In the search for a record of eustatic sea level change on glacial–interglacial timescales, the Seychelles ranks as one of the best places on the planet to study. Owing to its location with respect ...to the former margins of Northern Hemisphere ice sheets that wax and wane on orbital cycles, the local—or relative—sea level history is predicted to lie within a few meters of the globally averaged eustatic signal during the Last Interglacial period. We have surveyed and dated Last Interglacial fossil corals to ascertain peak sea level and hence infer maximum retreat of polar ice sheets during this time interval. We observe a pattern of gradually rising sea level in the Seychelles between ∼129 and 125 thousand years ago (ka), with peak eustatic sea level attained after 125 ka at 7.6 ± 1.7 m higher than present. After accounting for thermal expansion and loss of mountain glaciers, this sea-level budget would require ∼5–8 m of polar ice sheet contribution, relative to today's volume, of which only ∼2 m came from the Greenland ice sheet. This result clearly identifies the Antarctic ice sheet as a significant source of melt water, most likely derived from one of the unstable, marine-based sectors in the West and/or East Antarctic ice sheet. Furthermore, the establishment of a +5.9 ± 1.7 m eustatic sea level position by 128.6 ± 0.8 ka would require that partial AIS collapse was coincident with the onset of the sea level highstand.
•Peak eustatic sea level (ESL) for MIS 5e estimated at ∼7.6 ± 1.7 m above present.•Polar ice sheets contributed ∼5–8 m of ice-equivalent sea level to this peak.•Partial collapse of Antarctic ice sheet may have occurred early in MIS 5e.•Gradual sea-level rise of ∼0.2 m ka−1 recorded between ∼129 and 125 ka.
More accurate global volumetric estimations of shallow-water reef deposits are needed to better inform climate and carbon cycle models. Using recently acquired datasets and International Ocean ...Discovery Program (IODP) Expedition 325 cores, we calculated shallow-water CaCO
volumetrics and mass for the Great Barrier Reef region and extrapolated these results globally. In our estimates, we include deposits that have been neglected in global carbonate budgets: Holocene Halimeda bioherms located on the shelf, and postglacial pre-Holocene (now) drowned coral reefs located on the shelf edge. Our results show that in the Great Barrier Reef alone, these drowned reef deposits represent ca. 135 Gt CaCO
, comparatively representing 16-20% of the younger Holocene reef deposits. Globally, under plausible assumptions, we estimate the presence of ca. 8100 Gt CaCO
of Holocene reef deposits, ca. 1500 Gt CaCO
of drowned reef deposits and ca. 590 Gt CaCO
of Halimeda shelf bioherms. Significantly, we found that in our scenarios the periods of pronounced reefal mass accumulation broadly encompass the occurrence of the Younger Dryas and periods of CO
surge (14.9-14.4 ka, 13.0-11.5 ka) observed in Antarctic ice cores. Our estimations are consistent with reef accretion episodes inferred from previous global carbon cycle models and with the chronology from reef cores from the shelf edge of the Great Barrier Reef.
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.
The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2 rise to various processes. Here we show that the growth of the world's largest reef system, the Great Barrier ...Reef (GBR), is marked by a pronounced decrease in δ13C in absolutely dated fossil coral skeletons between 12.8 and 11.7 ka, which coincides with a prominent minimum in atmospheric δ13CO2 and the Younger Dryas. The event follows the flooding of a large shelf platform and initiation of an extensive barrier reef system at 13 ka. Carbon cycle simulations show the coral δ13C decrease was mainly caused by the combination of isotopic fractionation during reef carbonate production and the decomposition of organic land carbon on the newly flooded shallow‐water platform. The impacts of these processes on atmospheric CO2 and δ13CO2, however, are marginal. Thus, the GBR was not contributing to the last deglacial δ13CO2 minimum at ∼12.4 ka.
Plain Language Summary
An outstanding problem in our understanding of the global carbon cycle is unraveling the processes that were responsible for the rise of atmospheric CO2 during the last deglaciation (19,000–11,000 years ago). The carbon isotope 13C is commonly used to attribute the last deglacial atmospheric CO2 rise to various processes. The growth of tropical coral reefs has been controversially discussed in this context. To test this, well constrained reef carbonate records that span the last deglaciation are necessary, but such records are generally not available. Here we make use of a multi‐proxy coral reef record obtained at the Great Barrier Reef (GRB). The δ13C signal in the carbonate skeletons of fossil corals indicates a pronounced minimum that precisely coincides with a prominent minimum in atmospheric δ13CO2 as indicated by ice core records for the Younger Dryas cold period. We show, by carbon cycle simulations, that the GBR coral δ13C signal can be explained by changes in reef carbonate production and decomposition of organic land carbon on a newly flooded wide area. However, the simulations indicate that that the world's largest reef system in existence appears to have little effect on the last deglacial atmospheric CO2 and δ13CO2 changes.
Key Points
Great Barrier Reef corals indicate pronounced decrease in skeletal stable carbon isotopes between 12.8 and 11.7 ka during Younger Dryas
Event follows shelf flooding and barrier reef initiation at 13 ka and coincides with prominent atmospheric stable carbon isotope minimum
Carbon cycle modeling reveals marginal impact of changes in reef carbonate production and land carbon decomposition on atmospheric carbon
The Ribbon Reef 5 borehole offers a unique record of reef growth spanning the entire history of the northern Great Barrier Reef (GBR). Previous studies have reported the main stratigraphical, ...lithological and chronological patterns, as well as basic descriptions of the coralgal assemblages, but no detailed coral community analysis was undertaken. We present a quantitative analysis of the nature and distribution of Pleistocene coral communities and apply several statistical tools to define recurrent coral associations and compare the eight reef-building cycles recognized throughout the evolution of the GBR. The start of significant reef building occurs at 137m based on a major change in coral community structure and the inception of the reef cycles (Cy1–8). This revision, along with available stratigraphical and chronological data, suggests that barrier reef initiation may have occurred prior to MIS 11, earlier than previous reports. The coral assemblages at 137m reflect the transition from lower mesophotic (60–100m) to upper mesophotic (30–60m) settings, while the eight reef cycles above are characterized by three recurrent shallow-water reef-coral associations: Porites-Montipora-faviids (Po-Mo-Fa), pocilloporids (Poc), and Acropora-Isopora (Acro-Iso). Typically, these cycles begin with the Po-Mo-Fa association and end with the Acro-Iso association, reflecting shallowing and a catch-up growth mode. However, the first two cycles are characterized by a transitional phase dominated by the Poc association. The dominance of pocilloporids during the early stages of the GBR’s history and the long-term shift to an Acropora-Isopora-dominated community may reflect an increase in competitive pressure of acroporids over pocilloporids. Our findings are consistent with the view that reef coral community structure is predictable over 100-kyr time scale. However, variations within reef cycles highlight the importance of environmental changes operating at millennial time scales. Further studies are needed to better refine the reef chronology and clarify the influence of environmental variables (i.e. sea surface temperature, turbidity) on reef coral community structure.
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•The GBR initiation likely predates MIS 11 in the Ribbon Reef 5 borehole.•Transition from mesophotic to “shallow reef” corals reflects shallowing upward.•Reefal section composed of 8 reef cycles and 3 recurrent coral associations•Glacial-interglacial sea level changes control reef initiation-demise cycles.•Coral trends within reef cycles linked to millennial-scale environmental changes
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