Revision of the Messinian-Early Zanclean Sediments from ODP Hole 953C (Canary Island Archipelago, North-Eastern Atlantic): Biostratigraphy, Cyclostratigraphy, and Astronomical Tuning

A quantitative study was performed on calcareous plankton of the Messinian-early Zanclean succession recovered at ODP Leg 157 Hole 953C (Canary Island Archipelago, North-Eastern Atlantic).This revision allowed to recognize some events typically recorded in the Mediterranean region, highlighting affinities between the Mediterranean and North Atlantic Ocean, in the considered time interval. The presence of such events in an open-ocean succession provides the possibility to substantially improve the biostratigraphic resolution and supplies useful correlation tools between theMediterranean and oceanic areas.Moreover, to unravel cyclical patterns of deposition and given that the investigated succession shows no evident lithological pattern, cyclostratigraphic analyses have been based on abundance fluctuations of Globigerinoides-Orbulina group, neogloboquadrinids, and warm-water versus cool-water species ratio. As a result, forty-three precession-controlled cycles have been recognized spanning from 6.457Ma to 4.799Ma.

To understand the cause(s) and consequences of the onset of the MSC in detail, an age control is needed for the part of the Atlantic immediately adjacent to the Mediterranean. In this connection, the ODP Leg 157 Hole 953C (Canary Island Archipelago, North-Eastern Atlantic) is considered favourable to the aims of the work for (i) the location; (ii) the excellent paleomagnetic data; (iii) micropaleontological records, despite intervals of no recovery.
The aim of this paper is (i) to present an biostratigraphy for the Messinian-early Zanclean of the Hole 953C, based on quantitative data of planktonic foraminifera (previous studies by [67,68] are to be considered inadequate for this research); (ii) to compare the bioevents recognized, with those occurring in other astronomically calibrated sections; (iii) to recognize other cyclic patterns for using as further control of the obtained astronomical calibration.  [70]) were drilled in the volcaniclastic apron of Gran Canaria in the Canary Island Archipelago (North-Eastern Atlantic) ( Figure 1). ODP Hole 953C is situated at 28 ∘ 39 .012N, 15 ∘ 08 .670W, 68 km northeast of Gran Canaria, 90 km west of Fuerteventura, and 98 km east of Tenerife, at water depth of 3577.8 m. It is the most distally located site on the flanks of Gran Canaria and thus seems to have been spared some of the major slumped intervals and unconformities present at Sites 954, 955, and 956. The recovered sedimentary succession spans the interval from the middle Miocene to Holocene, and the dominant nonvolcanic lithologies were divided into seven major units and three subunits (see [69]). The present study focuses on a 51.11 m thick succession from the Unit III, embracing cores 18R to 13R from 346.89 to 295.78 mbsf ( Figure 1) and representing the Messinian-early Zanclean time interval. Core recovery was generally over 50%, and mainly consisting of gray to brownish-gray clayey nannofossil ooze, interbedded with minor graded nannofossil clay-silt, foraminifer lithic silt and sands, and foraminifer sands.

Sample Preparation and Micropaleontological Analysis
2.2.1. Foraminifera. Samples for foraminiferal analysis were disaggregated in normal water and washed through a 63 m sieve. The residues were oven dried at 40 ∘ C. Qualitative and quantitative planktonic foraminiferal analyses were performed, using a stereoscopic microscope, on 170 samples (average spacing 20 cm). The samples were divided with a microsplitter to obtain unbiased aliquots with about 300 planktonic foraminifera. They were (1) picked, (2) mounted onto faunal slides, (3) determined at specific and supraspecific level, and (4) counted (200-300 specimens) from the larger than 125 m fraction of the washed residue to collect quantitative faunal data. The assemblages are usually abundant and generally well preserved, although at times they are poor. Very few samples show a very scarce faunal content.
Raw data of microfossils were transformed into percentages over the total abundance, and percentage abundance curves were plotted. Species with phylogenetic affinities and similar environmental significance were also grouped to better interpret distribution patterns.
Furthermore, a semiquantitative analysis from the larger than 63 m fraction was carried out on all samples in order to pinpoint the actual position of the bioevents.

