Introduction
A major and much-discussed example of a potential global “megadrought” and
cooling during the Holocene occurred between ca. 4.2 and 3.9 ka cal BP
(the 4.2 ka event hereafter) . One of the
best-documented case studies of the occurrence of the 4.2 ka event in
the Mediterranean basin comes from the RL4 flowstone from the Renella Cave
. The Mg/Ca molar ratio,
δ13C and organic matter florescence records obtained from RL4
flowstone calcite indicated a prominent reduction in cave recharge between
ca. 4.3 and 3.8 ka BP, corresponding to a pronounced drier period shown by the
δ18O record. This was somewhat surprising considering the
cave's geographic position. Renella Cave is located in a narrow valley
draining the western side of the Apuan Alps (Fig. 1), a mountain belt that
receives its precipitation from air masses of North Atlantic origin
that interact with the most important cyclogenesis
center of the Mediterranean region, the Gulf of Genoa (Fig. 1). The Gulf of
Genoa cyclogenesis is most active between November and February
, but is a persistent feature over the whole year
. To a large extent, its location is determined by local
topography, with the Alps (but also the Apuan Alps and Apennines, which bound
the eastern side of the Gulf of Genoa) playing a major role by trapping air
masses moving eastward and triggering Genoa cyclones . The
combination of these factors produces high precipitation amounts, locally
reaching values up to 3000 mm yr-1 over
the Apuan Alps . Cyclogenesis in the Gulf of
Genoa seems to be further maintained by cold advection of air masses on the
western flank of the larger synoptic system towards the relatively warm
temperatures over the Mediterranean Sea. Numerical experiments indicate that
this is not a triggering factor, since Genoa cyclones are mainly
topographically induced. It may, however, be crucial for determining the
maximum intensity reached by the local low-pressure systems
e.g.,. The finding of a reduction
of cave recharge over Renella Cave at the time of the 4.2 ka event
implies a substantial reduction in the advection of moisture from the North
Atlantic and the cyclogenesis over the area, despite the fundamental
orographic role exerted by the Apuan Alps. In this paper, we further explore
the 4.2 ka event in the Apuan Region, presenting new data (stable
isotope, trace elements, Mg, P, Y and U) from a Corchia Cave speleothem.
Corchia Cave is located at a higher altitude compared to Renella Cave and it
is characterized by a deeper and more complex plumbing system
.
Discussion
Speleothem stable isotopes composition
δ18O values of calcite (δ18Oc) in the Apuan
speleothems have been mostly interpreted on the premise that the
δ18O in precipitation (δ18Op) in the western
Mediterranean is dominated by the “amount effect”; i.e., a positive shift
in δ18Op values corresponds to a lower precipitation amount
and vice versa, with a gradient of ca. -2 ‰ per 100 mm per month
of precipitation . On the other hand, the temperature effect
on precipitation ca.
+0.3 ‰ ∘C-1; is almost equal but
opposite in sign to the effect of a change in temperature on water–calcite
isotopic fractionation ca.
-0.2 ‰ ∘C-1;. Thus, as a
first-order interpretation, higher δ18Oc values are
considered to indicate a shift towards drier conditions, while lower values
are associated with more humid periods
. This “paradigm” holds
for central and eastern Mediterranean speleothems
e.g., and is also
utilized for interpreting lacustrine marls, land snail shells and pedogenic
carbonates in the region
e.g.,.
However, this picture can be complicated by changes in the seasonality of
rainfall distribution. For instance, observed that
years with an anomalous reduction of precipitation from spring to late autumn
may have annual average isotopic values lower than expected, adding weight to
the prevailing winter signal i.e., more negative δ18O
of precipitation;. This anomaly can be transferred to
continental carbonates , and especially to Corchia
speleothems, because cave recharge is mostly related to autumn–winter
precipitation . It has also been noted that changes in the
isotopic composition of the source of vapor (Atlantic Ocean vs. Mediterranean
Sea and changes in isotopic composition of the source) may have an additional
effect . Despite these
potential uncertainties, the interval with higher δ18O values
at 4.5–4.1 ka BP should primarily indicate a reduction in cave recharge
induced by a decrease in local precipitation, most probably related to
winter, i.e., the season of maximum cave recharge . This
may also indicate a change in the proportion of cave recharge between the
cold (autumn–winter) and the warm (spring–summer) half of the year.
