Overarching Theme of the project

​The Last Interglacial (125 000 years ago) was the last period of the Earth’s history when climate was slightly warmer than pre-industrial. As a result of higher global and polar atmospheric temperatures and slightly higher polar sea surface temperatures LIG ice sheets were smaller and global mean sea level was higher than today. The LIG represents a process-analog for future warming: sea level and ice sheets dynamics during this period are often regarded as indicators for the sensitivity of ice sheets to future melting.
Our knowledge of the Last Interglacial warmer coasts is characterized by three main research gaps:
  • Last Interglacial global mean sea level estimates are both inaccurate and imprecise.
  • Sea-level oscillations within the Last Interglacial have been inferred from a number of studies, but their existence is still debated and the rates of rapid sea level changes are poorly quantified.
  • The Last Interglacial may have been characterized by storms more intense than today, but this hypothesis is largely controversial and based on geological evidence from few sites.
​This project aims to fill the current research gaps related to sea level and extreme waves in the Last Interglacial.
Between 2019 and 2023 we will perform a range of research activities aimed at: 
  • Advancing current knowledge of LIG global mean sea level and associated uncertainties. 
  • Investigating the existence and pattern of sea level fluctuations within interglacials.
  • Test the hypothesis that Last Interglacial extreme wave events were stronger than today.

Global database of LAST INTERGLACIAL sea level and wave proxies (WP1)

Currently, there are few global databases collecting Last Interglacial RSL indicators, re-examining original geological studies. These are the starting point of this project: we want to use them as basis to build a new-generation database of LIG sea level and wave proxies. The new database must include data on RSL indicators and their ages reviewed with a unique standard, and it must report information on potential geological proxies of abrupt intra-interglacial sea level changes (i.e., ‘double-stepped’ reef stratigraphies) and of paleo wave intensity.

Achievements

We built WALIS, the World Atlas of Last Interglacial Shorelines, that was filled with 20 manuscripts (9 of which co-authored by WARMCOASTS team members) in a Special Issue of the journal Earth System Science data and adopted by other research groups as springboard for new research. All papers and data composing WALIS are open-access. We found little evidence in the literature of proxies of paleo wave intensity, therefore this part of the database was not implemented. WALIS includes a data insertion interface coded within the project, python scripts to export data and a visualisation interface, all available on GitHub.

Western Atlantic sea level data (WP2)

The global database will represent a unique step in Pleistocene sea level studies, but it will contain, by its own nature, data that were originally surveyed with very different approaches and measurement methodologies. As an example, elevation data collected a decade or two ago (before differential GPS systems became relatively cheap and portable) often report elevations with no associated measurement errors or no hydrographic reference datum. In the database, most measurement and datum errors will have to be estimated ex-post, and this will necessarily increase final RSL uncertainties. Therefore, the second step of WARMCOASTS will be the collection of new field data to complement the information contained in the database. We will perform fieldwork to measure RSL indicators across passive margins spanning the entire Western Atlantic.

Achievements

Following the plan of a pole-to-pole transect, we collected data in Argentina (Patagonia), south Brazil and on the US East Coast (Florida, South Carolina and Virginia). These were the areas which showed more potential according to the WALIS data compilation. In each area, we collaborated with local geologists to evaluate the presence of sites where samples for dating were already available, where we re-assessed precise elevation and modern analogues, and of potential new sites. In the Western Atlantic, we used Optically Stimulated Luminescence and Amino Acid Racemization to date samples from the sites where Last Interglacial sea-level. To improve the baseline for models to be employed in WP4, we performed specific fieldwork in SW Madagascar and Aruba, Curaçao and Bonaire. Both areas were then used as test beds for our stratigraphic forward models. Due to the uncertainties in travels caused by COVID pandemic, we repurposed some fieldwork to NW Italy, which led us to work on cave deposits from the Last Interglacial. The fieldwork done in different areas was published in 5 manuscripts (with one manuscript under consideration).

Global mean sea level and tectonics inversion (WP3)

The database and the field data will allow to estimate paleo RSL at, respectively, global and regional scales. The Western Atlantic transect will focus on passive margins, while the global database will contain data from both active and passive margins. We will exploit these data in conjunction with a newly created large set of GIA and DT predictions to first invert for eustasy at passive margins and then for tectonics at active margins.

Achievements

We collaborated with DT modellers to improve predictions of dynamic topography using shorelines older than the Last Interglacial, and highlighted the possible role of reef Isostatic Adjustment in Last Interglacial sea-level histories, which was hitherto neglected. We worked on understanding whether improving Last Interglacial sea-level histories might help best understand current Antarctic mass loss and consequent sea-level rise. We were part of a group who used GIA and DT models analysed with bayesian statistics on datafrom the Bahamian Archipelago to suggest that Last Interglacial sea-level was lower than assumed by previous works. This assumption is now being tested on data from the WALIS database and new field data, and the same group (including WARMCOASTS team members) is investigating the possibility to use the same bayesian techniques to invert for tectonics at selected sites. This work resulted in 6 manuscripts (and associated data and code).

Reef stratigraphic forward modelling (WP4)

We will investigate sea level variability within the LIG using reef models. The rationale behind this is that we need to advance the classic stratigraphic approach used so far to argue on the intra-interglacial sea level variability. We propose to try a different approach to discern between an eustatic vs non-eustatic origin of the ‘double-stepped’ stratigraphy of selected fossil reefs. We will focus on modelling the processes and sea level scenarios that may contribute to build a particular stratigraphy, and then we will evaluate the results of our modelling in light of what we observe in the LIG stratigraphy.

Achievements

We used two different kinds of models (to reproduce reef buildup and demise, and the formation of tidal notches) to investigate sea-level patterns in the Last Interglacial, also supplementing our research on possible intra-interglacial sea-level oscillations in Florida and Italy. The bulk of our work was directed at calibrating and running reef stratigraphic forward models, both commercial and open-source, on synthetic and real case studies to understand whether intra-LIG sea-level oscillations are necessary to explain the observed reef stratigraphies at different sites. This work produced 4 published manuscripts (including related data and code) and 2 manuscripts currently under consideration.

Extreme waves in the Last Interglacial (WP5)

While LIG coastal landforms and deposits have been widely used to reconstruct paleo RSL, the potential of some of these features to indicate more intense LIG wave conditions has been seldom exploited. Besides the records of Bermuda and Bahamas, the location of other sites that can carry information on LIG wave intensity is presently unknown. In this activity, we will analyze the our global database to review which geological facies can be used as proxies of LIG wave intensity, and where they are located globally. We will then use hydrodynamic models to gauge whether LIG storm ridges across the Western Atlantic can be used to infer past wave intensity.

Achievements

This wok proved among the most challenging within the project. In an attempt to transfer the knowledge to LIG reefs, we worked on wave attenuation on modern reefs, but with no avail. We then collaborated on a large, multi-project work to simulate storm surges under Last Interglacial climate conditions, which supported more local works in Cape Verde and the ABC islands. We also investigated the creation of beach ridges in Patagonia as an effect of wave buildup, improving approaches to correlate local wave energy to the elevation of these landforms. In Florida, we identified storm horizons in GPR data and work in ongoing to assess the implications of ridge morphology at one site in Patagonia in terms of wave direction. Overall, this work produced 2 published manuscripts and 2 manuscripts currently under consideration (alongside with a significant amount of data and code).
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