Calcareous Nannofossil.
Standard smear-slides were prepared for the nannofossil analyses for the intervals encompassing the principal bioevents (from core 17R Section 3 to core 13R Section 5). Qualitative analyses on a total of 30 samples have been performed by Dr. Baldassini N (Siena University) to determine additional biohorizons and bioevents to improve the biostratigraphic resolution.

Micropaleontological Data
The analysis of the planktonic foraminiferal fauna allowed the identification of 45 taxa, which sometimes were grouped into categories on the basis of morphological, phyletic, or ecological affinities. The categories are (Table 1 Figure 2 shows the quantitative distribution of those species (or categories) which generally display frequencies higher than 1.5%. The main components are Globoturborotalita nepenthes and Neogloboquadrina gr. which are continuously present throughout the investigated interval, reaching maxima frequencies of 90% and 80%, respectively.
Neogloboquadrina gr. includes also small-sized specimens, sometimes under poor preservation conditions that were difficult to distinguish. Different coiling directions of Paleontology Journal   the N. acostaensis (average value of 17%) have been counted separately as far as the study time interval coiling-change of this species has been proved to have potential biostratigraphic significance. Particularly, in the lower part of the studied interval, sinistrally coiled specimens prevail up to 344.57 mbsf, and after this level the dextrally coiled increases in abundance.
N. pachyderma comprises specimens with low trochospiral test, four to five and one-half chambers in the final whorl with the last chamber irregular often a kummerform, surface characterized by large euhedral calcite crystals, umbilicus from closed to open but always narrow, and aperture umbilical-extraumbilical with a thick apertural rim. This species is present from the base of the investigated G. decoraperta gr. and G. obliquus gr. show a continuous presence along the investigated interval with high abundance value of approximately 35.5% and 33%, respectively.
Dentoglobigerina gr. displays low frequencies (average value of 2.1%) intervals with isolated abundance peaks higher than 15%.
Sphaeroidinellopsis seminulina gr. shows a general trend very similar to that of Dentoglobigerina gr., displaying low values (approximately 2%) but a continuous distribution with peaks of abundance higher than 14%.
G. miotumida, which includes less inflated forms, is present from the bottom of the studied interval with low frequencies (average value of 7%) up to 344.35 mbsf where it disappears. From this depth three short influxes are recorded (average value of 0.5%). The coiling direction of G. miotumida is prevalently sinistral.
G. menardii gr. includes the species G. menardii and G. multicamerata. G. menardii shows low and scattered occurrence from the base of the investigated interval (average value of 1.3%). The coiling direction is prevalently dextral with short sinistral influxes. G. multicamerata, which includes the forms with seven to ten chambers in the final whorl, first occurs at 342.05 mbsf and displays a low a discontinuous distribution with a maximum of approximately 2%. Throughout the studied interval, it is exclusively dextral.
G. margaritae margaritae occurs firstly at 336.11 mbsf, displaying a continuous presence until the top of the studied interval with low percentage value (about 9%). The coiling direction of this species is prevalently sinistral.
G. cibaoensis first occurs at 325.25 mbsf, showing low (about 2%) and scattered distribution up to 297.28 mbsf where it disappears. Throughout the studied interval, it is prevalently sinistral.
Unkeeled globorotalids are represented by Globorotalia juanai, G. margaritae primitiva, and G. scitula. G. juanai is present from the base of the investigated interval, showing abundance of about 5% that decreases until its disappearance at 337.66 mbsf.
G. margaritae primitiva first occurs at 337.49 mbsf, displaying low but continuous distribution (about 9%) with isolated abundance peaks higher than 10%. Throughout the studied interval, it is exclusively sinistral.
G. scitula shows low (about 1.4%) and scattered occurrences from the base. The coiling direction of this species is exclusively dextral. Additional

