Calcareous microfossils as tracers of major palaeoceanographic
perturbations: the case of the onset of the Messinian salinity crisis
in the Tertiary Piedmont Basin (NW Italy)
Lozar F., Violanti D., Bernardi E., Dela Pierre F., Natalicchio M.
Dipartimento di Scienze della Terra, Università di Torino, via Valperga Caluso 35, 10125 TORINO, Italy
Summary
Different kind of sedimentary rocks (primary gypsum, laminated shale) record the onset of the Messinian Salinity Crisis in the Mediterranean successions; this event occurred synchronously at 5.96 Ma. The disappearance of calcareous microfossils is one of the proxies used to identify this timeline. In the Tertiary Piedmont Basin calcareous microfossils disappear before, at or after the onset but, when present, they show a very distinctive stratigraphic sequence of bioevents. This allows placing the onset of the MSC in distal, intermediate, and marginal settings independently from gypsum deposition.
Keywords: Calcareous microfossils, Tertiary Piedmont Basin, Messinian salinity crisis, climate and environmental change.
Calcareous microfossils, particularly foraminifers and calcareous nannofossils, have been a major tool for stratigraphic correlation since the XX century. They have been extensively used for biostratigraphic purposes in marine sedimentary successions worldwide, especially those of Late Mesozoic and Cenozoic age. More recently, they became very useful for tracking climate oscillations; in particular, calcareous microfossils allow recognition of orbitally-driven sedimentary cycles (Gibbs et al. 2004; Iaccarino et al. 2004), which in turn could provide a very accurate chronostratigraphic framework for marine successions. Moreover, their use as tracers of major palaeoceanographic events is becoming very widespread (Erba 2004; Stoll et al. 2007). As such, they are a very useful tool to unravel both small scale climate perturbations and major paleoceanographic events that usually are not recorded by unique lithological characteristics.
In this work, we focus on the Messinian Salinity Crisis (MSC; Hsu et al. 1973), a major palaeoceanographic event that occurred in the Mediterranean region during the late Neogene, involving both the marine and continental realms (e.g. Dela Pierre et al. 2011; Natalicchio et al. 2012 and references therein). In the last 40 years much effort has been devoted to unravel the sedimentary response and timing of this dramatic event in different depositional environments across the entire Mediterranean basin. Recently, a widely accepted two phases-three steps model has been proposed, according to which the onset of the MSC is synchronous at the basin scale, occurring at 5.96 Ma (Krijgsmann et al. 1999) and leading to the deposition
of primary evaporites on the basin margins and of laminated shales in the deep sea (CIESM 2008, with references therein). As a corollary, since gypsum deposition occurs in a very high salinity water column, inhospitable to calcareous plankton, the disappearance of calcareous microfossils has often been considered as a reliable proxy for the MSC onset (Sierro et al. 2001, Manzi et al. 2007 and references therein), despite the occurrence of gypsum is delayed in deeper depositional environment, suggesting that salinity increase was not synchronous in marginal and deep water environments.
In the last few years our research group provided new integrated data on the stratigraphic architecture of the Messinian record in the Tertiary Piedmont Basin (TPB; Dela Pierre et al., 2011, 2012; Lozar et al., 2010; Violanti et al. 2011); these data allowed to reconstruct an updated stratigraphic model for the MSC record in the TPB, where the MSC onset is preserved in distal (Govone section), intermediate (Pollenzo section), and marginal settings (Moncalvo section; Dela Pierre et al., 2011). In this context, calcareous microfossils have played a very important role in placing and correlate the onset of this event in different depositional environments across the basin.
Our data show that in the TPB the disappearance of calcareous nannofossils, planktonic and benthic foraminifers is not synchronous at the basin scale, occurring before (Govone), at (Moncalvo) or after (Pollenzo) the MSC onset timeline at 5.96 Ma; this discrepancy has been recognised by means of physical-stratigraphic and paleomagnetic correlation in marginal and deeper water depositional environments (Lozar et al. 2009, 2010; Dela Pierre et al., 2011; Violanti et al., 2011).