In the western Mediterranean, and in temperate settings more generally,
speleothem δ13C composition arises from the relative
contribution of 13C-enriched CO2 derived from bedrock
dissolution and 13C-depleted CO2 deriving from
biological activity in the soil
e.g.,. Higher soil CO2
production occurs under wetter and warmer conditions, and thus
δ13C values usually show a general negative correlation with
the amount of rainfall, similar to the relationship already discussed for
oxygen. Therefore, the concomitant increase in δ13C between
4.5 and 4.1 ka BP corroborates the reduction in cave recharge associated with
lower biological activity in the soil in the catchment, increasing residence
time in the aquifer and increasing the dissolution of bedrock, including by
sulfide dissolution .
Trace elements
Trace element data supply a fundamental additional control in the
interpretation of secular variations in speleothem δ18O
composition e.g.,, which can
complement and improve our partial knowledge on the long-term processes that
drive the evolution of the oxygen isotopic composition of meteoric precipitation.
Mg is generally transported to the speleothem in solution and, according to
, Mg/Ca changes in drip waters are due to four
different factors, which may act concomitantly in Mg-rich dolomite bedrock
, as in the case of Corchia Cave.
First, due to the rate of dolomite dissolution being slower than that of
calcite , a longer water–rock interaction time
would cause higher Mg concentrations in the drips. Second, prior calcite
precipitation (PCP) may be important. This involves calcite precipitation in
dewatered fractures along the flow path, upstream of the speleothem. Air
present in fractures encourages CO2 degassing, calcite deposition and,
indirectly, the enrichment of Mg/Ca in the solution. This is because the
partition coefficient of Mg is less than 1 . The
occurrence of PCP is further highlighted by a positive covariance between Mg
and δ13C because CO2 degassing causes 13C
enrichment in the solution and then in the speleothem calcite
e.g.,. Third, incongruent dolomite dissolution is
plausible given the extensive presence of dolomite beds. Finally, there is
the possibility of the selective leaching of Mg . The
cumulative effect of these processes is an overall increase in Mg content in
speleothem calcite during periods of lower cave recharge i.e., under
drier
conditions;.
P and Y are mostly transported as detrital phases
, which are a mixture of mineral
particulate and organic colloids produced by the weathering of bedrock and
leaching of soil (respectively), and deposited as microscopic particles
concentrated in individual layers of calcite or as
macroscopically visible clastic-rich layers, especially transported during
flood events . Entrainment of detrital phases in a very
deep environment like GdS in Corchia is unlikely, as supported by the very
pure calcite of the GdS speleothems (see low values in
232Th/230Th ratio; Table 1). For this reason, in our case the
transportation of organic colloids is more likely. Studies of temperate
ecosystems indicate that both P and Y in cave drip waters are principally
bound to soil organic colloids and are indicative of infiltration rates,
vegetation decomposition rates and soil development
. Thus, higher P and Y concentrations can be
related to higher water infiltration rates and more developed soil and
vegetation (i.e., warmer and wetter conditions), whereas lower values are
indicative of lower infiltration and less vegetation and soil productivity
i.e., drier and colder
conditions;.
