Toward an Astrochronology-Based Age-Model for a Messinian Pre-Evaporitic Succession: The Example of Torrente Vaccarizzo Section in Sicily (Italy)

Abstract

Tectonic, paleoenvironmental, and paleoclimatic unstable conditions preceding the onset of the Messinian Salinity Crisis (MSC) highly affected marine life. Changes in calcareous plankton association are overall registered in the Mediterranean. They consist of a general transition from abundant and well-diversified planktonic associations to strictly oligotypic assemblages that precede their total disappearance at the onset of evaporitic precipitation. In this work, an accurate quantitative analysis of calcareous plankton, both foraminifers and nannofossils, has been carried out in the Torrente Vaccarizzo Section of Sicily (southern Italy). The aim is to independently define a chronostratigraphic pattern of bioevents preceding the MSC in the absence of magnetostratigraphic or radiometric constraints. The fluctuating abundance of the genus Orbulina fits well with the 100 ky Eccentricity maxima, and it is successfully applied to build an astronomically calibrated age-model for the section. On this basis, all the biohorizons have been recalibrated and discussed with regard to the previous literature. Abundant influxes of selected species demonstrated to be of local significance since they are highly affected by paleoenvironmental and paleoclimatic conditions. A chronological sequence of foraminifer and nannofossil events marks the onset of the MSC with a derived age of 5.957 My, which agrees well with previous findings from other Mediterranean sections. This methodology and the new biostratigraphic events may be useful for future studies on pre-evaporitic successions of the Mediterranean.

1. Introduction

Several studies have already been performed on pre-evaporitic deposits (e.g., Tripoli Formation in Sicily, Italy) to understand the complex climatic and tectonic events that preceded the Messinian Salinity Crisis (MSC) in the Mediterranean. According to the wider literature available so far, the pre-evaporitic successions are linked to climatic, oceanographic, and environmental variations [1] with the input of tectonic and/or diagenetic processes [2]. Several high-resolution works indicate that these sequences are related to orbital variations ([3] and references therein, [4]).

Deposition took place along a convergent margin within deformed basins, which recorded a progressive deterioration of environmental conditions at the onset of Messinian times [5,6,7,8,9,10]. Additionally, during Messinian and Plio-Pleistocene times, cyclic episodes of anoxia led to the deposition of organic-rich deposits (sapropels; [11,12]).

The sapropels (organic-rich layers) were likely deposited under conditions of minimum precession (maximum summer insolation), reflected in the Mediterranean basin by an increase in precipitation and runoff, which may have driven an increased stratification of surface waters. The marls are deposited during the subsequent rise in the precession index (reduction in summer insolation) and are reflected by a dry and cold climate, which corresponds to an increase in evaporation. This resulted in the mixing and reoxygenation of deep and bottom waters, reducing the preservation of organic matter [13,14,15]. Thus, it explains the low record of Total Organic Carbon (TOC), high content of Total Inorganic Carbon (TIC), and reducing conditions reflected by sulfur content >2% [7]. However, this lithological pattern is not consistent during the late Miocene along the whole Mediterranean: in the eastern part of the basin, sapropels are replaced by diatomites [4,16,17,18,19,20,21], while in the western side, the alternation of sapropel/marl and diatomite layers is dominant, like in the Sorbas section [11,22].

The present study focuses on the Torrente Vaccarizzo (TVCZ) Section from Sicily mainland. It shows exceptional metric-decimetric to millimetric lithological cyclicity highlighted by sapropel layers [23,24,25]. Detailed analysis on calcareous plankton, both foraminifers and nannofossils, important components of the pre-evaporitic successions, is useful for biostratigraphic and paleoceanographic/paleoenvironmental reconstructions [8,11,22,26,27,28,29,30,31].

However, a firm chronostratigraphic and biostratigraphic record of the Messinian pre-evaporitic interval in the Mediterranean may be hampered by the lack of chronostratigraphic data (i.e., magnetostratigraphic and radiometric dating) and/or the absence or scarceness of biostratigraphic markers due to the deterioration of the environmental conditions. Additionally, changes in the environmental conditions linked to local tectonics, river runoff, current circulation, and productivity may be peculiar to each sub-basin and therefore influence calcareous plankton assemblages. Therefore, biohorizons may retain a local significance.

The present study explores the use of abundance variation of calcareous plankton, recording an astrochronological cyclicity, to build an astrochronology-based age-model in the absence of other time constraints.

