They are dominated by loss of ignition and CaO. Ignition loss includes
organic matters and loss of carbon dioxide during decomposition of
CaCO3 into CaO. These two species account for more than 90% of
the total weight with exception of one sediment sample from Dimani
(Table 2). Other chemical species present in significant amount is
MgO, silica and aluminum oxide. They account for less than 2% each
with exception of one sediment sample. This sediment has chemical
characteristics significantly different from the hard rock collected in the
same cave. It is suggested that the sediment is foreign material brought
in either by human activities or runoff during the rain event. Influence
of hydrogeological zone on chemical composition of the cave features
revealed that the contents of magnesium oxide were higher for hard
rock and sediment of the caves found in the zone III compared to zone
II. The trend was reversed for stalactite and stalagmite (Table 3).
Strontium and phosphorous were the only trace elements found
in all samples. The values ranged from 0.03 to 0.19% and 0.1 to 0.5 as
SrO and P2O5, respectively. One sediment sample showed enrichment
of SrO value of 0.6%. There is no clear difference between chemical
composition of trace elements in hard rock/sediments and spaleotherms
(stalactite and stalagmite). The contents were not statistical different
among the zone in which these caves were found. This might be due
to their low content and presence of heterogeneous geological features
among the zones.
Generally, the cave features are dominated by calcite minerals
reflecting the dominant carbonate terrine in the cave areas. Aragonite
was found in sediment samples contained the highest content of SrO
(0.6%) and phosphorous found in zone III. It is suggested that this
chemical species is preferential incorporate in the aragonite lattice than
calcite or has influence on the precipitation of aragonite. This results
support findings of Huang & Fairchild [7] which suggested that Sr/Ca
is affected by crystallography of the spaleotherm. Okumura, 1987 also
reported that PO43- is preferentially incorporated in the aragonite than
in calcite lattice. Quartz mineral was also found in those samples in
which the SiO content is high.
Spaleotherm formation
Trace element variations in cave water reflect water-rock interaction
time which is controlled by hydrological condition [7,8]. The link
between trace element variation in spaleotherms and cave water is
partition of the element during calcite precipitation. The ratio of Mg/
Ca, P/Ca and Sr/Ca are shown in table 3.
Stalagmite showed wide variation of Mg/Ca ratios ranging from
16×10-4 to 78×10-4 while P/Ca ratios were almost constant. On the other
hand stalactite showed wide variation in both Mg/Ca and P/Ca ratios.
Stalagmite is highly influenced by cave water since it is precipitated on
the surface of the cave and hence being in continuous interacting with
water. In contrary, stalactite is precipitated on the roof of the cave as
results of precipitation of pore water infiltrating through the soil and
near surface rock. This different hydrological pathway affect water
solute composition and hence the ratios.
Magnesium partitioning coefficient for stalactite range between
0.0028 and 0.0259 while for the stalagmite ranged between 0.0069 and
0.035. The values agree fairly well with range of Kd reported by Huang
& Fairchild at temperature of 25oC and that of Huang [7,8]. Magnesium
partitioning coefficient during precipitation of spaleotherm is affected
by temperature and ionic strength [9]. The temperature in our samples
varies slightly (26.8-27.8). This one degree change could not be expected
to course changes of one order of magnitude for stalactite and five fold
for stalagmite. Thoroughly examination of the results showed that the
lowest concentration of KD was found in the saline water emphasizing
the effect of ionic strength. Our results concur with those of Huang
and Fairchild which found that partitioning coefficient was low in high
ionic strength solution [7].
Conclusion
Hydrogeochemical characteristics of cave water and spaleotherms
from Zanzibar Island, Tanzania were investigated. The results showed that
chemistry of cave water varies from fresh water to saline water depending
on the distance from the ocean as well as degree of mixing with sea water.
Generally the chemistry of cave water can be explained by mixing of
carbonate water and seawater. Chemical characteristics of cave features
showed that they are dominated with calcium carbonate with relatively
uniform chemical composition. This was also reflected by high degree of
calcite minerals in all features. Aragonite and quartz mineral are found
only in few samples in trace level. It was observed that the quartz mineral
was related with SiO contents above 1%. Inclusion of Mg during stalactite
formation showed that it was hindered by phosphorous concentration. The
magnesium partitioning coefficient range agrees well with those reported
in other studies. Moreover, the partitioning coefficient found to be low in
the cave containing saline water emphasizing the influence of ionic strength
on the Mg partitioning.
Acknowledgement
I would like to acknowledge Mr. Hassan R. Ali and Ali Omar Ali of the State
University of Zanzibar for sample collection and processing and SEAMIC for
sample analysis.
Volume 1 • Issue 11 • 2012
Citation: Vuai SAH (2012) Geochemical Characteristics of Spaleotherm Formation in Caves from Zanzibar Island, Tanzania. 1:505. doi:10.4172/
scientificreports.505
Page 5 of 5
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