Joan Manuel Vilaplana

El creciente interés en espacios naturales de áreas de montaña vinculado a actividades de ocio invernales hace conveniente la elaboración de estudios que evalúen cuantitativamente el riesgo de aludes. En el presente trabajo se han aplicado varias metodologías destinadas a localizar y delimitar las zonas de aludes y valorar el riesgo a partir de un Índice de Riesgo de Aludes (IRA) en una de las rutas de montaña más fre‐ cuentadas de los Picos de Europa, el camino de acceso a la Vega de Urriellu desde el Collado Pandébano, el PR–PNPE 21. Una vez definidas las zonas de aludes según criterios geomorfológicos, de vegetación e his‐ tóricos se ha estimado la frecuencia de los aludes a partir de la observación de secuencias temporales de fotografías aéreas, observaciones de campo, encuesta a la población y pendiente media del recorrido de los aludes. En cuanto al cálculo del riesgo se han planteado varios escenarios probables en relación a la va‐ riabilidad de los datos tenidos en cuenta en el análisis del riesgo, obteniendo como resultado un IRA en ge‐ neral bajo para el conjunto de la temporada invernal, moderado en determinadas situaciones de exposición prolongada al peligro, y alto en la Semana Santa debido al especial tránsito de visitantes. Además, se pro‐ pone la información al usuario como medida mitigadora del riesgo.

A compilation and a critical assessment of the 14C data set available show that the chronology of glacial events in the Pyrenees is not well constrained. After reviewing the literature on glacial reconstruction, we suggest a simplified subdivision of the Pyrenean last glacial cycle record into Last Pleniglacial, Deglaciation, and Neoglacial. To improve the numerical glacial chronology, we provide 10Be surface exposure ages for 5 glacial erosion surfaces, 9 moraines and 2 erratics in the Upper Noguera Ribagorcana Valley (south-central Pyrenees). Published corrected 14C data and new 10Be exposure ages indicate that the major phase of moraine building recorded in this valley during the Last Pleniglacial probably occurred after 25 ka BP. This age calls in question the generally accepted hypothesis of a very early deglaciation of the Pyrenees ca 70–40 ka BP, and strongly suggests that the Pyrenees could have been in pleniglacial conditions during the Last Glacial Maximum (LGM). However, we do not exclude the possibility that the maximum glacier extent during the last glacial cycle had taken place much earlier than the LGM, as indicated by some published U-Th, AMS
14C and OSL data. We suggest that pleniglacial conditions could have taken place during a longer ( >30–20 ka) period than generally assumed, and that the Last Pleniglacial could include several glacier fluctuations recorded irregularly in different valleys, with a last major glacier readvance taking place around the LGM. In addition, the Deglaciation is represented by a series of moraines deposited between ca 13.7+/-0.9 and
10.1+/-0.6 ka. This moraine series indicates a highly variable climatic pattern that is partly correlated with Greenland Stadial 1 (the Younger Dryas), and suggests that the Deglaciation could have continued into the early Holocene.

