Several record-breaking precipitation events have struck the
mountainous area of the Emilia–Romagna region (northern Apennines, Italy) over
the last 10 years. As a consequence, severe geomorphological processes such
as debris avalanches and debris flows, shallow landslides, and overbank
flooding have affected the territory, causing severe damage to human-made
structures. The unusual intensity of these phenomena prompted an
investigation into their frequency in the past, beyond instrumental time. In
the quest for an understanding of whether these phenomena are unprecedented in
the region, peat bog and lake deposits were analyzed to infer the frequency
of extreme precipitation events that may have occurred in the past. We
present the results of a dedicated field campaign performed in summer 2017
at Lake Moo in the northern Apennines, a 0.15 km
High-intensity precipitation (HIP) events, also known as torrential rainstorms for their capacity to generate flash floods in small streams, represent a significant component of the Mediterranean water cycle. More than half of the annual precipitation is concentrated in a few major precipitation events (e.g., Frei and Schär, 1998; Isotta et al., 2014). Better knowledge of their expected frequency (and maximum intensity) is crucial for planning adequate hydraulic defenses and sustainable water resource management in the present and future climate.
Under the threat of global warming, a growing number of studies have investigated the link between a temperature rise in air masses and the intensity of extreme precipitation, highlighting spatial and seasonal differences across the globe (e.g., Lehmann et al., 2015; Papalexiou and Montanari, 2019). There is a consensus that HIP events are increasing with global warming, while mean precipitation could decrease in some regions (Berg et al., 2013). Myhre et al. (2019) show that HIP over Europe almost doubles per degree of warming due to the combined effect of increasing frequency (the major driver) and an increase in intensity.
At the local scale, different responses of weather patterns and limitations
in moisture availability can alter the uniform expected rise in HIP due to
the increase in saturation water vapor pressure (6 % K
The investigation of sedimentary archives, like peat bogs and lakes, allow for the verification of the hypothesized linkage between extreme precipitation and temperatures in the distant past, substantially widening the period during which we can verify this relationship.
In the literature there is ample documentation of the use of these sedimentary archives to infer chronologies of past flood events (Zavala et al., 2006, 2011; Giguet-Covex et al., 2012; Gilli et al., 2013; Glur et al., 2013; Stoffel et al., 2013, 2016; Wirth, 2013; Wirth et al., 2013; Anselmetti et al., 2014; Longman et al., 2017; Schillereff et al., 2014; Swierczynski et al., 2017; Ahlborn et al., 2018; Wilhelm et al., 2012, 2018; Zavala and Pan, 2018), alongside others such as tree rings (Ballesteros-Cánovas et al., 2015), speleothems (Regattieri et al., 2014; Zanchetta et al., 2011), and torrential fans and cones (Schneuwly-Bollschweiler et al., 2013).
Interestingly, over the Alpine area a synchronization between increasing flood frequency and Holocene cooling periods characterized by cold and wet summers, like the Little Ice Age (LIA), has been documented by several studies (e.g., Glur et al., 2013; Henne et al., 2018). Other authors reported that under warm paleoclimate conditions floods in the Alps became rarer but stronger in intensity, highlighting the complexity of this issue (Giguet-Covex et al., 2012; Brönnimann et al., 2018a). Alpine data mostly derive from lacustrine sedimentary successions that are known to be influenced by a combination of factors (e.g., precipitation intensity, duration, seasonality, and changes in atmospheric circulation and land use) that might overshadow the physically based relationship between temperature increase and extreme precipitation events (Utsumi et al., 2011; Brönnimann et al., 2018b).
In contrast, less attention was paid to the Holocene flood activity in the northern Apennines area (N Italy in Fig. 1), which has a different precipitation climatology compared to the inner Alpine region. Indeed, the link between cold summers and high values of flood frequency appears more logical for the Alpine region, which shows a precipitation maximum in the summer. In contrast, in the northern Apennines area, precipitation maxima occur in autumn, with more than 60 % of the annual total precipitation concentrated in a few days characterized by severe meteorological events (Isotta et al., 2014).