2. Geological Setting and Study Area

The study area is located in central Sicily (southern Italy) within the Caltanissetta Basin and is delimited by the villages of S. Caterina Villarmosa (west) and Villarosa (east). The Caltanissetta Basin [23,24,32,33] is made of a series of perched sub-basins (e.g., Corvillo, Pasquasia, etc.) developed on top of the orogenic Apenninic-Maghrebian Chain [34,35]. Here, the deformed substratum is made of Cretaceous-Oligocene Varicoloured Clays and Numidian Flysch quartz-arenites and clays (Figure 1). Thrust-top sedimentation started in the late Miocene with the deposition of deltaic silts, shallow-water fine sands, and prodelta marine clays of the Terravecchia Fm. The latter are usually shaped in badlands and at places overlaid by diatomaceous laminites of the Tripoli Fm. It consists of an alternation of clayey marls, laminated diatomites, and lime mudstones, forming the cyclic transitional deposits towards the onset of the Messinian Salinity Crisis (MSC). The succession, indeed, is followed by evaporitic deposits: gypsum, halite, and K- and Mg-salts, precipitated in the basin depocenters and passing laterally to carbonates, Calcare di Base Fm. (CdB) along the flanks of the basin margins [23,32,33,34,35,36]. The genesis of the CdB is either evaporitic or microbialitic/bacterial related [23,37,38,39,40]. Additionally, in the literature, the brecciated nature of the CdB is still a matter of debate. It has been interpreted as the product of in situ karstic/dissolution [23,32,33,36,39,41,42] and/or clastic re-sedimentation processes [38,43,44,45,46]. Despite the diverse facies, the CdB dominates the First Cycle of evaporites in outcrops [36] along with the gypsum, while thick halite layers and Mg- and K-salts are mainly preserved in the subsurface. There are several extractive salt mines among the districts of Agrigento, Caltanissetta, and Palermo (e.g., Petralia) within the Caltanissetta Basin. During the Messinian, an Erosional Surface (MES), linked with a drastic sea-level fall of the Mediterranean [36,38,43,47], separated the older First Cycle from the younger Second Cycle evaporites (“Upper Evaporites” of [48]). The latter is made of gypsum, interbedded with detrital mud and gypse-arenites [33,36,49]. Evaporites are capped by the pelagic chalks and marls of Trubi Fm. in Pliocene time, testifying to the restoration of open marine conditions [50]. The succession passes upwards into an alternation of marls and calcarenites of the Enna Fm., marking the Plio-Pleistocene forced regression and the final emergence of the area [33,51,52,53].

The overall sedimentary succession is highly affected by thrusts and folds, with thrust anticlines on the hanging wall (basin margins) and footwall synclines (deep basins). Here, we focus on the Tripoli Fm. cropping out along Torrente Vaccarizzo, close to Villarosa village (EN). The section is located on the northwestern limb of the Mucciarello anticline, plunging north-eastward.

3. The Stratigraphy of the TVCZ Section

According to the available literature, the thickness of the Tripoli Fm. varies from tens of meters, in the depocenter of some perched basins (e.g., Gaspa, Pasquasia), to almost zero, in the marginal high-standing areas (e.g., Sambuco). Four are the main lithologies that generally alternate within the Tripoli succession [23]: (i) clays and marls; (ii) carbonaceous beds; (iii) diatomaceous laminites; (iv) lime mudstones.

The study area lies along the southern bank of the Vaccarizzo Stream. Here, a 27-meter-thick measured section of Tripoli Fm. crops out, and it is overlain by more than 30 meters of CdB beds. The strata show an average strike and dip of N 215°/50° along the northwestern flank of the Mucciarello Anticline [36], which plunges north-eastward.

The developed stratigraphic log (Figure 2) can be divided into four different sectors:

(i)

In the first 6.5 m, dark grey to grey silts are dominant and related to the topmost part of the Terravecchia Fm. Their maximum thickness is about 2 m. In particular, the bottom part (0–4 m) is characterized by the alternation of black clays (sapropels), silts, and marls, which gradually pass upwards into silts. The latter are followed by a 25 cm layer of lime mudstone.

(ii)

From 8 to 19 m, the succession is dominated by the alternation of sapropels and marls. From 13 m upwards, three layers of laminated diatomites, of about 30 cm each, precede the deposition of a 20 cm thick gypsum bed that marks the first evaporitic bed of the section. On top of it, the succession shows a 1.4 m thick sapropel overlain by 60 cm of marls.

(iii)

From 19 m to 27 m, laminites and marls of different thicknesses are alternated. Specifically, the lower part (19–22 m) is dominated by a highly slumped dark grey laminite (over 1 m thick) with interbedded gypsum layers. A 20 cm black level interrupts the facies alternation of laminites and marls.

(iv)

From 27 m to the top, tens of meters thickness of CdB beds follow. They are alternated with thin layers of marls and sapropels. Overall, marls show a grey to dark grey color, which becomes greenish towards the CdB beds.

The section shows several black, organic-rich intervals that have been the object of a previous study and revealed a change from oxic/dysoxic to dysoxic/anoxic conditions towards the younger part of the section characterized by specific biomarkers (i.e., squalene produced by marine hypersaline organisms [7]). Previous works have already pointed out a connection between lithological and astronomical cycles [5,6,23,25,33,55,56]. Therefore, this section seems to be promising to explore also the biological events preceding the onset of the MSC in response to climatic/environmental changes.

4. Materials and Methods

4.1. Sampling Strategy

A total of 69 samples within the 29.4 m succession have been sampled during different field-work campaigns. For this reason, the original denomination of samples was renamed according to Table S1 in Supplementary Materials.

Sampling spacing varies between 20 and 50 cm and integrates a previously scattered sampling aimed at organic geochemistry analysis made by [7]. Two samples (960 cm and 2250 cm) for paleomagnetic analysis have also been collected. A first screening permitted the distinguishment of barren samples from oligotypical to diversified assemblages, as reported in Table S1 in Supplementary Materials.

4.2. Quantitative Analysis: Calcareous Nannofossils

For the study of calcareous nannofossils, a total of 69 smear slides, prepared according to standard methodology (e.g., [57]), were analyzed using a polarizing light microscope at 1000× magnification. A total of at least 300 specimens larger than 3 µm were counted in random fields of view. Specimens < 3 µm in size were not considered.

For species with low abundances, we extended the counting to 1 mm² of the slide, corresponding to about 100 fields of view. The frequency of Discoaster and Helicosphaera species was calculated within at least 30 specimens belonging to the Discoaster and Helicosphaera genera.

Results are thus presented as (i) percentages within the total assemblage; (ii) percentages within the Discoaster and Helicosphaera genera; (iii) the number of specimens per mm².

Nannofossils in the studied samples show preservation from moderate to good. Occasional overgrowths are present on some specimens of the Discoaster genus, but this feature never prevents their identification at the species level. Reworking may be significant on some levels. Few samples (TVCZ 24, 36, 42, and 46) yield scarce or oligotypical nannofossil assemblages. Barren samples are listed in Table S1 (Supplementary Materials).

