Late pleistocene coastal sea surface temperature and environmental reconstruction for a post global-warming climate modelling in the central mediterranean

Abstract

The Last Interglacial (MIS 5) is a warm period within the Late Pleistocene. In particular, its first substage (MIS 5e) is the last warmer than today period before the present-day Interglacial, therefore it is considered a good analogue for assessing future climate responses to global warming. The present work focuses on two shallow marine deposits cropping out along the Ionian Sea coastline, central Mediterranean: 1) a MIS 5e fossiliferous calcarenite from Taranto (Apulia), and 2) MIS 5a/c bioconstructed deposits from Capo Colonna (Calabria). These deposits were studied for a paleoenvironmental characterization, which included the paleoecology of benthic associations, paleotemperature estimates and early diagenetic phenomena, with the aim of providing new data on the response of these systems to climatic variations. The MIS 5e calcarenite from Taranto, sampled in three related sites cropping out along the Mar Piccolo (MP) coastline (Masseria La Penna, Mass. S. Pietro, and Il Fronte), returned a rich molluscan fauna consisting of 120 extant species and 5174 individuals, including four of the tropical Senegalese Fauna, today absent in the Mediterranean. The molluscan assemblages were used for a refined quantitative palaeoenvironmental reconstruction; as a whole, the fauna represents an autochthonous-parautochthonous assemblage coming from a sandy-vegetated infralittoral bottom, locally accompanied by coarser and less finer sediment fractions, and characterized by slight lateral variability, shifting from more sheltered and exposed areas, and from finer and coarser bottoms, suggesting a heterogeneous and well-structured palaeoenvironment during MIS 5e. The same molluscan assemblage was used for a multi-proxy palaeoclimatic study. The biogeographic-climatic affinity of the assemblage shows, compared to today, a double percentage of warm affinity species, while the cold affinity species are similarly represented, indicating a warmer but not strictly tropical Sea Surface Temperature (SST). This is confirmed by the most recurring preferred SST ranges of the assemblage, indicating an average of 20 °C. The skeletal compositions of five well-preserved molluscan and coral specimens were analyzed for trace elements and stable isotopes for further mean SST estimations. From the comparison of the results of several equations available in literature, it appears that only some SST estimations are realistic, converging into similar values of, on average, 20.8 ± 0.9 °C. Considering all the used proxies, the MIS 5e SST difference compared to today falls in the range 1.2 - 2.0 °C for the Gulf of Taranto (GT) (being a more reasonable scenario) and 2.0 - 2.8 °C for the MP. This is not a firmly tropical-like SST setting as suggested by the sole Senegalese fauna, indicating at least 2.7 °C to 3.5 °C more than to today’s GT and MP, respectively. The approximations and assumptions made for obtaining SST values with any single proxy indicates the need of a multi-proxy approach to define the best SST estimation. Cemented samples of the same MIS 5e calcarenite were analyzed through optical and SEM analyses. Clasts mostly consist in medium to coarse sandy size skeletons and fragments of skeletons of a variegated biota together with mainly large mollusks shells that inhabited the sediment and a very minor amount of siliciclastic. Micritization variably affects most of the bioclasts while cements are constituted by a microcrystalline texture with various micro-morphologies and fabrics: not-isopachous aphanitic and filamentous rims, vacuolar peloidal meniscus, aphanitic micro-mounds, and aphanitic porosity-filling matrix. Cements are constituted by sub-micron sized anhedral to nanospheroidal crystals of low Mg calcite, mixed with a minor amount of irregular platy crystals of saponite. All fabrics of cement are rich of mineralized filamentous, tubular, and sub-spherical bacteria bodies that imply the presence of a microbial community forming an epilithic to endolithic biofilm that stabilized the incoherent sediment and mediated the early precipitation of cements. This process led to the synsedimentary hardening of some parts the mobile sandy substrate, allowing the settlement of sessile taxa - such as Spondylus gaederopus, oysters, serpulids and barnacles. The Last Interglacial (MIS 5) transgressive-regressive deposits of the Capo Colonna marine terrace provide a good fossil example of a Central Mediterranean infra/circa-littoral setting, characterized by both calcareous coralline algae-dominated low-relief bioherms and biostromes, analogous respectively to the present coralligenous and mäerl habitats. The skeletal primary framework of the bioherms consists of laminar to massive encrusting coralline red algae acting as main bioconstructors, with minor bryozoans, encrusting foraminifera and serpulids as secondary frame-builders. Whereas the autochthonous mäerl tabular beds are mainly composed of free-branched coralline red algae rudstones. A variable amount of sandy bioclastic sediment is laterally interbedded with the bioconstructions and tends to be entrapped in their cavities and pockets. All sedimentary sub-facies of the bioconstructions and associated sediment, are rich in autochthonous syn-sedimentary microbial-mediated micrite, forming aphanitic, peloidal, clotted peloidal, and filamentous fabrics. Microbial micrite can also trap and bind a variable amount of grains or being a secondary component of the sandy detrital sediment with micritic rims surrounding the clasts. All these early-lithified micrites show the typical nanostructure of the primary microbial-mediated carbonates, rather than a detrital mud particles accumulation, as they consist of nanospheres coalescing into subhedral microcrystals, replacing, and mineralizing both microbial cells (present with several morphological types) and extracellular substances. This in turn implies the widespread presence of benthic lithifying microbial biofilms that colonized both the cavities of the skeletal framework of the bioconstructions, and the intergranular space of the associated sediment. These microbial communities, thanks to the metabolic processes of the microorganisms that induced the carbonate precipitation, significantly contributed to the early cementation of all the studied deposits. In conclusion, the results of the work were used to infer possible effects of climate change in similar Mediterranean contexts. The diversity and temperature preferences of the MIS 5e molluscan assemblage from Taranto show no significant differences compared to modern analogous faunas from some Southern Italy sites reported in the literature, which could show a resilient response to an SST increase of +2 °C, already experienced during MIS 5e and expected in the next decades. This, however, does not consider the different causes, the current higher warming rates, as well as the human impact, which could affect faunas more than the temperature itself. Today, microbial induced micritic cementation processes, similar to those reported in the studied MIS 5 deposits, are observed in tropical/subtropical carbonate platform realms and have not been indicated for similar infra-circalittoral beds of the modern Mediterranean. Their occurrence in MIS 5 deposits confirms the warmer SST during MIS 5e and suggests a persistently warm SST at the end of MIS 5. However, as these cements have been associated with an estimated SST of just +2 °C compared to modern times, it is suggested that early cementation could become common in the Mediterranean mobile bottoms already in the near future as an effect of the global warming.