Ocean-atmosphere interactions as drivers of mid-to-late Holocene rapid climate changes: Evidence from high-resolution stalagmite records at DeSoto Caverns, Southeast USA
Introduction
Whereas the climate of the mid-to-late Holocene appears relatively stable when viewed in the context of the large, rapid swings of the last deglaciation, a number of significant abrupt hydroclimate events have been documented in the time interval between 5 ka (1 ka = 1000 yrs) to present. The most dramatic and most intriguing abrupt climate events are two that occurred at or near the mid-Holocene. The first, named the “5 ka” event, occurred at the transition from the relatively warm Hypsithermal to the relatively cool Neoglacial (H/N transition) and its extent is considered global (Davis and Thompson, 2006, Hodell et al., 2001, Keigwin, 1996, Thompson et al., 2006). The other occurred during the early part of the late Holocene, the “4.2 ka” event but its global extent, expression and whether it represents a single or multiple events are subject to debate (Booth et al., 2005, Finné et al., 2011).
Two aspects of the prominent 5 ka and 4.2 ka climate shifts are particularly controversial: (i) their timing and duration, and (ii) the controlling factors of the hydroclimate shifts (Arz et al., 2006, Bar-Matthews and Ayalon, 2011, Booth et al., 2005, Cullen et al., 2000, Davis and Thompson, 2006, Dixit et al., 2014, Drysdale et al., 2006, Yang et al., 2015).
The precise timing and duration of the 5 ka and 4.2 ka events are equivocal because most dates are either based on the radiocarbon time scale using reservoir corrections with large uncertainties (Arz et al., 2006, Cullen et al., 2000, Dixit et al., 2014, Russell and Johnson, 2005) and/or questionable correlations (Gasse, 2000). Exceptional are speleothem archives from caves in Corchia, Italy (Drysdale et al., 2006), Soreq, Israel (Bar-Matthews and Ayalon, 2011), and Mawmluh, Northeast India (Berkelhammer et al., 2012) whose age models were derived from absolute U/Th dates.
The factors controlling multi-centuries climate variability at sub-Milankovitch timescales are poorly constrained (Wanner et al., 2008). A number of studies proposed solar variability as a possible forcing mechanism (Bond et al., 2001, Wanner et al., 2008) while others implicate the Atlantic Multidecadal Oscillation (AMO; Knudsen et al., 2011), a northward shift of the Westerlies (Zanchetta et al., 2014), or El-Niño-Southern Oscillation (ENSO) variability (Booth et al., 2005, Donders et al., 2005).
Here we report the results of a stalagmite investigation (DSSG-5) from DeSoto Caverns in the Inner Gulf Coast (IGC), Southeast US (SEUS) (Fig. 1 A) spanning the time interval from 6.0 to 1.1 cal ka BP (calendar kilo-anum Before Present, where present is 1950 CE). The stalagmite, whose chronology is anchored on 35 precise 230Th/234U absolute dates, yields stable oxygen and carbon isotope time-series at interannual resolution of 2–8 yrs that exhibit with unusual clarity the climate changes that occurred during mid-to-late Holocene. The geographic location of the cave in the IGC offers unusual opportunities to study hydroclimate variability around the mid-to-late Holocene time interval for the following reasons: (i) it is located at the southern limit of the winter polar jet-stream; (ii) it receives moisture predominantly from the Gulf of Mexico (GoM) that occupies a central position in the Atlantic Warm Pool (AWP), the second largest oceanic warm pool on Earth (Wang and Enfield, 2001); (iii) GoM is a significant source of moisture fueling the North American rainfall at present and its moisture-controlling role likely extended back in the Holocene and beyond, and (iv) global atmospheric circulation patterns, such as ENSO and the Bermuda High (BH), govern the interannual δ18O isotope trends discerned in the water cycle compartments (Lambert and Aharon, 2010). This study provides detailed climate proxy records whose chronology is well constrained, and assesses the dominant drivers of climate variability in mid-to-late Holocene in the SEUS where high-resolution continental paleo-climate records, such as those from speleothems, are notably lacking.
Section snippets
Study site and regional climate
DeSoto Caverns (86°16′36″ W, 33°18′26″ N) is located on the outskirts of Childersburg, AL in the IGC and is separated from its major moisture source in the GoM by 365 km of low elevation coastal plains (Fig. 1, inset). Details of the cave geomorphology can be found in Lambert and Aharon, 2010, Lambert and Aharon, 2011 and will not be repeated here. Pristine fossil and active speleothems within the cave consist primarily of metastable aragonite (Lambert and Aharon, 2010, Lambert and Aharon, 2011
Methods
An actively growing stalagmite, located in the center of the cave front-chamber, was cored along the growth axis (core dimensions: 75 cm long, 4 cm diameter) without causing permanent damage to the whole formation (DSSG-5, Fig. 2 A). The uppermost 40 mm of the stalagmite contains several hiatuses reported by Aharon et al. (2012) and therefore the paleoclimate reconstruction in this study focuses on the stalagmite interval below 40 mm. X-ray diffraction analysis coupled with petrographic
Stable oxygen and carbon isotopes
δ18O and δ13C measurements, graphed against depth below 40 mm of stalagmite (Fig. 2B and C), consist of 1884 samples of which 97% are primary aragonite (n = 1828) and 3% (n = 56) are replacement calcites. Phillips (2011) documented two types of neomorphic calcite fabrics replacing the metastable aragonite in the DSSG-5 stalagmite (Fig. 2 A): (i) equant, and (ii) columnar. Equant calcite typically inherits the textural and chemical features of the primary aragonite whereas the columnar calcite
Paleoclimate implications of the δ18O and δ13C time-series
The isotope time-series in Fig. 5 raise the question of their paleoclimate significance. Three prominent isotope events need to be address: (i) the exceptional 18O and 13C-enrichments at ∼4.8 cal ka BP (the “5 ka” event); (ii) the recovery of the isotopes to almost modern level at ∼4.6 cal ka BP followed by interdecadal and intercentennial δ18O and δ13C synchronous cycles, and (iii) the millennial 18O-enrichment trend that is absent in the contemporaneous δ13C time-series. We will first
Conclusions
- 1.
Investigation of an actively forming stalagmite at DeSoto Caverns by thin sections petrography, stable oxygen and carbon isotopes and high precision 230Th/234U radiometric dating of pristine aragonites render proxy hydroclimate time-series at interannual resolution and afford an assessment of the dominant climate drivers in the time interval ∼6-1 cal ka BP.
- 2.
Prior monitoring of the cave ambient environment, isotope chemistry determinations of multiple drips, decadal-long weekly rainfall δ18O and δ
Acknowledgements
The study of the mid-to-late Holocene stalagmite from DeSoto Caverns was funded by grants to the senior author from the Alabama State Climatologist Office and a faculty grant from the Gulf Coast Association of Geological Societies (GCAGS). Rajesh Dunghana was the recipient of graduate student grants from GCAGS and internal grants from the Department of Geological Sciences at the University of Alabama. U-series measurements were performed under contract by John Hellstrom at the School of Earth
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