For this study, we adopted the calcareous nannofossil biostratigraphic scheme for the Mediterranean area of Di Stefano et al. [58].

4.3. Quantitative Analysis: Planktonic Foraminifers

For the analysis of the planktonic foraminifers, 63 samples along the whole section (Table S1 in Supplementary Materials) were firstly dried and weighted and then washed through a 63 µm sieve. However, only the fraction greater than 125 µm was considered for quantitative analysis. From this, about 300 specimens of planktonic foraminifers were picked and counted in sample splits. The quantitative distribution pattern of selected planktonic foraminifers categories, having biostratigraphic and/or paleoclimatic significance, has been recorded. The planktonic foraminifer biostratigraphic scheme, the range chart, and the chronology of planktonic foraminifer biohorizons, adopted in the present work, are from [59].

Planktonic species were grouped into the following categories: (1) neogloboquadrinids: including Neogloboquadrina acostaensis (Blow) and N. humerosa (Takaianagi and Saito); (2) Orbulina spp.; (3) Turborotalita multiloba (Romeo); (4) Turborotalita quinqueloba (Natland); (5) Globorotalia scitula (Brady); (6) globigerinids: including Globigerina spp. and Globigerinoides spp.; (7) Globigerinita spp.: including G. glutinata (Egger), and G. uvula (Ehrenberg). Raw data of microfossils were transformed into percentages over the total abundance. The counted specimens of N. acostaensis have been separated on the basis of their coiling direction and plotted as ratios of dx/and sx/total number of N. acostaensis.

5. Calcareous Plankton Biostratigraphy Preceding MSC

The pre-evaporitic interval has been widely studied from the western to central and eastern Mediterranean, ranging from Spain through Apennines, Sicily, and Greece [5,6,27,38,60,61,62,63,64,65,66,67,68]. Thus, a biostratigraphic scheme based on calcareous plankton bioevents is well-established so far for the Messinian time in the Mediterranean area. It has been calibrated by integrating classical quantitative biostratigraphy with magnetostratigraphy and cyclostratigraphy, giving rise to a detailed bio-chronological framework of the interval preceding the MSC (

Table 1. Messinian biostratigraphic horizons from Mediterranean sections and inferred ages according to the available literature. (*) = Tie point used for the age-model. FCO = First Common Occurrence; FAI = First Abundance Influx.

Bioevents		Present Study	Other Mediterranean Sections
		Sample	Sorbas Basin	Falconara/Gibliscemi	Fanantello	Trave	M. dei Corvi	M. del Casino	Metochia	Pissouri	Piedmont Basin	Kalamaki (Ionian Sea)
E.(*)	Sphenolithus + Helicophaera peak I = “MSC onset event”	TVCZ-49	(*)5.99 [68]		5.97 [63]					5.99 [64]	5.98 [66,67]
D.	Last influx T. multiloba	TVCZ-36	6.07 [11,59]	6.07 [17,18,59]
C.	N. acostaensis sx/dx coil. change	TVCZ-17	6.36 [11]	6.337 [5] 6.35 [6] 6.34 [56]						6.342 [64]
	Neogloboquadrinids sx/dx coil. change						6.37 [69]
	FCO N. acostaensis dx			6.44 [17,18]
B.	Influx H. selli	TVCZ-14				6.49 [27]
	FCO H. selli					6.50 [65]		6.96 [63]		6.53 [64]
	FCO cf. H. selli								6.48 [62]
A.(*)	“FAI T. multiloba”	TVCZ-14	(*)6.412 [11]
	FCO T. multiloba			6.40 [6] 6.41 [17,18] 6.415 [56]					6.415 [70]			6.415 [31]

).

However, the already documented evolution of the calcareous plankton towards low diversity and oligotypic assemblages until their complete disappearance is still of great interest.

In Sicily, noteworthy successions documenting the conditions preceding the MSC are represented by the Falconara, Gibliscemi, Capodarso, Serra Pirciata, and Marianopoli Sections (Reference on Table 1). Reference sections fall within foraminifers Biozones MMi 13c and MMi 13d of [59] and nannofossil Biozone MNM11d of Di Stefano et al. [58].

Selected calcareous plankton bioevents have been recorded in the TVCZ Section as well and are discussed in detail in the following sections.

5.1. Calcareous Nannofossils in the TVCZ Section

The results of the quantitative analysis of calcareous nannofossils in the TVCZ Section are shown in Figure 3, which illustrates the frequencies of the main components within the total assemblage (average percentages > 1%, Supplementary Materials Tables S2 and S3) and in Figure 4, where the frequencies of ceratholiths and discoasters are presented.

Reticulofenestra medium-sized (3–7 µm) is the most abundant taxon, representing, in some cases, over 50% of the total assemblage (Figure 3). This category also includes Reticulofenestra rotaria, which was recorded in few samples with very low frequencies. Other relevant components are the helicoliths, mainly represented by Helicosphaera carteri and by H. sellii, which show a discontinuous sporadic presence and an abundance peak in the lower part of the section (“a” in Figure 3). Sphenolithus spp. (S. abies + S. neoabies), well represented along the whole section, shows a remarkable rhythmic pattern as well as the Calcidiscus genus (C. leptoporus + C. macintyrei) (Figure 3).

Similar behavior is shown by Geminilithella rotula and Rhabdosphaera spp. (mainly represented by R. clavigera; Figure 3); Pontosphaera spp. is present in the central part of the succession with maximum frequencies of 20%, and Syracosphaera spp. in the lowermost part with frequencies lower than 10% (Figure 3).

