Chronostratigraphic confirmation of MIS 5 age of a baymouth bar at Is Arenas (Cagliari, Italy)
Introduction
Sea-level change as imprinted in rocks and sediments along the coastline is the end product of eustatic, glacio–hydro-isostatic, and tectonic factors. Eustasy is global and time-dependent in quasi equilibrium with changing volume of polar ice sheets. Glacio-isostatic Adjustment (GIA) and tectonics vary globally, regionally, and locally. GIA effects vary with proximity of former ice sheets and depth and configuration of continental shelves upon water loading and unloading. Areas proximal to plate boundaries generally experience the greatest tectonic inquietude, but regional and local factors often complicate the tectonic picture. The GIA component along the Italian coast has been recently evaluated and predicted in comparison with field observations at sites not significantly affected by tectonic processes (Lambeck et al., 2004a, Lambeck et al., 2004b).
In the stable areas of Italy, the present sea-level position is the highest eustatic level reached during the Holocene. Thus, any coastal Holocene marine deposit (aside from those deposits created by tsunami) found above the modern sea level indicates tectonic uplift (Lambeck et al., 2004b, Antonioli et al., 2009).
In 2005, the National Geological Survey of Italy (APAT, 2005) published the new scale 1:50.000 sheet, “557 Cagliari” of the Geologic map of Italy. Along the Cagliari coastal plain, the 557 Cagliari map identifies Site 1 as Is Arenas (Fig. 1) where wide outcrops of littoral deposits up to 4–5 m above sea level (noted here as “+”, as in +4–5 m). The map identifies these deposits as Holocene, in disagreement with the extensive historical literature (summarized in Hearty, 1986, Hearty et al., 1986, Ulzega and Hearty, 1986), together with published documentation over the past century indicating a Tyrrhenian age (MIS 5.5, 125 ka). More recently, Ferranti et al. (2006) published an extensive review of the elevation of the last interglacial sea level indicators in Italy.
The designation of the Is Arenas deposits as Holocene (APAT, 2005) would indicate that all previous analyses and interpretations were incorrect. Further, a Holocene age of the Is Arenas marine deposits introduces notable implications for recent tectonic mobility of the area that can be reconciled only by radical vertical tectonic displacement, a suggestion contradicting observations that Sardinia is one of the most stable areas of the western Mediterranean.
This reinterpretation of the age of the sandy, fossiliferous baymouth bar at Is Arenas (APAT, 2005, Coltorti et al., in press) would designate southern Sardinia as one of the most tectonically active regions of the entire Mediterranean, comparable to areas such as Southern Calabria (Ferranti et al., 2008) and the Gulf of Corinth (Palyvos et al., 2010). As expected, this interpretation of Is Arenas aroused lively scientific debate in recent papers (Antonioli, 2007, Carraro et al., 2007, Coltorti et al., 2007), a public debate organized at the Geological Survey (ISPRA) on March 8, 2007, and ongoing discussion (Coltorti et al., in press, Thiel et al., in press).
The aim of this research is to discuss the new data and interpretations of a Holocene age of Is Arenas, as published in the illustrative notes of the 557 Cagliari sheet. It critically assesses the accuracy and interpretation of the data underlying this position. New ESR results performed on the deposits and fossils are discussed along with a synopsis of existing published AAR and U-series data.
Section snippets
Setting
The coastal sector of Cagliari and Campidano is influenced by multiple factors that can complicate the tectonic picture. The GIA component along the Italian coast has been recently evaluated and predicted through comparison with field observations at sites not significantly affected by tectonic processes (Lambeck et al., 2004a, Lambeck et al., 2004b).
Into the western coastal plain, which hosts the Laguna Santa Gilla system, and the eastern coastal plain represented by the Molentargius Pond
Sample collection
Intensive field investigations at Is Arenas focused on the distribution and elevation of sedimentary structures indicative of palaeo-sea level, and the collection of optimal fossil marine molluscs for dating. Samples for geochronological analyses were collected from several sample sites in the area of Is Arenas. In the Quarry of the Orto Botanico, marine shells were sampled (Fig. 5e) from the same stratigraphic level from which Coltorti et al. (in press) obtained Holocene ages. The elevation of
ESR results
Table 1 contains specification of the ESR analyses. The dates range between 150 and 80 ka with a mean of 112 ka, indicating that the deposit must be related to MIS 5, and confirming previously published data. These results agree with: i) OSL data by Thiel et al. (in press), ii) previously published AAR and iii) U-series data, and independently confirm a correlation of the Is Arenas deposits with the last interglacial period, most likely MIS 5.5.
Discussion
This discussion considers the scientific issues that prompted Coltorti et al. (in press) to write, “the younger synthem evolved during the Holocene. Radiocarbon dating confirms these attributions”. Ulzega and Hearty (1986) found in the marine deposit of Is Arenas (dated as MIS 5.5 using aminostratigraphy) an in situ S. bubonius (125 ka). Coltorti et al. (in press) wrote, “although Issel (1914) never mentioned the occurrence of Strombus bubonius at Is Arenas, Ulzega and Hearty (1986) did,
Conclusion
Coltorti et al.’s (in press) conclusion that the Is Arenas baymouth barrier is of Holocene age implies a tectonic uplift of 12 m. Several previous studies have established on biostratigraphic, morphostratigraphic, aminostratigraphic, U-series, and now ESR data that the barrier bar is of last interglacial, MIS 5.5 age. In addition the OSL results of Thiel et al. (in press) support this stratigraphic assessment and the MIS 5 age of the Is Arenas marine deposit. Well-defined geomorphological
Acknowledgments
This work was partially supported by the VECTOR project funded by the Italian Ministry of Education, University and Research. We acknowledge Mauro Agate and an anonymous reviewers for their constructive review.
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