Paleoclimate variability during the Blake geomagnetic excursion (MIS 5d) deduced from a speleothem record
Introduction
The possible relationship between the intensity of Earth's magnetic field and climate on glacial-interglacial timescales is a long debated issue (e.g. Wollin et al., 1971, Wagner et al., 2001, Christl et al., 2004, Courtillot et al., 2007). For instance, increases in cosmic-ray flux during periods of low geomagnetic intensity may hypothetically enhance cloud formation and eventually contribute to tropospheric cooling (Svensmark, 1998, Knudsen and Riisager, 2009). Geomagnetic excursions regularly occur during periods of low geomagnetic intensity in which shielding against cosmic rays was strongly reduced, as evidenced by the corresponding increases in cosmogenic 10Be in ice- and deep-sea cores (Frank, 2000). Pleistocene excursions and their associated geomagnetic intensity lows typically occurred during interglacials and interstadials, but shortly before the onset of cold intervals (Thouveny et al., 2004, Kitaba et al., 2013), suggesting a connection between low geomagnetic intensity and climatic cooling. However, unequivocal evaluation of this possible link is impeded by limitations in the resolution and dating of the usual paleoclimatic and paleomagnetic records. Furthermore, even if such a connection is confirmed, it could be circumstantial since the variations in geomagnetic field intensity may in fact be linked to variations in Earth's orbital parameters (Thouveny et al., 2008), which is considered the main climate-controlling factor in the Pleistocene (Hays et al., 1976).
The Blake geomagnetic excursion occurred at the onset of the Last Glacial period and during a prominent low in geomagnetic intensity that lasted longer than the excursion (Thouveny et al., 2004). This low geomagnetic intensity allowed increased cosmic-ray fluxes, as evidenced in the deep-sea 10Be record (Carcaillet et al., 2004). This record, and the relative paleointensity record of marine sediments, also indicates short term oscillations in geomagnetic intensity during the Blake excursion (Carcaillet et al., 2004, Thouveny et al., 2004, Osete et al., 2012). Therefore, the Blake excursion seems a good candidate to test possible links between geomagnetism and paleoclimate. However, this potential link is difficult to evaluate due to inherent limitations in the records in which the excursion is normally recognized (marine and eolian sediments: Lund et al., 2006, Fang et al., 1997). Recently, Osete et al. (2012) recognized the Blake excursion in a stalagmite from an alpine cave (Cobre) in northern Spain, providing a relative paleointensity record of the excursion and constraining its age by means of U–Th dating. Compared to other records, this new speleothem record of the Blake offers a better opportunity to test possible paleomagnetism-paleoclimate connections. This is because speleothems typically preserve a variety of high-resolution paleoclimate proxies (Fairchild and Baker, 2012) which can be tied to the same precise, U–Th-based chronology to which the paleomagnetic variations are referred.
Speleothems from european alpine caves are particularly useful to infer Quaternary paleoclimates in continental areas. This is partly because those speleothems are highly sensitive to climate changes (Holzkämper et al., 2005). In cave passages distant from large entrances, temperature is normally rather stable and reflects the mean annual air temperature outside the cave. Since subfreezing temperatures impede the precipitation of speleothem calcite, growth periods of alpine spelothems are excellent indicators of warm climatic phases, while their growth hiatuses typically reflect cold periods. Even if subfreezing temperatures are not reached during a cooling event in an alpine cave, the resulting changes in the overlying soil may have a notable impact on speleothem growth and chemistry (e.g. Scholz et al., 2012). Finally, an additional advantage of using european alpine speleothems for paleoclimate reconstruction derives from the fact that their δ18O trends are easily compared to Greenland ice δ18O records, because both respond similarly to regional temperature changes (Meyer et al., 2008, Boch et al., 2011).
