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Terminology – Definitions and explanations of palaeo-sea-level terminology References – download a spreadsheet of references
NOTE: all elevations are quoted relative to LMSL unless otherwise stated.
The relative sea level record at Bermuda during MIS 11
The highest unequivocal marine deposit found on Bermuda to date was discovered in the 1960s at +28m at Government Quarry (3). It was described as conglomerate, plastered against a cliff of Walsingham Formation limestone. It included rounded volcanic clasts. And, as there is no known exposure of in-situ volcanic rock on Bermuda today, it was from the outset considered to be relatively old deposit. Although it has, unfortunately, been quarried away, an equivalent marine deposit was subsequently found in a small cave in the same quarry at +22 m. It consisted of a basal conglomerate followed by a succession of shelly sediments and flowstone layers (7). The lower flowstones yielded a weighted average age of ~390 ka and the underlying marine event was, accordingly, attributed to Marine Isotope Stage 11. (42).
The considerable elevation of the Government Quarry marine deposits was credited, by Hearty et al. (1999) (7), to a eustatic sea level surge triggered by the warming-related “collapse” of the Greenland and Western Antarctic ice sheets as well as a significant contribution from the Eastern Antarctic Ice Sheet. (7). Three alternative explanations, to this ice-sheeet collapse scenario, are: 1. a megastunami; 2. glacio-hydro-isostatic adjustment; and 3. tectonic uplift. The megatsunami hypothesis (41) was rebutted by Hearty (42) on the basis of biological evidence found within the sediments, which was consistent with sustained marine submergence, as opposed to a catastrophic washover event.
The elevation of the Bermuda MIS11 sedimentary marine deposit when compared to much lower sea levels interpreted, elsewhere, from marine isotopes and MIS11 fossil reefs is a discrepancy that has been attributed to the unreliability of the isotope- and reef-sourced data (42). It is argued that the isotope record, as a sea-level proxy, does not provide direct evidence of sea level position. Meawhile, potential shortcomings of fossil reef derived data (corrected for uplift) has been imputed to the rapidity of the relative sea level rise. Reef growth, it is argued, simply got left behind (42) and did not record sea level maxima with same resolution as the clastic palaeo-sea-level record at Bermuda.
Notwithstanding these explanations, Bowen (2010) (40) challenged the significance of MIS 11 RSL elevations close to ~+ 20 m at Bermuda or elsewhere. He attributed the anomaly to site-specific vertical land motion and saw no need to invoke eustatic effects asssociated with rapid excess melting of the West Antarctic and Greenland ice sheets.
Raymo and Mitrovica (2012) (32) modelled the GIA effects at Bermuda during MIS 11 and concluded that a -10 m elevation correction should be applied to the relaitve palaeo-sea-level position in order to account for post-glacial crustal subsidence over the course of the anomalously long interglacial. Allowing for this correction, they estimated that eustatic sea level rose to ~+6m to +13 m which did not require any contribution from a putative collapse of polar ice sheets.
The potential for tectonic effects contributing to the MIS 11 RSL record at Bermuda was raised by Hay et al (2014) (3). Bowen (2010) (40), as alluded to above, was of the same mind and pointed to emergent MIS 7 and MIS 5a marine deposits on Bermuda which are, also, anomalously elevated. However, the uplift argument is not consistently supported by the palaeo-sea-level at Bermuda. It is belied by the elevation of MIS 5e sea-level imprint which, at ~+6m, conforms with the global consensus on eustatic sea level at the LIG (MIS 5e).
The global sea level record during MIS 11.
MIS 11 was one of the longest (~30 k years) and warmest interglacial periods of the entire Quaternary (39). Some estimates put the global temperature only slightly cooler than MIS 5e (36, 37). The eustatic elevation of sea level at MIS 11 remains highly controversial. Widely recognised, isotopic records, from deep sea cores, have yielded estimates ranging from around present sea level to +6m.
There is direct evidence from preserved plant-life that, unlike today, northwestern Greenland was ice free around 415 ka during MIS 11 (38). Ice sheet modeling indicates that such ice-free conditions would have coincided with at least 1.4 meters of sea level equivalent contribution from the Greenland Ice Sheet.
MIS 11 (~400 ka) marine deposits at ~ +20m in the Bahamas (47) and Bermuda(7), have been associated with polar ice sheet collapse (7) or a megasunami (41). More recently the high RSL elevations have been interpreted as the outcome, at least in part, of glacio-isostatic adjustment (GIA) (30). To account for post-glacial subsidence of these sites over the course of the anomalously long interglacial (40), it has been concluded that a 10m downward correction of the observed RSL elevations is required. Raymo and Mitrovica (2012)(32) calculated that MIS 11 eustatic sea level stood in the range of +6 to +13 m, which is consistent with other studies (71)(51)(39).
In their study area of Mossel Bay, South Africa, Roberts at al. (2012) (36) found that the MIS 5e sea level record corresponded well with the consensus GMSL value of approximately +6m. They cited this as evidence of tectonic stability which, in turn, justified Pleistocene palaeo-sea-level studies in the area. Based on an unambiguous sea level indicator marking the contact between shoreface and foreshore environments in MIS11 sediments, they reported a maximum sea level at the time of deposition of approximately + 13m. This, they noted, is consistent with the higher end of the downwardly revised values in the Bahamas and Bermuda (36).
Allowing for uplift on the Canary Islands, where MIS 5e marine deposits are preserved at +12m, Muhs et al. (2014)(56) calculated a likely range for MIS 11 GMSL of +9 m to +24 m, which they pointed out could only be achieved with significant contributions of meltwater from the Western and Eastern Antarctic Ice Sheets. A “very warm interglacial” was similarly advocated by Muhs et al. (2012) (71) , based on their studies of marine deposits on Curaçao from which a conservative MIS11 GMSL estimate of +8.4m to +10 m was made.
After an extensive review of MIS11 palaeo-sea-level imprints from around the globe Bowen (2010)(40) came to a somewhat different conclusion. His calculations certainly did not support an MIS 11 sea-level elevations anywhere near the ~+ 20 m. He saw no need to invoke rapid excess melting of the West Antarctic and Greenland ice sheets. After applying uplift corrections, he surmised that MIS 11 eustatic sea level stood close to the present level. Indeed, global ice volumes may have been broadly similar to those of today (40),(53).
Rohling et al. (2010) noted that their Red Sea record strongly supports Bowen’s conclusions.
Bermuda Geology 