Rapid glaciation and a two-step sea level plunge into the Last Glacial Maximum

Abstract

The approximately 10,000-year-long Last Glacial Maximum, before the termination of the last ice age, was the coldest period in Earth's recent climate history. Relative to the Holocene epoch, atmospheric carbon dioxide was about 100 parts per million lower and tropical sea surface temperatures were about 3 to 5 degrees Celsius lower. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago and was followed by about 10,000 years of rapid deglaciation into the Holocene. The masses of the melting polar ice sheets and the change in ocean volume, and hence in GMSL, are primary constraints for climate models constructed to describe the transition between the Last Glacial Maximum and the Holocene, and future changes; but the rate, timing and magnitude of this transition remain uncertain. Here we show that sea level at the shelf edge of the Great Barrier Reef dropped by around 20 metres between 21,900 and 20,500 years ago, to -118 metres relative to the modern level. Our findings are based on recovered and radiometrically dated fossil corals and coralline algae assemblages, and represent relative sea level at the Great Barrier Reef, rather than GMSL. Subsequently, relative sea level rose at a rate of about 3.5 millimetres per year for around 4,000 years. The rise is consistent with the warming previously observed at 19,000 years ago, but we now show that it occurred just after the 20-metre drop in relative sea level and the related increase in global ice volumes. The detailed structure of our record is robust because the Great Barrier Reef is remote from former ice sheets and tectonic activity. Relative sea level can be influenced by Earth's response to regional changes in ice and water loadings and may differ greatly from GMSL. Consequently, we used glacio-isostatic models to derive GMSL, and find that the Last Glacial Maximum culminated 20,500 years ago in a GMSL low of about -125 to -130 metres.

Reef-flat and back-reef development in the Great Barrier Reef caused by rapid sea-level fall during the Last Glacial Maximum (30–17 ka)

“…1 and 2) for HYD-01C, and 22-19 ka (reef 3a) for HYD-02A, based on our foraminiferal production ages and other published data (Webster et al, 2018;Humblet et al, 2019). The timing of reef-flat and back-reef formation is consistent with the two stepwise falls in sea level during the LGM (Yokoyama et al, 2018; Fig. 3).…”

“…More recently, sediment cores recovered from the shelf edge of the Great Barrier Reef during Integrated Ocean Drilling Project (IODP) Expedition 325 revealed that drowned shelf-edge reefs composed of in situ coralgal frameworks and associated microbialites developed during the LGM and the subsequent last deglaciation (Webster et al, 2018;Braga et al, 2019;Humblet et al, 2019). In addition, combined paleo-water depths and >580 radiometric ages of the coralgal assemblages revealed a two-step sea-level plunge, each of several tens of meters in magnitude and a change rate of 15-20 mm yr -1 , into the LGM (Yokoyama et al, 2018). In response to these sea-level changes, five reef sequences have migrated vertically and laterally since 30 ka (Webster et al, 2018).…”

The Influence of Carbonate Platforms on the Geomorphological Development of a Mixed Carbonate‐Siliciclastic Margin (Great Barrier Reef, Australia)

“…We opted for a curve which approximates global eustatic sea level rather than a locally derived one because the millennial‐scale factors that modulate local accommodation space (e.g., fluvial supply and marine deposition; Posamentier et al, 1988) are independently simulated in the model. Similarly, for this study we chose not to use sea level curves reconstructed solely from coral reef proxies (i.e., Yokoyama et al, 2018—IODP Exp. 325 from the Great Barrier Reef) to prevent a self‐confirming result in the carbonate accretion module.…”

Late Quaternary sea‐level change and evolution of Belfast Lough, Northern Ireland: new offshore evidence and implications for sea‐level reconstruction

“…Our suggested later start of the transgression is slightly out of sync with the onset of this event. However, recent results (Yokoyama et al, ) suggest that MWP‐1A occured as a series of steps with the last pulse coinciding closely with the latest possible onset of the first transgression (Fig. S3). …”