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 history1. 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 lower2,3. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago4 and was followed by about 10,000 years of rapid deglaciation into the Holocene1. 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 ago1,5, 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.
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We thank the IODP and ECORD (European Consortium for Ocean Research Drilling) for drilling the GBR, and the Bremen Core Repository for organizing the onshore sampling party. Financial support of this research was provided by the JSPS KAKENHI (grant numbers JP26247085, JP15KK0151, JP16H06309 and JP17H01168), the Australian Research Council (grant number DP1094001), ANZIC, NERC grant NE/H014136/1 and Institut Polytechnique de Bordeaux. This paper is a contribution to the INQUA commission on Coastal and Marine Processes and the PAGES PALSEA2 programme.
Nature thanks P. Whitehouse and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Fig. 1 Simplified classification of the main coralgal assemblages observed in the Expedition 325 cores.
Shallow reef habitats are represented by coral assemblages when associated with thick crusts of aA1 coralline algae and vermetid gastropods (8). Deep, fore-reef slope settings are defined by coral assemblages cD when associated with thin crusts of aA3 and in the absence of aA1 and aA2. cA, massive/robust branching Isopora (1) and corymbose Acropora gr. humilis (2); cB, branching Seriatopora (3) and Acropora sp. (4); cC, massive/encrusting meruliniids (5); cD, encrusting to massive Porites (6) and encrusting Montipora; aA3, Mesophyllum and Lithothamnion (10); aA1, Porolithon onkodes (7); and aA2, thin P. onkodes, Porolithon gardineri, Harveylithon gr. munitum (9).
The main lithologies are divided into coral reef framework, (1)–(3), and detrital sedimentary facies, (4)–(8).
The vertical patterns in coral recovery, lithologies, coralgal assemblages and vermetid gastropods are summarized here. These data define a major hiatus (pink line) in reef development as the top of Reef 2 was exposed following the sea level fall to the LGM-b low-stand (see main text for details). Following deglacial sea level rise, reef growth was re-established as Reef 3b turned on when the shelf reflooded before this reef drowned after about 14 kyr ago (ka). (‘bst.’, boundstone; ‘mb’, microbialite; ‘unconsol.’, unconsolidated sediment; and ‘gastro’, gastropod.)
Cores M00033A-10R and 11R (HYD-01C) and M00055A-4R (NOG-1B) clearly define the nature of the Reef 2/Reef 3b boundary, which represents a major hiatus in reef growth (see Extended Data Fig. 3). This boundary is characterized by major changes in lithologies, coralgal assemblages and diagenetic features, including fresh water and meteoric cements (blue star) indicating that the top of Reef 2 has been subaerially exposed. (LMC, low-magnesium calcite; RD, reef-death events.)
The blocky low-magnesium calcite meteoric cement that is related to subaerial exposure is initially precipitated in the intergranular voids of grainstone in 55A 5R1. Then peloids (p) of high-magnesium calcite formed under marine conditions and filled the remaining voids when they were submerged following re-flooding. Scale bar, 100 µm.