Coccolithus pelagicus is present along the section and shows a fluctuating trend, generally with low abundance (Figure 3).

Discoaster specimens are common in the lower part of the succession and almost absent from 12 m upward. Their distribution pattern is characterized by wide and rapid fluctuations ranging from very high percentages (in the central part of the section) to zero. Within the Discoaster genus, D. variabilis and D. brouweri are the most common. Discoaster pentaradiatus and D. asymmetricus are discontinuously present in low percentages. Discoaster quinqueramus and D. tamalis depict two peaks in the basal part of the section (Figure 4).

Ceratholiths are important components of the nannofossil assemblages in the Messinian time interval. In the TVCZ Section, they are represented by Amaurolithus primus, A. delicatus, and Nickilithus amplificus. These taxa are present from the base of the section and are characterized by a fluctuating trend.

The remarkably highly fluctuating presence of Lithostromation perdurum is noteworthy and already documented within an equivalent stratigraphic interval in the Sorbas Basin [12].

According to our results, the TVCZ section should be younger than the first occurrence of Nicklithus amplificus (thus falling within Zone MNN11c of Di Stefano et al. [58]), which is considered a sufficiently reliable event in the Messinian, with an attributed age of 6.69 My [62].

Helicosphaera carteri has been a main component of the helicoliths assemblages since the Early Miocene—e.g., [58,71,72]. On the contrary, the presence of Helicosphaera sellii is traditionally assigned to the early Pliocene. Nevertheless, the sporadic occurrence of this species in the Messinian is well documented in several Mediterranean sections—e.g., [27,62,65,73]. In the TVCZ section, the H. sellii abundance peak detected at about 5 m (sample TVCZ-14; “a” in Figure 3) is well comparable with the H. sellii influx described by Iaccarino et al. [65] with an attributed age of 6.50 My.

In the uppermost part of the section, an abundance peak of Sphenolithus spp. (“a” within the dashed rectangle in Figure 3) is slightly followed by G. rotula and H. carteri spikes. This precise sequence of abundance peaks was previously reported in different Messinian sections (“MSC onset bioevent”) [29,68]. The ages attributed to the base of this event are reported in Table 1.

The presence of Discoaster quinqueramus in Mediterranean sections was widely debated—e.g., [62]. Nevertheless, our findings testify that “true” D. quinqueramus occurs in the TVCZ Section as previously documented—e.g., [27,65,74]. In the specific, an abundance spike of the species is observable within sample TCVZ-16 (“c” in Figure 4). Discoaster tamalis has been a significant component of Discoaster assemblages since the late Zanclean. Nevertheless, the sporadic presence of the species is also documented in some Mediterranean sections since the Messinian time interval—e.g., [27,65]. Yet, the presence of D. tamalis in the lowermost part of the section (sample TVCZ-2; “a” in Figure 4) is not comparable with the similar event described by Iaccarino et al. [65] and Di Stefano et al. [27] (“D. tamalis Influx”) occurring below the first occurrence of N. amplificus and dated 6.9–6.79 My [65].

The interval between samples TVCZ-54 and TVCZ-64 does not contain calcareous nannofossils. Yet, few samples (TVCZ-65, TVCZ-69) from the clay horizons between CdB strata yield scarce, oligotypic assemblages mainly composed of specimens from the Sphenolithus and Helicosphaera genera.

5.2. Planktonic Foraminifers in the TVCZ Section

The preservation of the planktonic foraminifers assemblages is variable in the TVCZ Section. From the bottom of the section up to 320 cm, within the silts and marly silts, the plankton community is abundant and moderately to well preserved (Supplementary Materials Table S4). From 320 cm up to 1555 cm, within silts and grey-black shales, diatomitic laminites, and lime mudstones, preservation becomes moderate to poor, and planktonic association is generally common. The distribution of planktonic foraminifers is almost continuous up to 1650 cm, except for three barren intervals in the lower-middle part of the section, located respectively from bottom to top, at 685 cm, 1290–1365 cm, and 1470 cm. From 1650 cm upward, the samples are barren of planktonic foraminifers but contain only a few species of benthic foraminifers, such as Bulimina echinata, B. aculeata, Bolivina dilatata, and Uvigerina spp.

The results of the quantitative analysis of plankton foraminifers in the TVCZ Section are shown in Figure 5, where the frequencies of the main taxa within the total assemblage (average percentages > 1%) are plotted. Additionally, the main biohorizons with biostratigraphic significance are here presented, discussed, and compared with previous findings.

From the base of the section to about 450 cm, the planktonic association is quite diversified, made of abundant globigerinids (Globigerinoides obliquus and G. quadrilobatus group, Globigerina spp.), Orbulina spp., mainly sinistral coiling Neogloboquadrina acostaensis, followed in abundance by Neogloboquadrina spp. (mainly N. humerosa) and Globigerinita spp. (Figure 5). Rare specimens of the Globorotalia scitula group have also been recorded.

A remarkable event present in the TVCZ Section is the First Abundance Influx (FAI) of T. multiloba, renamed after Sierro et al. (“First Abundant influx” T. multiloba, [11]) in sample TVCZ-14 at 495 cm (Table 1), with a percentage of 14.7% (A in Figure 5). T. multiloba is an endemic form of the Mediterranean and has been considered an ecophenotypic variant of T. quinqueloba. The bioevent (named “First Abundant Influx” or “First Common Occurrence by different authors, e.g., Table 1) has been widely used for biostratigraphic correlations in the Mediterranean. It characterizes subzone MMi 13c [5,11,31,59,70] and is dated at 6.4 My (Table 1) at Falconara Section (Sicily), Sorbas and Abad sections (Spain), and Kalamaki (Ionian Sea) and Metochia (Gavdos Island, Greece). Bellanca et al. [6] and Blanc Valleron et al. [56] reported the second and last influxes at the Falconara section, respectively, dated at 6.28 My and 6.07 My.

