ARTICLE IN PRESS
Quaternary Science Reviews 23 (2004) 2337–2353
Lacustrine responses to tephra deposition: examples from Mexico
Richard J. Telforda, Philip Barkerb,*, Sarah Metcalfec, Anthony Newtond
b
a
Bjerknes Centre for Climate Research, Allégaten 55, N-5007 Bergen, Norway
Hysed, Department of Geography, Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YB, UK
c
School of Geography, University of Nottingham, Nottingham NG7 2RD, UK
d
Geography, School of Geosciences, University of Edinburgh, Edinburgh EH8 9XP, UK
Received 9 June 2003; accepted 24 March 2004
Abstract
One of the major problems with palaeoclimate investigations in volcanic regions is that tephra inputs to lakes can cause changes in
proxies analogous to those of climate forcing. We review the range of impacts thought to be associated with tephra deposition,
distinguishing between direct effects on lake ecosystems and indirect changes to catchment nutrient cycles. To achieve better
understanding of these complex responses, we have used high-resolution diatom analysis from around 17 tephra layers, in three
Mexican lakes. A positive response to the tephra inputs has been identified for over half of these layers. The most common response
was for diatom concentrations to increase after tephra deposition; also, in plankton dominated systems, Fragilaria spp. replaced
Stephanodiscus spp. and Aulacoseira spp. An increase in the supply of silica to the lakes through their catchments is probably the
cause of the diatom changes in the lakes studied. Direct effects of the tephra inputs to these lakes can be excluded as they would not
generate changes of sufficient longevity. The impact of these tephras, which lasted for several decades, was insufficient to perturb the
underlying long-term climate forcing of these lake systems.
r 2004 Elsevier Ltd. All rights reserved.
1. Introduction
Major volcanic eruptions are an important driver of
global climate change (Zielinski, 2000) and will have
profound effects on terrestrial and aquatic ecosystems at
a variety of scales (Lucht et al., 2002), yet changes to
ecosystems caused by tephra (volcanic ash) are poorly
understood. The deposition of tephra in lakes is
common in many parts of the world and a feature of
many lake sediment cores. The tephra layers, once
dated, have successfully been used to provide a
chronological framework for lacustrine records of
environmental change (Newnham and Lowe, 1999;
Newton and Metcalfe, 1999; Litt et al., 2001). There is
evidence however, that tephra deposits are not merely
passive markers, but cause important changes in lakes
and their catchments (Zielinski, 2000; Haberle et al.,
2000). At the local scale, deposition of large quantities
of hot tephra proximal to a volcano will have immediate
*Corresponding author. Tel.: 01524-65201; fax: 01524-847099.
E-mail address: p.barker@lancs.ac.uk (P. Barker).
0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2004.03.014
ecological impacts, for example in the Mt. St. Helen’s
1980 blast zone (Baross et al., 1982; Wissmar et al.,
1982; del Moral and Jones, 2002; Saucedo et al., 2002);
in the destruction of vegetation by the Volcan de Colima
in 1998/1999 (del Moral and Jones, 2002; Saucedo et al.,
2002); or in controls on vegetation recolonisation
(Foster et al., 1998; Whittaker et al., 1999). Much less
is known of the effects of the deposition of thin, cold,
distal tephra even though these are more frequent and
geographically important events.
The possible impact of distal tephra deposition on
aquatic ecosystems was recognised in the 1950s by
Eicher and Roundefell (1957) and has been directly
recorded by a small number of ecological studies
(Kurenkov, 1966; Collier, 2002; Fazlullin et al., 2000).
However, continuous ecological monitoring in active
volcanic terrains is rare, and few studies have the
decadal durations necessary to measure both chronic
and acute ecosystem changes. Palaeoenvironmental
studies are better equipped than neo-limnology to
record long-term temporal ecosystem change using
indicator species such as the siliceous algae diatoms
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R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
Fig. 1. Map of study area showing lakes sampled and known volcanoes with inset map showing whole of Mexico.
and chrysophytes. Many palaeoenvironmental studies
have attributed changes in diatom assemblage composition or diatom concentration to tephra falls (e.g. Lotter
et al., 1995; Barker et al., 2000; Eastwood et al., 2002).
The nature and duration of tephra impacts reported
from different lakes varies substantially: Hickman and
Reasoner (1994) report a 10-fold increase in diatom
concentrations in the sediment lasting up to 300 years in
alpine lakes in Alberta, Canada; Barker et al. (2000)
report long-term changes in the composition of the
diatom assemblages in a Tanzanian crater lake; Lotter
et al. (1995) find only minor, short-term changes in small
European craters. Other studies have reported no direct
response to tephra deposition (Telford and Lamb,
1999), or more often, have not identified tephra as an
important forcing factor relative to other processes.
Notwithstanding these differing responses, it is important to be able to recognise the impact of tephra on lakes
to prevent misinterpretation of palaeolimnological
records from volcanic regions. Current understanding
of the effects of tephra on aquatic ecosystems is
inadequate to discriminate between these contradictory
observations.
Most tephra deposited in lakes arises from localised
cinder cones that eject material at low levels into the
atmosphere and are unlikely to have a direct climatic
impact. However, a large variety of possible environmental responses can be induced due to the complex
interaction of the magnitude and characteristics of the
tephra fall and the lake catchment ecosystem. Few
comprehensive reviews of the impacts of tephra on lakes
exist, and we will begin with a critique of the
mechanisms proposed in the literature. We will then
examine diatom assemblages around 17 tephra layers
from three contrasting lakes in the Central Mexican
highlands (Fig. 1), to investigate any tephra-induced
ecosystem change, its nature and duration.
2. Mechanisms of tephra impacts
A wide variety of mechanisms explaining possible
tephra impacts on lakes and their catchments have been
reported. These include terrestrial and aquatic ecosystem adjustments, changes in nutrient cycles and
pathological effects on organisms. We will first consider
direct effects of tephra on lacustrine systems. Eruptions
usually emit volatile elements that are rapidly released
on contact with water (for example Fruchter et al. (1980)
estimated more than 30% of the total Cl and SO2
4 in
tephra from the 1980 Mt. St. Helens eruption was
leached within 1 h). These salts may make an appreciable, but short-lived addition to the solute budget of a
lake (Schulz et al., 1997). Abella (1988) dismissed the
possibility that toxicity of elements were responsible for
fish kills in Lake Washington as death occurred too long
after the eruption.
An important, and often hypothesised mechanism, is
that tephra falling through a lake will reduce light
availability for macrophytes and photosynthetic algae
(Abella, 1988). However, this is difficult to reconcile
with sedimentation calculations using Stoke’s law that
predict that tephra should sink quickly in still waters.