Focusing the analysis on the Emilia–Romagna region (ER region; Fig. 1), an increase in the interannual variability of torrential rainfall has been observed over the last 10 years with marked or even exceptional droughts, such as those occurring in 2012 and 2017 (Grazzini et al., 2012), followed by years of record-breaking rainfall (2014 and 2018). Between September 2014 and September 2015, the ER region was affected by three events of exceptional intensity with an estimated return period of several centuries (Grazzini et al., 2016).
To consolidate HIP trends on the N Apennines area, we analyzed the Holocene sedimentary succession of a small-sized peat bog (Lake Moo in Fig. 1) through the application of a multidisciplinary approach that integrates sedimentological and environmental data (e.g., pollen-derived paleovegetation patterns) with climatological observations and physical arguments.
Lake Moo is in the proximity of the main N Apennines crest that is very
exposed to moist maritime airflow coming from the central Mediterranean Sea.
The dimension of the drainage basin is small (total area Are the recent events unprecedented on a millennial timescale (i.e., the
Holocene period)? Is the frequency of HIP events coupled with (paleo)temperature variations?
The Lake Moo plain (44
Present-day climate conditions primarily reflect the interaction of the prevalent atmospheric flow and the mountain range surrounding the Lake Moo plain. The proximity to the Ligurian Sea (about 40 km SW) makes the location particularly favorable to orographic precipitation enhancement in the case of moisture-laden southerly flow. The most precipitation is usually observed in spring and autumn, with a marked peak in early autumn. In this period we observe a particular synergy of midlatitude synoptic disturbances, becoming more frequent towards the cold seasons, and strong convective systems still developing over the warm Mediterranean Sea at the end of summer and in autumn (Grazzini et al., 2020); this is a situation very favorable for the genesis of extreme precipitation events in the area.
The study area mainly consists of extensively fractured serpentinites that represent the accreted fragments of the Ligure–Piemonte oceanic basin separating the European and Adria plates during the Middle–Upper Jurassic (Marroni et al., 2010). The ultramafic rocks are bordered by polygenic breccias made of blocks of limestones or marly limestones embedded within a fine-grained matrix (Mt. Ragola Complex, late Santonian–early Campanian; Elter et al., 1997).
The geomorphological landscape includes flat areas and steep slopes located at different altitudes. On the former, marshy environments commonly occur recording the last filling phases of small lacustrine basins, some of which still exist as Lake Moo and Lake Bino (Figs. 1–2). However, the origin of Lake Moo is still a matter of debate as some authors point to a glacial origin (Elter et al., 1997; Marchetti and Fraccia, 1988; Carton and Panizza, 1988), while others have interpreted the basin as the expression of Holocene deep-seated gravitational slope deformations (Geological, Seismic and Soil Service of the Emilia–Romagna Region, 2012) mainly controlled by changes in the incision rates of the Nure stream (Elter et al., 1997). The complexity of past and present morphological processes is likely enhanced by the superposition of lithological units with strong mechanical contrast, such as ophiolites and the underlying a predominantly clayey unit (Elter et al., 1997).
Taking into account the criteria indicated by Gilli et al. (2013) and
Schillereff et al. (2014), Lake Moo shares several advantageous
characteristics for the reconstruction of Holocene flood activity:
steep slopes (average inclination of 24 absence of lacustrine basins in the upstream part of the catchment; small drainage basin area (1.94 km one dominant inflow into the lake; lack of regulated flow structures; and lack of natural pre-lake sediment storage zones.
Geomorphological map showing the main geological units and the hydrological elements of the Lake Moo plain. The detailed mapping of flood deposits formed during the recent HIP event (13–14 September 2015), the location of the geognostic investigations and the geophysical survey tracks are also reported. Original data from the field campaign are overlaid on the geological database of the Emilia–Romagna region.
The field campaign led to the acquisition of two sedimentary cores (S1 and S2), 14 and 6 m long, respectively, and one trench (T1) to investigate the sedimentary succession capped by the high-density flood deposits formed during the recent HIP event (13–14 September 2015). The location of the cores and the trench benefited from a detailed geomorphological map (Fig. 2) and a high-resolution reflection seismic survey (Fig. SUP1 in the Supplement), both originally produced for this research. The latter provided useful information about the lake-basin-floor morphology and the thickness of the infilling succession.
