Investigating the hydrological significance of stalagmite geochemistry (Mg, Sr) using Sr isotope and particulate element records across the Late Glacial-to-Holocene transition
Introduction
Trace element concentrations in speleothems are a proxy for past climate and ecosystem variability over a wide range of environmental conditions (Treble et al., 2003, Borsato et al., 2007, Griffiths et al., 2010, Moreno et al., 2010, Zhou et al., 2011). The incorporation of trace elements into speleothem calcite is a function of both the chemical composition of the parent water and the crystal growth mechanisms. Data for spatial and temporal trace element variability within layers composed of the same crystal fabric allow evaluation of the interaction between climate parameters and dripwater chemistry, the latter being controlled by soil processes and host rock dissolution (Fairchild and Baker, 2012).
Dripwater transports trace elements into the cave in both dissolved and undissolved (particulate-colloidally bound, PCB) form (Fairchild and Treble, 2009). PCB elements include Al, Fe, Mn and Th, which are commonly adsorbed onto organic or inorganic colloidal particles (Hartland et al., 2012). These generally immobile elements are thought to be liberated from soil silicates under low-soil-pH conditions and during high-infiltration events (Kabata-Pendias, 2011). In contrast, the dissolved constituents of dripwater, such as Mg and Sr, are generally mobile elements which are transported as simple inorganic complexes in percolating waters. The concentration of Sr and Mg solutes in cave dripwater is a function of the residence time of the water in the aquifer when the bedrock is their only source (Tremaine and Froelich, 2013, Rutlidge et al., 2014). Consequently, these elements are considered to be reliable hydrological proxies (Hellstrom and McCulloch, 2000, McMillan et al., 2005, Fairchild and McMillan, 2007). Co-variance of Sr/Ca and Mg/Ca ratios indicates increased incongruent dissolution in carbonate phases in the host rock and is related to the residence time of the water in the aquifer (Sinclair, 2001). However, studies invoke incongruent dissolution to explain anti-correlated behavior between stalagmite Sr/Ca and Mg/Ca (e.g. Roberts et al., 1998). Positive covariation between Sr/Ca and Mg/Ca can also be achieved through prior calcite precipitation, which is accompanied by a parallel shift in δ13C due to the changes in the amount of 12C lost through degassing (Fairchild et al., 2000, Tooth and Fairchild, 2003, Johnson et al., 2006, Mattey et al., 2008, Wong et al., 2011, Stoll et al., 2012, Tremaine and Froelich, 2013, Regattieri et al., 2014). Coupled increases in speleothem Mg/Ca and Sr/Ca would be interpreted as marking low infiltration conditions (dry periods) in both the prolonged ‘water-rock’ interaction (WRI) and prior calcite precipitation (PCP) scenarios.
In fossil speleothems, the concentration of trace elements in the parent solution is commonly unknown, and the ‘partition-coefficient’ approach, which implies equilibrium incorporation, cannot be tested because non-equilibrium processes cannot be ruled out. The assumption that modern dripwater composition reflects the past may be valid only when the general climate mode has remained similar (Schimpf et al., 2011). During major climate transitions, this assumption is obviously not applicable. An alternative approach is to assume that fossil dripwater Mg and Sr concentrations can be estimated from the composition of the host rock, when this is the sole source of these two solutes in the dripwater. This assumption is valid when karst soil geochemistry is the result of bedrock weathering, with minimal input from allochthonous materials (e.g., Belli et al., 2013). When the host rock is the major source of solutes, a “speleothem enrichment factor” (SEF) can be calculated by comparing the concentration of a given trace element in the speleothem and in the host rock (Frisia et al., 2012). SEF has been found to be a useful tool for estimating the contribution of dolomite and limestone dissolution to groundwater Mg and Sr, and thus stalagmite, concentrations (Borsato et al., 2016). However, SEF cannot account for a contribution from external sources, such as airborne dust.