U concentrations in speleothems can be challenging to interpret. In
near-neutral pH cave drip waters, stable uranyl UO22+ complexes
form with carbonate, phosphate and organics
. During rock alteration and
pedogenesis in oxidizing conditions, tetravalent U(IV) changes to hexavalent
U(VI), which is soluble in water . U concentration in water
may be influenced by changes in soil redox conditions. An increase in
microbial respiration would lead to reduced oxidation within the soil, and
this may cause less uranium to be oxidized to the hexavalent state and thus
lower uranium concentrations in the seepage water
. Soil and groundwater P
concentrations may also influence the transport of U through the strong
affinity between phosphate and uranyl ions . After
percolation through the karst network, U(VI) precipitates in calcite as
UO22+, probably as a replacement of Ca2+. Experimental
studies on synthetic calcite show that the partition coefficient of U between
solution and solid varies between 0.06 and 1.43 , with no
significant relationship with growth and drip rates or temperature. To
disentangle the predominant environmental driver of U concentration in CC27,
a comparison with the other proxies is useful. Our time series shows only a
weak positive correlation between U and P (Table 2), suggesting that some of
the U may be bound to phosphate and organic colloids. This is supported by
, whose observations confirmed the possible control of
colloids on the transport of U isotopes in freshwaters. A strong negative
correlation between U and Mg is also observed (Table 2), whilst the general
pattern of U changes appears to be negatively correlated with both
δ13C and δ18O (Fig. 4), suggesting that
higher–lower U concentration is related to wetter–drier conditions. This can
suggest both a stronger leaching from rocks overlying GdS and hosting high U
contents e.g., Brecce di Seravezza, BSE in Fig. 2; or
a partition coefficient for U in GdS water >1; in this case changes in the U
concentration of CC27 could be related to the degree of PCP, as for Mg but in
an opposite fashion. Overall, the most simple mechanism to control the U/Ca
ratio would be the cave recharge and consequently leaching mechanism.
Variations in soil redox conditions are likely to be negligible due to the
thick bedrock cover over GdS. Indeed, the increase in soil biological
activity, responsible for less oxidizing conditions and greater colloidal
transport, would have been favored by wetter and warmer conditions, thus
affecting U concentrations.
During the 4.2 ka period (Fig. 4), lower P and Y likely indicate reduced
colloidal production in soil and/or lower infiltration and transport into the
cave. This is indicative of reduced vegetation development and water
infiltration due to a reduction in precipitation. The latter also causes a
reduction in U content. These patterns are in agreement with the
concomitant increase in Mg related to the occurrence of PCP and thus to
drier conditions, as testified also by the increase in δ13C
related to soil CO2 supply but also to CO2 degassing
upstream of feeding drip.
Comparison among CC27, CC26 and Renella proxies. From bottom: CC27
δ13C, δ18O, CC26 δ18O, Renella
δ18O, CC27 Mg/Ca, CC26 Mg/Ca, Renella
Mg/Ca. The yellow box represents the 4.2 ka BP interval as recorded by
CC27 and Renella proxies. The Renella δ18O age model is
adapted from Drysdale et al. (2019), and the Mg/Ca age model is
adapted from .
Therefore, the shifts observed in all trace elements (higher Mg, lower U, P
and Y) and in both stable isotope compositions (higher δ13C
and δ18O) together define a period of reduced cave recharge
between 4.5 and 4.1 ka BP (Fig. 4). Assuming that the trace element behavior is
related to hydrological variations as discussed before, we can combine the
different individual trace element records to produce a composite mean
anomaly (MA) time series and filtered index (FI) series
. This approach detects coherent variability across
multiple speleothem properties, reducing the noise inherent in each series
and highlighting the environmental signal. To produce the MA, the individual
records were smoothed using a 10-point moving average, then normalized to
produce correspondent time series of anomalies (i.e., deviations from a zero
mean expressed in standard deviation units). Because the pattern of Mg is
reversed with respect to the other trace elements, the latter were multiplied
by -1. Standard scores of the individual series were then averaged for each
time increment to produce the MA wherein low (high) values correspond to
relatively wetter (drier) conditions (Fig. 4). The result reveals a prominent
drier interval in the MA between ca. 4.5 and 4.1 ka BP (Fig. 4). In order to
identify statistically significant events among these intervals, we then
calculated the standard deviation of these four scores for each time
increment. We thus set an upper (lower) threshold MA>(<)MAaverage+(-)SDaverage and such
that any data point that satisfies these conditions is deemed “significantly
dry” and “significantly wet” (respectively) relative to the mean state
over the period. This statistical approach identifies patterns of
simultaneous variations between different time series
and, in particular, highlights the drier period between 4.5 and 4.1 ka BP. This
period of drier conditions overlaps, within uncertainties, the drier phases
identified in the RL4 flowstone from Renella Cave
Fig. 5;. Figure 5 also shows the
comparison between the CC27 multi-proxy record and that of CC26, a previously
studied speleothem from the same chamber
.