The two cores, S1 and S2, were obtained using a continuous drilling system that guaranteed an undisturbed stratigraphy and a high recovery percentage (about 90 %). A trench (6 m long, 3 m wide, 2 m deep) was excavated between the two coring sites, allowing for a detailed analysis of the most recent deposits.
We focused our attention on the longest core (S1) that shows the complete
record of the Lake Moo infilling succession, as it reaches the ophiolite
bedrock. Therefore, all the laboratory analyses (grain size, radiocarbon and
palynological analysis) were undertaken on core S1, while the other core
(S2) and the trench were used to support the stratigraphic reconstruction.
Elevation and geographic coordinates of reference core S1 were acquired
using a Garmin eTrex 10 GPS receiver: 1120.2 m a.s.l. and 44
Facies characterization of the Lake Moo sedimentary succession was mainly performed by integrating the observable macroscopic physical characteristics (i.e., grain size, sedimentary structures, Munsell chart color and types of bounding surfaces) with the grain size data available for the reference core S1. The interpretation in terms of depositional environments also benefited from the precise position of the core(s) with respect to the spill point of Lake Moo and the application of the facies tract concept. The latter strongly supported the interpretation of the coarse-grained intervals encountered throughout the studied succession. Specifically, a facies tract is defined as the association of genetic facies that can be observed within the flood deposits along with the downslope motion transformation (Lowe, 1982; Mutti, 1992; Mutti et al., 1996). Thus, the application of the facies tract concept, if framed into the stratigraphy of the core(s), may allow for the reconstruction of how flood deposits changed through time at a fixed location (Figs. 3, 4).
Sedimentological features and lithofacies characterization of the reference core S1. Radiocarbon dates, palynological and grain size samples, and stratigraphic units (described in Sect. 4.2) are also reported.
The abundance of wood remains and peaty layers within the sedimentary
succession under examination supported the development of a robust
chronological framework. A total of 12 samples were selected from the reference
core S1 and radiocarbon dated at the CEDAD Laboratory of the University of
Salento (Italy). The data are available in Table TS1 (in the Supplement), and the conventional
An age–depth model was also constructed using linear interpolation. Coarse-grained intervals were not excluded from the processing because of the difficulty to evaluate the exact thickness of the deposits interpretable as instantaneous events. Indeed, the upper limit of coarse-grained intervals commonly appears transitional toward finer sediments, and, in a few cases, some intervals for a total thickness of 107 cm were not recovered.
In order to refine facies characterization and highlight past vegetation changes at the study site, pollen analyses were carried out on 14 samples collected from the core S1. We focused the analyses on two key stratigraphic intervals formed during periods of well-known different paleoclimate (and paleotemperature) conditions: (i) Interval 1 (ca. 10.5–9 m of core depth), covering the period between ca. 9600 and 7300 cal yr BP, thus centered on the Holocene Thermal Maximum – HTM (Renssen et al., 2012), and (ii) Interval 2 (ca. 5.5–4.5 m of core depth), recording the final stages of HTM and the following cooling period between ca. 5500 and 3800 cal yr BP. In these two stratigraphic intervals, samples were collected from fine-grained layers following a mean sampling resolution of ca. 30 cm.
A standard methodology already tested for pollen substrates was applied with
some minor modifications (Lowe et al., 1996). The method includes a series
of laboratory treatments: about 8–10 g of sample was treated in 10 %
Na pyrophosphate to deflocculate the sediment matrix; a
Identification of the samples was performed at
Determination of pollen grains was based on the Palinoteca of the “Centro Agricoltura Ambiente – CAA G. Nicoli” laboratory (Italy), atlases and a vast amount of specific morpho-palynological literature. Names of the families, genus and species of plants conform to the classifications of Italian Flora proposed by Pignatti (2017–2019) and European Flora (Tutin et al., 1964–1993). The pollen terminology was based on Berglund and Ralska-Jasiewiczowa (1986), Faegri and Iversen (1989), and Moore et al. (1991) with slight modifications that tend to simplify plant nomenclature. The term “taxa” is used in a broad sense to indicate both the systematic categories and the pollen morphological types (Beug, 2004). For each sample, at least 500 pollen grains were counted, and the identified taxa have been expressed as percentages of the total pollen sum that includes only terrestrial pollen, no fern spores and no aquatic plants.