Another remarkable biohorizon is the change from sinistral to dextral coiling of N. acostaensis. The event is identified by the dominance of dextral coiling N. acostaensis over the total of N. acostaensis (percentage greater than 50%). N. acostaensis sx/dx coiling change (C in Figure 5) is recorded between TVCZ-15 (550 cm) and TVCZ-17 (640 cm) since the latter bears a low number of specimens. The event has been reported in the literature from different sections with ages ranging from 6.337 to 6.36 My. Authors [17,18] describe a FCO of dextral N. acostaensis dated at 6.44 My from the Falconara/Gibliscemi Section.

Several additional influxes of T. multiloba have been recorded in the TVCZ Section in samples TVCZ-19 (16.19%), TVCZ-26 (97.04%), TVCZ-31 (61.40%), and TVCZ-36 (61.80%), respectively, at 670, 930, 1095, 1240 cm. The taxon occurs in poorly diversified and oligotypic associations together with T. quinqueloba (Table 1). This points to stressed environmental conditions preceding the Messinian Salinity Crisis [59].

An influx of sinistral N. acostaensis is recorded at TVCZ between samples TVCZ-24 (855 cm) and TVCZ-23 (795 cm).

In the TVCZ Section, up to 9 peaks in abundance of Orbulina spp. have been recorded and will be described and commented in the next paragraph. Relative abundance fluctuations of Orbulina spp. and high peaks (80–90% “orbulinites” event, [59]) have been previously recorded in pre-evaporitic deposits.

The disappearance of planktonic foraminifers preceding the onset of the MSC is reported in correspondence with a first gypsum layer (1650 cm, Figure 5) within sample TVCZ-46. This event is still a matter of debate and is considered either diachronous or synchronous throughout the Mediterranean and will be discussed in the next paragraph [6,11,17,18,38,56,75,76,77].

6. Age-Models

A chronostratigraphic frame of the events preceding the MSC in the TVCZ Section was first attempted through biostratigraphic data. Unluckily, the test on paleomagnetic properties revealed the absence of natural remanent magnetization (NRM) in the sampled intervals. For such reason, two bioevents, considered synchronous and widespread in the Mediterranean (Table 2), have been then selected as Tie Points (TPs):

(1).

The First abundance influx (FAI) of T. multiloba in Sample TVCZ-14 (495 cm from the base) and dated at 6.412 My, according to Sierro et al. [11], also described as “FCO of T. multiloba” and dated at 6.40 My [6], 6.41 My [17,18], and 6.415 My [56]. Therefore, an age of 6.42 My [5,11,17,18,56] has been fixed as TP for the preliminary age-model.

(2).

Sphenolithus + Helicosphaera peak I coincident with the base of the “MSC onset event” (“a” within the dashed rectangle in Figure 3) in sample TVCZ-49 (1750 cm from the base), dated at 5.99 My by Mancini et al. [68].

In the absence of further fixed TPs, the resulting age-model (Figure 6) is presented by a straight line that defines a homogeneous sedimentation rate of 2.92 cm/ky along the considered tract of the section. This age-model does not prove to be entirely reliable, considering the lithological variations affecting the examined section. However, it allows the assignment of numerical ages to the other bioevents in between (Figure 6). Their ages are comparable with the ones reported in the literature.

Yet, the depth-age graph of Figure 6 may be used as an initial chronological framework to retrieve the Milankovitch cyclicity that could derive from the distribution pattern displayed in the TVCZ Section by Orbulina spp. (Figure 5), which seems to provide a better response than other taxa.

In fact, Orbulina thrives in relatively warm and oligotrophic surface waters [78,79], and is tolerant to high salinity conditions [80,81,82]. According to the existing literature, Orbulina is dominant in Mediterranean Messinian successions, thus, representing a good proxy in stressed environments [1,17,18,56,83,84,85]. Furthermore, it seems to fit well with Milankovitch cyclicity—e.g., [21,22,28]. In the specific, Orbulina seems to display a similar pattern recorded by the genus Globigerinoides, which shows an in-phase correlation with the Eccentricity curve: Globigerinoides maxima—carbonate minima, Eccentricity maxima [28] as also reported from several Pliocene successions of Sicily [86]. In this paper, Globigerinoides spp. and Globigerina spp. have been grouped and counted within globigerinids (Figure 5). We focused instead on Orbulina spp. variation abundance, since several Authors [18,59] report from the Mediterranean area, starting from 6.40 My, the well-known “orbulinites” event characterized by 80–90% of Orbulina spp.

For spectral analysis, the Orbulina spp. original data from the TVCZ Section were linearly interpolated through the software Past 4.11 [87] and equally spaced at intervals of 10 ky (Figure 7a), according to the preliminary age-model (Figure 6). The autocorrelation test on the Orbulina spp. curve (Figure 7b) proves that it contains a periodicity after 10 points (~100 ky) that corresponds to a frequency of 0.0957 cycles/10 ky (Figure 7c). This frequency is very similar to the high-frequency Eccentricity for the considered time interval, which is 0.1035 cycles/10 ky (La2004 solution by Laskar et al. [88], available on the IMCCE website—https://www.imcce.fr/ (accessed on 24 March 2023)). Moreover, the cross-correlation test (Figure 7d) between the Orbulina spp. and the Eccentricity shows a phase relationship between the two curves.

