Stalagmite-inferred European westerly drift in the early Weichselian with centennial-scale variability in marine isotope stage 5a
Introduction
The last glacial cycle, starting from marine isotope stage (MIS) 5e at ∼130 thousand years ago (ka, relative to AD 1950; hereafter), features rapid and recurrent millennial climate variations, as documented in Greenland ice cores with relatively warm interstadials (GI) and cold stadials (GS) (Johnsen et al., 1992; Dansgaard et al., 1993; NorthGRIP-Members, 2004) (Fig. 1a). Northern hemisphere summer insolation (NHSI) peaks (Fig. 1b) brought warm temperatures and ice volume reductions, interrupting the Earth's transition from the last interglacial to last glacial maximum (Lambeck and Chappell, 2001). In the early Weichselian (∼115–74 ka), two warm intervals, MIS 5c and 5a (Fig. 1) started following the NHSI maximum at 105 and 85 ka (Fig. 1b), with the global sea-level maximum reaching −10 to −20 m at 100 and 83 ka, respectively (Fig. 1e). These two warm intervals featured unstable ice-sheet configurations in Fennoscandia, Greenland, and North America (Chapman and Shackleton, 1999; Mokeddem and McManus, 2016; Batchelor et al., 2019). Ice-sheet instability and the associated meltwater input in the North Atlantic may have resulted in multi-centennial to millennial oscillations in Asian stalagmite δ18O values associated with Asian monsoon intensity variations (Cheng et al., 2016) (Fig. 1b). Fluctuations in Atlantic planktic foraminiferal δ18O (δ18Opf) values (Fig. 1c; de Abreu et al., 2003) and sea surface temperatures (SST) off the Iberian margin (Fig. 1c; Martrat et al., 2004) during MIS 5c and 5a also suggest variations in the Atlantic meridional overturning circulation (AMOC), which can modulate the hydroclimate of the North Atlantic and Europe (Stouffer et al., 2006; Margari et al., 2010; Kageyama et al., 2013; Jackson et al., 2015; Stockhecke et al., 2016; Tzedakis et al., 2018). For example, abrupt δ18O shifts in Alpine stalagmite records (NALPS; Fig. 1d) which correspond to the δ18O shifts in Greenland ice cores (Fig. 1a; NorthGRIP-Members, 2004) could have been affected by ocean circulation changes (Boch et al., 2011). At the end of MIS 5b (∼85 ka), the abrupt positive shift in NALPS δ18O has been linked to the end of GS22, accompanying warming in the North Atlantic (Boch et al., 2011).
The circum-Mediterranean region has a classical Mediterranean climate featuring hot/dry summers and mild/wet winters (Beck et al., 2018). Over the past two decades, droughts in southern Europe have threatened water supply, ecosystem, and agricultural instabilities over the past two decades (Hoerling et al., 2012; Naumann et al., 2021). Complex forcings from rising greenhouse gases, ice-sheet meltwater input, and changes in the strength of AMOC hinder our ability to reliably predict Mediterranean hydroclimate over the next century. As NHSI values during MIS 5a are as high as those in the Holocene (Berger, 1978; Laskar et al., 2011), highly-resolved proxy records during MIS 5a offer important clues to better understand the future climate. Previous studies have highlighted centennial-to-millennial scale climatic variability in the North Atlantic during MIS 5 (e.g., Oppo et al., 1997; Mokeddem and McManus, 2016), especially in southern Europe (Denniston et al., 2018; Tzedakis et al., 2018; Budsky et al., 2019). Most cases, however, have focused on the Eemian warm period (MIS 5e; e.g., Tzedakis et al., 2003, 2018; Drysdale et al., 2005; Allen and Huntley, 2009; Milner et al., 2013;). Less attention has been given to subcentennial-to centennial-scale variability during MIS 5a due to the limitation of archive resolution and dating precision.
Here, we present a subdecadal-to multidecadal-resolved stalagmite-inferred precipitation record with robust chronology, from Monaco (northern Mediterranean) to understand hydroclimate variability associated with the westerly changes during 88.7–80.3 ka in the early Weichselian, especially focusing on MIS 5a. This is complemented by results from a transient experiment for the period 86–80 ka performed with the Earth system model (LOVECLIM), to assess the potential hydroclimatic variability drivers in southern Europe.
Section snippets
Cave and regional settings
Observatoire cave ([43°43′ N, 7°24’ E], 103 m above sea level) (Fig. 2, Fig. 3) preserves evidence of the oldest human occupations in Monaco, southern Europe (Rossoni-Notter et al., 2016). The cave is located along the northwestern Mediterranean coastline, in upper Jurassic bedrock - a limestone block from the “Arc de Nice” subalpine mountain chain (Gilli, 1999). This 600 m-long cave opens to the south and its entrance hall is 17 m in length, 6 m in width, and 7 m in height (Fig. 3a). After the
U–Th data and age model
Detailed U–Th isotopic and concentration data and dating results of Observatoire stalagmites OV12-1 and 12-5 are given in Table S1. Stalagmite OV12-1 has 238U content of 0.4–0.7 × 10−6 g/g and 232Th context of 10–2200 × 10−9 g/g. Stalagmite OV12-5 features high 238U contents of 1.5–8.5 × 10−6 g/g and low 232Th contents of 0.01–2.1 × 10−9 g/g. The uncertainties of corrected 230Th dates are from ± 368 to ± 597 years on OV12-1 and from ± 19 to ± 180 years on OV12-5. The dates are in stratigraphic
Tests for stalagmite isotopic equilibrium conditions
Factors affecting isotopic compositions in stalagmites depend on a variety of isotopic fractionation processes during rainfall condensation and water infiltration in karst systems (McDermott, 2004). Unexpected kinetic effects that are usually associated with degassing during carbonate precipitation could bias the isotope signals from the climatic imprint. The Hendy test (Hendy, 1971) and duplication test (Dorale and Liu, 2009) have been widely used for evaluating isotopic equilibrium. The
Conclusions
We present stalagmite δ18O and δ13C-inferred precipitation records from 88.7 ± 0.4 to 80.3 ± 0.1 ka covering parts of MIS 5b to 5a from Observatoire cave, Monaco, southern Europe. The inferred precipitation record features four multi-centennial arid events during MIS 5a, suggesting that the westerlies moved away from Monaco. New transient simulation suggests that AMOC slowdowns can divert the westerlies from the Mediterranean region, which would result in stalagmite-inferred dry conditions in
Author contributions
C.-C.S. directed this research. C.-C.S., Y.-C.C. and H.-M.H conceived the project. C.-C.S., V.M., P.V., P.S., E.R.-N., A.M. and C.-C.W. conducted field surveys and collected stalagmites. Y.-C.C. and H.-M.H. conducted subsample preparation. Y.-C.C. and H.-M.H performed U–Th dating. H.-S.M. and X.J. conducted carbon and oxygen stable isotope analyses. L.M. conducted model simulations. A.M. conducted statistical analyses. Y.-C.C., C.-C.S., H.-M.H., L.M., and H.S. prepared the draft and
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to deeply thank G.S. Burr of the Research Center for Future Earth, National Taiwan University, for his constructive suggestions. We are thankful for the financial support provided by grants from the Science Vanguard Research Program of the Ministry of Science and Technology, Taiwan, ROC (110-2123-M-002-009), the Higher Education Sprout Project of the Ministry of Education, Taiwan, ROC (110L901001 and 110L8907), and the National Taiwan University (109L8926). We are also grateful
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