Biostratigraphy
Calcareous plankton biostratigraphy of the Hole 953C is based on the stratigraphic distribution of several Late Miocene-early Pliocene marker species. Ten biostratigraphic events, which have previously been dated astronomically both in the Atlantic Ocean and in the Mediterranean Sea, are also recognized in this work. In the following, distribution patterns of the main biostratigraphic marker species are discussed in detail and compared with other land or deep-sea successions both from the Mediterranean Sea and Atlantic Ocean. The first, last regular, and last occurrences (FO, LRO, and LO) are shown in Table 2.
According to the literature, these events directly postdate the sinistral to dextral coiling change of N. acostaensis. In the Atlantic Ocean, they have been recorded within subchron C3An.1r; the LRO in the Ain el Beida section was found in the range of 6.308-6.311 Ma [48]. In the Mediterranean area, these events occur in the upper part of subchron C3An.2n; at Falconara section the LO was found at 6.506 Ma [52] and at 6.51 Ma by Blanc-Valleron et al. [73], the LRO in the Sorbas Basin at 6.504 Ma [45].
The progressively restricted connection between the Atlantic and Mediterranean during the Messinian was probably responsible for the earlier disappearance of this species from the Mediterranean.
Based on the previously mentioned micropaleontological observations, (i) the change in coiling direction of N. acostaensis from left to right (6.37 Ma, [56]) and (ii) the LRO of G. miotumida (6.31 Ma, [48]) occurred in the same level. It follows that a hiatus is recorded in the Hole 953C at 344.57 mbsf with an age span about 6.436 Ma to 6.252 Ma. According to Brunner et al. [67], the top of Chron C3An.2n (6.436 Ma, [56]) and the base of Chron C3An.1n (6.252 Ma, [56]) were apparently removed by slumping.

First Occurrence of Globorotalia margaritae margaritae.
The First Occurrence of Globorotalia margaritae margaritae has been more widely disputed, because of the taxonomical problems related to its identification near the evolutionary transition from its ancestor.
Additional bioevent has been also recognized in the Globorotalia juanai LO at 337.66 mbsf, which was recorded in the Ain el Beida section in AEB 29 cycle by Krijgsman et al. [48].

Last Occurrence of Nicklithus amplificus.
The importance of the Last occurrence of Nicklithus amplificus as a biostratigraphic marker in the upper Miocene and its reliability for time stratigraphic correlation has been confirmed in studies of numerous deep-sea succession [8,[121][122][123]; the LO was found at 5.978 Ma in the ODP Site 925 and 926 ( [124] and reference therein), at 5.939 Ma in the eastern Mediterranean ( [124] and reference therein) and in the age range of 5.972-5.975 Ma in the Ain el Beida section [48]. This event occurs very close to the C3An/C3r Chron boundary and marks the beginning of the Messinian Salinity Crisis [8] in the Mediterranean basins. In the Hole 953C, the LO of this species is recorded at 334.52 mbsf (sample 157-953C-17R-3, 42-44 cm).

Last Occurrence of Triquetrorhabdulus rugosus. The
Last Occurrence of Triquetrorhabdulus rugosus is traditionally used for identifying the Miocene/Pliocene boundary in open ocean areas and is considered an additional event to recognize the CN10a/CN10b subzonal boundary [125]; it is astronomically dated at 5.279 Ma and at 5.255 Ma in the lowlatitude Atlantic [124] and in the Loulja section [34], respectively. In the Mediterranean basin, the T. rugosus LO is not considered a useful biostratigraphic event, because it is too rare in the samples. Nevertheless, it was identified in several successions just below the base of the Thvera subchron, and this bioevent is astronomically dated at 5.279 Ma [25,32,124,[126][127][128][129]. According to the paleomagnetic stratigraphy, T. rugosus LO has a closely comparable stratigraphic position in Atlantic and Mediterranean records, and it can be used to correlate the base of the Pliocene in its type area (Eraclea Minoa section-Sicily, [130]) to the oceanic record. At Hole 953C, the LO of this species occurred at 312.73 mbsf (samples 157-953C-15R-2, 63-65 cm).
In the absence of an evident lithological rhythm, biotic proxies may be taken into account to highlight the astronomical cyclicity of a sedimentary succession. In this framework, cyclostratigraphy and astronomical tuning to the ATNTS [56] has been carried out by means of foraminifera-based proxies (e.g., [62-66, 131, 132]). In particular, the abundances curves of Globigerinoides/Orbulina gr., neogloboquadrinids, and the ratio of warm-water versus cool-water species show Milankovitch periodicities. The relationship between selected climate-sensitive planktonic taxa and lithological cyclicity, correlated in turn to astronomical cycles, is well expressed in the lower Messinian deposits of Sicily, Spain, and Greece ( Figure 3).