Assuming a particle size of fine sand, tephra will have a
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R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
settling velocity of over 40 m h1 (Julien, 1995) and is
unlikely to explain ecological responses >1 year. Even
pumice can settle quickly through water once it becomes
saturated (Manville et al., 2002). A related phenomenon
is ingestion of the falling tephra by fish and trapping of
fine particles within their gills (McDowall, 1996). If
sufficient fish die, the lake may experience more widespread change through a reduction in predation.
Similarly, if aquatic macrophytes are damaged or killed
by tephra, there will be less available habitat for
epiphytic algae and a dislocation in the nutrient cycles
may be expected. Harper et al. (1986) found an increase
in overall diatom concentrations above two tephra
layers and recorded variations in the responses of
different diatom life-forms. In both cases, the concentration of epiphytic diatoms peaked after that of
planktonic species and epipelic species, possibly due to
the destruction of vegetation by tephra and the
consequent limitation of epiphytic habitats (Harper
et al., 1986). Conversely, epipelic (or epipsammic if
tephra is sand-sized) species can colonise the newly
deposited tephra and it is noticeable that these are often
the species to show a positive response to tephra falls.
It is possible that a thick tephra creates an impermeable barrier over the lake’s sediment, preventing the
regeneration of nutrients such as phosphorus (Barker
et al., 2000; Barker et al., 2003). Tephra presents a
physical barrier to the transport of P into the water
column by preventing resuspension and irrigation of the
sediment by bioturbation and wave action, as well as a
barrier to P diffusion depending on its thickness. If there
are any P binding sites on the tephra, these will further
reduce the P flux, however, given the size of tephra
grains, the surface area will be low compared to iron
hydroxides, so this process may be relatively unimportant. A diffusional barrier can only be important in
sediments where a P concentration gradient between the
pore water and the water column exists and will usually
be less important in oligotrophic systems.
Probably the most obvious chemical effect of the
deposition of a tephra layer into a lake is a huge,
essentially instantaneous, addition of silica. However,
this particulate silica is not immediately available to the
biota, rather it dissolves slowly and is confounded by the
rapid sedimentation of the tephra through the water
column. Mixing processes will reduce the sinking rate
2339
and could be significant in enabling the dissolution of
tephra in the water-column of large lakes (Haberyan,
1998). Once the tephra reaches the lake bed the silica
will continue to be released, producing a net increase in
the benthic silica flux if the supply from the tephra is
greater than that from the pre-existing lake sediment,
whose most reactive component is likely to be diatom
frustules. The rate of silica dissolution depends on the
specific dissolution rate, the specific surface area, and
the fraction of different silica-rich sediments (Table 1).
Dissolution rates are pH sensitive, but for crystalline
silicates, dissolution is largely pH indifferent up to pH 8,
thereafter dissolution rate increased by 0.3 log units per
pH unit (Brady and Walther, 1989). Since dissolution is
a function of the surface chemistry, this relationship
probably also holds for amorphous silica such as
diatoms and tephra. The diffusion rate of Si as described
by Frick’s law may change slightly between tephra and
lake mud due to small differences in porosity. In both
cases the porosity will be high (>70%), so the difference
in diffusion rates will be minimal. These considerations
suggest that direct silica inputs do not provide the
necessary sustained stimulus to explain the enhanced
productivity of siliceous algae often reported after
tephra falls.
Tephra-induced changes in the silica and phosphorus
budgets of lakes can also be brought about by the
modification of catchment processes. The diversity of
catchments makes it difficult to generalise, nevertheless
there are a number of potential mechanisms through
which cold, distal tephra falling over the lake’s catchment could influence the lake. Most chemical weathering involves the solution of silica by leaching,
therefore the addition of tephra to a catchment increases
the rate of weathering and hence the amount of Si
leached into the lake. Shoji et al. (1981) estimated that
25% of the silica in a felsic tephra was eventually
leached, and in anything other than a very thin tephra,
or a lake which receives silica-saturated groundwater,
this represents a large addition to the Si budget
(Dahlgren et al., 1999). Weathering will proceed rapidly
until either the tephra is converted to a more stable
state, or is buried. Clearly, lakes with very small
catchments relative to lake area, e.g. crater lakes and
pools within bogs, will be less sensitive to this
mechanism than those with extensive catchments.
Table 1
Solubility of different silica bearing sedimentary components
Diatom silica
Specific Si dissolution rate
Specific area
15
9.8 10
100 (c)
(a)
Tephra
2.8 10
3 (d)
Quartz silt
14
(b)
16
1.3 10
10 (e)
Units
(e)
(mol cm2 s1)
(m2 g1)
Values (a) and (c) are from Lawson et al. (1978); (b) is from Gislason and Eugster (1987) for a basaltic tephra, but note that rhyolitic tephras have a
slower rate; (d) and (e) are from Brady and Walther (1989).
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Although the precise mechanisms will be site specific,
the slow release of tephra-derived silica from catchments
would provide a sustained flux of silica to a lake on a
time scale comparable to the century-scale responses
found in some systems.
3. Site descriptions
The Trans-Mexican Volcanic Belt (TMVB) stretches
across central Mexico at approximately 19 N for over
1100 km (Fig. 1). It is largely Quaternary in age
(Demant, 1981) and includes two major stratovolcanoes
which are active at the present day: Popocatépetl and
the Volcan de Colima. Much volcanic activity, however,
has been in the form of monogenetic cinder cones
(Newton et al., under revision). The best known of these
is the Volcan Paricut!ın that appeared in a maize field in
February 1943. Within the TMVB are a large number of
closed basin lakes, many of which have been the subject
of palaeoenvironmental research (Metcalfe et al., 2000).
Here we focus on three lake basins within the modern
state of Michoaca! n: Zirahuén, Pa! tzcuaro, and Zacapu
(Fig. 1). The basins are all in the Michoaca! n-Guanajuato Volcanic Field (MGVF) where monogenetic
cinder cones and shield volcanoes predominate (Hasenaka, 1992). Hasenaka (1994) has estimated that there
are some 900 cinder cones, 100 lava cones and more
than 300 medium-sized shield volcanoes in the MGVF,
in addition to the two stratovolcanoes. It has been
estimated that there are, on average, two eruptions per
1000 years in this area, with 16 Holocene cinder cones
(Hasenaka and Carmichael, 1987). Over the historical
period the MGVF has seen the eruption of Paricut!ın
(1943–1952) and of Volcan Jorullo (1759–1774). There
are documentary records of both these eruptions,
including reports of lava flows and tephra falls. Perhaps
unsurprisingly, sediment sequences from the three lake
basins chosen for this study all contain multiple tephra
layers.
