Lay Summary authored by Paul Blanchon. Read the full paper here: https://www.openquaternary.com/articles/10.5334/oq.87/
The precise timing and rate of postglacial sea-level (SL) rise not only provides key insight into the nature of ice sheet disintegration during the last deglacial warming ~15 thousand years ago, but also helps constrain the response to future warming. One of the most complete reconstructions of postglacial SL rise is from Barbados where thick, back-stepping, sequences of the reef-crest coral Acropora palmata have been recovered in cores from the shelf and slope off the south coast. As seen in the figure below, successive reconstructions of SL using the Barbados data show rapid accelerations in SL rise at ~14 and 11 thousand years ago, and these have been linked to pulses of ice and meltwater discharge during the most rapid phase of ice-sheet decay. Similar meltwater pulses have subsequently been found in other SL reconstructions, but their timing, depth and magnitudes show significant differences with those from Barbados.
To help resolve these differences, we re-examine the stratigraphy and growth history of the reef-crest sequences at Barbados and find that rather than consisting only of in-place coral colonies (as previously claimed), they are in-fact composed of a mixture of in-place colonies and large clasts generated by skeletal fragmentation and transport during hurricanes. This finding is important because the downslope transport of clasts complicates the assumption that the reef-crest corals grew within a few metres of SL, and so necessitates a reappraisal of the SL reconstruction and how the depth and magnitude of melt-water pulses are defined.
By accounting for the bias created by downslope clast transport, we provide a revised SL reconstruction and show that the deepest A. palmata units were deposited on a slope next to deep-water corals, not on a shallow elevated reef-crest structure as previously thought. As a consequence, the onset of the first meltwater pulse, MWP-1a, cannot be identified from Barbados, which explains it’s earlier timing in other SL reconstructions. We also show that the depth and magnitude of the second meltwater pulse, MWP-1b, is best defined from the difference in elevation between the top of the reef-crest that was drowned by the pulse, and the base of the reef-crest that back-stepped after the pulse ended. Using these elevations, with adjustments for water depth, we estimate that the second meltwater pulse was 5 m smaller, shallower, and occurred 150 years later than previously claimed. This smaller magnitude may explain why MWP-1b has not been recognized in the other major SL reconstruction at Tahiti: it may not have been large enough to shift the reef crest into waters with deeper corals, thereby allowing it to simply regrow and catch-up with sea level. So although these new findings help reconcile differences with other reconstructions, they also highlight the limitations and uncertainties in reconstructing SL from more complex reef sequences.
Full paper: Blanchon, P., Medina-Valmaseda, A. and Hibbert, F.D., 2021. Revised Postglacial Sea-Level Rise and Meltwater Pulses from Barbados. Open Quaternary, 7(1), p.1. DOI: http://doi.org/10.5334/oq.87