Five peaks with a frequency of ~0.1 cycles/10 ky are confidentially recognized within the Orbulina spp. distribution pattern (Figure 8a). Thus, the ages of these Orbulina peaks (Table 2) correspond to the Eccentricity maxima in the ~100 ky period (Figure 8b). These can be used as TPs for the construction of the age-model illustrated in Figure 8c. Based on this age-model, the ages of all sampled points may be re-calculated (Table 2).

Figure 8. (a) Abundance variation curve of Orbulina spp. vs. depth. (b) Abundance variation curve of Orbulina spp. vs. time. (c) Age-model for the TVCZ Section based on astrochronology, where maximum amplitude peaks of Orbulina spp. were correlated with maximum amplitude peaks of Eccentricity. Data are interpolated and spaced with a step of 10 ky. The yellow bands correlate with the Orbulina spp. curve vs. depth and the Orbulina spp. and Eccentricity curve vs. time.

Figure 8. (a) Abundance variation curve of Orbulina spp. vs. depth. (b) Abundance variation curve of Orbulina spp. vs. time. (c) Age-model for the TVCZ Section based on astrochronology, where maximum amplitude peaks of Orbulina spp. were correlated with maximum amplitude peaks of Eccentricity. Data are interpolated and spaced with a step of 10 ky. The yellow bands correlate with the Orbulina spp. curve vs. depth and the Orbulina spp. and Eccentricity curve vs. time.

Table 2. Derived ages for each sampled point in the TVCZ Section, according to the age-model of Figure 8c. The yellow labels correspond to the ages of the samples where the Orbulina spp. maxima (used as TPs) occur.

New Sample Numeration		Original Sample Numeration	Position (cm)	Sedim. Rate (cm/ky)	Age (ky)	New Sample Numeration		Original Sample Numeration	Position (cm)	Sedim. Rate (cm/ky)	Age (ky)
TVCZ	53	22	1875	3.11	5957.74	TVCZ	26	45	930	2.75	6269.93
TVCZ	52	25	1855		5964.16	TVCZ	25	6B	920		6273.56
TVCZ	51	24	1820		5975.41	TVCZ	24	6	855	2.32	6297.20
TVCZ	50	21	1780		5988.27	TVCZ	23	5	795		6323.08
TVCZ	49	23	1750		5997.91	TVCZ	22	4	760		6338.18
TVCZ	48	20	1710		6010.77	TVCZ	21	3	710		6359.75
TVCZ	47	19 bis	1670		6023.63	TVCZ	20	2	685		6370.53
TVCZ	46	19	1650		6030.06	TVCZ	19	43	670		6377.00
TVCZ	45	18B	1590		6049.34	TVCZ	18	1	655		6383.47
TVCZ	44	18 bis	1555		6060.59	TVCZ	17	42	640		6389.95
TVCZ	43	18	1510		6075.06	TVCZ	16	46	600	4.63	6407.20
TVCZ	42	17	1470		6087.91	TVCZ	15	47	550		6418.01
TVCZ	41	16B	1440		6097.56	TVCZ	14	48	495		6429.90
TVCZ	40	16	1410		6107.20	TVCZ	13	49	455		6438.55
TVCZ	39	15	1365		6121.66	TVCZ	12	50	425		6445.04
TVCZ	38	14B	1320		6136.13	TVCZ	11	51	395		6451.52
TVCZ	37	14	1290		6145.77	TVCZ	10	52	375		6455.85
TVCZ	36	13	1240		6161.84	TVCZ	9	53	320		6467.74
TVCZ	35	12B	1215		6169.88	TVCZ	8	54	260		6480.71
TVCZ	34	12	1190		6177.91	TVCZ	7	55	230		6487.20
TVCZ	33	11	1130	2.75	6197.20	TVCZ	6	56	185		6496.93
TVCZ	32	10	1100		6208.11	TVCZ	5	57	150		6504.50
TVCZ	31	44	1095		6209.93	TVCZ	4	58	130		6508.82
TVCZ	30	9	1060		6222.65	TVCZ	3	59	90		6517.47
TVCZ	28	7B	995		6246.29	TVCZ	2	60	45		6527.20
TVCZ	27	7	940		6266.29	TVCZ	1	61	10		6534.77

Table 2. Derived ages for each sampled point in the TVCZ Section, according to the age-model of Figure 8c. The yellow labels correspond to the ages of the samples where the Orbulina spp. maxima (used as TPs) occur.

New Sample Numeration		Original Sample Numeration	Position (cm)	Sedim. Rate (cm/ky)	Age (ky)	New Sample Numeration		Original Sample Numeration	Position (cm)	Sedim. Rate (cm/ky)	Age (ky)
TVCZ	53	22	1875	3.11	5957.74	TVCZ	26	45	930	2.75	6269.93
TVCZ	52	25	1855		5964.16	TVCZ	25	6B	920		6273.56
TVCZ	51	24	1820		5975.41	TVCZ	24	6	855	2.32	6297.20
TVCZ	50	21	1780		5988.27	TVCZ	23	5	795		6323.08
TVCZ	49	23	1750		5997.91	TVCZ	22	4	760		6338.18
TVCZ	48	20	1710		6010.77	TVCZ	21	3	710		6359.75
TVCZ	47	19 bis	1670		6023.63	TVCZ	20	2	685		6370.53
TVCZ	46	19	1650		6030.06	TVCZ	19	43	670		6377.00
TVCZ	45	18B	1590		6049.34	TVCZ	18	1	655		6383.47
TVCZ	44	18 bis	1555		6060.59	TVCZ	17	42	640		6389.95
TVCZ	43	18	1510		6075.06	TVCZ	16	46	600	4.63	6407.20
TVCZ	42	17	1470		6087.91	TVCZ	15	47	550		6418.01
TVCZ	41	16B	1440		6097.56	TVCZ	14	48	495		6429.90
TVCZ	40	16	1410		6107.20	TVCZ	13	49	455		6438.55
TVCZ	39	15	1365		6121.66	TVCZ	12	50	425		6445.04
TVCZ	38	14B	1320		6136.13	TVCZ	11	51	395		6451.52
TVCZ	37	14	1290		6145.77	TVCZ	10	52	375		6455.85
TVCZ	36	13	1240		6161.84	TVCZ	9	53	320		6467.74
TVCZ	35	12B	1215		6169.88	TVCZ	8	54	260		6480.71
TVCZ	34	12	1190		6177.91	TVCZ	7	55	230		6487.20
TVCZ	33	11	1130	2.75	6197.20	TVCZ	6	56	185		6496.93
TVCZ	32	10	1100		6208.11	TVCZ	5	57	150		6504.50
TVCZ	31	44	1095		6209.93	TVCZ	4	58	130		6508.82
TVCZ	30	9	1060		6222.65	TVCZ	3	59	90		6517.47
TVCZ	28	7B	995		6246.29	TVCZ	2	60	45		6527.20
TVCZ	27	7	940		6266.29	TVCZ	1	61	10		6534.77

