Geology Reference
In-Depth Information
Several recent studies of speleothem records at
high resolutions have revealed climate features on
seasonal (Treble et al. 2003; Johnson et al. 2006;
Banner et al. 2007; Mattey et al. 2008) or even
synoptic time scales (Frappier et al. 2002) which
provide the critical direct link between the local
weather and how it is recorded during the spe-
leothem deposition process. Speleothem calcite
deposition in caves is commonly seen to be cyclical,
resulting in the development of laminae defined by
alternating pairs of fabrics (Baker et al. 2008;
Genty & Quinif 1996). Using constraints from
growth on dated artefacts and
14
C analyses
(Baldini et al. 2005; Genty et al. 2001; Tan et al.
2006; Mattey et al. 2008) cyclic laminae have some-
times been shown to be annual features and related
to strong seasonality of the local climate. Annual
growth laminae provide a means of deriving a
chronology at the best possible precision, and may
also preserve trace element and stable isotope pat-
terns that can be related to the local climate and
hydrological cycle.
Our work in Gibraltar combines comprehensive
multi-annual cave monitoring with high resolution
analyses of fabric, trace elements and stable iso-
topes in modern speleothem. A recent study of a
modern stalagmite from New St. Michaels Cave
(Mattey et al. 2008) revealed annual growth
laminae which preserve exceptionally well-defined
seasonal d
13
C and d
18
O cycles linked to ventilation.
We were able to identify the d
18
O of winter
dripwater from the complex seasonally resolved
speleothem record and show excellent inter-annual
correspondence with the d
18
O of winter precipi-
tation. In the present paper we present a more
detailed overview of the results of the first 4 years
of cave environment monitoring which includes
local meteorology, cave and soil temperature,
humidity and pCO
2
, and of drip discharge and
monthly analysis of drip water for trace element
and isotopic analysis. The monitoring data enable
the seasonally resolved speleothem fabric, trace
element and isotope record to be precisely linked
to the nature and timing of local processes in the
soil, cave air and local climate. We propose a
coupled CO
2
degassing - calcite precipitation
model which links the development of annual
cycles in d
13
C and Sr with the effects of seasonal
cave ventilation.
2.5 km long with a maximum elevation of 423 m
(Fig. 1). The ridge is asymmetric, having a steep
to near-vertical eastern slope which is partly
banked by Pleistocene sand-dune deposits, and a
western slope falling more steadily at 358 towards
the town of Gibraltar near sea level. Above 100 m
altitude the western slopes are covered in Mediterra-
nean scrub forest with low rock outcrops of the
Gibraltar limestone. The peninsula of Gibraltar
links the Betic and Rif mountain chains, which
form the southwestern end of the Mediterranean
Alpine belt and is mainly composed of early Jurassic
age limestone and dolomite which form the lower
limb of an overturned nappe (Rose & Rosenbaum
1991). These beds dip steeply to the west and
although there are no surface streams, swallets or
resurgence features, the dolomites and limestones
contain many solution caves located at altitudes
ranging from below present sea level to near the
summit ridge at over 400 m. Many caves have
natural entrances exposed by erosion, but other
significant caves have also been revealed though
tunneling (Rosenbaum & Rose 1991).
The location and a plan of St Michaels Cave
is shown on Figures 1 and 2. Old St Michaels
Cave (OSM) (Shaw 1955) has been known since
Roman times and is open to tourists as a
show cave. The cave has developed in faulted
dolomitic limestone creating a large main
chamber. Dissolution has also followed bedding
planes, creating minor caves linked to OSM and
forming natural entrances to the system. During
World War II, a new access tunnel was driven into
the lowest part of the show cave, known as the Hos-
pital, exposing a lower series of solution rifts
leading southwards along the strike of the Gibraltar
limestone at an altitude of around 325 m (Fig. 2).
This system, New St Michaels Cave (NSM) (Shaw
1954), rivals the old show cave system in terms
of the scale of speleothem decoration, and also
contains a 6 m deep lake which accumulates water
from seepages and drips entering the southern part
of NSM.
Gibraltar caves such as the St Michaels system
preserve evidence of phreatic origins and have
since undergone several phases of draining and
decoration with secondary speleothem deposits
(Tratman 1971). Because the present altitude of
the large St Michaels system is over 300 m asl, the
phreatic features indicate that these caves have
undergone significant uplift to their present posi-
tion (Tratman 1971; Rose & Rosenbaum 1991;
Rodrigues-Vidal et al. 2004). Tunnelling near sea
level in the 19th century revealed more large
natural caves such as the Ragged Staff system
with similar overall morphology but with far less
speleothem deposition. Ragged Staff Cave contains
brackish lakes with water filled passages extending
Regional setting, monitoring techniques
and analytical methods
Old and New St Michaels Caves
The Rock of Gibraltar, located where the Atlantic
meets the Mediterranean at the junction of Europe
and Africa, forms a North - South trending ridge
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