On the basis of vegetational and ecological characteristics, the following
main pollen groups were identified: conifers (
The 14 samples prepared for pollen analysis were also investigated for the
identification of microcharcoals. Microanthracological analysis has been
used to track past changes in fire history, likely connected to
anthropogenic activities (Vescovi et al., 2010). A point-count estimation of
microscopic charcoal abundance was carried out, and charcoal fragments
encountered during pollen counting were recorded in four size classes based
on the long axis length: 10–50, 50–125, 125–250,
Pollen and microcharcoal results are presented in Sect. 5 to support facies stratigraphy and to discuss controlling factors on flood activation.
Central to our study is the availability of a reliable (paleo)temperature dataset for the chronological period recorded by the Lake Moo sedimentary succession. In this respect, the Holocene paleoclimate reconstruction conducted by Samartin et al. (2017) on the nearby Lake Verdarolo represents a very important reference. The Lake Verdarolo site is located at 1390 m a.s.l., 270 m higher and 54 km SE from Lake Moo (Fig. 1), in a very similar climatic context. The authors reconstructed the mean July air temperature using a chironomid-based inference model developed through a combination of data extracted from more than 200 lakes in Norway and the Swiss Alps (Heiri et al., 2011). This vegetation-independent paleotemperature reconstruction is the first for the northern Apennines, and it agrees with that of Gemini Lake (1350 m a.s.l. elevation and 48 km SE from Lake Moo) as well as several other records coming from central Italy (Samartin et al., 2017).
Modern temperature and precipitation time series (1961–2018) at Lake Verdarolo and Lake Moo were derived from the gridded high-resolution dataset of Emilia–Romagna (Eraclito4), described in Antolini et al. (2016). Trend estimation and a Mann–Kendall significance trend test were computed with the pyMannKendall package (Hussain et al., 2019).
A total of 14 lithofacies (LM1 to LM14) have been identified within the infilling
succession of the Lake Moo basin. A detailed lithological description of each
lithofacies is reported below.
LM1 to LM2 includes a clast-supported deposit with a thickness ranging between 5 and 15 cm. Fine boulders to coarse cobbles with abundant coarse- to fine-grained
sand matrix, showing a low degree of sorting, occur. Polygenic clasts have
low sphericity and a very angular shape. Wood fragments locally occur. The
basal and top contacts are sharp, and the basal one occasionally shows evidence
of erosion. LM3 to LM4 includes a clast-supported deposit with a thickness ranging between 5 and 15 cm. It is composed of fine cobbles to coarse pebbles with a very fine sand
to silt matrix. Polygenic clasts have low sphericity and a subangular shape.
Scraps of wood are encountered. The basal and top contacts are sharp, and the
basal one occasionally shows evidence of erosion. LM5 is a massive matrix-supported deposit composed of medium pebbles to granules
embedded within a very fine sand to silt matrix. Polygenic clasts have low
sphericity and a subangular shape. Scraps of wood are encountered. The basal
and top contacts are sharp, and the basal one occasionally shows evidence of
erosion. LM6 to LM8 is generally a clast-supported, poorly sorted deposit with a
thickness ranging between 10 and 30 cm. Polygenic clasts and massive or crudely
graded angular to subangular fine pebbles to very fine granules occur.
