Environmental changes across the onset of the Messinian salinity crisis: insights from the Piedmont Basin (NW Italy).

<p>Since the discovery of the late Miocene (Messinian) Mediterranean Salt Giant more than 50 years ago, the environmental conditions that caused its formation have been debated. Such reconstruction suffers from the absence of modern analogues, the lack or scarcity of fossils (calcareous plankton and benthos, but also pollens), and the inaccessibility of the evaporites buried beneath the present-day Mediterranean seafloor. We investigate the palaeoenvironmental changes, which drove the formation of the Mediterranean Salt Giant at the onset of the Messinian salinity crisis (MSC) through high resolution sedimentological, petrographical, and geochemical (lipid biomarkers, major and trace elements) analyses of sedimentary successions of the Piedmont Basin (NW Italy). Shale/marl couplets deposited in intermediate to deep-water settings (200 – 1000 m) are targeted, representing the lateral equivalent of primary sulphate evaporites from shallow-water settings that accumulated between 5.97 and 5.60 Ma. We suggest that climate and hydrological changes affected the northern Mediterranean in the earliest stage of the MSC event, leading to an intensification of water column stratification. An upper water layer of marine water influenced by freshwater input was separated through a pycnocline from more evaporated, denser and oxygen-depleted bottom waters. The water column structure and pycnocline oscillation exerted pivotal control over the sedimentary products pertaining to the first stage of the MSC.</p>

A new reverse subchron (C33n.1r) in the Campanian: astronomical duration estimate and geomagnetic/chronostratigraphic implications

<p>The Cretaceous Normal Polarity Superchron (CNPS, chron C34n, Aptian–Santonian, ~83–118 Ma in CK95 GPTS) is followed in the Campanian by two relatively long chrons (chron C33r, 3.925 My duration and then the normal chron C33n, 5.456 My duration) straddling most of the Campanian stage. The analysis of the geomagnetic reversal history has classically determined two nearly linear segments for the late Cretaceous–Cenozoic interval divided at chron C12r. The length of chrons in the younger interval has no systematic trend and henceforth is considered stationary for statistical analysis with a mean chron length of 0.248–0.219 My while the older segment has 0.749 My mean chron length. The stationarity for this latter interval is attained, however, when the two long polarity chrons C33r and C33n adjacent to the CNPS are omitted. Studies in the weakly magnetized southern England chalk succession and marine Bearpaw Shale in the Canadian Rockies from Alberta have argued about the presence of a number of reversals within C33r and C33n (and C34n). However, all these remain ambiguously established and not incorporated in the standard GPTS despite their significance for theories of geodynamo behavior and potentiality for high-resolution stratigraphic correlations that could notably impact, for instance, the chronostratigraphy of dinosaur-bearing terrestrial Upper Cretaceous of the Western Interior of North America. In any case, no polarity subchrons within C33r or C33n have been reported in any deep-sea record or in the landmark pelagic “scaglia” sections from the Gubbio area in the central Italian Apennines, for which a good integrated biostratigraphy and a thorough paleomagnetic record exists.</p><p>Here, we report on a new reverse subchron in the lower part of C33n, informally named the Postalm Fall Subchron (PFS), retrieved in the Postalm section (Gosau Group, Northern Calcareous Alps of Austria). The Postalm section shows a deepening trend from upper Santonian conglomerates and grey shelf marls to pelagic bathyal red marly limestones of Campanian age. The section has previously been studied in the frame of an integrated multi-proxy stratigraphic study that includes high-resolution calcareous plankton biostratigraphy, magnetostratigraphy, stable isotopes, strontium stratigraphy and Fe content. Robust paleomagnetic data has pinpointed the top of C34n that defines the Santonian-Campanian boundary together with key biostratigraphic markers in the lower part of the red unit. The integrated study extents upwards for about 170 m up to calcareous nannofossil zone UC16 in chron C32 in the late Campanian. The cyclic nature of the pelagic sequence has been studied by means of spectral analysis on the limestone/marl couplet thickness data and geochemical proxies that allows identifying the short and long eccentricity cycles and to establish a cyclostratigraphic framework. From 33 new tightly collected samples in this study, 23 display unambiguous reverse polarity and conform the PFS subchron that straddles 3–4 precession cycles (~70 ky duration) within the UC15b calcareous nannofossil biozone. The average sediment accumulation rate at Postalm (~2 cm/ky compared to ~0.6–1 cm/ky at Gubbio) and the high-quality paleomagnetic signal have favored this discovery. The absolute age calibration and related geomagnetic and chronostratigraphic implications would be discussed.</p>

A global mean sea-surface temperature dataset for the Last Interglacial (129–116 kyr) and contribution of thermal expansion to sea-level change

A valuable analogue for assessing Earth’s sensitivity to warming is the Last Interglacial (LIG; 129–116 kyr), when global temperatures (0−+2 °C) and mean sea level (+6–11 m) were higher than today. The direct contribution of warmer conditions to global sea level (thermosteric) are uncertain. We report here a global network of LIG sea surface temperatures (SST) obtained from various published temperature proxies (e.g. faunal/floral assemblages, Mg/Ca ratios of calcareous plankton, alkenone UK’37). Each reconstruction is averaged across the LIG (anomalies relative to 1981–2010), corrected for ocean drift and with varying seasonality (189 annual, 99 December-February, and 92 June–August records). We summarise the current limitations of SST reconstructions for the LIG and the spatial temperature features of a naturally warmer world. Because of local δ18O seawater changes, uncertainty in the age models of marine cores, and differences in sampling resolution and/or sedimentation rates, the reconstructions are restricted to mean conditions. To avoid bias towards individual LIG SSTs based on only a single (and potentially erroneous) measurement or a single interpolated data point, here we average across the entire LIG. To investigate the sensitivity of the reconstruction to high temperatures, we also report maximum values during the first 5 ka of the LIG (129–124 kyr). The global dataset provides a remarkably coherent pattern of higher SST increases at polar latitudes than in the tropics, with comparable estimates between different SST proxies. We report mean global annual SST anomalies of 0.2 ± 0.1 °C and a maximum of 0.9 ± 0.2 °C respectively. Using the reconstructed SSTs suggests a mean thermosteric sea level rise of 0.01 ± 0.1 m and a maximum of 0.13 ± 0.1 m respectively. The data provide an important natural baseline for a warmer world, constraining the contributions of Greenland and Antarctic ice sheets to global sea level during a geographically widespread expression of high sea level, and can be used to test the next inter-comparison of models for projecting future climate change. The dataset described in this paper, including summary temperature and thermosteric sea-level reconstructions, are available at https://doi.org/10.1594/PANGAEA.904381 (Turney et al., 2019).