Paleo sea levels reconsidered from direct observation of paleoshoreline position during Glacial Maxima (for the last 500,000 yr)

Abstract

The drastic climatic changes which characterise the cooling trend of the last few million years of Earth history led to variations in eustatic sea level that had tremendous impact on the geology and ecology of continental margins. Reconstructing a sea-level curve back in time is not an easy task. Observations of shoreline positions are always a local measurement of Relative Sea Level that needs to be corrected from the effect of tectonic and thermal subsidence, sediment loading, compaction and glacio-hydro isostasy. Extensive studies have been done for the last deglaciation and for the last 100,000 yr cycle. But very few studies deal with position of sea level during earlier cycles, simply because conditions are very rarely favourable for the preservation of such witnesses. The shelf of the Golfe du Lion (Western Mediterranean) reveals a unique record of shoreline paleopositions during glacial maxima of at least the last five circa 100 kyr glacial/interglacial cycles. In fact it is the entire glacial deltaic lobe of up to 50 m thick (from delta front or shoreface to prodelta) that has been preserved in place and which provides direct and independent constraints for relative sea-level minima. We measure a relative sea level of: − 112m, − 128, − 134, − 246 and − 262 m for MIS 2, 6, 8, 10 and 12 respectively. After corrections taking into account postdepositional movement of strata (subsidence), we find, that sea level dropped to a depth of − 102 ± 6 m during the last three glaciations (MIS2, MIS6, MIS8) but reached exceptionally low values of more than − 150 ± 10 m during the preceding glaciations MIS10 and MIS 12 at about 340 and 434 kyr BP. This general time framework and sedimentological interpretation has been confirmed by preliminary results from two deep drillings during the PROMESS cruise (july 2004), which validate our methodology. However, no detailed and absolute datings of such witnesses are available so far, so that we cannot prove that these levels are the lowest ever reached during each glacials, but they correspond undoubtedly to the last preserved shoreface before rapid sea-level rise. We also suggest that the abrupt change in sealevel maxima might be the overprint of 400 kyr orbital periodicity cycles. Last but not least, these results prove that the Golfe du Lion is indeed a unique laboratory to study paleoclimates and sea-level variations on a larger time scale. Further work is needed for a complete glacio-hydro-sedimento isostatic modelling of each sequence and each glacial to further constraint local sea level versus global sea level and quantify, in particular the relative effect of glacio-hydro isostatic effect (which differ according to ice sheet extend) but also of erosion-sedimentation isostatic effect (erosion on land and deposition on the outer shelf and slope).

High-Frequency Sequences in the Quaternary of Pelotas Basin (coastal plain): a record of degradational stacking as a function of longer-term base-level fall

“…The minimum highest sea-level altitude estimated to System II is 9.5 ± 1 m, to System III is 7.7 ± 1 m, and to System IV is 3 ± 1 m. Rohling et al (1998) and Rabineau et al (2006) established magnitudes of lowstands to the past 500 kyr. Considering the correlation with oxygen isotope stages to determine the age of sequences lowstand, it is possible to determine that sea level was more than 100 m below its current position during each lowstand (Fig.…”

Differences between the last two glacial maxima and implications for ice-sheet, δ18O, and sea-level reconstructions

“…Here we show that subsequent improvements to the Red Sea record firmly indicate a greater sea-level drop during the LGM than during the PGM. Independent evidence from western Mediterranean palaeoshorelines also suggests that the LGM sea-level drop exceeded the PGM sea-level drop by about 10 m (Rabineau et al, 2006).…”

“…The fourth source for our assessment of a PGMeLGM sealevel offset concerns fossil coral position data (Z cp ) from a comprehensive database that has been harmonised in terms of dating and uplift-correction protocols (Hibbert et al, 2016). The fifth source consists of western Mediterranean palaeo-shorelines (Rabineau et al, 2006). The latter was discussed before, while the other four sources are detailed below.…”

An Arctic Ocean ice shelf during MIS 6 constrained by new geophysical and geological data

“…The MIS 6 Arctic Ocean likely received substantially less freshwater because the hydrological cycle is generally weaker in colder climates (Held and Soden, 2006) and specifically the rivers ending in the Barents and Kara Seas were blocked by a very large Eurasian Ice Sheet (Svendsen et al, 2004). Furthermore, w0.8 Sv of fresher water (32.5 psu) is presently entering through the Bering Strait (Woodgate and Aagaard, 2005) which, with a threshold of approximately 52 m, was closed during the peak of MIS 6 (Rabineau et al, 2006). Provided that the wind-induced vertical mixing was comparable to that in the present climate, the cold halocline would deepen from 130 to 510 m if the freshwater supply was reduced from today's 0.28 Sv to 0.1 (Appendix 1).…”