The Zirahuén basin (19 210 N, 101 460 W) is the most
southerly and smallest of the study sites covering an area
of about 260 km2 at 2075 m.a.s.l. The lake was created
by a lava dam, probably of late Pleistocene age, but is
undated. Lago de Zirahuén has a maximum depth of
about 40 m and is a freshwater, alkaline (pH 8.4),
calcium-magnesium bicarbonate lake (Davies et al.,
2002). The lake is monomictic and stratifies between
April and October. Zirahuén has traditionally been
regarded as an oligotrophic or oligo-mesotrophic lake,
but water chemistry data indicate an increase in trophic
status over the last 15 years (Bernal-Brooks and
MacCrimmon, 2000). The Pa! tzcuaro basin (19 400 N,
101 350 W) is much larger, with a catchment area of
927 km2. It has been suggested that this was formerly
part of the R!ıo Lerma drainage system and that the
basin was isolated from the river by Pleistocene volcanic
activity (Davies et al., 2004). Links between the
Pa! tzcuaro and Zirahuén basins have also been proposed. Bradbury (2000) suggests that the modern basin
configuration post-dates 44,000 yr BP. The basin floor
lies at an altitude of 2034 m a.s.l., but the surrounding
highlands reach more than 3000 m. The topography of
the basin is steep and highly dissected in the north, but
gentler in the south. The lake itself is shallow (maximum
depth o12 m), moderately alkaline (pH 8.8) and
sodium-carbonate dominated. Pa! tzcuaro is well mixed,
with high turbidity and has been classified as eutrophic
!
!
to hyper-eutrophic (Chacon-Torres,
1993; ChaconTorres and Muzquiz-Iribe, 1997). This basin has been
a focus for palaeoenvironmental studies since the 1950s
(Hutchinson et al., 1956; Watts and Bradbury, 1982;
O’Hara et al., 1993; Bradbury, 2000). The Zacapu basin
(19 510 N, 101 400 W) is the most northerly of the study
sites, and at the lowest elevation (1980 m a.s.l.). Unlike
Zirahuén and Pa! tzcuaro, this basin has been artificially
drained, with a series of schemes initiated in the late
19th century. As a result, although lake sediments cover
some 261 km2, there is only a small, eutrophic, remnant
lake in the south-west corner of the basin and a series of
small, spring fed pools around the basin margins. The
pH of the main lake is 8.8 and that of the smaller pools
somewhat lower. There are roughly equal proportions of
the major cations, and bicarbonate is the dominant
anion. Cores have been collected from both the drained
basin floor and the Laguna de Zacapu (e.g. Metcalfe,
1995), and there are discontinuous records extending
back more than 50,000 years (Ortega, pers. comm.).
4. Methods
The approach adopted in this study has been to
integrate a detailed analysis of diatom response to
tephra inputs, within a broader investigation of longterm environmental change in the region. The geochemistry of the tephra layers found in the cores has been
determined by electron microprobe analysis of glass
shards and additional analyses of tephra from known
sources (see also Newton and Metcalfe, 1999). Highresolution diatom analysis (every 0.5 cm) above and
below individual tephra layers, has been complemented
by lower resolution sampling through the entire core
sequences (Adby Collins, 2000; Terrett, 2000; Davies
et al., 2004). Diatom preparation followed standard
techniques, using H2O2 and HCl. Polystyrene microspheres of a known concentration were added to permit
the calculation of absolute diatom abundance (Battarbee and Kneen, 1982). A range of other sediment
characteristics has been determined including mineral
magnetic measurements, major element analysis (XRF)
and available P. Age control is based on radiocarbon
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2341
Table 2
Radiocarbon dates from Zacapu and P!atzcuaro
d13C (%)
Lab code
2840790
4580760
5140770
15,4707120
5765750z
26.3
26.5
26.4
22.6
26.1
GU-8722
GU-9027
GU-9026
AA-39100
SRR-6514
960745
1325745
2765770
3790750
8345755
9395770
10,770770
12,225780
13,585785
14,9407100
18,9507145
22.1
21.5
24.3
19.8
17.9
18.5
25.1
23.6
25
23.4
23.1
AA-37993
AA-37994
AA-37995
AA-37996
AA-37997
AA-37998
AA-37999
AA-38000
AA-38001
AA-36546
SRR-6513
Site
Sample
Type
Date
Zacapu (Cantabria)
1/1
1/1
1/2
1/6
1/6
Conventional
Conventional
Conventional
AMS
Conventional
P!atzcuaro
KD/72–74
C4/D1/11–13
C4/D3/72.5–74.5
C4/D3/86–88
C4/D5/30.5–32.5
C4/D5/39–41
C4/D5/49.5–52.5
C4/D6/16.5–18.5
C4/D6/53–55
C4/D7/28.5–33
C4/D8/58–62
AMS
AMS
AMS
AMS
AMS
AMS
AMS
AMS
AMS
AMS
Conventional
37.5–41
87–92
14–19
59–62
94–100
Asterisk indicates an assumed d13C value.
z
Date is assumed erroneous.
14
C yr BP
210
Pb dates from Zirahuen are given in Davies et al. (2004).
dates from Pa! tzcuaro and Zacapu (conventional and
AMS see Table 2) and 210Pb dating for the younger
Zirahuén material (Davies et al., 2004).
5. Results
5.1. Tephra chemistry
Virtually all of the tephra layers found were the result of
basaltic-andesitic to andesitic activity and were produced
by monogenetic cinder cones. Their chemistry is summarised in Fig. 2 and the details are available through the
Tephrabase web site (University of Edinburgh, 2004).
More silicic tephras are rare, with SiO2 abundances
ranging from 52% to 67%. The most silicic tephra (SiO2
67%) occurs in the Zacapu Cantabria core, dated to 5140
14
C yr BP. Tephra from the eruption of Paricut!ın was
found in all cores taken from Zirahuén and in the
uppermost sediments from Pa! tzcuaro (not described here).
Tephra from Jorullo was found in cores from Zirahuén
and in the highlands between Zacapu and Pa! tzcuaro. The
tephrastratigraphy of this region will be described in detail
elsewhere (Newton et al., under revision).
5.2. Diatom analyses around tephra layers
Results of the detailed diatom analyses undertaken
around the tephra layers are presented below in the
context of the other sediment analyses (lithology,
available P and magnetic susceptibility) and coarser
resolution whole core diatom analysis. The basins are
described from south to north, as above.
