MIS 5e relative sea-level changes in the Mediterranean Sea: Contribution of isostatic disequilibrium
Introduction
Sea-level changes are primarily a reflection of water mass transfer between continents, where water is stored as ice during cold periods, and oceans, where meltwater is introduced during warmer periods. This process is known as glacial eustasy (Suess, 1906) and occurs in response to changes in atmosphere and ocean temperatures related to variations in atmospheric CO2 concentrations and Milankovitch-driven insolation (Stocker et al., 2013). A fundamental aspect for the study of past climate change over glacial-interglacial time scales is the collection, analysis and interpretation of Relative Sea Level (RSL) indicators, that are fossil landforms, deposits or biological assemblages with a known relationship with a paleo sea level (Hibbert et al., 2016, Rovere et al., 2016a). Once vertical movements associated with Glacial Isostatic Adjustment (GIA) (Lambeck and Purcell, 2005), tectonics (Simms et al., 2016) or other post-depositional processes (Rovere et al., 2016b) are taken into account, paleo RSL indicators can be used to constrain ice-mass variations in response to changes in atmospheric and ocean temperatures during past interglacials (Dutton et al., 2015). In turn, estimates of paleo global mean sea level can be used to constrain processes regulating ice melting in paleo ice-sheet models, which eventually may be used to gauge the sensitivity of present-day polar ice sheets to future scenarios of global warming (e.g. Deconto and Pollard, 2016).
The most studied past interglacial is the Marine Isotopic Stage 5e (MIS 5e, 117–127 ka), which is the last period of the Earth's history when climate was warmer than today. Generally, MIS 5e sea-level studies are oriented towards two main goals. The first is to understand how to account for processes causing departures from eustasy (e.g., GIA, tectonics) in order to produce reliable estimates of past global mean sea levels. The second consists on the calculation of tectonic movements starting from the elevation of RSL indicators and assumptions on eustatic sea-level changes. This aspect is particularly relevant for the understanding of the long-term vertical movement of coastal areas, which is in turn important for the planning of coastal infrastructures in active geodynamic settings and need to be accounted for to correct future climate-related rates of RSL change (Antonioli et al., 2017).
Despite the common consideration in isolation, the two aims outlined above are mutually dependent and they are both tied to GIA predictions. In fact, to achieve the second goal, one must calculate the climate-related and GIA-modulated RSL elevations, which are the result of the first goal. The latter, however, stems from a priori information on long-term tectonic motions, which is the result of this second goal. Studies on MIS 5e RSL change in the Mediterranean Sea have often either adopted standard ESL values to calculate vertical tectonic rates at active sites or neglected the GIA overprint in the calculation of the ESL signal (Ferranti et al., 2006).
In this paper we focus on MIS 5e sea-level variations in the Mediterranean Sea. We investigate the GIA contributions to the spatiotemporal variability of RSL change during MIS 5e within the basin using GIA numerical simulations that incorporate the solid Earth and gravitational response to three glacial-interglacial cycles prior to MIS 5e and that evolve towards present. We also evaluate the GIA-modulated contribution of four scenarios for GrIS and AIS melting during MIS 5e. We compare our RSL predictions to observations from 11 sites that have been previously hypothesized as tectonically stable based on the low elevation of the MIS 5e shoreline.
We use field data and numerical GIA predictions at these sites to address the following questions:
- 1.
How much of the observed MIS 5e RSL variability in the Mediterranean can be explained by GIA?
- 2.
How significant are the uncertainties in GIA, as well as GrIS and AIS melting scenarios when using MIS 5e shorelines to calculate tectonic vertical motions?
Section snippets
Paleo relative sea-level indicators
The Mediterranean Sea has been a central focus for studies on sea level changes for over two centuries (Benjamin et al., 2017). The basin is characterized by different tectonic regimes (Fig. 1, see Supplementary Text for a brief outline) and its relatively low tidal amplitudes and low wave energy helped to preserve RSL indicators almost ubiquitously (see Fig. 1 in Ferranti et al., 2006 for an overview and detailed reports in Anzidei et al., 2014, Ferranti et al., 2006, Galili et al., 2007, Mauz
RSL data
The difference between the measured elevation of the RSL indicators and the actual paleo sea level can be significant once the indicative meaning is properly accounted for (Fig. 2g, see Supplementary Materials for details on the calculation of the indicative meaning at each site and the Supplementary Text for a working example). The set of 11 revised RSL sites from supposedly stable areas in the Mediterranean shows a MIS 5e RSL highstand in the range of 2–10 m above present-day sea level (Fig. 2
Discussion
Our numerical simulations show that the Earth is not in isostatic equilibrium during the MIS 5e. The GIA processes that accompany and follow the melting of GrIS and AIS during the MIS 5e (scenarios 1–4) add up to the background GIA to increase the regional RSL variability. Each location, within the Mediterranean Sea and during MIS 5e, is characterized by a local RSL curve that can be significantly different from the eustatic.
The GIA-induced spatial variability of the RSL change is small if
Conclusions
- 1.
The observed range of MIS 5e RSL highstand from 11 tectonically stable sites in the Mediterranean is comprised between 2 and 10 m above present msl. The observed highstands are not necessarily coeval. Evidences of two MIS 5e RSL stands are found in Mallorca, northern Tyrrhenian coast of Italy, southeastern Sardinia and Tunisia.
- 2.
The GIA-induced RSL changes across the Mediterranean are characterized by a significant regional variability throughout the MIS 5e. The Earth is in isostatic imbalance
Acknowledgments
AR and TL's research is financially supported by the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative [ABPZuK-03/2014] the ZMT, the Leibniz Centre for Tropical Marine Research. BdB is funded by NWO Earth and Life Sciences (ALW), project 863.15.019. The authors acknowledge USSP Urbino Summer School in Paleoclimatology (Urbino, Italy), MOPP-MEDFLOOD - Modeling Paleo Processes (INQUA CMP projects 1203P and 1603P), PALSEA (PAGES/INQUA working group) and
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