According to the new age-model, the spectral analysis was performed again on the Orbulina spp. distribution pattern in the TVCZ Section, obtaining the results illustrated in Figure 9 that can be compared with the one in Figure 7. The autocorrelation test of Orbulina spp. (Figure 9a) is much clearer and linear than the previous one (Figure 7b), showing a repetitiveness every 10 points. The simple periodogram of Orbulina spp. (Figure 9b) shows a frequency peak at 0.1035 cycles/10 ky that is well aligned with that of Eccentricity, while the two peaks were slightly misaligned according to the preliminary age-model (Figure 7c). In addition, the cross-correlation test between Orbulina spp. and Eccentricity still shows a good phase relationship between the two curves (Figure 9b).

Finally, the evolutive harmonic analysis (short-time Fourier Transform in Past 4.11) on Orbulina spp. (Figure 9d) shows the already recognized significant frequency of ~0.1 cycles/10 ky but also indicates that this signal is not homogeneous along the section. This frequency is not well visible along the youngest part of the succession since Orbulina spp. is absent.

According to the inferred age of the considered interval (6.536–5.958 My), the low-frequency Eccentricity signal of about 0.025 cycles/10 ky (periodicity of ~400 ky) is difficult to detect, considering that the section interval deposited in 580 ky, thus recording only almost 1.5 cycles.

7. Discussion

The astrochronological age-model (Figure 8c;

Table 3. Bioevents and calculated ages from preliminary and astrochronological age-models. * Ages calculated according to the preliminary age-model (Figure 6). ** Ages calculated according to the astrochronological age-model (Figure 8c and Table 2).

Sample (cm)	Events in the TVCZ Section	Age * (My)	Age ** (My)
TVCZ-53 (1875)	MSC onset	---	5.957
TVCZ-53 (1875)	Top CN_MSC_OE	---	5.957
TVCZ-49 (1750)	Base CN_MSC_OE	5.99 (TP)	5.997
TVCZ-46 (1650)	Disappearance of planktonic foraminifers	6.024	6.030
TVCZ-36 (1240)	Last local influx T. multiloba	6.165	6.161
TVCZ-31 (1095)	IV local Influx T. multiloba	6.214	6.209
TVCZ-26 (930)	III local Influx T. multiloba	6.271	6.269
TVCZ-24 (855)	local Influx sx N. acostaensis	6.297	6.297
TVCZ-19 (670)	II local Influx T. multiloba	6.360	6.377
TVCZ-17 (640)	sx/dx coiling N. acostaensis	6.370	6.389
TVCZ-16 (600)	Influx D. quinqueramus	6.384	6.407
TVCZ-14 (495)	FAI T. multiloba	6.41 (TP)	6.429
TVCZ-14 (495)	Influx H. selli	6.420	6.429
TVCZ-2 (45)	II Influx D. tamalis	6.574	6.527

) clearly shows the variation in the sedimentation rate along the succession. As expected from field observations, it is higher in the lower part (4.63 cm/ky), where terrigenous lithologies of the Terravecchia Fm. crop out, and much lower in the upper part (from 2.32 to 3.11 cm/ky) where diatomitic laminites and marls, assigned to the Tripoli Fm., occur.

In addition, it is now possible to assign ages to the calcareous plankton events detected along the section (Table 3) and to compare them with the same or similar events described in other Mediterranean sections.

7.1. Age of Calcareous Nannofossils Bioevents in the TVCZ Section

According to the age-model reported in Figure 8c, the base of the section has an age of 6.534 My. This datum is in good agreement with the presence of Nicklithus amplificus from the oldest sample, as the inferred age for the First Occurrence (FO) of this species is 6.69 My [62].

The spike of Discoaster tamalis occurring in the lowermost part of the section (sample TVCZ-2) has an age of 6.527 My; thus, it cannot correspond to the influx of the species described by Di Stefano et al. [27] at Trave Section with an age of 6.768 My. As already supposed on a biostratigraphic basis, it should represent a further younger influx of the species never before detected.

On the contrary, the influx of Helicosphaera sellii occurring in sample TVCZ-14 with an inferred age of 6.429 My, is comparable with the similar event reported in the literature in several Mediterranean sections (Table 1).

The presence of an abundance spike of D. quinqueramus in sample TVZC-16 (6.407 My) is noteworthy and may represent a useful tool for stratigraphic correlation.