5.2.1. Lake Zirahuén
Four short cores were obtained from this basin in
1998 using a micro-Kullenberg corer and have yielded
records of environmental change over the last 1000
years. The diatom, magnetic susceptibility, heavy metal
chemistry, tephra and dating of these cores are discussed
in some detail in Davies et al. (2004). The short cores
from Zirahuén contained four tephras, including those
from both Jorullo and Paricut!ın (Davies et al., 2004;
Newton et al., under revision) (Table 3). Core ZD/98
was taken from the deepest part of Zirahuén and had
four tephras, of which the surface layer was identified as
Paricut!ın (AD 1943–1952) and a layer from 28 cm as
Jorullo (AD 1759–1774). At 40–41 cm there was a more
silicic tephra layer from an unidentified source (ZD9841). A two-fold increase in diatom concentration
occurred after this tephra (Fig. 3a). The Fragilaria spp.
dominated diatom assemblage, did not return to pretephra concentrations within the sampled section. No
detectable species change accompanied this tephra,
although this sampling interval lay within a portion of
the core already dominated by Fragilaria spp., whereas
the base of the core was dominated by Aulacoseira spp.
and the top 15 cm by Cyclotella spp. A second core
ZR/98, was taken from near the mouth of the main
stream feeding Zirahuen, and included only the Paricut!ın tephra, as the rate of sediment accumulation was
much more rapid at this site. The diatom stratigraphy
showed no response to this 0.5 cm horizon ZR98-55
(Fig. 3b). The Paricut!ın tephra was found at the very top
of core ZD/98 and it was not possible to determine
whether any post-tephra change occurred. The diatom
flora in Fig. 3b is consistent with that in the basal 15 cm
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2342
5
High K
(High-K Calc-alkaline series)
4
K2O (wt%)
3
Medium K
(Calc-alkaline series)
2
Low K
1
(Low-K, tholeiite, series)
basalt
basaltic
andesite
andesite
dacite and rhyolite
0
45
50
55
60
65
70
75
80
Si2O (wt%)
Fig. 2. Summary of the tephra geochemistry. Solid black triangles=Cantabria tephras; open circles=P!atzcuaro tephra (core 4); crosses=Zirahuen
tephras.
Table 3
Response for different analysed tephra-lake pairs
Lake
Tephra code
Depth (cm)
Thickness (cm)
% Silica
Y/N
Species
Conc.
D/S ratio
P!atzcuaro
C4/T404
C4/T464
C4/T479
C4/T495
C4/T499
C4/T502
C4/T520
C4/T636
C4/T643
403–404
463.5–464
470–479
494–495
498–499
501–502
519–520
634.5–636
642–642.5
1
0.5
9
0.8
1
1
1
1.5
0.5
58.49
63.58
60.4
53.33
58.69
59.39
57.17
62.31
61.67
No
?
Yes
Yes
No
Yes
No
Yes
No
—
—
Yes
Yes
—
?
—
Yes
—
—
?
?
Yes
—
?yes
—
Yes
—
—
—
—
—
—
—
—
—
—
Cantabria, Zacapu Swamp
CA1/T37
CA1/T58
CA1/T87
CA1/T114
CA1/T176
CA1/T558
37–37.5
57.5–59
86–87
112–114
174.5–176
156.5–158
0.5
1.5
1
2
1.5
1.5
60.68
55.79
61.26
67.41
61.81
57.63
Yes
?
Yes
No
Yes
Yes
—
?
—
—
Yes
—
Yes
—
Yes
—
Yes
Yes
Yes
—
Yes
Yes
—
—
Zirahuén
ZD98-1 5
ZD98-3
ZD98-28
ZD98-41
ZR98-55
55
0–1.5
3
28
40–41
1.5
1
1
1
56.5
58.11
57.37
60.7
No
No
No
Yes
—
—
—
—
—
—
—
Yes
—
—
—
—
0.5
B58
No
—
—
—
Column ‘Y/N’ describes whether a diatom response is shown or not; ‘Species’ records if this response was a change in species composition; ‘Conc.’ if
the response was a change in diatom concentration. The ‘D/S ratio’ is that between diatoms and chrysophyte statocysts.
of ZR/98. Major diatom changes in this core occur
only in the top 45 cm of the sequence (Davies et al.,
2004).
!
5.2.2. Lake Patzcuaro
A number of cores have been taken from Pa! tzcuaro,
but here we focus on a 7.35 m core (C4) taken in the
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2343
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ph u cu cu
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hn chn ula ula occ yclo ym iplo pith rag rag rag
ne nkn iato
ag
o
c
r
y
F
A A A
F
A C C C
G N N N
D
S u
D E F F
a
l
ttu
38
39
Depth (cm)
40
ZD98-41
41
42
43
44
20
(a)
40
20
10 20 30
la
e
nc
ttu
a
a
a
a
z4
la
im ta
al
at
rl
la
is
ha sp
el
tri ens
a
a
va
ar
ph
ra
iss eola
p
s
n
e
t
n
i
e
e
r
e
ul tion
a
a
la ttul
e e
a
u
u c
t
n
c
t
t
t
w
i
g
c
r
c
i
c
.
o
i
n
i
i
t
o
a
o in to no tic ce
a
i
n sp s v ell
t
g
l
pt
ea m tra
m la
icr ip
i
ar ons inn
t
nn
al ver cen
ry hya cryp unk fon lan
m ell
pi
m
p
c
p
c
es hes thes one lla s
s
s
a
a
a
h
a
a
a
t
t
ll
a
a ula hia hi
ia ia on
e
ri
e
ei lari lari
ei
ul
ul ul
c
an an an o lot
c
be n
ila
ch ch l c
i
i
on
vic avic avic avic itzs itzs
m iplo
hn chn chn nom yc
zs itzs ota
pl rag rag
ag
i
a
y
t
r
c
i
C
F
D
A
D
N
C
N
F
T
F
N
N N
N N
A
A
A
N
50
Depth (cm)
52
54
ZR98-55
56
58
(b)
40
20
20
20
20
0
16
32
Fig. 3. High-resolution diatom sampling around tephra in Zirahuén cores (a) ZD/98 and (b) ZR/98. Diatom values are percentages of total diatom
sum.
north-east part of the basin which covers some 19,000
years. The full diatom sequence from this core is
described by Terrett (2000). The magnetic susceptibility
is relatively low from the base of the core to 350 cm
when an abrupt increase is observed following a horizon
rich in ostracods thought to represent a dry interval
ARTICLE IN PRESS
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
2344
D
ep
th
(m
Li
)
th
ol
og
y
14
C
da
t
es
Lake Pátzcuaro core 4
Magnetic susceptibility
Low Field
0
5
10
0
Available P (ppm)
20
40
60
Tephra
layers
80
0
1325±45
1
2
2765±70 3
C4/T287
4
C4/T404
C4/T424
C4/T434
C4/T464
C4/T479
C4/T495
C4/T499
C4/T502
C4/T520
3790±50
8345±55
9395±70
10770±70 5
12225±80
13585±85
C4/T561
6
C4/T604
C4/T620
C4/T636
C4/T643
14940±100
7
18950±145
Ostracod rich
Silty clay
Clay
Fig. 4. Complete stratigraphy of Lake P!atzcuaro core 4 together with magnetic susceptibility and available P. The 15 tephra layers are marked as
black horizontal lines on the lithology column. Tephra C4/T479 is a 9 cm thick coarse-grained tephra and is distinguished by a thick dashed line. Nine
of the tephra layers have been studied in detail (see Table 3) and all have been scanned.
