Understanding sea-level changes in the Mediterranean over the Common Era

Despite coastal zones representing only 10% of the Earth’s total land area, the rich resources at the land-ocean interface result in a concentration of population at, or near, to coasts, where many live below current and projected annual flood levels (e.g., Kulp and Strauss, 2019). This can make these communities extremely vulnerable to future sea-level rise. For the Mediterranean specifically, more than one third of the population lives in coastal areas and on deltas, with a doubling in population in the region from 1960-2010 (UNEP/MAP, 2016). Therefore, we need to better constrain the long-term patterns that controlled relative sea-level (RSL) changes in this area over centennial and millennial timescales in response to climatic changes (e.g., the Roman Warm Period, the Medieval Climate Anomaly, and the Little Ice Age, Vacchi et al., 2021). By assessing sea-level changes during the last century in the context of a millennial-scale perspective, we can evaluate the novelty of current trends. These results aid in predicting the magnitude and rates of future sea-level change and can inform coastal management.

To date, many RSL reconstructions in the Mediterranean during the Common Era (last 2000 years) have been derived from archaeological remains of maritime structures (e.g., Lambeck et al., 2004). However, these records are not ideal as their relationship with the contemporary sea level is often unclear. Furthermore, archaeological sea-level data lack the resolution and temporal coverage needed to investigate climate-driven changes in RSL which are typically no more than 20 cm and occur over multiple centuries (e.g., Grinsted et al., 2009). An alternative approach is to utilize sequences of salt-marsh sediments, and the microfossils preserved within them, to constrain the position of past RSL tightly and continuously (e.g., Walker et al., 2021). These sediment archives have been used extensively globally, with a particular focus for high-resolution reconstructions on the Atlantic coast of the USA (Brain et al., 2017). Recent work on the Adriatic Coast (central Mediterranean; Shaw et al., 2016; 2018) suggests that these records are applicable across the Mediterranean. The use of biomarkers provides a new technique that has the potential to assist with identified limitations in using microfossils (e.g., van Soelen et al., 2010). Finally, the microtidal regime and rich salt-marsh sediment archives of the Mediterranean make it an ideal region to explore climate-related sea-level variability.

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Image Captions

Aerial image of a Mediterranean salt marsh,A typical vegetation sequence in a Mediterranean Salt marsh


The project will be broken into two complementary components focusing on 1) understanding of the modern environment and 2) applying that understanding to fossil cores to reconstruct RSL changes. To address the first component, there will be a detailed investigation of the modern salt marsh and tidal-flat environments of the Mediterranean coast. The objective of these analyses is to document the distribution of microfossils (e.g., foraminifera), as well as sedimentary (e.g., grain size) and geochemical (e.g., carbon isotopes, biomarkers) indicators in relation to tidal elevation. These analyses will provide modern analogues that are applied to the second component. Samples will also be collected to characterize the geotechnical properties of the sediments to enable the removal of the effects of sediment compaction (e.g., Brain et al., 2017). Development of a Bayesian Transfer Function (BTF) will establish the functional relationship between microfossil assemblages and/or biomarker data and tidal elevation.

Secondly, the student will undertake multiple field seasons to conduct detailed stratigraphic investigations at tens of salt marshes across the Mediterranean region. This will identify the most promising sites that contain the longest record of salt-marsh sediment that can be used to reconstruct relative sea level over the past 2000-3000 years, and across multiple climatic oscillations. The chosen sediment cores will undergo the same analyses as described above for the modern samples. The BTF will be applied to the cores to reconstruct the relationship between each sample and former tidal levels. The microfossil assemblages will supply the primary information for the reconstruction; however, the model will also incorporate secondary information in the form of the litho- and chemo-stratigraphic data (collected as part of the first component).

The final stage of analyses will involve the development of high-resolution age models for the chosen cores, aiming to achieve decadal-to-centennial resolution. The primary method for this will be radiocarbon, but this will be supplemented (where possible) using radiometric isotopes (e.g., 137Cs, 210Pb) and known pollution horizons (e.g., changes in lead concentration). This is an approach that is particularly useful during time periods where radiocarbon is less effective due to plateaus. Once this is complete, the two fossil components (BTF-derived elevation estimates and chronological model) will be combined to produce RSL reconstructions for multiple sites in the Mediterranean.

