In the last decade a renewed interest on the onset and developement of the Messinian Salinity Crisis (MSC) led to a general revision of several classic sections in the Mediterranean area (see review in Rouchy & Caruso, 2006). New stratigraphical, palaeontological, sedimentological, geochemical and petrographical data were collected in the Messinian sediments cropping out along the Tanaro river, near Pollenzo, in order to re-examine the onset and development of the MSC in this critical area of the Tertiary Piedmont Basin (BTP). This work is focused on the calcareous nannofossil (CN) assemblages of the lower part of the section (Marne di Sant'Agata Fossili Formation, SAF), while integrated biostratigraphical, palaeoecological, sedimentological, geochemical and petrographical analyses are in progress. The sampled section consists of cyclically alternating hemipelagic bioturbated marls and laminated clays, cropping out for a total thickness of 130 m, belonging to the SAF formation and overlain by the Vena del Gesso formation. A 90 m thick slumped interval occurs 25 m above the base of the section, probably related to a strong intramessinian tectonic phase. Above the slump, the marly and pelitic layers continue, but several thin arenaceous beds are intercalated. The CN assemblage is moderately to well preserved in the lower part of the section (below the slump) and contains Reticulofenestra sp., Discoaster sp., Umbilicosphaera sp., Helicoshaera carteri, Calcidiscus leptoporus, Scyphosphaera sp., Syracosphaera sp., together with rare Amaurolithus sp.. The occurrence of Messinian marker species, such as Amaurolithus primus/delicatus, A. primus, A. tricorniculatus and the absence of R. rotaria, identify the MNN11c subzone of Raffi et al. (2003), even if the marker species, Nicklithus amplificus, was not found in our material. The topmost 3 m of the SAF fm are barren of CN. The lithologic cycles below the slump, consisting of pelitic and marly couplets (about 3-4 m thick), contain different CN assemblages, cyclically repetitious. Therefore, we distinguished different cyclic assemblages, characterizing different lithologies. In general, the marly semicouplets (1-2 m thick) record a marked decrease of Discoaster sp. from bottom to top, whereas Umbilicosphaera sp. and small Reticulofenestra sp. increase from bottom to top. The top of the marly semicouplets are characterized by very high abundance of “small” Reticulofenestra (<5m), up to 90%, and the assemblage is nearly oligospecific. The pelitic semicouplets (about 1 m thick) record a “normal” assemblage; but “small” Reticulofenestra sp. decreases from bottom to top, whereas Discoaster sp. Helicosphaera carteri, Calcidiscus leptoporus, Pontosphaera sp. increase. Moreover, abnormally high frequences of Syracosphaera pulchra and Scyphosphaera sp. occur. The increased frequences of Discoaster sp. in the pelitc semicouplets suggest warm oligotrophic waters (Flores et al. 2005); here the co-occurrence of high abundances of H. carteri and Pontosphaera sp. suggests a fluctuation in salinity in the upper water column, maybe in relation to water stratification. The very high abundance of “small” Reticulofenestra (<5m) corresponds to period of eutrophication of the upper water column (as testified elsewhere by the common co-occurrence of Diatoms; Flores et al., 2005); here the increase in nutrients starts at the base of the marly semicouplets and it is completed before the top of the bed, where the decline in nutrients availability is marked by the recovery of typically meso to oligotrophic taxa, such as Discoaster sp. As already proposed for coheval sediments of the western Mediterranean, these cyclic assemblages may reflect climatic cycles consisting of alternation of warmer/wetter and colder/dryer periods, probably related to precessional cyles (Flores et al.; 2005). Above the slump, CN abundance decreases, but the composition of the assemblages reflects the progressive deterioration of the marine environment prior to the onset of the MSC. In general, the upper 12.5 metres of the SAF contain several strongly oligotypic CN assemblages, with changing composition at different stratigraphic position; “normal” messinian CN assemblages also occur. In particular, 12 and 3 metres below the first selenitic gypsum bed, two sharp peaks of “small” Reticulofenestra (R. minuta and R. haqi combined, up