Invited reviewThe analysis of Last Interglacial (MIS 5e) relative sea-level indicators: Reconstructing sea-level in a warmer world
Introduction
Past interglacials are of interest to the scientific community as they can be used to study the behavior of the climate system during times as warm as or slightly warmer than today. Of particular interest is the degree to which relatively small perturbations to climate forcing variables such as atmospheric or sea surface temperature, insolation, or CO2 can lead to polar ice volume and sea-level changes. For instance, during marine isotope stage (MIS) 5e, the Last Interglacial (LIG, ~ 128 to 116 ka , Stirling et al., 1998), ice core evidence suggests that greenhouse gas concentrations were slightly higher than pre-industrial levels (Petit et al., 1999) and summer insolation at high latitudes was also higher by ~ 10%. These small changes in climate forcing were apparently sufficient to warm polar temperatures (> 66° latitude) in both hemispheres by about 3–5 °C relative to today (Otto-Bliesner et al., 2006) and global mean temperature by an estimated 1.5 °C (Turney and Jones, 2010, Lunt et al., 2013). By comparison, global mean temperature has increased by about half this, or by ~ 0.85 °C, since 1880 (IPCC, 2013) and an additional global warming of 1 °C, that could be expected to raise polar temperatures by 3–6 °C (Kattsov et al., 2005), is likely to occur by the end of this century. Indeed, the Antarctic Peninsula has been warming by an average of 0.5 °C per decade over the last 60 years (Mulvaney et al., 2013).
There is increasing evidence suggesting that the MIS 5e climatic conditions resulted in smaller ice sheets and, therefore, higher than present sea-levels (e.g., Kopp et al., 2009). The study of sea-level indicators dating from the Last Interglacial, therefore, is fundamental to unravel potential patterns of future sea-level rise caused by global warming (IPCC, 2013). The only direct observations that allow reconstruction of MIS 5e sea-levels are features associated with paleo sea-levels such as, for example, fossil coral reef terraces (Murray-Wallace and Woodroffe, 2014). However, reconstructing MIS 5e sea-level from such observations carries uncertainties related to age attribution and to how sea-level indicators are measured and interpreted by field geologists.
Two main issues are related to. i) the methods used to establish the elevation of a sea-level indicator, ii) how precisely those measurements are referred to modern mean sea-level. Standard topographic techniques (e.g. differential GPS, with vertical accuracy down to a few centimeters) have been employed in Pleistocene and Pliocene field studies only recently and therefore measurement errors reported by older studies need to be re-assessed. A fundamental issue relates to how paleo sea-level is calculated from the elevation of an indicator. Indeed, most MIS 5e (and older) markers cannot be correlated precisely to a tidal datum as happens, for example, with particular foraminifera assemblages in Holocene salt marshes (Shennan and Horton, 2002) or with coral microatolls (Woodroffe et al., 2012, Mann et al., 2016). Most MIS 5e sea-level indicators carry with them large sea-level uncertainties that are often not reported or properly defined.
The overall aim of this paper is to give a complete account of the best field practices that should be adopted when surveying MIS 5e and older sea-level indicators. In this study we aim to:
- i)
Present a set of definitions and standardizations that should be adopted in MIS 5e sea-level studies. Adopting such definitions both in studies reporting new sea-level indicators as well as in literature reviews will ensure that the results will be easily integrated in sea-level databases (Düsterhus et al., 2016).
- ii)
Describe the most common landforms and deposits used as MIS 5e sea-level indicators, together with their upper and lower limits of formation under modern conditions.
- iii)
Present an example of how the standard methodology described in this paper can be applied to a real study case.
- iv)
Discuss the implications for paleoclimate reconstructions of adopting correct procedures in the measurement and reporting of MIS 5e datasets.
Section snippets
Definitions
Today, processes acting near modern mean sea-level (MSL) are shaping a set of landforms on both rocky and sedimentary coasts. These features include, for example, shore platforms or cobble beaches. When these features are found in the geologic record, disconnected from their environment of formation (for instance, a shore platform observed several meters above present-day sea-level), we infer that a Relative Sea-Level (RSL) change has occurred. Any elevation difference between the original and
Last Interglacial RSL indicators
Scientific observations of late Quaternary, and particularly MIS 5e, shorelines higher than present date back almost two centuries (Darwin, 1846, Hutton, 1885, Lyell, 1837). Since then, numerous papers have addressed Last Interglacial relative sea-levels. Pedoja et al., 2014 compiled the most extensive review of paleo sea-level studies to date, identifying 987 studies that reported at least the elevation of an MIS 5e site. It is worth noting that the number of such studies increased
Dating methods
Together with the measurement and interpretation of the elevation of MIS 5e RSL indicators, it is essential to establish their age as precisely as possible with absolute or relative dating methods. Among the absolute dating techniques most often used in Last Interglacial studies, only 230Th/U dating of corals can resolve timing of deposition within the interglacial (Dutton and Lambeck, 2012, O’Leary et al., 2013). Other techniques, such as electron spin resonance (ESR), optically stimulated and
From field measurement to paleo RSL
As described in the previous sections, the most accurate way to calculate the paleo RSL associated with a MIS 5e deposit is to study its modern analog. An example of how studies and datasets on modern coastal dynamics can be used to derive the indicative meaning of a MIS 5e beach deposit is presented in Fig. 15. The example is that of Cala Millor, Mallorca Island, Spain. Cala Millor is an ~ 1.7 km-wide sandy beach (Fig. 15a). The beach profile is bounded on its low end by the presence of a
Discussion
How did the polar ice sheets, and hence sea-level, respond to MIS 5e warm conditions? Last Interglacial RSL indicators are often used to infer paleo RSL at one location. In turn, observations at many sites can contribute to a global understanding how polar ice sheets responded to moderate climate warming. Despite the long tradition of MIS 5e studies, often the methods used to measure the markers are not sufficiently described, are of low accuracy, or are not referred to a known tidal datum.
Conclusions
Although MIS 5e is the most studied period of the Earth's past, at least in terms of paleo sea-level, much research still need to be directed towards obtaining better paleo RSL elevations from field data. In this review we addressed all the relevant observations that are needed when studying MIS 5e RSL markers. Most of the concepts reviewed here can also be applied to other interglacials. In conclusion, we highlight the following points:
- •
Measurement. The measured elevation of a RSL indicator
Acknowledgments
AR, DH and TL's research is supported by the Institutional Strategy of the University of Bremen, funded by the German Excellence Initiative (ABPZuK-03/2014) and by ZMT, the Center for Tropical Marine Ecology. The authors acknowledge NSF grant OCE-1202632 ‘PLIOMAX’ for support, as well as MEDFLOOD - Modelling Paleo Processes (INQUA CMP projects 1203P and 1603P) and PALSEA (PAGES/INQUA/WUN) working groups for useful discussions. MV contributes to the Labex OT-Med (ANR-11-LABX-0061) and to the
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