The almost concomitant presence of abundance spikes of Sphenolithus spp., Helicosphaera spp., and G. rotula defines the so-called “MSC onset event” as defined by Mancini et al. [29,68], here redefined as CN_MSC_OE (=Calcareous Nannofossil MSC Onset Event) (Table 3). In the TCVZ section, the base of this event corresponds to the Sphenolithus spp. peak occurring in sample TVCZ-23 and has an age of 5.997 My, in good agreement with the previous literature. The top of the same events falls within sample TVCZ-53 (5.957 My), which also coincides with the deposition of the CdB and the beginning of the barren interval and, thus, the onset of the MSC in the considered area.

Finally, the abundance spike of Sphenolithus spp. and Helicosphaera spp. occurring above the barren interval (Figure 3) may be compared with the second Sphenolithus Influx reported by Manzi et al. [63] at Fanantello, dated at 5.860 My. Yet, it is not possible to assign an age to this bioevent, as the age-model is not available for this tract of the section.

7.2. Age of Foraminiferal Bioevents in the TVCZ Section

The calculated age of main events preceding the MSC from the astrochronological age-model is discussed here with respect to previous findings from other Mediterranean sections (Table 3).

The main events are chronologically listed as follows:

The first abundance influx (FAI) of T. multiloba (495 cm, sample TVCZ-14) dated at 6.429 My, generally considered a reliable and synchronous event, fits quite well with previous ages reported by other authors—ranging between 6.40 and 6.415 My.

In the TVCZ section, N. acostaensis is relatively abundant from the base; thus, its shift from sinistral to dextral coiling (derived age 6.389 My) is in good agreement with the same event dated at 6.373 Ma at Ain el Beida Section (Morocco) [89] and 6.36 Ma [11] at Sorbas, as well with sx/dx coiling change in Neogloboquadrinids at M. dei Corvi—Sardella Section, whose astronomical age is 6.37 My [69].

After the FAI, three other local influxes of T. multiloba have been recorded at 670 cm (sample TVCZ-19, 6.377 My), 930 cm (sample TVCZ-26, 6.269 My), and 1095 cm (sample TVCZ-31, 6.209 My). As already published by [11,56], several peaks of T. multiloba are recorded between 6.42 My and 6.07 My.

The fourth and last local influx of T. multiloba, recorded in the TVCZ section in sample TVCZ-36 (1240 cm) with a derived age of 6.161 My, fits well with a similar event reported by [17,18,56] at 6.17 My. However, after this event, other abundance peaks have been found in younger levels of the Falconara section at 6.09 My and the last one at 6.07 My [56]. Therefore, it is not excluded that younger spikes can be detected in the TVCZ section as well, with a denser sampling resolution towards the CdB. Such influxes are characterized by strictly oligotypic associations, mainly dominated by few genera (e.g., Turborotalita and benthic foraminifers of the genera Bolivina and Bulimina), testifying to cold-eutrophic waters and increase in salinity [1,31,70,76,84]. Although several authors referred to such events for biostratigraphic correlation in the Mediterranean, it should be pointed out that these peaks may have a local significance. Thus, each basin registers salinity variations through time, strictly dependent on local tectonic, climatic, and hydrological changes.

The dominance of dextral N. acostaensis is briefly interrupted by a local influx (66%) of sinistral N. acostaensis at 855 cm (Table 3). Its age has been astronomically derived at 6.297 My and is quite different from the two already known sinistral influxes of N. acostaensis, respectively at 6.13–6.14 My and 6.08–6.09 My [5,11,31,56,70].

The disappearance of planktonic foraminifera, due to the inhospitality of the marine environment, at Torrente Vaccarizzo is preceded by a short barren interval between samples TVCZ-37 and TVCZ-39 (1290–1365 cm), with astrochronological ages of 6.145 to 6.121 My. A similar pattern was already found by Bellanca et al. [6] in the same section. Such an event is not abrupt but rather shows a cyclic transition made of an alternation between barren samples (TVCZ-37, TVCZ-39, and TVCZ-42) and rich association samples (TVCZ-40, TVCZ-43, and TVCZ-44), the result of normal marine influxes within the basin. The definitive disappearance of planktonic foraminifera, in sample TVCZ-46 (1650 cm) at 6.03 My, precedes the “MSC onset bioevent” [29,68] shown by the abundance peak of Sphenolitus spp., slightly followed by H. carteri s and G. rotula spikes. Such an age agrees well with the same event reported by Blanc-Valleron et al. [56] from the Falconara composite section.

8. Conclusions

The main goals deriving from the present study are:

(1)

The building of an astrochronology-based age-model for the Messinian pre-evaporitic TVCZ Section. It is based on the abundance variation of Orbulina spp. (Orbulina peaks), which proved to fit well the 100ky-Eccentricity cycles. This methodology is useful in the absence of other chronostratigraphic constraints, such as magnetostratigraphy and radiometric dating.

(2)

Based on the astrochronological age-model, all the collected samples have been dated. Therefore, the age of all recognized bioevents, both nannofossils and foraminifers, was calculated. Some well-known bioevents were confirmed to be reliable markers for biostratigraphic correlation, and new ones were detected for the first time, improving the biostratigraphic resolution at the MSC onset. In particular, the I Sphenolitus + Helicosphaera peak (here renamed CN_MSC_OE), with an inferred age of 5.997–5.957 My, represents the last plankton event preceding the CdB deposition. This event is preceded by the disappearance of planktonic foraminifers, which is not abrupt but records an alternation of evaporitic and normal marine phases in the basin.

(3)

Many taxa show characteristic “peak-abundance distribution” reflecting stressed conditions in the basin, highlighted by rapid changes in oxygen content, nutrient, salinity, and temperature of the water mass. This trend was already described elsewhere in the stratigraphic levels preceding the MSC.

(4)

The age of the MSC onset calculated in the TVCZ section is 5.957 My, in good agreement with previous literature.