to 90% of the total assemblage) are recorded and support the hypothesis of eutrophic surface waters and stressed environment. An increase in abundance of P. japonica, a species recorded in the clays interbedded to the selenitic gypsum in Cyprus (Wade & Bown, 2006), suggests that episodes of stressed and highly saline waters started 10.5 m below the first selenitic gypsum bed. Abundance peak of Sphenolithus abies (up to 60% of the total assemblage) and high relative abundance of H. carteri (up to 20% of the total assemblage) are also recorded 7.5 metres below the first selenitic gypsum; the concomitant abundance of both species in our material suggests a shallow water environment (supported by H. carteri), rich in nutrient and increasing salinity (S. abies being interpreted as a schizohaline species and H. carteri as tolerating high salinity waters; Flores et al., 2005; Kowenhoven et al., 2006). The occurrence of discrete oligotypic CN assemblages in a succession of “normal” messinian CN assemblages documents surface waters relatively rich in nutrient and progressively increasing in salinity towards the top of the SAF. The presence of shallow water markers supports the idea of a decreasing depth of the basin during the pre-evaporitic phase and a progressively restricted environment. The occurrence of “normal” messinian CN assemblages also testify that this trend of increasing salinity and restricted environment was interrupted by several episodes of re-establishment of salt concentration close to the global ocean values during the pre-evaporitic phase. The occurrence of euryhaline fishes (Aphanius sp.) just 1.5 m below the selenitic gypsum beds also testifies that the increase in salinity occurred during the pre-evaporitic phase, thus preceding the precipitation of gypsum and the onset of the Messinian Salinity Crisis.

Calcareous nanofossils herald the Messinian Salinity Crisis: the Pollenzo section (NW Italy)

LOZAR, Francesca;CAVAGNA, Simona;CLARI, Pierangelo;DELA PIERRE, Francesco;MARTINETTO, Edoardo;VIOLANTI, Donata
2008

Abstract

In the last decade a renewed interest on the onset and developement of the Messinian Salinity Crisis (MSC) led to a general revision of several classic sections in the Mediterranean area (see review in Rouchy & Caruso, 2006). New stratigraphical, palaeontological, sedimentological, geochemical and petrographical data were collected in the Messinian sediments cropping out along the Tanaro river, near Pollenzo, in order to re-examine the onset and development of the MSC in this critical area of the Tertiary Piedmont Basin (BTP). This work is focused on the calcareous nannofossil (CN) assemblages of the lower part of the section (Marne di Sant'Agata Fossili Formation, SAF), while integrated biostratigraphical, palaeoecological, sedimentological, geochemical and petrographical analyses are in progress. The sampled section consists of cyclically alternating hemipelagic bioturbated marls and laminated clays, cropping out for a total thickness of 130 m, belonging to the SAF formation and overlain by the Vena del Gesso formation. A 90 m thick slumped interval occurs 25 m above the base of the section, probably related to a strong intramessinian tectonic phase. Above the slump, the marly and pelitic layers continue, but several thin arenaceous beds are intercalated. The CN assemblage is moderately to well preserved in the lower part of the section (below the slump) and contains Reticulofenestra sp., Discoaster sp., Umbilicosphaera sp., Helicoshaera carteri, Calcidiscus leptoporus, Scyphosphaera sp., Syracosphaera sp., together with rare Amaurolithus sp.. The occurrence of Messinian marker species, such as Amaurolithus primus/delicatus, A. primus, A. tricorniculatus and the absence of R. rotaria, identify the MNN11c subzone of Raffi et al. (2003), even if the marker species, Nicklithus amplificus, was not found in our material. The topmost 3 m of the SAF fm are barren of CN. The lithologic cycles below the slump, consisting of pelitic and marly couplets (about 3-4 m thick), contain different CN assemblages, cyclically repetitious. Therefore, we distinguished different cyclic assemblages, characterizing different lithologies. In general, the marly semicouplets (1-2 m thick) record a marked decrease of Discoaster sp. from bottom to top, whereas Umbilicosphaera sp. and small Reticulofenestra