The relatively detailed chronology provided by the
La Massana ice-dammed palaeolake allows
a tentative correlation with the climatic event chronologies
that have been recorded for the North
Atlantic and the Greenland ice sheet,
with consequences already documented in many
parts of Western Europe. The rapid fluctuations
recorded by Andorran glaciers during the second
half of the Wurm suggest links in response to abrupt
global events, perhaps in relation to Heinrich events
(Elliot et al. 2002; Hemming 2004). These are sudden
but relatively brief cooling spikes caused by the release of large masses of ice and meltwater into the
North Atlantic.The periodicity of sequence units detected in
the La Massana palaeolake is greater than that of
Heinrich events during the last 50 ka, as
likewise reported in the sedimentary record of
Lake Banyoles, which is situated farther east, near
the Mediterranean (Hobig et al. 2012). The evidence
thus provided suggests that the Valira glaciers did
respond to global forcing events but that their
response to global climatic events did not occur
systematically. At least the sedimentary record at
La Massana, as currently understood, is equivocal.
Perhaps local factors also presided over glacier
behaviour, as observed in the case of modern glaciers
(Pratt-Sitaula et al. 2011). Based on the Andorran
chronostratigraphy summarized in the Wheeler
diagram and on currently established marine
chronologies of Heinrich events (Hemming 2004),
event H3 (31 ka, mostly of European origin; could hypothetically have generated the
depositional sequence SD1 (the age of SD1, based
on sample 5, is 30.48–29.35 ka b2k).
Event H2 (24 ka, mostly of Laurentide origin; likewise correlates with SD3 (H2 occurred
between samples 9 and 11, i.e. between 26.02–
25.71 ka b2k and 22.38–21.90 ka b2k).
Event H4 (38 ka, also mostly of Laurentide origin) was approximately coeval with the age of
the slope deposits dated at Canillo (samples
2 and 3; i.e. 39.68–38.15 ka b2k and 35.21–
34.27 ka b2k) and therefore correlates with a period
of general ice retreat. This is contrary to the expectation
of a great ice advance. Likewise, at the time
of H1 (16.8 ka, which also corresponds to the
top of depositional sequence SD5),
the Ordino glacier had already begun its retreat
from La Massana (as evidenced by sample 16; 17.03–16.55 ka b2k), and the Valira d’Orient glacier had definitively vacated the palaeolake area (Till 5). In summary, apart from H3 and H2, the Andorran record yields a mismatch with H4 and H1 (both of Laurentide origin). Based on the sequence of Greenland stadials (GS) and interstadials (GI), many of the climatic fluctuations recorded by the Greenland ice sheet
for the last glacial cycle were also abrupt (Rasmussen et al. 2014). During MIS 3, the frequency of GI–GS cycles was greater than the cycles recorded by the La Massana sequence stratigraphy. One of the early slope deposits dated at Canillo (sample 3; 35.21–34.27 ka b2k) coincides with GI-7 and hypothetically correlates with those present at the base of unit SD0. The slope deposits at the base of SD1 at La Massana
could coincide with interstadial GI-5. The base of SD2 (slope deposits corresponding to sample 7, 28.79–28.07 ka b2k) correlates with GI-4. In contrast, the slope deposits
(which should correspond to a thinner or receding
glacier) at the base of SD3 coincide with stadial GS-3 (sample 9; 26.02–25.71 ka b2k), not with an interstadial. Likewise, the oldest dated slope deposits at Canillo (Planas et al. 2011)
coincide exactly with stadial GS-9 (sample 2, 39.67–38.15 ka b2k), with no indication of glacier presence in the valleys between GS-9 and GI-7. The glacial advance as far as
Cal Tolse (erratic boulders south of Sant Julia de
Loria) before 32.78+1.18 ka (sample 4) matches the timing of GS-6. For comparison, the advance recorded at c. 35.3+
8.6 ka based on 10Be in the Ariege valley falls statistically
between GS-11 and the beginning of GS-3; the advance dated to 36+3 ka on the Gallego falls between GS-9 and GS-6.
During MIS 2, the Greenland sequences were much longer and less sharp than those recorded in the La Massana palaeolake. The base of the SD4 slope-deposit unit coincides clearly with the beginning of GS-2.1b, i.e. with the
start of a period of moderate warming that Rasmussen
et al. (2014) chose not to classify as an interstadial.
The Sorna`s slope deposits (sample 16), which mark the onset of definitive Ordino glacial recession, coincide with the peak of phase GS-2.1a, a cooler period. The glacial recession,
however, should logically have occurred before deposition of the Sornas slope deposits. We also note that the rapid deglaciation in the upper Tet valley correlates with the relative global warming event recorded by GS 2.1b, as perhaps likewise farther west at Lake Redo d’Aigues Tortes (Noguera de Tor) and at Bious (Ossau). GS 2.1b (21 to
17.5 ka) is also the currently accepted time interval
of rapid collapse of the Rhone glacier in the Swiss
Alps (Ivy-Ochs et al. 2004 2006).