(Fig. 4). Higher susceptibility probably represents
increased soil erosion in the Late Holocene, as has been
previously detected in this basin (O’Hara et al., 1993).
Peaks in available P are found around 650 and 450 cm,
but no systematic relationships are found between either
available P or magnetic susceptibility, and the tephra
horizons (Fig. 4).
From the base of the core to ca. 500 cm, the diatom
flora was dominated by large Stephanodiscus spp., A.
ambigua var. robusta and A. granulata. Between ca. 500
and 350 cm, A. ambigua dominates. In the upper part of
the core, A. granulata is accompanied by more
periphytic and epiphytic taxa indicative of a shallowing
of the lake. A sequence of 15 tephra layers has been
found in this core, the majority of which were deposited
in the late Pleistocene and early Holocene. Some of these
were diffuse, others occurred at the end of drives and
although all were scanned, we have only studied the best
defined tephras in detail (Table 3).
A fine black tephra (C4/T643) and a white tephra
comprising a lower coarse and upper fine unit (C4/T636)
were deposited in Lake Pa! tzcuaro around about
14,9407100 14C yr BP. The origins of these tephras
are unknown (Newton et al., under revision). The
Aulacoseira granulata and Stephanodiscus spp. dominated diatom assemblage showed no discernable response to the first 0.5 cm-thick tephra C4/T643 (Fig. 5a).
Instead, diatom concentrations fell to levels lower than
those before its deposition and the assemblage was
unchanged. In contrast, pronounced changes in the
ARTICLE IN PRESS
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
C
De da
pt tes
h
(c
Cy
m
cl
)
ot
el
la
m
Au
ic
hi
la
ga
co
ni
se
an
ira
a
gr
an
St
ul
ep
at
ha
a
no
St
di
ep
sc
ha
no us n
di
sc iaga
us
ra
e
af
Fr
.m
ag
in
ila
ut
ria
ul
us
el
lip
ti c
a
Fr
ag
ila
ria
Fr
pi
ag
nn
ila
at
ria
Fr
a
ag
co
Co ila
ns
t
c ri
Ac co a b rue
hn ne rev ns
Ni an is is
tz t pl tr
Na sch hes ace iata
v ia l n
Di icu la anc tula
at la cu eo
om su um la
ta
co bm
nc ur
en iali
tra s
ti o
Se
n
d.
P
co
nc
en
tra
tio
n
2345
14
630
632
634
C4/T636
14,940 – 100
636
638
640
642
C4/T643
644
40
40
20
60
60
20
20
1
0
la
tu
t
ce
a
80
ppm
us
ul
ut
in
n
n
n
ns ata
s
la
n
m
ta
tio
l
v
f.
io
tio
ra tria
en
a
u
a
a
r
a
e
u
a
at
1
r
t
n
t
t
r
r
t
g
s
s
.
t
t
n
i
a
i
.
s
u
ra
n
e
na na .
en
am
sp m
gr tell rev
af
in pin pp isc
on
ce
ne
nc
a
) es es
a ira
s
nc
s
p
r
b
c
n
e
r
d
o
i
s
i
t
o
e
a ia
c
t
c
th
cm th
co
a
se
ria ria la no
r
ria
se se ell
da h ( an an
m
ge
dr
P.
co
ila gila icu pha
ila
co aco lot gila
t
o
n
.
n
e
n
t
C
a
g
g
a
p
l
v
h
l
h
a
a
a
a e
e
yc ra
yn Dia
ul
po
ed
Au
Fr Fr
N St
D Ac Ac
S
F
Fr
S
S
C
Au A
m
si
is
ut
in
14
2
109 valves/g
(a)
ua
ta
g
bi
s
di
8345–55
460
C4/T464
465
470
9395–70
475
C4/T479
- 480
485
NOT COUNTED
10770–70
490
C4/T495
495
-
C4/T499
500
C4/T502
505
CORE GAP
510
515
C4/T520
520
525
20
(b)
40
100
20
20
20
80
40
20
20
20
60
40
4
8
1 2 3
108 valves/g106 points/g
20 40 60
ppm
Fig. 5. Diatom response to older tephras in Lake P!atzcuaro core 4. Diatom values are percentages of total diatom sum: (a) shows two of the older
tephras C4/T636 and C4/T643, while (b) shows the five tephra layers between 460 and 525 cm. This latter section spans two core drives separated by a
12 cm gap.
ARTICLE IN PRESS
2346
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
diatom stratigraphy were associated with the second
finer tephra C4/T636 (Fig. 5b). Prior to this 1.5 cm-thick
tephra, A. granulata and Stephanodiscus spp. had
dominated the diatom flora, although their concentration in the sediment was relatively low. Above the
tephra, the diatom concentration increased 10-fold and
the assemblage became dominated by Fragilaria spp.
(especially F. elliptica), A. granulata was absent and
Stephanodiscus spp. were rare in the post-tephra
sediments. The diatom concentration declined exponentially away from the tephra and Fragilaria dominance
was reduced, with Cyclotella michiganiana and subsequently Stephanodiscus spp. taking their place. Diatom
concentration eventually returned to its pre-tephra
levels, and the Fragilaria spp. apparently stimulated by
the tephra, declined to less than 25% of the diatom
assemblage. Although the full core diatom record shows
the presence of Fragilaria spp. in this part of the core,
they are present at p10%, whereas the percentages
shown in Fig. 5a and b are significantly higher. The
chronology does not allow precise calculation of
the duration of this event, but we estimate it was of
the order of several decades derived from linear
accumulation rates between 14C dates.
No clear diatom response was shown to tephra
C4/T520 at about 12,200 14C yr BP (Fig. 5b). Following
this tephra Stephanodiscus spp. did increase in frequency
at the expense of Cyclotella stelligera and A. granulata.
Fragilaria spp. increased somewhat later, but diatom
concentration was variable, and not obviously higher
after the tephra. These lagged diatom changes cannot be
unambiguously attributed to the tephra.
Three tephra layers occurred within 10 cm at about
10,800 14C yr BP. The sediments around the diffuse first
tephra (C4/T502), mixed with lacustrine mud, were
dominated by Aulacoseira ambigua (Fig. 5b). This taxon
was not found immediately above the tephra, where
Fragilaria spp. were dominant, but does generally
dominate early to mid-Holocene sediments in this core.
