Salt-marsh based records of Common Era relative sea-level changes from the Mediterranean

Overview

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). 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 understand the long-term patterns that control relative sea-level (RSL) changes in this area over centennial and millennial timescales including the response to climatic changes and predict future change that can inform coastal management.

To date, the majority of RSL reconstructions in the Mediterranean during the Common Era (last 2000 years) were largely based on archaeological records (e.g., Lambeck et al., 2004). However, these records can be problematic as they require an interpretation of the relationship between the archaeological remains and former sea level, which is often controversial and varies between groups (Evelpidou et al., 2011; Lambeck et al., 2018). Further, samples are not closely spaced in time and that limits our ability to reconstruct the evolution of RSL over the decadal to centennial timescales that are relevant to climate-related RSL variations (e.g., Grinsted et al., 2009). An alternative approach is to utilize sequences of salt-marsh deposits that tightly constrain the position of past relative sea level (REFS). These sediment archives have been used extensively globally, with a particular focus for high-resolution reconstructions on the Atlantic coast of the USA (Kemp et al., 2009; Brain et al., 2017). Recent work in the Mediterranean (Shaw et al., 2016, 2018) suggests that these records can also be used in the Mediterranean. The microtidal regime and rich salt-marsh sediment archives make it an ideal region to explore climate-related sea-level variability.

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Image Captions

Marsh1.JPG Figure 1. Aerial view of a typical salt-marsh environment in the study region. Photo credit: M. Vacchi

Marsh2.JPG Figure 2. A detailed view of the salt-marsh environments to be targeted by this project demonstrating dominance by Salicornia and Juncus species. Photo credit: M. Vacchi

Methodology

The project will be broken into two complementary components that focus on 1) the 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 contemporary salt-marsh and tidal-flat environments of the Mediterranean coast. The objective of these analyses is to document the modern distribution of microfossils (e.g., foraminifera) as well as sedimentary (e.g., grain size) and geochemical (e.g., carbon isotopes) indicators in relation to tidal elevation. These analyses will be used as modern analogues that are then applied to the second component of the study. The student will additionally collect samples to characterize the geotechnical properties of the sediments to enable the removal of compaction in fossil cores (e.g., Brain et al., 2016). The student will apply recently developed techniques in quantitative reconstructions of relative sea level to develop a Bayesian Transfer Function (BTF). The BTF will establish the functional relationship between the information in the microfossil assemblages and tidal elevation.

Secondly, the student will undertake detailed stratigraphic investigations at tens of salt marshes within the Mediterranean region through the collection of sediment cores. The objective of this is to identify the most promising sites that contain the longest record of salt-marsh peat that can be used to reconstruct relative sea level over the past 2000 years and across multiple climate oscillations (e.g., the little ice age and medieval climate anomaly). The chosen sediment cores will undergo the same analyses in terms of bio-, litho-, and chemo-stratigraphy as described above for the contemporary samples. The developed BTF will be applied to the core 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 to provide the BTF with an informed “priorâ€ distribution for the core sample elevations and thus provide further constraints for the resulting reconstruction. These priors will be based on information collected as part of the first component of the project.

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 (e.g., 137Cs from above ground nuclear weapons testing) and known pollution horizons (e.g., changes in lead concentration), an approach that is particularly useful during time periods where radiocarbon is less effective due to plateaus in the calibration curve. Once this is complete, the two fossil components (BTF-derived elevation estimates and chronological model) will be combined to produce RSL curves 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 the initial study sites for collection of modern samples and initial sediment cores during completion of the first fieldwork season. Characterization of the sedimentological, microfossil, geotechnical, and geochemical characteristics of the modern sediments collected from the first field season, including secondment to BGS Keyworth.

Year 2

Further fieldwork to collect core samples from a second site and to fill in identified gaps within the modern record from the 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 BGS Keyworth.

Year 3

Finalize laboratory work and develop Bayesian transfer functions and appropriate priors during secondment to Maynooth University.

Year 3.5

Completion of thesis write-up and preparation of manuscripts.

Training
& Skills

This project is a Collaborative Studentship between Durham University and the British Geological Survey (BGS) Keyworth. The project will also benefit from external supervisory support from Dr. Matteo Vacchi (University of Pisa, https://unimap.unipi.it/cercapersone/dettaglio.php?ri=135482) 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 and analysis (BGS, Keyworth); microscopy and interpretation of salt-marsh microfossil assemblages (Durham); modelling sediment compaction (Durham); radiometric and pollution-marker dating methods (Durham and BGS Keyworth); 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 have the opportunity to present their results at national and international conferences to develop presentation and communication skills and disseminate results.

References & further reading

Brain, M.J., Kemp, A.C., Hawkes, A.D, Engelhart, S.E, Vane, C.H., Cahill, N., Hill, T.D., Donnelly, J.P., and Horton, B.P., 2017. Exploring mechanisms of compaction in salt-marsh sediments using Common Era relative sea-level reconstructions. Quaternary Science Reviews 156, 96-111.

Evelpidou, N., Pirazzoli, P., Vassilopoulos, A., Spada, G., Ruggieri, G., & Tomasin, A. (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., Moore, J.C., and Jevrejeva, S., 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., Anzidei, M., Antonioli, F., Benini, A., & Esposito, A. (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., Anzidei, M., Antonioli, F., Benini, A., & Verrubbi, V. (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., Kirby, J.R., Holgate, S., Tutman, P., and Plater, A.J., 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., Plater, A.J., Kirby, J.R., Roy, K., Holgate, S., Tutman, P., Cahill, N., and Horton, B.P., 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., Marriner, N., Morhange, C., Spada, G., Fontana, A., and Rovere, A., 2016. Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: Sea-level variability and improvements in the definition of the isostatic signal. Earth-Science Reviews 155, 172-197.

Further Information

Dr. Simon E. Engelhart
simon.e.engelhart@durham.ac.uk
+44 (0) 191 33 43509
@SeaLevelChange

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