A data-model comparison pinpoints Holocene spatiotemporal pattern of East Asian summer monsoon

https://doi.org/10.1016/j.quascirev.2021.106911Get rights and content

Highlights

  • Holocene speleothem δ18O and rainfall show heterogeneous pattern in eastern China.

  • East Asian speleothem δ18O reflects large-scale circulation and monsoon rain belt shift.

  • Divergent phases between speleothem and other proxy records are not contradictory.

Abstract

Conflicting reconstructions of Holocene variability of the East Asian summer monsoon (EASM) from speleothem versus other types of proxy records have yielded widely divergent estimates of its phase relationship with the Indian summer monsoon (ISM) and Northern Hemisphere summer insolation (NHSI). This apparent discrepancy has been partly attributed to the uncertainties in the climatic representation of Chinese speleothem oxygen isotope18O) records. Here we present a composite speleothem δ18O record of the last ∼14 kyr from Shennong Cave in southeastern China and model-simulated data of rainfall and meteoric δ18O over eastern China. Our synthesis of the proxy-model data suggests that the spatial patterns in both speleothem δ18O and paleo-rainfall over eastern China during the Holocene are diverse at orbital and multi-millennial scales. Our findings imply that: 1) speleothem δ18O in the EASM regime is largely controlled by the large-scale circulation and concomitant latitudinal shifts of the monsoon rain belt; notwithstanding the heterogeneous spatiotemporal pattern of Holocene rainfall as inferred from various proxy records, a coherent orbital-scale speleothem δ18O variability across most Asian monsoon regions (except southeastern China) indeed stems from the NHSI-forced changes in overall monsoon intensity; overall monsoon intensity is not equivalent to monsoon rainfall amount but a manifestation of the large-scale atmospheric circulation; 2) divergent phase relationships with NHSI between speleothem δ18O and other proxy records are consistent with—rather than contradictory to—the NHSI forcing mechanism. Speleothem δ18O and rainfall records reflect two different aspects of the monsoon dynamics. These results may thus, largely help to reconcile the divergent views of the Holocene Asian monsoon variability.

Introduction

Over the past two decades, speleothem oxygen-isotope (δ18O) records from eastern China have been widely used for characterizing and understanding East Asian summer monsoon (EASM) variability on a wide range of timescales (Cheng et al., 2016, 2019; Wang et al., 2001, 2005; Zhang et al., 2019). However, the climatic significance of speleothem δ18O has been persistently debated, as exemplified by the widely contrasting estimates of phase relationships between the EASM, Indian summer monsoon (ISM), and Northern Hemisphere summer insolation (NHSI) when inferred from speleothems versus other types of proxy records (Chen et al., 2015, 2016; Cheng et al., 2021; Clemens et al, 2010, 2018; Gebregiorgis et al., 2018; Maher, 2008; Maher and Thompson, 2012; Ruddiman, 2006). With regard to the Holocene in particular, this debate is centred on two main issues. (1) The timings of the strongest EASM phase during the Holocene inferred from speleothem and other proxy records differ considerably. For example, the loess-paleosol and lake-sediment proxy records place the wettest period between ∼5 and 2.4 kyr BP (thousand years before present, where present = 1950 CE) on the western Chinese Loess Plateau (Maher, 2008) and at ∼6 kyr BP in northern China (Chen et al., 2015), respectively. In contrast, the speleothem δ18O records from southern China suggest a strongest EASM occurred in the Early-Middle Holocene (10-7 kyr BP) (Cheng et al., 2019, 2021; Dong et al., 2010; Wang et al., 2005; Zhang et al., 2019). These temporal inconsistencies have led to a proposition that variations in speleothem δ18O may reflect changes in moisture source (or circulation regime), rather than EASM intensity (Chen et al., 2016; Maher, 2008; Maher and Thompson, 2012). (2) Orbital-scale variability of the broader Asian monsoon reconstructed from Chinese speleothem δ18O and from marine records in the Arabian Sea (Clemens et al., 2010) lag NHSI (June 21) by ∼3 kyrs and ∼9 kyrs, respectively, at the precession band. This discrepancy has been attributed to the confounding influence of summer, winter, and spring rainfall δ18O signals in speleothem records (Clemens et al., 2010). Furthermore, unlike the Chinese speleothem δ18O records, the planktonic foraminifera δ18O-based reconstruction of the EASM from the East China Sea lacks precession cycles and the seawater δ18O record from the Andaman Sea (Gebregiorgis et al., 2018) lags NHSI by ∼9 kyrs at precession band, suggesting that orbital-scale EASM variations are more sensitive to greenhouse-gas and high-latitude ice-sheet forcing rather than insolation (Clemens et al., 2018). Contrasting views of the Holocene EASM history may reflect how various proxies record different aspects of EASM dynamics (Cheng et al., 2019, 2021; Zhang et al., 2020a). For instance, speleothem δ18O might reflect the overall EASM strength, if defined by summer monsoon wind strength and/or spatial-scale of the monsoon circulation (Cheng et al., 2016; Liu et al., 2014b), whereas the marine/lacustrine reconstructed rainfall records are possibly more sensitive to local or regional precipitation-evaporation balance (Cheng et al., 2019; Gebregiorgis et al., 2018; Li et al., 2020d; Zhang et al., 2020a). To address these differences, a comprehensive comparison of speleothem δ18O with other proxy records and model simulation data is warranted.

