The “4.2 ka event” is frequently described as a major global
climate anomaly between 4.2 and 3.9 ka, which defines the beginning of
the current Meghalayan age in the Holocene epoch. The “event” has been
disproportionately reported from proxy records from the Northern Hemisphere, but
its climatic manifestation remains much less clear in the Southern Hemisphere.
Here, we present highly resolved and chronologically well-constrained
speleothem oxygen and carbon isotopes records between
Proxy locations and climatology.
The “4.2 ka event” is considered to be a widespread climate event between 4.2 and 3.9 ka (thousand years before present, where the present is 1950 CE) (e.g. Weiss et al., 1993, 2016). Many paleoclimate records from the Northern Hemisphere (NH) have characterised the event as a multi-decadal to multi-centennial period of arid and cooler conditions across the Mediterranean, Middle East, South Asia and North Africa (e.g. Finné et al., 2011; Marchant and Hooghiemstra, 2004; Migowski et al., 2006; Mayewski et al., 2004; Staubwasser et al., 2003; Arz et al., 2006; Zielhofer et al., 2017; Stanley et al., 2003; Kathayat et al., 2017). The structure of the 4.2 ka event from many proxy records, such as peat cellulose records from the eastern Tibetan Plateau (Hong et al., 2003, 2018), speleothem from northeastern India (Berkelhammer et al., 2012) and southern Italy (Drysdale et al., 2006), marine sediments from the Gulf of Oman (Cullen et al., 2000) and the northern Red Sea (Arz et al., 2006), and the dust record in the Kilimanjaro ice core (Thompson et al., 2002), typically characterised it as a single pulse-like signal in the long-term context of these records. In contrast, the structure of the 4.2 ka event in the Southern Hemisphere (SH) remains unclear. Some proxy records from the tropical and subtropical regions of Africa and Australia show a shift towards drier conditions around 4 ka (e.g. Russell et al., 2003; Marchant and Hooghiemstra, 2004; Griffiths et al., 2009; Denniston et al., 2013; Berke et al., 2012; De Boer et al., 2013, 2014, 2015; Schefuß et al., 2011; Rijsdijk et al., 2009, 2011). Other records show virtually unchanged hydrological conditions (e.g. Tierney et al., 2008, 2011; Konecky et al., 2011) or two-pulsed multi-decadal length wet events (Railsback et al., 2018) during the period contemporaneous with the 4.2 ka event.
The goal of this study is to investigate a time period that spans the
4.2 ka event in a key region of the SH via highly resolved and precisely
dated proxy records. Here, we present speleothem oxygen (
Rodrigues (
Modern observations of
Two stalagmites, LAVI-4 and PATA-1, from La Vierge and Patate caves,
respectively, were used in this study. La Vierge (19
Subsamples (80–130 mg) for
LAVI-4 and PATA-1 stable isotope (
We obtained 26 and 7
The time interval from 6 to 3 ka in LAVI-4 speleothem corresponds to a
sample depth of 274 to 81 mm below the top, respectively. A drip-water
relocation occurred at a depth of 124 mm, which is associated with a Type L
surface characterised by slow growth and narrow layers under progressively
drier conditions (Railsback et al., 2013) (Figs. S3 and S4). It cannot be
ruled out that there is also a hiatus at this depth (
Conventional criteria to assess the isotopic equilibrium of stalagmites are
provided by the Hendy Test (Hendy, 1971), which requires no correlation
between
The temporal resolution of the LAVI-4
Inferred hydro-climatic variability at Rodrigues from 6 to 3 ka.
LAVI-4
Comparison of LAVI-4 with climate proxy records from the eastern
Indian Ocean. From top to bottom, z-score-transformed speleothem
The z-score-transformed profiles of LAVI-4
Overall, the climate variations recorded at Rodrigues from 4.2 to 3.9 ka are
characterised by high-frequency (decadal to multi-decadal) fluctuations,
including the major arid/wet events mentioned above. Notably, however, the
mean hydro-climatic state of this time interval inferred from both
The most prominent feature of our record is a switch from an interval
characterised by high-frequency
The LAVI-4
To sum up, our Rodrigues records show evidence of multi-decadal–decadal
hydro-climate fluctuations around the mean state between 6 and 3 ka. After
3.9 ka, the hydro-climate was characterised by a multi-centennial trend
toward much drier conditions, which ended with a return at
Comparison of LAVI-4 with climate proxy records from India and east
Africa. From top to bottom:
A close examination of our Rodrigues
In parallel with drier condition along the southern limit of the austral summer ITCZ, proxy records from Lake Edward (Russell et al., 2003), Lake Victoria (Berke et al., 2012) and Tangga Cave (Wurtzel et al., 2018), which are located near the northern limit of the contemporary austral summer ITCZ, also exhibit drier conditions. In contrast, records within the core location of the austral summer ITCZ, such as Lake Challa (Tierney et al., 2011), Lake Tanganyika (Tierney et al., 2008), Lake Malawi (Konecky et al., 2011) and Makassar Strait (Tierney et al., 2012), show either slightly wetter or virtually unchanged hydro-climatic conditions (Figs. 4 and 5). Based on the observed spatial patterns, we suggest that the contraction of the ITCZ both in terms of a north–south meridional shift as well as with respect to its overall width may have played an important role in modulating the hydro-climate in our study area during and after the 4.2 ka event.
All data needed to evaluate the conclusions in the paper
are presented in the paper. Additional data related to this paper may be
requested from the authors. The data will be archived at the National Climate
Data Center
(
The supplement related to this article is available online at:
HC, AS and HYL designed the research and experiments; HC, AS,
JB, YFN, AAA, AM and HYL completed the fieldwork; HYL, HC, YFN and CS
performed stable isotope measurements and
The authors declare that they have no conflict of interest.
This article is part of the special issue “The 4.2 ka BP climatic event”. It is a result of “The 4.2 ka BP Event: An International Workshop”, Pisa, Italy, 10–12 January 2018.
We thank Nick Scroxton and another anonymous reviewer for their contribution to the peer review of this work. We very much appreciate editorial help from Raymond Bradley. This work was supported by grants from the NSFC (41472140, 41731174 and 41561144003), US NSF grant 1702816 and a grant from the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS (SKLLQG1414). Edited by: Raymond Bradley Reviewed by: Nick Scroxton and one anonymous referee