Interestingly, the prominent shift in the δ18Oc of CC27 is
not apparent in the CC26 δ18Oc record. However, during the
4.2 period, CC26 shows a pronounced increase in the Mg/Ca ratio
, consistent with Mg (and other trace elements)
variation measured in the CC27 record (Fig. 5). The difference between the CC26 and
CC27 δ18Oc records deserves further comment. For deep,
large caves like Corchia, the complexity of the recharging system leads to
the presence of different reservoir compartments characterized by different
infiltration points and by different hydrological routings. In addition, GdS
has the peculiarity to receive drip recharge from lateral paths because the
chamber is overlain by the impermeable phyllitic basement
. This increases the drip-path complexity. Different paths
and mixing of water may create small differences in the geochemistry of
different drip points e.g.,, though located in
close proximity (i.e., in the same cave chamber). In addition, the steep
slopes of Mt. Corchia can produce important changes in the mean altitudes of
infiltration for different drips (up to several hundred meters over a
relatively short distance), inducing the “altitude isotope effect”
. derived an altitudinal effect of ca.
-0.15‰ 100 m for the western Apuan slopes, which is significant
considering the minor isotopic changes recorded in this cave during the
Holocene . Differences in the long-term isotopic records
of different speleothems from the same chambers may be related to long-term
drip-path evolution and, particularly, to changes in the mean altitude of
recharge and/or to mixing of different reservoir compartments having
different recharge areas. Another mechanism that can explain the different
δ18O trends is the precipitation of calcite out of isotopic
equilibrium conditions. Indeed, kinetic fractionation causes unpredictable
isotopic enrichment . However, this is unlikely for both
CC26 and CC27, as they are both composed of monotonous columnar calcite
, the fabric thought to occur when speleothems are
continuously wet and under relatively constant flow with calcite precipitation
from fluids at or near isotopic equilibrium
e.g.,.
CC27 δ18O and trace element mean anomaly filtered
index records compared with paleoclimate regional proxies. From bottom:
(a) Calderone Glacier reactivation ;
(b) CC27 δ18O; (c) CC27 trace element mean
anomaly filtered index; (d) chironomid midge assemblage
temperature derived from Verdarolo Lake fossil ;
(e) chironomid midge assemblage temperature derived from Gemini
Lake fossil ; (f) Accesa Lake summer
precipitation ; (g) Accesa Lake winter
precipitation ; (h) Accesa Lake mean annual
temperature ; (i) Accesa Lake mean annual
precipitation ; (j) Gulf of Lions storm activity
record ; (k) alkenone temperature record from
Gulf of Lions ; (l) SST Mg/Ca temperature
record from Alboran Sea ; (m) SST alkenone
temperature record from Alboran Sea ; (n) SST
alkenone temperature record from MD04-2726 in the east Mediterranean
; (o) NAO index reconstruction ;
(p) mean anomaly filtered index of the regional proxies as
discussed in the main text.
Other evidence of environmental changes over the Apennine and central Italy
Environmental and climate change during the drier period identified over the
Apuan Alps between ca. 4.5 and ca. 3.8 ka BP (considering the whole range of
ages present at Corchia and Renella caves) was previously reported for
several sites on the Apennines and the surrounding coastal plains. In the
central Apennines, geomorphological evidence of cooling and the
reformation of Calderone Glacier were reported
, and this event is stratigraphically
constrained by the presence of Agnano Mt. Spina (ca. 4.4 ka cal BP) and
Avellino (ca. 3.8 ka cal BP) tephra layers
. This suggests a reduction of either
summer and/or winter temperature that may have caused a
prevalence of snow precipitation during winter, higher winter accumulation
and/or a reduced summer ablation. Cooler conditions during
summer are also supported by fossil chironomid assemblages in the nearby
northern Apennine Verdarolo and Gemini lakes Figs. 1 and
6;. In further geomorphological evidence, a period of
reduced alluvial activity is recognizable over the northern Apennines between
ca. 4.4 and 3.6 ka cal BP , consistent with drier
conditions or rainfall patterns less prone to produce alluvial layers.