To date, most Chinese Holocene speleothem δ18O records are distributed along a SW-NE transect in monsoonal China, but coeval time series from southeastern China (110–120°E; 20–30°N) were hitherto not available (Fig. 1A) (Comas-Bru et al., 2020; Zhang et al., 2019). In this study, we present a composite 14-kyr speleothem δ18O record from Shennong Cave, Jiangxi Province, southeastern China (Fig. 1A), where δ18O variations in both precipitation and speleothem calcite are influenced almost equally by both EASM and non-summer monsoon precipitation (Zhang et al., 2020b). The Shennong δ18O record fills a critical gap, allowing us to characterize more thoroughly the spatiotemporal variability of Holocene rainfall and speleothem δ18O in the EASM domain. We compared paleo-rainfall and speleothem δ18O records in eastern China with the published simulation results from Community Atmosphere Model version 5 (CAM5) and TRACE (Kong et al., 2017; Liu et al., 2009, 2014b). Additionally, three new snapshot experiments for 9 kyr BP, 6 kyr BP and the Pre-Industrial (PI) were performed by an isotope-enabled atmosphere-ocean fully coupled climate model COSMOS-wiso (ECHAM5-MPIOM-JSBACH) (Werner et al., 2016) to interpret reconstructed paleo-rainfall and speleothem δ18O records. Our results, together with previous observational and model results, characterize the spatiotemporal patterns of speleothem δ18O and other proxy records in the context of model simulations, and in turn provide new insights into the underlying mechanism(s), especially with regard to their orbital-scale phases, thus reconciling the long-standing debates about EASM variability and its relationship to ISM and NHSI during the Holocene.

Section snippets

Regional setting and sampling

Shennong Cave (117°15′N, 28°42′E, 383 m a.s.l.) is located in the northern Jiangxi Province, southeastern China (Fig. 1A). In the study area, EASM rainfall (spanning mid-May to September, MJJAS) only accounts for ∼50% of annual precipitation (Zhang et al., 2020b) (Fig. 1A). A large fraction (∼40%) of annual precipitation occurs from March to mid-May—a period termed the ‘spring persistent rain’ (Wan and Wu, 2009) that precedes the regional onset of EASM rainfall. The amount-weighted mean δ18O

Chronology and stable isotope records from Shennong Cave

A total of 52 230Th dates provide robust chronological controls for three stalagmite age models (Fig. 2). All 230Th dates are in stratigraphic order, and the age models were established by linear interpolation between dates. The three stalagmites provide δ18O and δ13C records across the interval 0.9–14 kyr BP with four short hiatus in the Middle-Late Holocene (Fig. 2, Fig. 3). During the overlapping growth periods (4.5–8.0 kyr BP), the three δ18O records replicate well after correcting

Significance of speleothem δ18O and δ13C in Shennong Cave

By using precipitation and δ18O data from the nearest GNIP station (Changsha; 28.2°N 113.1°E), simulated data from Isotope-incorporated Global Spectral Model version 2 (IsoGSM2) (Yoshimura et al., 2008), and a seasonally resolved 200-year speleothem δ18O record from E’mei Cave (160 km NW of Shennong Cave), we previously demonstrated that precipitation δ18Ow or speleothem δ18O in the study area is primarily controlled by the precipitation seasonality (i.e., the ratio of MJJAS/annual

Conclusions

We present the first composite speleothem δ18O record spanning the last 14 kyr from Shennong Cave in southeastern China, which fills an important spatial gap in the suite of Holocene speleothem δ18O records across eastern China. Our new speleothem record and model simulations demonstrate that the patterns of both Holocene speleothem δ18O and precipitation, including the Holocene rainfall optimum, are indeed spatiotemporally diverse across monsoonal China. A close model-data comparison

Author statements

H.W.Z. and H.C. proposed and directed the study. X.Z. conducted COSMOS-wiso simulations, analysed the modelling results and assisted H.W.Z with dynamical interpretation of records. H.W.Z. and H.C. led the writing, and X.Z., Y.J.C., A.S., C.S. Z.Y.L. and J.B. contributed most to the discussion and editing. G.K. and Y.T. participated in sampling. H.W.Z., Y.T., X.X.J. and R.L.E. contributed to 230Th dating. H.W.Z., J.Y.L., W.J.D. and Y.F.N. contributed stable isotope analysis. All authors

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.

Acknowledgements

We would also like to thank Editor Miryam Bar-Mattews and three anonymous reviewers for their comments and suggestions. Thanks to Dr. Wenwen Kong and Dr. Jun Hu for their suggestions of interpretting. We would like to thank Editor Miryam Bar-Mattews and three anonymous reviewers for their comments and constructive suggestions. We also thank Dr. Wenwen Kong for her help of using snapshot experiments data and Dr. Jun Hu for his suggestions of the interpretation of convective precipitation. This

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    These authors contributed equally to this work.

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