Crudely horizontal laminae are locally recorded on top. Fragments of wood can
be encountered at the base of the layers. The basal contact is sharp and
occasionally shows evidence of erosion; the upper boundary is transitional. LM9 to LM10 shows massive or crudely fining-upward graded very coarse to fine
sands with a low degree of sorting. The total thickness ranges between
5 and 20 cm. Evidence of a planar lamination and scattered wood fragments is found
close to the upper boundary, which is transitional. The basal contact is
sharp. LM11 is fine sandy loam to clayey loam, showing a dark grayish brown color
(10YR 4/2 or 10YR 3/2). Polygenic fine to medium pebbles aligned with low
sphericity and a very angular shape occur. Both basal and top contacts are
sharp. LM12 is a silty deposit of dark color (10YR 3/1) with a high content of
decomposed organic matter. Occasional fine to medium pebbles aligned with
low sphericity and very angular shape occasionally occur. LM13 is a loam to silty clay deposit showing a coarse angular blocky structure
and dark olive gray color (5Y3/2), with yellowish brown (10YR5/6) due to the
presence of iron oxides. The deposit is deprived of calcium carbonate, and a
field pH 5.5 value was recorded. LM14 includes massive clayey silts with a low amount of decomposed organic matter
and a dark greenish gray color (5G 4
In terms of sedimentary processes, the coarse-grained LM1–LM10 lithofacies have been interpreted as flood deposits triggered by HIP events in the catchment area (Milliman and Syvitski, 1992; Mulder and Syvitski, 1995; Mutti et al., 1996). Particularly, the identified lithofacies were grouped according to their features (i.e., grain size, color, sedimentary structures) and the facies tract concept (Lowe, 1982; Mutti, 1992; Mutti et al., 1996), distinguishing three main depositional settings along an idealized transect (Fig. 4): subaerial (LM1 to LM4), marginal (LM5) and subaqueous–lacustrine (LM6 to LM10).
Idealized genetic facies tract showing the 14 lithofacies (LM1–LM14) identified within the reference core S1, whose stratigraphy is reported in Fig. 3.
The fine-grained facies (LM11-LM14 in Figs. 3, 4) also form two main groups. The first group includes LM11 and LM13 (paleosol) that are characterized by features indicative of subaerial conditions; the second group consists of LM12 (i.e., peaty deposits) and LM14, deposited along the marginal zone of the (paleo)lake under low-energy conditions and different degrees of organic matter enrichment.
Above the ophiolite bedrock, five informal stratigraphic units have been distinguished within core S1 and are described as follows (Fig. 3). Each unit is interpreted in terms of depositional environment.
the disappearance of truly lacustrine deposits that are replaced by
subaerial ones through an erosional surface that marks the lower boundary of
Unit 4 (the stratigraphic depth of this boundary is in accordance with
the altitude of the spill point of Lake Moo at 1116 m a.s.l.); and the return of several centimeter-thick flood deposits (LM3–LM4).
As a whole, we interpret the lacustrine succession as an infill of a structural depression produced by gravitational block sliding that was induced by post-glacial fluvial incision (Elter et al., 1997).
Representative photograph of the trench (see Fig. 2 for location), showing the uppermost 50 cm thick sedimentary succession. Though the exposure is quite small, the graded pebble–sand couplets belonging to Unit 5 can be interpreted as a sheet-flood deposit.
The age–depth model, based on 8 out of 12
S1 core stratigraphy and the age–depth model obtained from radiocarbon dates. Red and orange lines represent the 2-sigma probability envelope.
The resulting age–depth model suggests the occurrence of four main
stratigraphic intervals in terms of sedimentation rates. The lowermost
interval (about 9.5–7.3 cal kyr BP) denotes a mean accumulation rate of ca.
