Deglacial Meltwater Pulse 1B and Younger Dryas Sea Levels Revisited with Boreholes at Tahiti

REPORTS and/or collisional scattering in the compressed capsule 27. This work was supported in part by the U.S. Department Rochester (412761-G), and General Atomics (DE-AC52- core (Figs. 1 to 3). of Energy and the Laboratory for Laser Energetics (LLE) 06NA27279). 23. R. D. Petrasso et al., Phys. Rev. Lett. 90, 095002 (2003). National Laser User’s Facility (DE-FG52-07NA28059 and 24. C. K. Li et al., Phys. Rev. E 80, 016407 (2009). DE-FG03-03SF22691), Lawrence Livermore National 9 December 2009; accepted 15 January 2010 25. T. A. Lasinski et al., Rev. Mod. Phys. 45, S1 (1973). Laboratory (B543881 and LDRD-ER-898988), CEA/DIF Published online 28 January 2010; 26. J. J. MacFarlane, J. Quant. Spectrosc. Radiat. Transf. 81, (France, Cooperative agreement DE-FC52-08NA28302), 10.1126/science.1185747 287 (2003). LLE (414090-G), Fusion Science Center at University of Include this information when citing this paper. persisted in the reef at the levels corresponding Deglacial Meltwater Pulse 1B and to the age of MWP-1B. To settle the issue, we dated by U-Th 47 Younger Dryas Sea Levels Revisited pristine coral samples from three new reef cores (P8, P9, and P10) drilled onshore of the Papeete barrier reef in Tahiti, close to the location of our with Boreholes at Tahiti previous study (23 U-Th dated corals from P6 and P7 cores) (12). P8 is at about the same Edouard Bard,* Bruno Hamelin, Doriane Delanghe-Sabatier position as P7 (12) but was drilled at an angle of 33° toward the sea, whereas P9 and P10 were Reconstructing sea-level changes during the last deglaciation provides a way of understanding the ice collected in the inner part of the barrier reef dynamics that can perturb large continental ice sheets. The resolution of the few sea-level records toward the Papeete Pass (fig. S1). covering the critical time interval between 14,000 and 9,000 calendar years before the present is still Figure 1B shows a comparison of the new insufficient to draw conclusions about sea-level changes associated with the Younger Dryas cold U-Th data from P8, P9, and P10 cores with the event and the meltwater pulse 1B (MWP-1B). We used the uranium-thorium method to date previous Tahiti record (12). Unlike the Barbados shallow-living corals from three new cores drilled onshore in the Tahiti barrier reef. No significant cores, each of these Tahiti cores yields an discontinuity can be detected in the sea-level rise during the MWP-1B period. The new Tahiti sea-level uninterrupted record of the time window record shows that the sea-level rise slowed down during the Younger Dryas before accelerating again corresponding to MWP-1B. The new U-Th data during the Holocene. (Fig. 1B and table S1) provide an unprecedented on March 4, 2010 resolution and can be compared to the other sea- nderstanding the behavior and predict- hiatuses between individual drill cores collected level records from Barbados (7, 8), Papua New ing the fate of large ice sheets can be at different depths off Barbados (see SOM text 2 Guinea (Huon Peninsula) (18, 20), and Vanuatu Udone in parallel by studying recent and and map in fig. S2). Several other records have (Urelapa) (21) (Fig. 1, C and D). The North ongoing changes in the climate system (1, 2) been interpreted later as direct or indirect evidence Greenland Ice Core Project (NorthGRIP) iso- and by studying the dramatic sea-level changes of the occurrence of MWP-1A (12–16). One of tope record is also plotted in Fig. 1A using its that occurred during the last deglaciation the main goals of the recent Integrated Ocean most recent time scale (22). This is done to [21,000 to 5000 years before the present (yr B. Drilling Program (IODP) Leg 310 at Tahiti was compare the sea-level records with climatic P.)]. To date, the most complete record of specifically to collect an additional coral record transitions such as the inception and the end of www.sciencemag.org deglacial sea level is based on reef cores drilled over the MWP-1A time window. The new suite the Younger Dryas (YD), which marks the start at Barbados, which have yielded ages from both of coral samples collected during this IODP of the Holocene period. 