Beside the host rock, aeolian dust is a potential source of additional Mg and Sr, particularly in semi-arid regions. Based on data from infiltration experiments in the Wellington caves (NSW Australia), Rutlidge et al. (2014) demonstrated that Mg has both soil and bedrock components, whereas Sr is solely bedrock-derived. Their study highlights that interpretation of speleothem Sr and Mg elemental records should differentiate between seasonally dry and hot (semi-arid) settings such as Wellington caves and more temperate settings. The conclusion that Mg–Sr systematics in speleothems from semi-arid settings are unlikely to follow the single-source model probably also applies to stalagmites formed in periglacial settings, e.g. during a deglaciation when aeolian dust accumulated during glacial stages is mobilized. Wind-blown dust contributions can often be recognized from 87Sr/86Sr studies (e.g. Goede et al., 1998, Ayalon et al., 1999, Musgrove and Banner, 2004).
To contribute to this debate, we report trace element and Sr isotopic records for two stalagmites (SV1 and SV7) from Grotta Savi, in the southern foothills of the Alps, NE Italy. The Grotta Savi site was not glaciated during the Last Glacial Maximum but, similar to most periglacial locations in the Italian Alps, was influenced by deposition of aeolian dust during glacial conditions (Monegato et al., 2011). The proxy records in stalagmites SV1 and SV7 are thought to capture regional temperature and hydrological conditions during the Late Glacial to Holocene transition in the northern Adriatic region. The two stalagmites are characterized by different fabrics which reflect diverse infiltration systems, and are thus good materials to test a novel approach for interpreting anti-correlated Mg/Ca–Sr/Ca elemental records. This approach uses observed relationships between trace element concentrations, mineral fabrics and variations in 87Sr/86Sr to identify if Mg and Sr have multiple sources and allows separation of the various source terms to isolate and model the hydrological-climatic component.
We use this approach to define a hydrological index which is thought to be relevant to stalagmites grown under both wet-temperate and semi-arid to arid conditions, provided the different contributions to observed Mg–Sr records from different sources can be identified from microstratigraphy, PCB element variability and Sr isotopes. Savi speleothems are known to have recorded hydrological changes (Belli et al., 2013) and we use the Mg–Sr based hydrological index to search for significant changes in moisture source or amount in the δ18Oc records. In the case of Savi, this approach helps to clarify the response of climate in SE Alps of the large-scale reorganization of atmospheric and oceanic circulation that characterized the Younger Dryas (YD) (Brauer et al., 2008, Bakke et al., 2009, Rach et al., 2014), which caused important hydrological variability and South to North changes of wind direction (Baldini et al., 2015, Bartolomé et al., 2015).
Section snippets
Study site and speleothem description
Stalagmites SV1 and SV7 were collected 500 m from the main entrance in Grotta Savi, located near the Adriatic coast in NE Italy (45°37′N and 13°53′E). Today the site is characterized by temperate Mediterranean climate (Belli et al., 2013), but in the Late Glacial, proximity to the Alps and a more continental location (a result of sea levels 80–60 m below present) (Lambeck and Bard, 2000) probably supported a different pattern of orographically forced mesoscale circulation (Pichler and Steinacker,
Petrographic observation
Microstratigraphic observations were conducted on 30 micrometer-thick thin sections using a Zeiss Axioplan polarizing microscope at the School of Environmental and Life Sciences of the University of Newcastle. The SV1 microstratigraphic logging is based on the method of Frisia (2015), where fabrics are coded, transformed into numbers and logged against depth/ages. This method allows direct comparison of chemical and morphological data, highlighting when chemical changes are related to crystal
Host rock and dripwater chemical signature
The trace element composition of the limestone host rock is reported in Table 1. The Opicina Limestone is mostly composed of low-Mg calcite (Mg concentration ∼5100 ppm) with a relatively high Sr content (Sr concentration ∼550 ppm). Minor bioclasts (up to 0.5 mm in size) comprise about 8 vol%, are most likely red algae and are characterized by Mg and Sr concentrations almost twice the average of the low-Mg calcite matrix (Fig. S2). This suggests that the original Mg-calcite composition of red algae
Particulate and colloidally-bound (PCB) elements
The concentration profiles of PCB elements (Al, Mn, Th) in stalagmite SV1 show distinctive patterns of centennial-scale abundance peaks (up to 100× baseline levels) superimposed on segments with very low and more or less constant concentrations. In the remainder of this paper, the abundance peaks in the PCB element profiles are referred to as mode-P, defined as intervals where Th concentrations exceed the relevant mean baseline by a factor of 2 or more. On this basis, the profile for SV1
Hydrological index (HI) based on Mg and Sr time series
The variability of Srcg can be used to complement hydrological information from Mg concentrations. For low to moderate flow regimes (mode-F), the two series have a similar but antipathetic pattern. With the maximum bedrock-derived Mg threshold at about 260 ppm (see Section 5.2), we obtain Mgc, the series with minimum interference of particle-associated Mg. Normalizing Srcg and Mgc series and plotting normalized Mgc inversely (Fig. 5A), we can directly compare corrected Sr–Mg variation patterns.