Age versus depth plot showing the key in situ RSL data points from Hydrographer’s Passage (HYD-01C) (in blue) and Noggin Pass (NOG-01B) (in red). Accelerator mass spectrometer 14C ages derived from corals are indicated by open circles and those derived from coralline algae are indicated by crossed circles. U–Th coral ages are indicated by filled circles. Inflection points defining the maximum position of the RSL at HYD-01C and NOG-01B are also shown (see labels NO-5, 8, 9, and HY-3, 5, 6 in Supplementary Information, corresponding respectively to data points 11, 8, 1 and 9, 7, 5 on this figure; see also Fig. 3). The combined RSL envelope represented by the black lines (maximum and minimum positions) takes into account the uncertainties in the age (2σ), palaeowater depth and position in the core of each data point, which is illustrated by a coloured rectangle (blue for Hydrographer’s Passage and red for Noggin Pass) (see Methods for more details). If we were to omit the 14C data owing to possible unaccounted-for variability in local reservoir ages63, the LGM-b sea level drop defined by coral U–Th ages would be 1.5 kyr earlier at 23.5 ka, corresponding to an extended sea level drop over about 3 kyr at a rate of about 7 m kyr−1. Key RSL index points are as follows. 1, 325-M0053A-13R-1W 21-25 (20.51 ka, 117.93 m; NO-9). 2, 325-M0054B-06R-1W 64-67 (20.50 ka, 124.39 m). 3, 325-M0054B-07R-1W 5-9 (20.47 ka, 125.3 m). 4, 325-M0035A-18R-1W 10-15 (20.43 ka, 127.11 m). 5, 325-M0036A-18R-2W 8-10 (20.70 ka, 128.75 m; HY-6). 6, 325-M0054B-08R-2W 73-75 (22.13 ka, 128.54 m). 7, 325-M0033A-11R-CCW 5-11 (22.11 ka, 106.83 m; HY-5). 8, 325-M0055A-04R-1W 35-40b (21.87 ka, 103.13 m; NO-8). 9, 325-M0032A-10R-1W 18-20 (23.49 ka, 107.95 m, HY-3). 10, 325-M0033A-13R-CCW 1-3 (23.62 ka, 109.46 m). 11, 325-M0055A-04R-2W 99-105 (23.97 ka, 104.84 m; NO-5).
Calculations, for selected far-field sites (a–f), using the ice model based on the lower-bound of RSL from GBR. GIA calculations implemented using parameters of upper-mantle viscosity and lithospheric thickness of 1020 Pa s and 70 km and lower-mantle viscosity of 1022 Pa s. However, the differences between data and calculations for the far-field sites of Tahiti (b), Bonaparte (c) and Sunda (d) indicate a better match with GIA when the higher RSL obtained from this study is used (Fig. 4) in calculating the global deglacial sea levels. The grey band represents the range of RSL predictions using GIA modelling with various earth parameters (lithospheric thickness H = 70 km, upper-mantle viscosity 1020–1021 Pa s, lower-mantle viscosity 1021–1023 Pa s), and a melting model (of ice history), which, in this case, was the MIN model. The red lines are for H = 70km, upper-mantle viscosity 2 × 1020 Pa s and lower-mantle viscosity 1022 Pa s. The blue lines show the case for H = 70 km, upper-mantle viscosity 2 × 1020 Pa s and lower-mantle viscosity 1023 Pa s. Vertical uncertainties are depth ranges for each sea level indicators and horizontal error bars are 2σ uncertainties for age estimations reported from the literature.
a–d, Total (a), North America (b), Eurasia (c) and Antarctic (d) ice sheet growth and melting histories for the past 35 kyr inferred from the present study (red bands) are compared with previously reported model (blue line). The blue curves represent the ANU GMSL results4, whereas the red bands, covering a range of GBR-RSL-based GMSL, are from this work. During the transition from LGM-a to LGM-b, around 21 kyr ago, there is enhanced precipitation over the North American Ice Sheet and to a lesser extent over Antarctica, although, for the latter, the ice volume continues to build up, for longer, until the termination of LGM-b approximately 17 kyr ago. The manually adjusted nominal ANU ice model will influence our inferred melting history for each ice sheet. Furthermore, although the ANU model differs in some respects from other ice sheet reconstructions64, the results from these simulations are not very different.
MAX (a) and MIN (b) represent the maximum and minimum extremes of GBR RSL. Blue and red bands are RSL ranges derived from our study for Hydrographer’s Passage (HYD) and Noggin Pass (NOG). Grey bands represent the range of predicted sea levels using the new ice model. Lithospheric thickness is fixed at 70 km, whereas mantle viscosities for the upper and lower mantle varied between 1020–1021 Pa s and 1021–1023 Pa s. RSL predictions for representative viscosities for HYD-01C and NOG-01B are shown as red and blue solid and dotted lines respectively. Vup and Vlow represent the upper- and lower-mantle viscosity values. (obs., observations.)
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Yokoyama, Y., Esat, T.M., Thompson, W.G. et al. Rapid glaciation and a two-step sea level plunge into the Last Glacial Maximum. Nature 559, 603–607 (2018). https://doi.org/10.1038/s41586-018-0335-4
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