Further, a general phase of reduced valley sedimentary infilling occurs
between ca. 4.2 and 3.6 ka cal BP in the upper Turano River drainage
basin, 60 km northeast of Rome, although the origin of this phase is
interpreted to be related to a general phase of biostasy related to a
well-developed vegetation cover preventing runoff and sediment transport
. At lower altitudes, stratigraphic data from the Pisa
alluvial plain (Fig. 1) indicate river channel avulsion
during this period, suggesting that occasionally some large floods may have
occurred. In contrast, show an increase in
alluvial events between ca. 4.3 and 4.1 ka cal BP in some valleys of
Basilicata (southern Italy). Quantitative reconstruction of climate
parameters using pollen concentrations from Accesa Lake southern
Tuscany; Figs. 1 and 6; shows reduced mean annual temperature
over the considered period. Precipitation, reconstructed from the same
record, shows high variability but no significant variations in mean annual
values . In terms of seasonality, the Accesa Lake records
show a winter precipitation increase between 4.4 and 4.1 ka cal BP and a
decrease between 4.1 and 3.7 ka cal BP, as well as opposing summer
precipitation patterns over both intervals (Fig. 6). The winter precipitation
reconstruction from Accesa may thus not be in agreement with proxy data from
Corchia speleothems, which indicate a reduction in cave recharge. Overall,
Apennines and central Italy records show some contrasting patterns. However,
the interpretation of some proxies for environmental change in this region
during the middle to late Holocene needs care for the potential human impact
. Indeed, this period corresponds to the beginning and
definitive affirmation of the Bronze Age in Italy . The
strong impact on the environment of the Bronze Age population makes it
mandatory to consider each recorded environmental change as the complex
result of both human activity and climate variability. This is particularly
true for pollen records , as Bronze Age
communities strongly modified the composition of the vegetation. According to
, many Apennine catchments underwent a shift from
generally prevailing biostasy to conditions of anthropic rhexistasy starting
at around 4.2 ka.
Regional comparison
Looking at the wider regional palaeoclimate, both Mg/Ca and alkenone
sea-surface temperature (SST) data from the Alboran Sea (core ODP-976, SW
Mediterranean; Fig. 1) show a temperature decrease around 4 ka cal BP
Fig. 6;. In the Gulf of Lions (NW
Mediterranean; Fig. 1), the alkenone SST record from the KSGC31 core shows marked
oscillations with minimum temperature values around 3.9 ka cal BP
. Interestingly, and also from the Gulf of Lions, a record
of storm activity shows a period of increased storminess between 4.4 and
4.05 ka cal BP . High storm activity in the Gulf of
Lions has been proposed to be related to cooling over the North Atlantic and
the western Mediterranean . The alkenone SST signal from
MD04-2726 in the east Mediterranean (Fig. 1) shows three marked oscillations
between 4.6 and 4.1 ka cal BP .
To perform a statistical even if simple analysis for the
abovementioned regional time series (i.e., all the time series in Fig. 6
except the CC27 data), we used the same approach used for CC27 proxies. We derived the
value for each point of each curve with 20-year binning, calculated the MA
and, on the detrended MA curve, computed the FI square-wave curve, setting
an MA threshold of +0.1 and -0.1. The obtained FI curve (Fig. 6)
represents “climatic anomaly” time series irrespective of the specific
meaning. It shows between ca. 4.5 and 4.2 ka BP a well-expressed “negative”
FI value, which roughly implies that this interval is a tendentially cold,
dry and stormy period, which is comparable with MA for CC27.
