Radiocarbon constraints on past positive cave ice mass balance during the las millennium, Julian Alps (NW Slovenia)

Ice caves are natural cave that contains important amounts of perennial (year-round) ice. Among them, Sag-type ice caves are bedrock hosted and present a vertical geometry. Such caves contain stratified ice deposits formed as much by the accumulation and diagenesis of winter snow in the entrance area, as the percolation and freezing of liquid precipitation into the forming firn. The regular infall of rock and organic debris provides material embedded in the ice whose age can be determined to estimate the accumulation history of the cave.

Radiocarbon dating of organic matter embedded in the cave ice is the most widely used method for investigating the age-depth relationships of underground ice deposits. In many caves, ice exposures are not continuous and the complex history of retreat and re-advance of the underground ice deposit requires a careful consideration of the stratigraphic context from which organic inclusions are sampled. Such radiocarbon datasets can be summarized using Kernel Density Estimates of the calibrated age-probability density distributions, highlighting either periods of probable past positive or negative mass balance.

In this paper, we investigate the ice accumulation history of M-17, a sag-type ice cave opening at 1879 m asl in the Tolminski Migovec massif of the Julian Alps (NW Slovenia). We constrained the past mass balance of this cave by dating 18 wood samples embedded in ice, building the largest currently available dataset for a single cave in the southern European Alps.

Figure 1: Ice stratigraphy in the deep ice pit of M-17, (A) detailed sketch of the exposed outcrop, (B) location of the outcrop within the cave, and (C) summary of the stratigraphy recorded along the ice and sampling transect for woody macro remains. Filled circles denote sampled macro-remains. (D) Rescaled photographs of selected wood fragments that were used for radiocarbon dating.

The reconstructed chronostratigraphy reveals three main phases of likely positive ice balance. In the deepest parts of the ice pit, samples are dated around 900–1100 AD, 1200–1300 AD, while below the main ice chamber, one sample provides evidence of positive mass balance around 1700–1800 AD. Samples derived from prominent detrital layers indicate a period of negative mass balance around 1300–1400 AD.

The onset of cave glaciation is deemed to have occurred no later than about 900 AD. In a paleo-climatic context, we find evidence of overall positive ice mass balance during multi-decadal periods characterized by cooler-than-average summers and wetter-than-average springs. Conversely, negative mass balance is recorded during a period warmer-than-average summers and dry springs.

Figure 2: Comparison of the radiocarbon-dated records from selected caves of the Dinarides and Eastern Alps. Kernel Density Estimates of the age distribution of ice sections are plotted in blue (orange) when they denote periods of positive (negative) mass balance. Low KDE values denote the absence of information for that period. Sections marked with an asterisk denote calibrated posterior probability distributions for individual samples. Red markers denote the individual posterior median ages underlying the KDE estimates.

The cave has experienced negative ice mass balance since its discovery in the 1980s. Large multi-annual ice speleothems present in the 1990s have melted away since. The height of the perennial snow cones has reduced enough to allow a second entrance to be connected to the main ice bearing chamber of the cave.

The cave's record of past positive mass balance compares well with southern European records from the Dinarides, but other well-dated replicated records are needed to derive quantitative estimates of past mass balance changes.

Racine, T., Spötl, C., Reimer, P., & Čarga, J. (2022). Radiocarbon Constraints on Periods of Positive Cave Ice Mass Balance During the Last Millennium, Julian Alps (NW Slovenia). Radiocarbon, 1-24.  

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