sp. increase from bottom to top. The top of the marly semicouplets are characterized by very high abundance of “small” Reticulofenestra (<5m), up to 90%, and the assemblage is nearly oligospecific. The pelitic semicouplets (about 1 m thick) record a “normal” assemblage; but “small” Reticulofenestra sp. decreases from bottom to top, whereas Discoaster sp. Helicosphaera carteri, Calcidiscus leptoporus, Pontosphaera sp. increase. Moreover, abnormally high frequences of Syracosphaera pulchra and Scyphosphaera sp. occur. The increased frequences of Discoaster sp. in the pelitc semicouplets suggest warm oligotrophic waters (Flores et al. 2005); here the co-occurrence of high abundances of H. carteri and Pontosphaera sp. suggests a fluctuation in salinity in the upper water column, maybe in relation to water stratification. The very high abundance of “small” Reticulofenestra (<5m) corresponds to period of eutrophication of the upper water column (as testified elsewhere by the common co-occurrence of Diatoms; Flores et al., 2005); here the increase in nutrients starts at the base of the marly semicouplets and it is completed before the top of the bed, where the decline in nutrients availability is marked by the recovery of typically meso to oligotrophic taxa, such as Discoaster sp. As already proposed for coheval sediments of the western Mediterranean, these cyclic assemblages may reflect climatic cycles consisting of alternation of warmer/wetter and colder/dryer periods, probably related to precessional cyles (Flores et al.; 2005). Above the slump, CN abundance decreases, but the composition of the assemblages reflects the progressive deterioration of the marine environment prior to the onset of the MSC. In general, the upper 12.5 metres of the SAF contain several strongly oligotypic CN assemblages, with changing composition at different stratigraphic position; “normal” messinian CN assemblages also occur. In particular, 12 and 3 metres below the first selenitic gypsum bed, two sharp peaks of “small” Reticulofenestra (R. minuta and R. haqi combined, up to 90% of the total assemblage) are recorded and support the hypothesis of eutrophic surface waters and stressed environment. An increase in abundance of P. japonica, a species recorded in the clays interbedded to the selenitic gypsum in Cyprus (Wade & Bown, 2006), suggests that episodes of stressed and highly saline waters started 10.5 m below the first selenitic gypsum bed. Abundance peak of Sphenolithus abies (up to 60% of the total assemblage) and high relative abundance of H. carteri (up to 20% of the total assemblage) are also recorded 7.5 metres below the first selenitic gypsum; the concomitant abundance of both species in our material suggests a shallow water environment (supported by H. carteri), rich in nutrient and increasing salinity (S. abies being interpreted as a schizohaline species and H. carteri as tolerating high salinity waters; Flores et al., 2005; Kowenhoven et al., 2006). The occurrence of discrete oligotypic CN assemblages in a succession of “normal” messinian CN assemblages documents surface waters relatively rich in nutrient and progressively increasing in salinity towards the top of the SAF. The presence of shallow water markers supports the idea of a decreasing depth of the basin during the pre-evaporitic phase and a progressively restricted environment. The occurrence of “normal” messinian CN assemblages also testify that this trend of increasing salinity and restricted environment was interrupted by several episodes of re-establishment of salt concentration close to the global ocean values during the pre-evaporitic phase. The occurrence of euryhaline fishes (Aphanius sp.) just 1.5 m below the selenitic gypsum beds also testifies that the increase in salinity occurred during the pre-evaporitic phase, thus preceding the precipitation of gypsum and the onset of the Messinian Salinity Crisis.
Alba e Tramonto della Crisi Messiniana-Alba, 10-11 ottobre 2008
Alba (CN)
10-11 ottobre 2008
Alba e Tramonto della Crisi Messiniana - Riassunti
Università degli Studi di Torino
43
44
Calcareous Nannofossils; Messinian; Tertiary Piedmont Basin; Alba
Lozar F.; Cavagna S.; Clari P.; Dela Pierre F.; Irace A.; Martinetto E.; Trenkwalder S.; Violanti D.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2318/68460
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