Project Timeline

Year 1

Assessment of potential sites using aerial imagery to determine the spatial distribution of salt marshes within the Mediterranean. Selection of initial study sites for modern sampling and initial sediment cores during first fieldwork season. Characterization of the sedimentological, microfossil, geotechnical, and geochemical characteristics of the modern sediments collected, including secondment to Glasgow.

Year 2

Further fieldwork to collect core samples from a second site and to fill identified gaps within the modern samples from first field season. Characterization of the fossil core sediments including sedimentological, microfossil, and geochemical characteristics, as well as pollution markers. This will partly be conducted during an additional secondment to Glasgow.

Year 3

Finalize laboratory work and develop Bayesian transfer functions and appropriate priors during secondment to Maynooth University. Presentation of results at a national meeting.

Year 3.5

Completion of thesis write-up and preparation of manuscripts.

& Skills

This project is a Collaborative Studentship between Durham University and the University of Glasgow. The project will also benefit from external supervisory support from Dr. Matteo Vacchi (University of Pisa, https://people.unipi.it/matteo_vacchi/) and Dr. Niamh Cahill of Maynooth University (https://www.maynoothuniversity.ie/people/niamh-cahill).

During the project, the successful candidate will obtain training to develop necessary key skills, including those required for field investigations of coastal stratigraphy (Durham); organic geochemistry/biomarkers and analyses (Glasgow and Durham); microscopy and interpretation of salt-marsh microfossil assemblages (Durham); modelling sediment compaction (Durham); radiometric and pollution-marker dating methods (Durham); and development of Bayesian transfer functions (Maynooth University).

The project will involve two field seasons in the Mediterranean to obtain data and samples for analyses. The student will also present their results at national and international conferences to develop presentation and communication skills and disseminate results.

References & further reading

Brain, M.J. et al. 2017. Exploring mechanisms of compaction in salt-marsh sediments using Common Era relative sea-level reconstructions. Quaternary Science Reviews 156, 96-111.

Evelpidou, N. et al. 2012. Late Holocene Sea Level Reconstructions Based on Observations of Roman Fish Tanks, T yrrhenian Coast of Italy. Geoarchaeology, 27(3), 259-277.

Grinsted, A. et al. 2010. Reconstructing sea level from paleo and projected temperatures 200 to 2100 AD. Climate Dynamics 34, 461-472.

Kulp, S.A., and Strauss, B.H., 2019. New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding. Nature Communications 10, 4844.

Lambeck, K. et al. 2004. Sea level in Roman time in the Central Mediterranean and implications for recent change. Earth and Planetary Science Letters, 224(3-4), 563-575.

Lambeck, K. et al. 2018. Tyrrhenian sea level at 2000 BP: evidence from Roman age fish tanks and their geological calibration. Rendiconti Lincei. Scienze Fisiche e Naturali, 29(1), 69-80.

Shaw, T.A. et al. 2016. Contemporary salt-marsh foraminiferal distribution from the Adriatic coast of Croatia and its potential for sea-level studies. Journal of Foraminiferal Research 43(3), 314-332.

Shaw, T.A. et al. 2018. Tectonic influences on late Holocene relative sea levels from the central-eastern Adriatic coast of Croatia. Quaternary Science Reviews 200, 262-275.

UNEP/MAP (2016). Mediterranean strategy for sustainable development 2016-2015. Plan Bleu, Regional Activity Centre.

Vacchi, M., Joyse, K. M., Kopp, R. E., Marriner, N., Kaniewski, D., & Rovere, A. (2021). Climate pacing of millennial sea-level change variability in the central and western Mediterranean. Nature communications, 12(1), 1-9.

van Soelen, E.E. et al. 2010. Late Holocene sea-level rise in Tampa Bay: Integrated reconstruction using biomarkers, pollen, organic-walled dinoflagellate cysts, and diatoms. Estuarine, Coastal and Shelf Science 86, 216-224.

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