A large increase in sponge spicule concentration may
indicate a change in benthic habitat conditions following
the tephra. Unfortunately, this tephra terminated the
drive and there is no sediment available from directly
below it. A second tephra (C4/T499), just 1 cm above the
first, showed no further impact on the Fragilariadominated assemblage. Following tephra C4/T499,
Stephanodiscus spp. became important in the assemblage
until the deposition of a third tephra layer (C4/T495)
after which its superiority was curtailed. Tephra C4/
T495 was followed by a large increase in diatom
concentration and also in Fragilaria brevistriata frequency a few cm above the tephra. A. ambigua
percentages then increased as F. brevistriata dominance
declined, but Stephanodiscus spp. did not recover. It is a
feature of the core that Stephanodiscus spp. are a
significant component of the Pleistocene flora, but are
rare in the Holocene sediments. Therefore, percentages
of Fragilaria shown by the detailed analyses around the
tephras described here, are well above those in the full
diatom record.
Tephra C4/T479 is a 9 cm-thick, coarse-grained black
tephra (Figs. 4 and 5b). The diatom assemblage was
dominated by A. ambigua both before and after this
tephra horizon, consistent with the coarser resolution
full core diatom record (Terrett, 2000). Within this thick
tephra, Synedra tenera and Achnanthes minutissima were
important at some levels. Diatom concentration values
increased substantially from those below the tephra
layer, and then abruptly declined at 466 cm when A.
distans became important. There were no major changes
in diatom assemblage at tephra C4/T464 (Fig. 5b), and
diatom concentration continued an increase that began
before the tephra influx. Similarly, the diffuse tephra
C4/T404 (not illustrated), had no impact on the diatom
concentration and the notable decline in A. distans in
favour of the dominant A. ambigua was initiated before
the tephra. In the context of the sequence as a whole, the
abundance of A. distans and its varieties seems to reflect
changes in lake chemistry associated with the accumulation of ostracod rich sediments in shallow water
conditions, i.e. water depth changes unrelated to tephra
inputs.
5.2.3. Cantabria, Zacapu main basin
The results presented here come from a 6 m core
(Cantabria), covering more than 15,000 years, taken
from the drained basin floor using a percussion corer.
The stratigraphy of this sequence was quite different
from that in the cores from the other basins. The upper
350 cm comprise black, fibrous peaty sediments with low
magnetic susceptibility (Fig. 6). In common with other
sequences from the drained area, the last B3000 yr were
missing as a result of desiccation and deflation of the
peaty surface material. A diatom record for the whole
sequence was studied by Adby Collins (2000). The
Cantabria sequence included six tephra layers (Table 3
and Fig. 6). The top 3.3 m of the core contained five
tephra layers, with the sixth incorporated into the
270 cm of grey silty-clay, lying below the surface peatlike sediments. Available P declined progressively
through the core, with lowest values in the peat rich
sediments, corresponding to the succession from lake to
swamp environment.
There was no change in the diatom assemblage
composition around the basal tephra in the sequence
CA1/T558, the only tephra in the silt-clay section (not
illustrated). Diatom concentration changes were erratic,
and were generally higher before the tephra than after it.
Within the peat-like sediments, some of the tephra
horizons produced responses, while others were complacent. Diatom concentrations around tephra CA1/
T176 were also low and preservation was not very good.
ARTICLE IN PRESS
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
2347
y
og
h
ol
th
Li
ep
t
D
14
C
da
(m
)
te
s
Zacapu (Cantabria)
Complete profile
Magnetic susceptibility
(χ10-9 m3 kg-1)
0
2840±90
3
6
0
10
Available P
Tephra
(ppm)
20
studied
30
40
50
0
CA1/T37
CA1/T58
CA1/T87
4580±60 1
CA1/T114
5140±70
CA1/T176
2
3
4
5
CA1/T558
15470±120
6
Organic rich peat-like sediment
Grey silty clay
Grey clay
Fig. 6. Complete stratigraphy of Zacapu (Cantabria) with mass specific magnetic susceptibility and available P. The six tephra layers are marked as
black horizontal lines on the lithology column. Diatoms from around all of these tephra layers have been studied in detail.
An increase in diatom concentration occurred after
CA1/T176 but declined to pre-tephra levels within 2 cm.
Moreover, N. amphibia increased at the expense of
Amphora veneta (not illustrated). Another species shift
occurred when Fragilaria spp. eventually became
dominant following tephra CA1/T114 at the expense
of Gomphonema spp. (Fig. 7a). Evidence for a delayed
response may also be shown by a peak in Fragilaria spp.
9 cm above CA/T176 (Adby Collins, 2000), although the
analytical resolution of the full core diatom sequence is
inadequate to confirm cause and effect. Similarly, there
was an apparent delay before the diatom concentration
increased after CA1/T114, although the diatom/chrysophyte statocyst ratio did increase immediately. The
composition of the diverse littoral diatom assemblage
living during the deposition of the next tephra CA1/T87
(Fig. 7b) showed no change attributable to the tephra.
There was a substantial increase in diatom concentrations and once again in the diatom/chrysophyte
statocyst ratio immediately after the tephra; these values
returned to pre-tephra levels within 3 cm. The full
core diatom record indicates that the shift to Fragilaria
spp. above CA1/T114 persisted until after CA1/T87,
when a change to Nitzschia and Gomphonema was
found.
Diatom concentration fell immediately before tephra
CA1/T58, but increased again 2 cm after the tephra.
There was also a change in assemblage composition
above this tephra: Cocconeis placentula and Gomphonema gracile declined and Nitzschia amphibia and Navicula
spp. increased in relative frequency (Fig. 7c). These
changes were not reversed back to pre-tephra conditions. No change in the composition of the diverse
littoral assemblage (with important contributions from
Eunotia spp., Gomphonema spp. and Nitzschia amphibia), occurred after tephra CA1/T37 (Fig 7d). However,
there was a substantial rise in diatom concentration and
in the diatom/chrysophyte statocyst ratio, but both of
these values dropped to their pre-tephra levels within
3 cm, and then increased for a second time.