Globigerinoides.
In agreement with their unanimously declared warm-climate affinity (e.g., [62][63][64]131]), the highest abundances of the Globigerinoides population seem to correspond to maxima of the insolation cycles. In the Sicilian "Tripoli Fm. " (Auct.), consisting of a repetition of the same lithological triplet ("sapropel type" brown laminatediatomite-marl), high frequencies of Globigerinoides gr. concentrate within brown laminates and diatomitic layers, (Modified after [73,74].) corresponding to insolation maxima [73,133,134], whereas lowest frequencies fall within the middle part of the marls. A similar situation is recorded in the coeval sediments of the Gavdos Island (Greece, [45,46,74,135]) where the basic cycle is three-folded as well (sapropel-diatomite-marl).
Here, the highest abundances of Globigerinoides spp. are recorded within the sapropel and in lower-middle part of the diatomites. In the Sorbas Basin (Southern Spain, [46]), the lower Messinian sediments of the upper Abad section display quadruplet-type lithological cycles (sapropel-marldiatomite-marl). In this case, maxima abundances of Globigerinoides spp. + Orbulina spp. are recorded in the middleupper part of the sapropels or at the transition between the sapropel layers and the first marly horizons (Figure 3).

Neogloboquadrina.
Usually reported as cool taxa [136][137][138], neogloboquadrinids show an evident preference for stratified waters within high-productivity zone associated with the deep chlorophyll maximum (DCM), at the base of the euphotic zone [139][140][141]. The neogloboquadrinids show maxima frequencies in correspondence of the topmost part of the diatomite and marl layers in the Sicilian Tripoli [73,134]; they generally reach the highest percentages at the base of the first marl layer in the upper Abad section [46,142] and within the diatomites at Gavdos [74] (Figure 3), thus implying different parameters than temperature controlling theirs distribution.
G. decoraperta gr. (G. decoraperta and G. apertura) is not living today but its descendant, G. rubescens, is currently living in subtropical and tropical environments. This species usually grows in the surface nutrient-depleted waters of the mixed layer in oceanographic settings with a strong stratification in the water column [136,143,144].
In the Sorbas basin, this group is abundant in the upper part of the sapropels and the lower part of the diatomites on Gavdos [74].
O. universa behaves as G. rubescens in the Sorbas basin and on Gavdos [74].
G. falconensis is a species whose geographic, depth, and seasonal distribution patterns are poorly known, because in the literature it is grouped with G. bulloides. However, studies of planktonic foraminiferal assemblages from the western North Atlantic [145], surface sediment data from the South Atlantic [146], and southern Indian Ocean [147] indicate that this species prefers to live in the cool subtropical regions.
G. glutinata is a ubiquitous species living in subarctic to subantarctic waters, low in absolute amount but with a high percentage in waters with very low total foraminiferal concentrations [143]. This species is abundant in the homogeneous marl-sapropel transitions both in the western and eastern Messinian Mediterranean [46,74].
A close dependence between lithofacies and microfossil assemblages has been proved for sapropel-bearing successions, with by definition, and the same microfossil assemblages can be considered as sensitive to astronomical periodicity also in successions lacking sapropels. Consequently, the abundance variations of the after-mentioned taxa allowed the astronomically tuned with the insolation curve of Laskar et al. ([104], La04 1:1 solution).
Particularly, insolation maxima-precession minima correspond to (i) maxima in the distribution patterns of both Globigerinoides/Orbulina gr. and warm/cold ratio; (ii) minima in the distribution patterns of neogloboquadrinids (Figure 4).