0.50 mm yr
As a basis for discussion we assemble, and temporally synchronize, the Lake Moo data with the most relevant paleoclimate proxy available from the literature, focusing on the N Apennines paleoarchives (Figs. 1, 7). As a whole, the Holocene infilling succession of the lacustrine basin (i.e., Units 2–3) is composed of an alternation of centimeters- to meters-thick, coarse-grained flood deposits and silts characterized by a variable amount of organic matter (Sect. 4.1. and 4.2.; Figs. 3, 4, 7). The coarse intervals are particularly well-developed and frequent within the portion of stratigraphic Unit 2, which is chronologically correspondent to the HTM (Holocene Thermal Optimum), constrained between ca. 9 and 5 kyr cal BP in the nearby Lake Verdarolo (Samartin et al., 2017, in Fig. 7a; Sect. 3.4). Interestingly, stable oxygen isotope records from the nearby Apuan Alps (Corchia caves; Fig. 1) document that, within the same time interval, the wettest conditions were observed (I and II humid periods in Fig. 7b) with a peak between ca. 8500 and 7500 cal yr BP (I humid period; Regattieri et al., 2014; Zanchetta et al., 2011). This precipitation peak is in phase with a clastic layer dated to 8.2 and 7.1 cal kyr BP, indicative of a period of enhanced cave flooding triggered by high-magnitude precipitation events (Zhornyak et al., 2011). The authors attribute this maximum fluvial activity to an increase in strong convective episodes like the one that affected the Versilia region in 1996 (see Cacciamani et al., 2000, for a description of the Versilia flood event).
Stratigraphy of core S1, main palynological features and
microcharcoal content. Relative abundances of the pollen groups that are explained in the
text (Sect. 3.3) are reported along with the frequencies of
hygrophilous herbs and aquatics. Asterisks point to samples containing
coprophilous fungi and other spores like
Climatic simulations (monthly temperature and precipitation) for the mid-Holocene climatic conditions (Tinner et al., 2013) show higher precipitation in autumn over the northern Apennines. We speculate that the precipitation increases, also reflected in high-frequency flood activity, could have occurred during a phase of a progressive reduction in the Hadley cell, paralleled by a reduction in subtropical high pressure after the insolation peak (Fig. 7c). This would lead to enhanced meteorological activity over the Mediterranean area due to the midlatitude weather systems, especially at the transition towards the cold season. Although in decline, the presence of the African monsoon (Skinner and Poulsen, 2016) could also have contributed to enriching subtropical air masses in the water vapor potentially extractable by midlatitude synoptic disturbances. Accordingly, Krichak et al. (2015) documented how this mechanism is relevant for modern extremes in precipitation in the Mediterranean; moreover, the events occurring in the autumn months are characterized by a greater transport of water vapor from the subtropical Atlantic, even across North Africa.
With the end of the HTM, a drastic decrease in flood activity is documented at Lake Moo, as the coarse intervals are abruptly reduced in number and thickness within the uppermost portion of Unit 2 (ca. 5.5–3.8 kyr BP). This stratigraphic trend is reasonably interpreted as the expression of a decrease in the frequency of HIP events over the study area under cooler and less humid climate conditions (Fig. 7). The limit between Unit 2 and Unit 3 corresponds to an unconformity that marks the passage to a marginal lacustrine succession of uncertain age, at least for the lowermost portion (subunit 3a). This interpretation is supported by palynological data, which show high values of herbaceous hygrophytes (mainly Cyperaceae) and aquatic species.
The upper subunit 3b, dated around 146–14 cal yr BP, is almost deprived of coarse materials, documenting a period of apparent flood inactivity at Lake Moo (Figs. 3, 7). This is followed by a reactivation of flood processes from ca. 1800 CE (i.e., coarse-grained flood deposits within Units 4–5; Figs. 3, 7) with a minimum of 5 events to a maximum of 12 events every 100 years, as the confidence range associated with the calibrated age at 2.80 m is rather wide (146–14 cal yr BP; Fig. 7). Despite the low degree of precision affecting radiocarbon ages younger than 200 years, this renewed increase in flood activity fits well with the instrumental record that points to a significant increase in record-breaking (from 3 to 24 h accumulations) precipitation events in the northern Apennines (Libertino et al., 2019) starting from the second half of the last century. These phenomena seem to be responsible for the replacement of lake deposits by subaerial ones through an erosional contact (Unit 4 lower boundary).