Depth (cm)
Cy
m
Eu be
no lla
Eu tia sp.
n fl
Eu oti exu
no a m os
tia o a
in nod
G
te o
om
rm n
ph
ed
on
ia
G
e
te
m
om
a
ph
gr
ac
on
ile
em
Ha
a
n
pa
Ni tzs
rv
tz ch
s i
ul
um
Pi chia a a
m
nn
a
p
m
u
Pi la
h
nn ri ph iox
ul a a ibi ys
ar f. a
ia h
m em
aj ip
Di
or te
at
ra
om
co
nc
en
Ch
tra
tio
ry
s
n
o
co p
nc hy
t
en e
tra sta
Sp
tio to
on
n cy
st
(m ge
c
eg o
as nce
cle n
Sp
re trat
on
s) io
g
n
(b e
iro co
tu nc
la en
Di
te t
) ra
at
tio
om
n
/s
ta
to
cy
st
ra
tio
Depth (cm)
(b)
20
20
20
40
on
em
Ha
a
sp
nt
Na zs
p.
c
vic hi
ul a a
a
Ni
s mp
tz
sc pp. hio
Pi hi
xy
nn a
s
Pi ula frus
nn ria tu
Un ula h lum
i d r i a em
e
pe nti su ipte
nn fie bc ra
Di at d & ap
at e c b ita
om e ro ta
co ntra ken
nc l a
en r e
tra as
Ch
tio
ry
n
co sop
nc hy
en te
tra sta
Di
tio to
at
n cy
om
st
/s
ta
to
cy
st
ra
tio
ph
2348
G
om
Am
An pho
o ra
Au mo ve
l a e o ne
Co cos ne ta
e is
Cy cco ira sp
m n e i sp ha
Eu bel s p p. ero
ph
n la la
or
Fr otia sp cen
a
ag s p. tu
la
i l a pp
r ia .
sp
p.
ARTICLE IN PRESS
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
108
110
112
CA1/T114
114
116
(a)
60
40
20
20
20
40
106valves (or cysts) /g
107valves (or cysts) /g
2
4
10
0.5 1 1.5
12
1
12
2
15
83
84
85
86
87
CA1/T87
88
89
106points/g
12
3
Fig. 7. Diatom and sponge responses in Zacapu (Cantabria) to four tephras: (a) CA1/T114, (b) CA1/T87, (c) CA1/T58, (d) CA1/T37. Diatom values
are percentages of total diatom sum. Sponge spicule and chrysophyte cyst concentrations are given as number per g of dry sediment.
Depth (cm)
Eu own
no
t
Eu ia f
le
n
Ni oti xuo
tz a m s
a
sc
hi on
a od
a
Na
m on
ph ta
G vicu
ib
om la
ia
p p
G ho up
om ne ul
m a
p
Cy ho a g
m nem ra
Pi be
a cile
n ll
Ha nul a s par
a p
Auntz ria p. vulu
sc af
m
l
a
Au co hi f h
l s a e
Co aco eiraam mip
s
p
Fr cco eira sp. hio tera
ag ne i
xy
t
s
Fr ilari is p alic
ag a la a
c
b
i
e
l
Ac a re n
v t
h ri
Ac na a p istr ula
nt inn iat
h
n
h
Di a e a a
at nt s ta
om he ex
co s h igua
nc un
en ga
tra ric
Ch
tio a
ry
n
co sop
nc hy
en te
tra st
tio ato
Sp
n cy
on
st
(b ge
s
iro c
o
tu n
la ce
te n
) tra
Di
tio
at
om
n
/s
ta
to
cy
st
ra
tio
kn
Un
Depth (cm)
Eu
no
t ia
i
N
itz nter
sc
m
ed
hi
a
am iate
ph
N
av
ib
ia
ic
ul
a
G
om line
ol
ph
at
on
a
sp
em
p.
a
G
gr
om
a
ci
ph
le
Au on
em
la
co
a
p
se
C
ira arv
oc
co
sp ulu
m
ne
.
is
pl
ac
en
Fr
tu
ag
la
ila
r
i
Ep
a
pi
it
n
D hem na
ia
ta
t o ia
m tu
r
co gi
nc da
en
tra
t io
C
n
hr
y
co so
nc ph
en yte
tra s
tio tat
n oc
ys
D
ts
ia
to
m
/s
ta
to
cy
st
ra
t io
ARTICLE IN PRESS
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
(a)
20
20
40
20
20
40
40
20
20
20
20
40
20
(b)
20
Fig. 7 (continued).
10
10
20
20
30
2
1
4
2349
54
55
56
57
58
59
CA1/T58
60
61
62
107valves (or cysts) /g
2
7
10 valves, points, or cysts /g
0.1
3
8
38
39
0.2
2 4 6
16
33
34
35
36
37
CA1/T37
ARTICLE IN PRESS
2350
R.J. Telford et al. / Quaternary Science Reviews 23 (2004) 2337–2353
6. Discussion
6.1. Are these events caused by tephra deposition?
In discussing these results, it is necessary to attempt to
distinguish between changes in the diatom stratigraphy
caused by tephra and those caused by other processes.
Ideally, this distinction would be addressed with
repetition and control: investigating similar lakes that
were either subjected to the tephra or not. In the TMVB,
it is difficult to find lakes similar in all aspects other than
exposure to tephra, and without the tephra layer to give
a synchronous horizon, impossible to get the same time
period in the control lakes. The multiple proxies used in
this study help in the isolation of the changes due to the
tephra, but in many other studies the discrimination of
tephra impacts will have to rely on other controls.
Most importantly, the recognition of tephra impacts
should meet these four criteria:
1. There should be a change in the diatom stratigraphy
greater than natural variation and distinguishable
from pre-tephra assemblages in either species composition or diatom concentration. The clearest
example we found where both concentration and
species abundance changed was tephra C4/T636 from
Pa! tzcuaro (Fig. 5a).
2. Diatom assemblage change should begin ‘immediately’ after the tephra given the smoothing effects of
sample resolution and sediment mixing processes.
Many of the tephra layers studied were diffuse, that is
they did not have a discrete top and base and in many
cases had mud admixed throughout. In a few cases
we noticed an apparent lagged response in diatom
species composition, e.g. C4/T495 (Fig. 5b), although
this could be a result of sediment mixing processes.
3. Theoretically, there should be recovery towards the
pre-tephra state (in the absence of other perturbations). However, if responses last for decades, as in
most of the examples here, the pre-tephra conditions
may not be a suitable reference.
4. Greater confidence in identifying a responsive lake is
gained if a similar response is shown by more than
one tephra event (cf. Barker et al., 2003). This
criterion is an attempt to reduce the probability that
an event co-incident with the tephra (within the
sampling resolution), but unrelated to it, will be
interpreted as a tephra response, however it is a filter
that may exclude responses not fitting the expected
patterns.
6.2. Interpretation of the different responses
The literature suggests a variety of mechanisms
through which inputs of tephra into lakes might cause
a change in the abundance and/or species composition
of the diatom flora. Where inputs of tephra are not thick
enough to cause fundamental changes in basin bathymetry and habitat availability, these floristic changes are
likely to be due to shifts in the silica loading, and
especially the Si:P ratio, which is considered to be an
important determinant of diatom species composition
(Kilham et al., 1986). The responses of the diatom
assemblages to tephra inputs identified in this study are
summarised in Table 3.