Conclusions
We have demonstrated that dated Mg and Sr concentration profiles in speleothems can be utilized as hydrological proxies provided that contributions from multiple sources to their concentrations in the speleothems can be deconvolved and trace element concentration profiles are tested against fabric microstratigraphy. The SV1 record provides good evidence of multiple sources of Mg and Sr. Exogenic contributions to their concentration variability was removed to obtain the solute uptake by calcite
Acknowledgments
The authors are grateful to F. Corradini (Istituto Agrario S. Michele all’Adige, Italy) for water analyses and F. Cucchi (Università di Trieste, Italy) for cave access and support during sampling. P. Bence (Melbourne University, Australia) is thanked for LA-ICP-MS analysis of host rock and stalagmite thin sections and S. Moggio (Newcastle University, Australia) is acknowledged for technical assistance and contributions to the discussion. This research was supported by a University of Newcastle
References (89)
- et al.
Regional temperature, atmospheric circulation, and sea-ice variability within the Younger Dryas Event constrained using a speleothem from northern Iberia
Earth and Planetary Science Letters
(2015) - et al.
Regional climate variability and ecosystem responses to the Northern Hemisphere last deglaciation from multi-proxy records in two northern Adriatic stalagmites
Quaternary Science Reviews
(2013) - et al.
Trace element distribution in annual stalagmite laminae mapped by micrometer-resolution X-ray fluorescence: implication for incorporation of environmentally significant species
Geochimica et Cosmochimica Acta
(2007) - et al.
Temperature and altitudinal influence on karst dripwater chemistry: Implications for regional-scale palaeoclimate reconstructions from speleothems
Geochimica et Cosmochimica Acta
(2016) - et al.
Comparative study of the kinetics and mechanisms of dissolution of carbonate minerals
Chemical Geology
(1989) - et al.
Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events
Quaternary Science Reviews
(2012) - et al.
Controls on trace-element partitioning in cave-analogue calcite
Geochimica et Cosmochimica Acta
(2013) - et al.
Cave aerosols: distribution and contribution to speleothem geochemistry
Quaternary Science Reviews
(2013) - et al.
A simple method for the precise determination of ⩾40 trace elements in geological samples by ICPMS using enriched isotope internal standardisation
Chemical Geology
(1997) - et al.