To compare the most recent stratigraphic units (Units 4–5) with quantitative
climate data, the instrumental temperature values, available since the
second half of the last century from the Eraclito ER dataset, has been added
to the Verdarolo curve. The overlap with the latest part of the Verdarolo curve
confirms the good accuracy of the reconstruction technique in this region
(Fig. 8). The recent sharp temperature increase is striking if compared with
the whole Holocene, and the current July temperature is comparable with the
maximum temperature reached at HTM over the study area. The actual trend of
July temperature, estimated over the period 1961–2018 from monthly mean
values, is
Comparison of current data and reconstructed July mean temperature
at the Lake Verdarolo site (see Fig. 1 for location). The blue line is the
reconstructed temperature, and the shaded area is the sample-specific
estimated standard error associated with the temperature reconstruction
based on the chironomid assemblage from Samartin et al. (2017). The orange line
represents the July mean temperature (1961–2018) retrieved for the grid cell
of Lake Verdarolo from the Emilia–Romagna climate reanalysis Eraclito (11-year
running average). The shaded orange area is
A positive link emerges correlating Holocene (July) paleotemperatures, precipitation intensity reconstructions and flood history (Fig. 7) at the study site, as the stratigraphic units showing the highest frequency of coarse-grained flood deposits (i.e., Units 2, 4 and 5) invariably fall into periods of higher temperatures (i.e., HTM and the post-LIA time). This strongly supports the hypothesis that greater warmth favors the occurrence of extreme precipitation events, probably more frequently at the end of the summer–autumn as already reported by other authors (e.g., Marcott et al., 2013; Giguet-Covex et al., 2012). Although the process attribution and the geographical uniformity of the HTM wet phase are still a matter of debate, the abundance of coarse-grained flood deposits within the Lake Moo record around 9.5–5.5 kyr BP suggests a precipitation increase also identified in other sites of the central and southern Mediterranean (Magny et al., 2012b).
However, local factors such as changes in vegetation cover (i.e., tree cover
percentage) due to human disturbance and fires could have significantly
contributed to enhancing slope erosion and then influenced the Lake Moo
sedimentary record, partly weakening our reconstruction of the
temperature–HIP–flood activity relationship at the Holocene scale. In order
to overcome this issue, we explored and compared the pollen and
microcharcoal content of two key intervals (I
For the post-LIA interval, in particular since 1950 CE, no significant contribution to debris mobilization can be attributed to vegetation landscape changes along the slopes as Lake Moo is a protected area characterized by dense forests (Corticelli et al., 2011; Sect. 2.1). Consistently, the most recent pollen sample (P14 yielding an age of 146–14 cal yr BP; Fig. 7) shows a tree cover of about 80 %.
The main considerations arising from the in-depth analysis of I For I Palynological data, combined with the stratigraphic interpretation of the S1
core, support our hypothesis that warmer and wetter conditions typical of
the HTM led to a high amount of HIP and then high flood activity in the
basin, independently of local factors. The presence of an extensive
vegetation cover along the slopes implies the need for even more intense
rainfall to trigger a debris mobilization that leads to the deposition of
thick, coarse-grained layers containing pebbles with remarkable diameters (LM6,
LM7 and LM8 in Fig. 10). For I Although pollen-derived vegetation features (i.e., decrease in tree cover
and a moderate degree of human impact) document local conditions more
favorable for debris flow activation with respect to those
characterizing I
Monthly mean 2 m temperature
Sedimentological and palynological features of two key
stratigraphic intervals (Intervals 1 and 2) from the reference core S1.
These features are compared with the most relevant paleoclimate data
available from the literature for the area of interest (for references, please
see Fig. 7). The anthropogenic index (AI) has been calculated as
(anthropogenic indicator
The paleoenvironmental–paleovegetation features characterizing the Interval
2 persists up-core, in correspondence to Unit 3 that records apparent flood
inactivity (Figs. 3, 7). The uppermost sample P14 likely tracks a strong
increment of
Placing present climate conditions into a geological perspective, beyond the instrumental era, is essential for understanding changes in the hydrological cycle induced by anthropogenic climate warming, complementing projections of model simulations. Physical reasons and regional climate reconstructions are consistently pointing to an increase in precipitation intensity when water vapor is not limited. In the last decade, record-breaking rainfall events have frequently occurred around the world. This trend is emerging with variable strength in different areas. Therefore, in the attempt to build confidence in the extreme precipitation trend, we extended our analysis, passing from the instrumental to the geological timescale (i.e., the Holocene period). This choice implies a robust multidisciplinary approach, which includes, in addition to climate and meteorological data, proxy and expertise coming from the geological, geomorphological, stratigraphic and palynological–microanthracological area.