For this study we have obtained several speleothem paleoclimate proxies over the interval covered by the Blake geomagnetic excursion from the same stalagmite in which Osete et al. (2012) characterized the excursion. The studied stalagmite is remarkable in that it contains fluorescent laminae, whose counting has allowed us to improve the chronology of the proxies and to derive a detailed growth-rate record. Apart from δ18O and δ13C in calcite, which are widely used paleoclimate proxies in speleothems, we also obtained trace element concentrations using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. This technique is still not so widely used in speleothem paleoclimate studies, but it is powerful for detailed reconstructions given its excellent spatial resolution (e.g. Treble et al., 2003). The combination of high resolution trace element analyses with δ18O and δ13C allows minimizing some of the uncertainties inherent in the interpretation of the proxies. Finally, the climate-sensitive alpine cave setting of Cobre cave has also contributed to improve the confidence of the paleoclimatic interpretation.
The aims of this paper are: (1) to provide a well dated high-resolution multi-proxy paleoclimate record of northern Spain during part of MIS 5d, discussing how the proxies contain useful paleoenvironmental information; and (2) to evaluate possible relationships between paleoclimate and geomagnetic variations during the Blake excursion. This is performed by comparing the paleoclimate proxy data with the paleomagnetic data of Osete et al. (2012), after synchronizing both records using a lamina-counted chronology constrained by U–Th dates.
Section snippets
Setting
Cobre cave is located in Sierra de Peñalabra, in the Cantabrian Mountains of northern Spain (Fig. 1A). The cave is developed in locally dolomitized Carboniferous limestones, it is well ventilated, and contains more than 10 km of surveyed passages that range from 1610 m to ∼1800 m in elevation above sea level (Fig. 1B). The main passage is an active low-gradient canyon at the water table, which resurges at the cave entrance. The catchment area of the cave waters mainly consists of outcrops of
Methods
A 1.5 cm-thick axial slab of PA-8 (named slab “C”), immediately adjacent to the slab studied by Osete et al. (2012) (slab “F”), was cut with a diamond saw, polished and scanned (Fig. 2). Fifteen blocks, each 4 cm by 2.5 cm, were cut from the polished slab using a 300 μm-thick diamond blade to minimize the loss of material between blocks. Double-polished thin sections were cut from the blocks using a 400 μm-thick diamond blade, so that the thin sections and the surfaces of the remaining blocks
Fluorescent lamination
A remarkable feature of stalagmite PA-8 is the presence of well developed fluorescent laminae over most of its length, clearly showing that the speleothem calcite is unaffected by significant diagenetic recrystallization. Lamination is defined by the alternation of relatively thin, highly fluorescing bands, and thicker, less fluorescent intervals (Fig. 3). The thickness of individual pairs of laminae is typically in the range of 50–100 μm. In most sections, FL lamination is very well defined,
Growth rate and fluorescence banding
FL annual banding in stalagmites is normally related to seasonal increases in soil-derived dissolved organic matter in drip waters, linked to increases in discharge (Baker et al., 2008). The good development of FL annual layers in PA-8 suggests that the pre-glacial landscape was significantly different from the present day configuration: the massive till deposits that cover the limestone today were probably absent during the growth of PA-8. PA-8 was then probably closer to the surface, so that
Conclusions
Trace-element, δ13C, δ18O, δ234U, fluorescent lamination, and growth-rate data from stalagmite PA-8, synchronized using a floating lamina-counted chronology constrained by U–Th dates, provide a detailed paleoclimate record of northern Spain during the Blake geomagnetic excursion, from ∼112 ka to ∼116.4 ka, corresponding to Greenland interstadial 25 of MIS 5d.
Most of the observed trace element variations in PA-8 can be explained by changes in groundwater residence time, which were coeval to
Acknowledgments
Adriano Cortel, Araceli Sariá and Carmen Pino helped to retrieve and transport the studied stalagmite from Cobre Cave. Previous work on Cobre Cave speleothems by Belén Muñoz greatly contributed to improve our understanding of the cave system. José M. Martínez-Egea and Pablo Zuazua, from the Castilla and León regional government, facilitated our research in the “Fuentes Carrionas y Fuente Cobre-Montaña Palentina” natural park. Heinrich Taubald kindly performed the stable-isotope measurements.
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