14C(3–5) and mass spectrometric U-Th campaign successfully confirms the existence of The large number of data points derived from methods (5–8). The Barbados record is charac- MWP-1A and leads to a reassessment of its age the four cores provides a very accurate con- terized by two periods of sea-level acceleration and amplitude (17). straint on the sea-level rise during this period, [meltwater pulses (MWP) 1A and 1B] that However, MWP-1B is even more controver- defined by the coherent upper envelope of the occurred around 14,000 calendar yr B.P. (cal yr sial and still needs to be confirmed, both at paleo-depths of the samples. The small scatter Downloaded from B.P.) and 11,300 cal yr B.P., respectively. Barbados and at other far-field sites. Indeed, of the data reflects the inherent uncertainty During each MWP event, the sea level appar- subsequent coral studies at Huon Peninsula (18) linked to the paleo-bathymetry of corals and ently rose by several meters per century, leading and Tahiti (12) questioned the timing and associated shallow-living biological assemblages to a major hydrological perturbation that prob- amplitude of this freshwater pulse. Additional (23, 24). Part of this overall scatter is also ably impacted the ocean circulation [e.g., (9, 10)]. doubts were also raised about the existence of related to the different positions of the drill cores Both the amplitude and the localization of MWP-1B by a study of sea level in northwest on the barrier reef (SOM text 2 and fig. S1). P9 injection into the ocean are crucial in understand- Scotland based on the so-called “marginal basin and P10 record the upper reef crest on the inner ing the climatic impact of a MWP event [e.g., isolation” technique (19). However, the inter- part of the barrier. By contrast, P8 was drilled on (11)]. However, several first-order questions re- pretation of this Scottish record is complex due the outer part of the barrier reef, with a deviation main unresolved on the precise characterization of to its proximity to former ice sheets in a region of 33° toward the sea. Therefore, in the lower these events, despite the intensive research carried where the postglacial rebound contribution is sections of P8 below 65 m, the corals (red out over the last decade (see SOM text 1). dominant, which explains why the local sea points in Fig. 1B) plot slightly lower than those The precise timing and amplitude of MWP- level continued to fall during most of the from P7, P9, and P10, a difference that remains 1A and 1B are still open questions, because deglaciation. So far, the sample coverage and small (<6 m) but fairly systematic. both of these events were originally detected as depth resolution of these different studies are The rate of sea-level rise at Tahiti can be still insufficient to reach a definitive conclusion calculated by means of linear fits over the three Centre Européen de Recherche et d’Enseignement des about MWP-1B. Unfortunately, the new IODP specific climatic intervals: before, during, and ’ Géosciences de l Environnement (CEREGE), UMR 6635 sample collection from Tahiti is of little help in after the YD event (thick lines in Fig. 1B) (see CNRS, University Aix-Marseille, Institut de Recherche pour le Développement, Collège de France, Europôle de l’Arbois, studying MWP-1B, because the depth range of SOM text 2 and table S2 for details). Taken BP 80, F-13545 Aix-en-Provence Cedex 4, France. the drill cores was targeted on MWP1-A and the together, the Tahiti data define a relatively *To whom correspondence should be addressed. E-mail: earliest part of the deglaciation (i.e., 90 to 120 m). smooth sea-level rise, with no significant accel- [email protected] At these depths, only deep-living coral species eration during the time interval corresponding to www.sciencemag.org SCIENCE VOL 327 5 MARCH 2010 1235 REPORTS MWP-1B at Barbados (11,400 to 11,100 cal yr ~40 mm/year (Fig. 1D). In the Huon record, the far-field sites agree with the higher resolution B.P.; area shaded green in Fig. 1). This time interval of MWP-1B falls within a time record from Tahiti: None of these reconstruc- conclusion is based on Acropora and Pocillopora gap of about a millennium (12,100 to 11,100 cal tions shows an abrupt 15-m step around 11,300 samples from the four cores P7, P8, P9, and P10 yr B.P.). The Vanuatu record has only two coral cal yr B.P., in contrast to the Barbados record. (table S1), which exhibit a rather small scatter samples in this interval, but even considering The new Tahiti record includes many (<6 m) (see Fig. 1B). In contrast to the Tahiti the few samples below (older) and above samples covering the YD cold period, in record, the MWP-1B event appears as a (younger), it remains difficult to pick out a step particular from cores drilled in the inner part prominent step of ~15 m between two drill or pause during the 13,000 to 10,000 yr B.P. of the outer reef (P9 and P10). An important cores at Barbados, implying an apparent rise of interval.

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