Most tephras showing a response are >1 cm thick,
although there are exceptions, e.g. CA1/T37 (Fig. 7d). It
is also apparent that there is no simple relationship
between the percentage silica content of the tephra
and diatom response, at least for the silica contents
(53–68%) of these tephras. Modern silica contents in
surface waters are quite high and it seems unlikely that
direct inputs of silica from the tephra to the lake would
be sufficient, in most cases, to shift the Si:P ratio in a
significant manner. However, increased loading to the
catchments could provide a large supply of silica that
would be slowly released and be more readily available.
Diatom samples that show a possible silica response
include those from within a peat unit at the top of the
Zacapu Cantabria sequence, where tephras CA1/T37
and CA1/T87 result in an increase in diatom concentration, but no change occurred in species composition
(Figs. 7b and d). (Note—available P at these levels gave
zero readings). In this case it seems that Si was a limiting
nutrient to diatom productivity, but other niche parameters were not changed sufficiently by the tephra to
cause species replacement. We can discount the possibility that the concentration increase was an artefact of
decreased diatom dissolution rates, since no increase in
delicate taxa or improvement in valve preservation was
found.
An increase in the diatom/chrysophyte statocyst ratio,
associated with a rise in diatom concentrations, was
found around some of the tephras where statocysts were
a significant proportion of the assemblage. Hickman
and Reasoner (1994) have previously found this
response mode, which must represent ecological changes
beneficial to diatoms but not to chrysophytes. If it is
assumed that the increase in diatom concentrations is a
response to increased silica availability, then the
chrysophytes must be insensitive to this change, despite
their siliceous statocysts. About 10% of chrysophyte
species have silica scales in their reproductive stage
(Duff and Smol, 1995), but ambient silica concentrations may not be a major variable in determining their
abundance (Duff and Smol, 1995).
Stronger ecological changes are produced when the P
cycle is also disrupted. Several interrelated mechanisms
are possible, but the most extreme is where the tephra
provides a barrier to internal P recycling. This process
is most acutely shown in crater lakes with small
catchments where sediment sources are a significant
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component of the P budget (Barker et al., 2000). The
limited diatom responses to the historical tephras in the
Zirahuén cores may be associated with high external
loading of P entering these lake systems in the recent
past, again muting the tephra response and/or to high
background silica yields from these erosive catchments
(Davies et al., 2004). At Lake Pa! tzcuaro, the last 3500
years witnessed increasing catchment disturbance
(Watts and Bradbury, 1982), probably leading to more
turbid conditions in the lake and a change in the
dominant diatom taxa (Terrett, 2000). Stronger diatom
responses are shown in the older sediments from Lake
Pa! tzcuaro that may be linked to shifts in the Si:P ratio
and nutrient competition between species. Some changes
are independent of the tephra deposition, for example, a
rise in sedimentary P began prior to deposition of the
lower tephra (C4/T643) and produced a rise in
Stephanodiscus spp. (Fig. 5a), probably in response to
increased P availability (Tilman et al., 1982). Whereas,
the deposition of the second, thicker tephra (C4/T636),
suppressed or diluted available P, causing a period of
dominance of the diatom record by Fragilaria elliptica,
and an increase in total diatom concentration. The rise
of Fragilaria species after the tephra has been found
elsewhere in this and other sequences from Mexico, and
has also been reported from studies of tephra impacts
elsewhere (e.g. Harper et al., 1986). Unfortunately
Fragilaria have wide tolerances of phosphate concentrations and their abundance may be linked more to
habitat availability than nutrient conditions (Sayer,
2001).
In a Quaternary context, the response of diatoms to
thin tephra inputs from cinder cones appears to be
relatively short lived (150–200 yr). The long-term evolutionary trend of the lake ecosystem is briefly perturbed,
but not fundamentally affected, and does not override
long-term changes due to climate. The longest duration
impacts occur when tephra makes a significant contribution to the catchment Si pool and/or disrupts the
internal recycling of P. Elsewhere in the region, the
Toluca basin has tephra layers measured in tens of cm or
even metres, resulting from eruptions of the nearby
Nevado de Toluca (Fig. 1). In this extreme case, these
thick pumice deposits will have an impact on long-term
lake ontogeny, probably disrupting ecosystems and lake
hydrology for millennia.
It is important to note that just less than half of the
tephra horizons show no detectable diatom response
and pre-tephra trends continue. There are three possibilities for this lack of response. Firstly, insufficient
tephra was deposited to shift nutrient cycles. Secondly,
the conditions or nutrients supplied by tephra were not
limiting the pre-tephra ecosystem, perhaps because the
system was already responding to an earlier tephra, and
although there was no change reported, an existing
response may have been prolonged. Thirdly, the system
2351
into which the tephra fell had high variability so the
tephra effects cannot be distinguished from natural
change.
7. Conclusions
This study has explored the response of lakes to
tephra deposition through high-resolution diatom analysis of lake cores from the central highlands of Mexico.
The diatoms have shown their sensitivity to tephra
impacts and their utility in tracing ecosystem changes
over long time periods. Tephra layers are common in the
lake sediments of this region, but the lake systems are
also complex, being affected by a range of forcing
factors.
Detectable responses to tephra deposition were
common in over half of the tephra layers investigated.
These responses were recorded by shifts in species
composition and diatom concentration, probably
brought about by changes in silica and phosphorus
availability. According to linear accumulation rates
between 14C ages, the diatom responses studied here
lasted for at least several decades, a duration that could
not have been detected in most modern, relatively shortterm limnological studies, but also one that may be
undetected in low resolution palaeoenvironmental studies. The thin tephra layers we studied had no apparent
long-term impact on lake evolution, rather they
represent short perturbations to underlying trends, that
in central Mexico are driven mainly by climate forcing
and human impact during recent millennia (Metcalfe
et al., 2000). This finding is important to palaeoclimate
studies from volcanic regions where tephra forcing is
often suggested and has to be isolated from climatic
interpretations, and yet has rarely been examined
directly. It is also likely that accelerated silica fluxes to
Mexican lakes caused by recent human activities
(O’Hara et al., 1993), have reduced the sensitivity of
lake ecosystems to future tephra inputs.
Acknowledgements
A Leverhulme grant (F158/BL) supported this work.
Thanks are owed to colleagues at the Universidad
Nacional Autonoma de México and the Universidad
Michoaca! na de San Nicolas de Hidalgo for assistance
during fieldwork; Gordon Cook at the Scottish Universities Environmental Research Centre (East Kilbride)
and the NERC Radiocarbon Laboratory (File 14.36) for
radiocarbon dates; Steve Juggins and Jane Entwistle
(Newcastle University) for statistical help and for access
to the Pa! tzcuaro core.
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