In Messinian sediments deposited just before the MSC onset (MNN 11 b/c CN Zone; foraminifer Non Distinctive Zone or MMi13c Zone), calcareous plankton fossil assemblages show a cyclic stacking pattern recording precessional fluctuations of warm/oligotrophic and temperate/mesotrophic conditions in both marginal and deeper water environments of the TPB. The cyclicity is best recorded by the opposite abundance peaks of Discoaster spp. versus C. pelagicus among calcareous nannofossils and O. universa versus N. acostaensis among foraminifers.
At the base of the lithologic cycle containing the MSC onset, calcareous microfossil assemblages show a very clear and regular sequence of bioevents. Among calcareous nannofossils, high abundance peaks of stress-tolerant taxa (S. abies, H. carteri, R. procera, U.
rotula) are recorded in the same stratigraphic order in different sections, predating the onset
of MSC by less than 10 kyr. As for foraminifers, the Last Recovery of planktonic and benthic foraminifer taxa in the >250 m size fraction occurs in the underlying lithologic cycle and
testifies a sharp size decrease of planktonic and benthic tests. In fact, foraminifer abundance and diversity decrease above the MSC onset, and only small (250-125 m) and very small size (125-64 m) tests are recorded, suggesting unfavourable environmental conditions. Moreover, the dominance of T. quinqueloba among planktonic foraminifers as well as
Bolivina spp., B. echinata, and C. crassa among benthic foraminifers testifies an increasingly
stressed palaeoenvironment, both in the water column and at the sea bottom.
In intermediate depositional environments (Pollenzo section) the Last Recovery of planktonic foraminifers in the 250-125 m size fraction occurs at the MSC onset, whereas benthic foraminifers survive for about 21 kyr; smaller planktonic foraminifer tests disappear 42 kyr after the MSC onset, shortly followed by calcareous nannofossils disappearance.
Planktonic foraminifers appear to have been more sensitive to unsuitable water column conditions than calcareous nannofossils, since the latter survived longer and healthier after the beginning of the MSC (Pollenzo section; Lozar et al., 2010; Violanti et al., 2011).
According to biostratigraphic correlation based on the last peak of G. multiloba, occurring at the Mediterranean scale four lithologic cycles below the MSC onset (i.e. ca. 84 kyr; Sierro et al., 2001, Kouwenhoven et al., 2006), in the Govone section calcareous microfossilis disappear some 60 kyr before the 5.96 timeline (Bernardi, 2013). Several hypotheses could be proposed to explain calcareous microfossil absence; it could be related to localized unfavourable environmental conditions, or to trophic competition with other microorganisms, possibly because the proximity to the shore and/or to a river mouth could have induced enhanced nutrient supply triggering siliceous plankton blooms, or to high acidity in the water column and at the bottom, preventing calcareous tests preservation.
Moreover, carbonate-rich layers resulting from lithification of chemotrophic mats dominated by sulphide oxidizing bacteria, are recognized from the MSC onset upwards, supporting a severe environmental change, as suggested by calcareous microfossil assemblages.
According to the TPB record, the biological response to the MSC onset involved at least both eucaryotes and prokaryotes; some bioevents, recorded in the same stratigraphic position across the basin, are thus synchronous at the TPB scale and mark the MSC onset. In particular calcareous microfossils, when recorded in the stratigraphic successions, are very helpful to identify the stratigraphic position of this palaeocenographic event.
The sequence of events, reliable at the TPB scale, its time frame and vertical stacking pattern need to be checked both in the Western and Eastern Mediterranean basins, in order to assess their reliability at the regional scale and their usefulness as a correlation tool for the onset of
the MSC in the entire Mediterranean. This will allow placing the onset of the MSC in distal, intermediate, and marginal settings independently from gypsum deposition, as it is now possible in the TPB.
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