Early Last Interglacial environmental changes recorded by speleothems

The Last Interglacial (LIG) was the time period about ~129–116 thousand years (ka) before present when global temperatures were as high as or even higher than in the current Interglacial. Both green-house gas concentrations and sea level were higher than today, while the extent of high‐latitude ice sheets was smaller. Today, the climate is being overprinted by Anthropocene warming and the modern global temperature is now approaching the warmth of the LIG. There is therefore a growing interest in the paleoclimate community to study and document this most recent interglacial before the Holocene as a ‘test bed’ for climate model projections in a purely natural climate forcing scenario.

In the foreland of the European Alps, the LIG has been studied using pollen and plant remains preserved in mires and lake sediments. These records provide the most detailed and comprehensive record of vegetation changes during the LIG but are poorly constrained chronologically. On the other hand speleothems offer superior age control and can contain sufficient pollen to provide information on the vegetation outside the cave.

In this paper, we present a replicated, precisely dated stable isotope record of the beginning and warmest phase of the LIG alongside pollen records from two speleothems from Katerloch Cave, located in the south‐eastern fringe of the Alps. The isotope record is first compared to  Alpine  speleothems  and  then  with  other European records. Subsequently, both the isotope and pollen records are compared to peri‐Alpine lake sediments, whose chronology is however less robust than speleothems.

The beginning of the interglacial is marked by a rapid rise of oxygen isotope values precisely dated at 129.4±0.4 ka and, with a brief delay, the carbon isotope values decrease significantly, reflecting vegetation re‐growth. The timing and magnitude of this event (Termination II) is coherent with other precisely dated speleothems from the Alps (figure 1).

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Figure 1: Comparison of speleothem oxygen (red/orange) and carbon(dark/light blue) isotope records from the Alps. The age uncertainty on Termination II for each dataset is represented by a star and an error bar (mean±2σ). Upper panel: This study Katerloch (SE Alps ~900 m a.s.l); then Sieben Hengste (N Alps ~1700 ma.s.l.) Schneckenloch‐Hölloch (N Alps ~1200 m a.s.l.) and lower panel: Melchsee‐Frutt (N Alps ~2000 m a.s.l.).

The comparably low oxygen stable isotope values of Katerloch suggest reduced advection of Mediterranean moisture to the southern fringe of the Alps and probably stronger westerlies during the LIG compared to the Holocene.

Pollen spectra extracted from the Katerloch speleothems capture the vegetation optimum between 128.6±0.5 and 128.0±0.4 ka by the occurrence of warm‐demanding taxa typical of the Eemian, such as Quercus, Ulmus, Tilia, as well as Ilex and Hedera (figure 2). The carbon isotope data and the pollen spectra record a delay in vegetation response to rising temperatures of about 250 years (soil bioproductivity) and 1000 years (arrival of warm‐demanding taxa), respectively.

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Figure 2: Percentage pollen diagram for stalagmites K2 (on top) and K4 (bottom); Only main pollen types and spores are presented. In the main diagram: in green AP=arboreal pollen (trees and shrubs); in yellow NAP=non‐arboreal‐pollen (herbs). The width of the sample bars corresponds to the timespan covered by each sample.

Although the pollen concentration in these speleothems is low, useful information on vegetation changes can still be extracted. This study illustrates the challenge of finding speleothems that contain sufficient pollen and at the same time are clean enough to allow precise dating. Our findings highlight that more speleothems should be tested for pollen presence and these data should be integrated in the vegetation reconstruction based on pollen records from sediments. These will provide a broader perspective on the past vegetation, allowing the acquisition of better information on the occurrence and importance of taxa that are poorly represented in the sediments, e.g., Hedera. Clearly, records with higher pollen concentrations are critically required to provide an absolutely dated chronological framework spanning the entire LIG in the greater Alpine realm.

 

Honiat, C., Festi, D., Wilcox, P.S., Edwards, R.L., Cheng, H., Spötl, C., 2021. Early Last Interglacial environmental changes recorded by speleothems from Katerloch (south‐east Austria). J. Quaternary Sci jqs.3398. https://doi.org/10.1002/jqs.3398


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