CORAL REEF RESPONSE TO EXTREME SEA-LEVEL CHANGE: THE MELTWATER PULSE 1A (14.65 ka and 14.3 ka BP). IODP EXPEDITION #310 ‘TAHITI SEA LEVEL’

Coral reef records related to past higher and/or rising sea levels provide an important baseline for developing projections regarding the response of modern coastal systems to future sea-level rise. Sea-level rise at the end of the current century is expected to range between 5.5 and 10 mm. yr^-1 on average, depending on the various scenarios of  global warming [IPCC, 2019]. The Last Deglaciation (23 to 6 kyr B.P.) is seen as a potential recent analogue of the environmental changes that the Earth may face in the near future as a consequence of ocean thermal expansion and the melting of polar ice sheets. The last deglacial record from Barbados suggests a non-monotonous sea-level rise averaging 10 mm.yr^-1 and punctuated by two ‘meltwater pulses’ (MWP) characterized by several centuries of extremely rapid sea-level rise related to catastrophic ice-sheet collapse [Fairbanks, 1989, Nature, 342, 637; Bard et al., 1990, Nature, 346, 456; Peltier & Fairbanks, 2006, Quat. Sci. Rev., 25, 3322].

IODP Expedition 310 ‘Tahiti Sea Level’ and land drilling on the modern barrier reef of Papeete have provided unparalleled coral reef records encompassing the period covered by the two MWP identified previously in Barbados. Reefs accreted continuously between 16 and 10 kyr B.P. in Tahiti, mostly through aggradational processes, at growth rates averaging 10 mm yr^–1. Changes in the composition of coralgal assemblages coincide with abrupt variations in reef growth rates and characterize the response of the upward-growing reef pile to non-monotonous sea-level rise and coeval environmental changes [Camoin et al., 2012; Geology, 40, 643; Camoin & Webster, 2015; Sedimentology, 62, 401].

While the MWP-1B at approximately 11.3 kyr B.P. in Barbados is absent or very small in Tahiti [Bard et al. 1996; Nature, 382, 241; Bard et al., 2010; Science, 327, 1235; Bard et al., 2016; Paleoceanography, 31], the Tahiti offshore record has provided the opportunity to document the MWP-1A at several drill sites. A sea-level rise of 16±2 m in amplitude has been evidenced between 14.65 and 14.3 kyr B.P., coeval with the Bølling warming [Deschamps et al., 2012, Nature, 483, 559]. The rate of eustatic sea-level rise ranged from 40 to 50 mm.yr^-1 during MWP-1A, implying that this episode corresponds to one of the fastest rises in sea level ever documented in Earth history.

This paper documents in unprecedented detail the reef response to extreme sea-level rise during MWP-1A in Tahiti. It is based on new accurate U-series and ¹⁴C AMS ages of corals and algae and the reappraisal of the environmental significance and paleowater depth interpretation of various coralgal assemblages. The succession in time and space of successive reef assemblages involved in reef accretion during the MWP-1A leads, for the first time, to reconstruct reef accretion patterns during this dramatic period, which is of prime importance to help forecasting coral reef response to future sea-level change.