Synoptic atmospheric conditions
We have already discussed the fact that speleothems over the Apuan Alps show
consistent evidence of reduced cave recharge in a period ranging from ca. 4.5
to 3.8 ka BP. This implies a reduction of cyclones of Atlantic origin
and of secondary cyclogenesis over the Gulf of
Genoa, considering the relation between cyclogenesis and rainfall in the area
. A reduction in local cyclogenesis can
be associated with a decrease in the arrival of North Atlantic air masses to
the Gulf of Genoa and also reduced transit of North Atlantic cyclones
and the reduction of vapor advection from the
western Mediterranean, which acts as a moisture source for the central
Mediterranean . An important point to consider, however, is
that it is not just a reduction in the cyclones reaching the Apuan Alps but
also a reduction of precipitation associated with each single cyclone. Model
simulations show that projected Mediterranean precipitation reduction in
winter is strongly related to a decrease in the number of Mediterranean
cyclones, but local changes in precipitation generated by each cyclone are
also important . For the central Apennines, winter
precipitation is negatively correlated with the North Atlantic Oscillation
(NAO) index , with a negative NAO index associated
with elevated precipitation during winter months. This is due to the lower
pressure gradient between the Azores High and the Icelandic Low, which causes
a southward shift in westerly trajectories, leading to increased penetration
of moist air masses from the Atlantic to the Mediterranean and stimulating
local cyclogenesis. The Genoa GNIP station shows a significant negative
correlation between the amount of winter precipitation and the NAO index (r=-0.74). At the same station, a significant positive correlation is found
between δ18Op and the NAO index (r=0.41)
. This is in agreement with rainfall amount and
seasonality effects, as enhanced winter precipitation during NAO phases
should be characterized by lower δ18O values. In central
Europe, where the amount effect is negligible, a relatively strong positive
impact of the winter NAO index on precipitation δ18O has been
attributed to the higher frequency of cold easterly winds carrying
18O-depleted moisture during NAO negative phases compared to warmer
westerly winds, which carry 18O-enriched moisture from the North
Atlantic Ocean and Mediterranean Sea into central Europe during positive NAO
winters . However, persistent penetration of easterly
moisture sources in the Gulf of Genoa is prevented by the Alpine ridge and
there is a more important contribution of the local recycling of vapor from
the Mediterranean Sea . Thus, the correlation observed at
the Genoa GNIP station suggests that during periods of reduced North Atlantic
moisture (i.e., NAO positive phase causing northward shift of storm-track
trajectories), precipitation from vapor of Mediterranean origin may dominate
locally. The higher δ18O values should be in response to the
effect of lower rainout of vapor masses and the
higher seawater isotopic composition of the Mediterranean compared to the
North Atlantic . From this point of view, the drier period
recorded in the Apuan speleothems can be related to persistent positive
NAO-like conditions, as invoked for some centennial- to multidecadal-scale
climatic changes over the western Mediterranean
e.g.,, including during the Medieval
Climate Anomaly . However, the reconstructed NAO index for
the studied interval does not show a pronounced “positive mode”
e.g., and thus this mechanism is not an
entirely satisfactory explanation (Fig. 6). Evidence from the tropics
suggests that, at the time of 4.2 ka event, there was a near-synchronous
disruption of the Indian and African summer monsoon systems
Fig. 6; resulting from a southward shift
of the Intertropical Convergence Zone (ITCZ)
. The position of the ITCZ and the
resulting strength of the boreal summer monsoon both influence summer aridity
and temperature in the Mediterranean region and are driven by atmospheric
subsidence and strengthening of the summer high-pressure systems over the
basin . A southward shift in the ITCZ
can weaken summer aridity, producing milder (cooler) summer conditions over
the Mediterranean. This is consistent with the precipitation reconstruction
from Accesa Lake , which suggests increasing rainfall
during summer, and with a temperature reconstruction from chironomids
indicating a reduction in summer temperature. On the basis of these
observations we suggest that a weakening of high-pressure cells over the
western Mediterranean during summer as the ITCZ shifts southward may have had
a pronounced effect on Mediterranean Sea temperatures. Reduced seawater
temperature at the end of summer can reduce evaporation during autumn–winter
in the western–central Mediterranean Sea, causing reduced advection towards
the central Mediterranean and decreased cyclogenesis in the
Gulf of Genoa. This effect cannot be completely captured from available SST
data, which record spring and/or annual average temperature and not specific
late summer–autumn temperature e.g.,
Fig. 6;. From the available data, it
seems possible that a general reduction in precipitation, as reflected in the
CC27 stable isotope and trace element data for the Apuan Alps, must have
involved reduced advection of vapor masses during winter and a reduction in
cyclogenesis over the Gulf of Genoa. However, during the summer, tropical
weakening of the monsoon systems may have triggered cooler and wetter
condition and this may have been one of the causes for the 4.2 ka event.