Influence of low-frequency variability on high and low groundwater levels: example of aquifers in Paris Basin

Groundwater levels (GWL) very often fluctuate over a wide range of timescales (infra-annual, annual, multi-annual, decadal). In many instances, aquifers act as low-pass filters, dampening the high-frequency variability and amplifying low-frequency variations (from multi-annual to decadal timescales) which basically originate from large-scale climate variability. In the aim of better understanding and ultimately anticipating groundwater droughts and floods, it appears crucial to evaluate whether (and how much) the very high or very low GWLs are sensitive to such low-frequency variability (LFV), which was the main objective of the study presented here. As an example, we focused on exceedance and non-exceedance of the 80 % and 20 % GWL percentiles respectively, in the Paris Basin aquifers over the 1976–2019 period. GWL time series were extracted from a database consisting of relatively undisturbed GWL time series regarding anthropogenic influence (water abstraction by either continuous or periodic pumping) over Metropolitan France. Based on this dataset, our approach consisted of exploring the effect of GWL low-frequency components on threshold exceedance and non-exceedance by successively filtering out low-frequency components of GWL signals using maximum overlap discrete wavelet transform (MODWT). Multi-annual (~7-yr) and decadal (~17-yr) variabilities were found to be the predominant LFVs in GWL signals, in accordance with previous studies in the northern France area. Filtering out these components (either independently or jointly) to (i) examine the proportion of high level (HL) and low level (LL) occurrences generated by these variabilities, (ii) estimate the contribution of each of these variabilities in explaining the occurrence of major historical events associated to well-recognized societal impacts. A typology of GWL variations in Paris Basin aquifers was first determined by quantifying the variance distribution across timescales. Four GWL variation types could be found according to the predominance of annual, multi-annual or/and decadal variabilities in these signals: decadal dominant (type iD), multi-annual and decadal dominant (type iMD), annual dominant (type cA), annual and multi-annual dominant (type cAM). We observed a clear dependence of high and low GWL to LFV for aquifers exhibiting these four GWL variation types. In addition, the respective contribution of multi-annual and decadal variabilities in the threshold exceedance varied according to the event. In numerous aquifers, it also appeared that the sensitivity to LFV was higher for LL than HL. A similar analysis was conducted on the only available long-term GWL time series which covered a hundred years. This allowed us to highlight a potential influence of multidecadal variability on HL and LL too. This study underlined the key role of LFV in the occurrence of HL and LL. Since LFV originates from large-scale stochastic climate variability as demonstrated in many previous studies in the Paris Basin or nearby regions, our results point out that i) poor representation of LFV in General Circulation Models (GCM) outputs used afterwards for developing hydrological projections can result in strong uncertainty in the assessment of future groundwater extremes (GWE), ii) potential changes in the amplitude of LFV, be they natural or induced by global climate change, may lead to substantial changes in the occurrence and severity of GWE for the next decades. Finally, this study also stresses the fact that due to the stochastic nature of LFV, no deterministic prediction of future GWE for the mid- or long term horizons can be achieved even though LFV may look periodic.

Comparison of temperature and humidity during MIS 11 and MIS 5e interglacials with the Holocene using stable isotopes in tufa deposits from northern France

Many recent palaeoclimatic studies have focused on Pleistocene interglacials, especially Marine Isotopic Stages (MIS) 5e and 11, as analogs to our modern interglacial (MIS 1). In continental area, archives allowing comparison between interglacials remain scarce. Calcareous tufa deposits, as they are characteristic of these periods and can provide long, almost continuous, palaeoclimatic records through their isotopic content, appear highly suitable for such investigation. In this paper, δ18O and δ13C values from the three well-dated tufas of Saint-Germain-le-Vasson, Caours, and La Celle are combined to compare temperature and moisture conditions prevailing during MIS 1, 5e, and 11, in the Paris Basin. Both Pleistocene interglacials, and especially their optima, appear stronger than the Holocene: MIS 11 was wetter and warmer than both the Holocene and MIS 5e, which itself experienced wetter conditions than the Holocene. These observations are consistent with palaeontological data from the studied sites, especially malacological assemblages, which record, as at other European tufa sites, a relative depletion of molluscan diversity during the Holocene compared with the Pleistocene (MIS 5 and 11) interglacials.

On chronostratigraphic calibration using Bryozoa: suggestions from two Paleogene case studies

The bryozoan species present in each Paleocene and Eocene chronostratigraphic unit of the Atlantic and Gulf Coastal Plains in North America, and the Paris Basin in France are analysed. The comparison between species exclusive to each unit considered (EX), non-exclusive species, but appearing for the first (FA) or last time (LA), species existing before and after the unit considered (BA), and, finally, the total number of species present (T), shows the chronostratigraphic discriminating value of this fauna within the Paleogene. The same analysis was previously applied to bryozoan Paleocene and Eocene families and genera, according to the inormation. It is also interesting to note the significant renewal of bryozoan fauna in both basins during the Middle Eocene.

The illustrations of Brongniart and Cuvier illuminate paleontology in the early nineteenth century

ABSTRACT The concept of biostratigraphy was a significant step in the evolution of geoscience. Alexandre Brongniart (1770–1847) and Georges Cuvier (1769–1832) were key contributors to developing the subdiscipline as they worked to decode the stratigraphy of the Paris Basin in the first decades of the nineteenth century. Their illustrations of fossils, local geologic columns, and a regional geologic map played a decisive role in furthering an understanding of the value of paleontology in the service of illuminating Earth history.

New insights on the Late Paleocene-Early Eocene Dinoflagellate cyst zonation for the Paris and Dieppe Basins

The Anglo-Belgo-Paris Basin, historical cradle of the Paleogene stratigraphy since the XVIIIth century, is known by the presence of very specific so-called “Sparnacian” deposits (very diverse and laterally highly variable, predominantly lagoonal to terrestrial facies), which encompass the short stratigraphic interval of the Paleocene-Eocene Thermal Maximum (PETM). Due to the insufficient paleontological record, the “Sparnacian” succession of the Paris and Dieppe-Hampshire Basins still needs a robust chronostratigraphic correlation with other Paleogene records worldwide. In order to refine the stratigraphy of the Thanetian-Lower Ypresian succession in northern France a number of cores and outcrop sections have been investigated palynologically. As a result, an updated version of the Dinoflagellate cyst zonation for the Paris and Dieppe Basins is proposed and contains six new or revised Biozones for this stratigraphical interval: Alisocysta margarita, Apectodinium hyperacanthum, Apectodinium-extreme acme, Biconidinium longissimum-acme, Dracodinium astra, and Axiodinium lunare/Stenodinium meckelfeldense. Based on combined bio-, litho- and chemostratigraphic data, it appears that the dinocyst assemblages, corresponding to the PETM event interval (“Sparnacian” deposits, Soissonnais and upper Mortemer Formations with the most negative 13δCorg values between -27 and -32 ‰ PDB), are characterized by an extreme acme of Apectodinium spp. (70-98%) in both basins, sometimes alternating with an extreme acme of a few gonyaulacoid groups in the Dieppe Basin. Dinocyst assemblages from the PETM interval contain a significant number of atypical, longer specimens of Apectodinium parvum, which could represent an ecological onshore substitute of species Axiodinium augustum in the Paris and Dieppe-Hampshire Basins. The establishment of a new Biconidinium longissimum-acme Zone suggests the absence of an important stratigraphical hiatus previously inferred for the Paris Basin.