Deposition and element fractionation processes during atmospheric pressure laser sampling for analysis by ICP-MS
Applied Surface Science
(1998)
Trace elements in speleothems as recorders of environmental change
Quaternary Science Reviews
Controls on trace element (Sr–Mg) compositions of carbonate cave waters: implications for speleothem climatic records
Chemical Geology
Loess in Europe—mass accumulation rates during the Last Glacial Period
Quaternary Science Reviews
Late Holocene annual growth in three Alpine stalagmites records the influence of solar activity and the North Atlentic Oscillation on winter climate
Earth and Planetary Science Letters
Trace elements in winter snow of the Dolomites (Italy): a statistical study of natural and anthropogenic contributions
Chemosphere
Timing and dynamics of the last deglaciation from European and North African δ13C stalagmite profiles-comparison with Chinese and South Hemisphere stalagmites
Quaternary Science Reviews
Aeolian contribution to Sr and Sr isotope variation in a Tasmanian speleothem
Chemical Geology
Evidence for Holocene changes in Australian–Indonesian monsoon rainfall from stalagmite trace element and stable isotope ratios
Earth and Planetary Science Letters
From soil to cave: transport of trace metals by natural organic matter in karst dripwaters
Chemical Geology
Multi-proxy constraints on the climatic significance of trace element records from a New Zealand speleothem
Earth and Planetary Science Letters
Partitioning of Sr2+ and Mg2+ into calcite under karst-analogue experimental conditions
Geochimica et Cosmochimica Acta
Seasonal trace-element and stable-isotope variations in a Chinese speleothem: the potential for high-resolution paleomonsoon reconstruction
Earth and Planetary Science Letters
Stalagmite growth and palaeo-climate: an inverse approach
Earth and Planetary Science Letters
Sea-level change along the French Mediterranean coast for the past 30,000 years
Earth and Planetary Science Letters
Late-glacial chironomid-based temperature reconstructions for Lago Piccolo di Avigliana in the southwestern Alps (Italy)
Palaeogeography, Palaeoclimatology, Palaeoecology
Sr, Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate
Geochimica et Cosmochimica Acta
A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem: reconstructed drip water and relationship to local precipitation
Earth and Planetary Science Letters
A revised Pliocene record for marine-87Sr/86Sr used to date an interglacial event recorded in the Cockburn Formation, Antarctic Peninsula
Palaeogeography, Palaeoclimatology, Palaeoecology
A speleothem record of glacial (25–11.6 kyr BP) rapid climatic changes from northern Iberian Peninsula
Global and Planetary Change
The dissolution kinetics of major sedimentary carbonate minerals
Earth Science Reviews
Controls on the spatial and temporal variability of vadose dripwater geochemistry: Edwards Aquifer, central Texas
Geochimica et Cosmochimica Acta
Concomitant separation of strontium and samarium-neodymium for isotopic analysis in silicate samples, based on specific extraction chromatography
Analytica Chimica Acta
The solubilities of calcite, aragonite and vaterite in CO2–H2O solutions between 0 and 90 °C, and an evaluation of the aqueous model for the system CaCO3–CO2–H2O
Geochimica et Cosmochimica Acta
Interaction of divalent cobalt, zinc, cadmium, and barium with the calcite surface during layer growth
Geochimica et Cosmochimica Acta
Annual trace element variation in a Holocene speleothem
Earth and Planetary Science Letters
Dripwater organic matter and trace element geochemistry in a semi-arid karst environment: Implications for speleothem paleoclimatology
Geochimica et Cosmochimica Acta
The significance of chemical, isotopic, and detrital components in three coeval stalagmites from the superhumid southernmost Andes (53°S) as high-resolution palaeo-climate proxies
Quaternary Science Reviews
Two mathematical models of Mg and Sr partitioning into solution during incongruent calcite dissolution: implications for dripwater and speleothem studies
Chemical Geology
Sr2+/Ca2+ and 44Ca/40Ca fractionation during inorganic calcite formation: I. Sr incorporation
Geochimica et Cosmochimica Acta
Solid solution partitioning of Sr2+, Ba2+ and Cd2+ into calcite
Geochimica et Cosmochimica Acta
Soil and karst aquifer hydrological controls on the geochemical evolution of speleothem-forming drip waters, Crag Cave, southwest Ireland
Journal of Hydrology
Comparison of high resolution sub-annual records of trace elements in a modern (1911–1992) speleothem with instrumental climate data from southwest Australia
Earth and Planetary Science Letters
Nanoscale effects of strontium on calcite growth: an in situ AFM study in the absence of vital effects
Geochimica et Cosmochimica Acta
Seasonal dripwater Mg/Ca and Sr/Ca variations driven by cave ventilation: Implications for and modeling of speleothem paleoclimate records
Geochimica et Cosmochimica Acta
Cited by (41)
Exploring soluble and colloidally transported trace elements in stalagmites: The strontium-yttrium connection
2023, Geochimica et Cosmochimica ActaCrystallization pathways, fabrics and the capture of climate proxies in speleothems: Examples from the tropics
2022, Quaternary Science ReviewsWhat we talk about when we talk about seasonality – A transdisciplinary review
2022, Earth-Science Reviews
- 1
Present address: Centre for Integrative Biology, University of Trento, 38123 Povo, Italy.