The major outcomes of our work can be summarized as follows.
The matching of instrumental temperature data and paleoenvironmental data
allow us to affirm that current summer temperature is comparable with the
temperature recorded in the northern Apennines during the HTM. The current
temperature trend computed from the monthly values of July temperature at
the Lake Verdarolo site over the period 1961–2018 is The stratigraphic units showing a high frequency of coarse-grained flood
deposits are Unit 2, deposited in the HTM, and Units 4 and 5, which belong
to the post-LIA time. Both periods are characterized by higher temperatures.
The human impact is almost absent in the HTM and very low in the post-LIA
time, especially in the last part of the 20th century. This strongly
supports the hypothesis that greater warmth favors the occurrence of
extreme precipitation events. In particular, the recent sedimentation rate
computed since ca. 1800 CE (post-LIA) is at least 1 m per 100 years. This high
value of the sedimentation rate has to be correlated with the presence of
numerous coarse-grained levels within Units 4 and 5, after a period of
absence of flood deposits in subunit 3b. This difference in flood layers must
be linked to an increase in HIP over the Lake Moo basin since we could not
attribute it to any other changes in physiographic or vegetation dynamics
(i.e., tree cover). During the HTM and current time, the forested area coverage
around Lake Moo has persisted at high values, and this factor is considered
disadvantageous for debris mobilization. On the contrary, during about 6–4 kyr BP we did not find any increase in coarse-grained layers from flood
events, even though the forested area declined significantly and the
anthropogenic impact increased. HIP increase in response to higher temperature is already detectable in
observation series. We found evidence that this also occurred in the past,
especially during the HTM, as testified by the higher deposition of coarse-grained
levels. A comparison with the past helps to understand future projections for
the area. However, we are aware that past evolution cannot be taken as a
perfect analogy for the future due to the different forcing and consequent
response of the climate system (D'Agostino et al., 2019). As temperature
will continue to increase in the Mediterranean area, precipitation intensity
would keep increasing over the northern Apennines. We hypothesize that
precipitation intensity increase will be more evident in months with cooler
and moist air masses, like in autumn and in winter, when moisture
availability is not limited. An increase in precipitation maxima in autumn
months is already emerging on the northern Apennines.
The Lake Moo basin proves to be an ideal study area to achieve a reconstruction between high-intensity precipitation and debris flow due to its position relative to the dominant atmospheric flow and favorable geological, geomorphological and vegetation characteristics. Further investigations, in association with other analogous sites along the northern Apennines crest, are planned to provide a multi-site assessment of the dynamics of past extreme precipitation events.
Original data acquired during the field campaign (including pollen counts, radiocarbon analysis and photos) are
available on the open data repository of ARPAE Emilia–Romagna
(
The supplement related to this article is available online at:
SS and FG developed the idea and the research activity planning and execution. SN, MA and MTDN have taken care of the administrative aspects. All authors, except for SN and MTDN, contributed to field activity. Granulometric analysis was conducted by AC. Pollen analysis was conducted by SM and MM; data elaboration and figures were created by VR, SM and MM. SS, FG and VR prepared the paper with contributions from all co-authors. FG provided management and coordination.
The authors declare that they have no conflict of interest.
We are very grateful for all comments received in the public discussion; in particular, we thank reviewers Picotti and Tinner for their useful suggestions. Finally, we thank Francesca Staffilani (Geological, Seismic and Soil Service, Emilia–Romagna region) for the pedological data interpretation.
This research has been supported by the Regional Agency of Civil Protection and the Geological, Seismic and Soil Survey of the Emilia-Romagna Region in the framework of the cooperation agreement with ARPAE-SIMC.
This paper was edited by Keely Mills and reviewed by Vincenzo Picotti and Willy Tinner.