DATE2018-05-21 04:08:37
AUTHORSF Lozar (1,2), D Violanti (1), F Dela Pierre (1,2), M Natalicchio (1,2), R Gennari (1,2), E Nallino (1)
  1. Dep. of Earth Sciences - University of Torino, Torino, Italy
  2. ALP Alpine Laboratory of Paleomagnetism, Peveragno (CN), Italy
ABSTRACTThe Messinian salinity crisis (MSC) is a short lived (5.97-5.33 Ma) dramatic paleo-oceanographic event occurred in the Mediterranean area as result of the reduced hydrological connection with the Atlantic ocean. The beginning of the MSC, marked by the deposition of gypsum in the marginal basins, triggered an immediate response of the biota, and particularly of the calcareous microfossils. In detail, the first 21 kyr of the crisis were characterized by very peculiar calcareous nannofossil and foraminiferal assemblages preserved in the first Primary Lower Gypsum (PLG) cycle (Sphenolithus abies, Umbilicosphaera rotula, and Rhabdosphaera procera peaks, Globorotalia scitula and G. suterae peaks; Lozar et al. in press). Also, in the lower PLG cycles some foraminiferal taxa exhibit a sharp size decrease (e.g. Turborotalita quinqueloba), the significance of which is still unclear. In addition, no information regarding the size of the calcareous nannofossils is available so far. Samples from the Piedmont Basin (northern Italy) and straddling the MSC onset yielded the Sphenolithus abies and Helicosphara carteri specimens measured for this work. Total length and basal width for S. abies and major and minor axes for H. carteri specimens were measured. The measures sharply decrease at the onset of the MSC but show a slow recovery to average values in the following cycles, despite their group abundance decrease. As for other calcareous microfossils, among the foraminiferal assemblages from the same cycles, the >125 micron fraction is devoid of foraminifers, whereas the 45-63 micron and 63-125 micron fractions contain abundant Turborotalita quinqueloba and minor Bulimina spp., thus showing a sudden size decrease in the cycle where most foraminiferal taxa abruptly disappear. The disappearance of foraminifers and nannofossils at the beginning of the crisis was traditionally related to a fast salinity increase, in turn responsible for the deposition of the evaporites recorded in the marginal basins of the Mediterranean Sea. Conversely, the size decrease among calcareous nannofossils and foraminifers described in this study suggest that the surface water was characterized by high nutrient supply and possibly fresh water input. The eutrophication is also supported by the presence of Umbilicosphaera spp. blooms, a calcareous nannofossil genus considered to better flourish when nutrient supply is high, by the presence of diatom frustules, and by other independent geochemical data (e.g. high Ba concentration). The assemblage composition and the microfossil size decrease recorded in this study thus suggest that the MSC could have been triggered by high nutrient input to the basin, possibly derived by increased runoff from the continent.