Astronomical
Tuning. The magnetic polarity reversal ( [67,70] and reference therein) and the bioevents recognized in the Hole 953C have been used to correlate the studied sedimentary records with Ain el Beida and Loulja sections (Morocco), astronomically calibrated by Krijgsman et al. [48] and van der Laan et al. [34], respectively. The sinistral/dextral N. acostaensis coiling change, the LO of N. amplificus, and the LO of T. rugosus are considered synchronous events, which were used as tie points to calibrate astronomically the Hole 953C sedimentary sequence. The S/D N. acostaensis and the LO of N. amplificus calibrated at 6.378 Ma and 5.975 Ma by Krijgsman et al. [48] and the LO of T. rugosus calibrated at 5.255 Ma in cycle LA 15R (and [34]) were used as the starting point for tuning the sedimentary cycles at the Hole 953C. In addition, the LO of Helicosphaera stalis ovata LO (recorded in the Ain el Beida section by [48]) and the first sinistral influx of N. acostaensis (recorded in Loulja section by [34]) are here considered.
Forty-three sedimentary cycles were recognized in the Hole 953C ( Figure 4) and are informally and progressively numbered from the base to the top of the studied interval.  [48], maxima in eccentricity enhance climate fluctuations on a precessional scale, because the eccentricity modulates the precession amplitude favouring the formation of distinct cycles.
In this interval, the recorded bioevents are (i) the LO of N. amplificus in the cycle 17 at the astronomical age of 5.979 Ma; (ii) the LO of H. stalis ovata in the cycle 24 at the Paleontology Journal Figure 6 astronomical age of 5.681 Ma; (iii) the presence of Discoaster quinqueramus in the cycle 28, which corresponds to cycle AEB 42 of Ain el Beida section [48]; (iv) the first sinistral influx of N. acostaensis in the cycle 30 at the astronomical age of 5.297 Ma; (v) the LO of T. rugosus in the cycle 33 at the astronomical age of 5.244 Ma. The recognition of these last two bioevents and the base of the Thvera magnetic event are a useful guide to identify the Miocene/Pliocene boundary in the interval studied. Since, from a cyclostratigraphic perspective, the boundary falls within the insolation cycle 510, five precessional cycles below the Thvera magnetic event, and has an astrochronological age of 5.332 Ma [53].

Chron
C3n.4n. The Chron C3n.4n (4.997 Ma-5.235 Ma, [56]) comprises a total number of ten sedimentary cycles, corresponding to 238 kyrs according to the ATNTS2004. In this interval, the relation between warm/cold ratio maxima versus neogloboquadrinids minima, allowed to identified only five precession cycles (35)(36)(37)(38)(39), because of no-recovery interval in the core 14 (6.97 m thick) that covers a time interval of 0.12 Ma. In particular, cycles 36-37 fit excellently with the high-amplitude fluctuations of the eccentricity curve. In this interval, the Globigerinoides/Orbulina gr. signal is not clear or completely absent. In the cycle 37, the FO of A. bizzarus is recognized, and it has an astronomical age of 5.038 Ma, corresponding to cycle LB 2R of Loulja section [34].  [56]) comprises a total number of 2.5 sedimentary cycles and corresponds to 97 kyrs according to the ATNTS2004. The cycles (42)(43) are highlighted by two negative fluctuations of Neogloboquadrina gr., which are in opposite phase with those of the warm/cold ratio Globigerinoides/Orbulina gr. signals. Finally, the increase in thickness of the cycles has been interpreted as a change in the sedimentation rate.

Conclusion
This paper presents the results of qualitative and quantitative analyses of the calcareous plankton assemblages, carried out in tract of the Hole 953C succession encompassing the Messinian to early Zanclean. The study allowed to recognize ten important bioevents, well calibrated and widely used in the Mediterranean and Atlantic correlation. Moreover, a cyclostratigraphic approach has been attempted to obtain a detailed chronostratigraphic subdivision. Lacking any evident lithological cyclicity and the no-recovery intervals along the succession, the combination of climate sensitive records allowed to extract signals of the astronomical cyclicities. The fluctuations curves of Globigerinoides/Orbulina gr., neogloboquadrinids, and the ratio of warm-water versus cool-water species highlight a frequency indicative of the Earth's precession periodicity. Particularly, the insolation curve is in phase with the Globigerinoides/Orbulina gr. and warm/cold ratio distribution and is in antiphase with the Neogloboquadrina gr. curve. As result, forty-three precession-controlled cycles have been recognized spanning from 6.457 Ma to 4.799 Ma.

Description of Plates 1 and 2
Plate 1 (see Figure 5). For quantitative data of ODP Hole 953C planktonic foraminifera, see Tables 1 and 2