Identification of a Quaternary rock avalanche deposit (Central Apennines, Italy): Significance for recognition of fossil catastrophic mass-wasting.

Whereas deposits of extremely-rapid, 'catastrophic' mass wastings >105 m3 in volume (for example, the Marocche di Dro rock avalanche in the Southern Alps and the Flims rockslide in the Western Alps) are easily recognized by their sheer mass and blocky surface, the identification of fossil catastrophic mass wastings partly removed by erosion must be based on deposit characteristics. Herein, a 'fossil' (pre-last glacial) rock avalanche, previously interpreted as either a till or debris flow, is described. The deposit, informally called 'Rubble Breccia', is located in the intramontane Campo Imperatore halfgraben that is bounded by a master fault with up to ca 900 m topographic throw. Based on documentation from field to thin section, and by comparative analysis with post-glacial rock avalanches, tills and debris flows, the Rubble Breccia is reinterpreted as a rock avalanche. The Rubble Breccia consists of an extremely-poorly sorted, disordered mixture of angular clasts from sand to block size. Many clasts show fitted subclast boundaries in crackle, jigsaw and mosaic fabrics, as diagnostic of catastrophic mass wasting deposits. Intercalated layers of angular to well-rounded clasts of coarse sand to fine pebble size, and deformed into open to recumbent folds, may represent shear belts folded during terminal avalanche propagation. The clast spectrum of the Rubble Breccia - mainly shallow-water bioclastic limestones, Saccocoma wackestones and other deep-water limestones and dolostones - is derived from the front range along the northern margin of the basin. Calcite cement found within the Rubble Breccia was dated with the U/Th disequilibrium method to 124.25 ± 2.76 ka bp, providing an ante-quam age constraint to the rock avalanche event. Because catastrophic mass wasting is a common erosional process, fossil deposits thereof should be more widespread than have been identified to date, although this may be a consequence of misidentification. The criteria outlined here provide a template to identify fossil catastrophic mass wasting deposits of any age.

230Th dating of flowstone from Ignatievskaya Cave, Russia: Age constraints of rock art and paleoclimate inferences.

Paleolithic antiquity of parietal art in Ignatievskaya cave, Southern Ural, is supported by its subject (Late Pleistocene animals) as well as by paleontological and palynological data, and 14C dates from cultural layers associated with artistic activity (17.8-16.3 cal ka BP; association is established by finds of ochre in these layers). However, three 14C dates of charcoal motifs yielded younger, Holocene ages (7.4-6.0 cal ka BP). In this study, we constrain the age of parietal art in the cave by 230Th dating of flowstone that brackets the paintings. Flowstone did not form in the cave between c. 78 and 10 ka BP, due to widespread permafrost in northern Eurasia at that time. Our 230Th dates do not support the middle Holocene age of art in Ignatievskaya cave and are consistent with its Upper Paleolithic antiquity instead.

Moisture availability in the southwest United States over the last three glacial-interglacial cycles.

The projected long-term drying of the southwest (SW) United States in response to climate warming raises a sobering alarm for this already water-limited region, yet the climatic controls on moisture availability over longer time scales remain a topic of debate. Here, we present a 350,000-year record of past water table fluctuations in Devils Hole 2 cave that are driven by variations in recharge amount to the local groundwater flow system. Because of the unprecedented length and precision of our record, we can observe variations in regional moisture availability over the last three glacial-interglacial cycles at a millennial-scale resolution. The timing of past water table rises and falls (>9 m in amplitude) closely coincides with the expansion and reduction of Northern Hemisphere ice volume, which in turn influences the position and intensity of westerly winter storms on orbital time scales. Superimposed on this long-term trend are millennial-scale highstands recorded during the last glaciation that coincide with North Atlantic Heinrich events. Earlier millennial-scale highstands provide the first evidence of multiple short-lived wet periods in the SW United States linked to coeval cooling intervals in the North Atlantic during marine isotope stages 6 and 8. The Devils Hole 2 water table record is currently the longest independently dated paleomoisture record in the SW United States and thus provides a critical testbed to examine the controls on regional moisture availability over larger time scales.

Late Palaeolithic cave art and permafrost in the Southern Ural.

Shulgan-Tash (also known as Kapova) cave located on the western slope of the Ural Mountains (Russia) is the easternmost European cave art monument of late Palaeolithic age. Radiocarbon dates from cultural layers in the cave suggest an age of about 16.3 to 19.6 ka (cal BP), but dates directly on the paintings were not obtained. In order to constrain the age of this art using an independent method, we performed detailed 230Th-U dating of calcite flowstone underlying and overgrowing the paintings at 22 sites in three halls of the cave. The youngest age for the underlying calcite (i.e., the maximum age of the cave art) is 36.4 ± 0.1 ka, and the oldest overlying calcite (constraining the minimum age of the cave art) is 14.5 ± 0.04 ka. The ca. 21.9 ka-long hiatus in calcite deposition during which the paintings were made is attributed to regional permafrost conditions and sub-zero temperatures inside the cave during Marine Isotope Stage (MIS) 2. This is supported by samples of cryogenic cave calcite, which document seven episodes of freezing and thawing of permafrost associated with stadials and interstadials of MIS 3, respectively.

Response to Comments on "Reconciliation of the Devils Hole climate record with orbital forcing".

Winograd and Coplen question the thorium-230 distribution model proposed to explain the age bias observed with increasing depth during Termination II. We have evaluated both criticisms and find that all samples display virtually identical fabrics, argue that the modern setting is not analogous to the conditions during Termination II, and reiterate the robustness of our age models. Our conclusions remain unchanged.

Timing and causes of North African wet phases during the last glacial period and implications for modern human migration.

We present the first speleothem-derived central North Africa rainfall record for the last glacial period. The record reveals three main wet periods at 65-61 ka, 52.5-50.5 ka and 37.5-33 ka that lead obliquity maxima and precession minima. We find additional minor wet episodes that are synchronous with Greenland interstadials. Our results demonstrate that sub-tropical hydrology is forced by both orbital cyclicity and North Atlantic moisture sources. The record shows that after the end of a Saharan wet phase around 70 ka ago, North Africa continued to intermittently receive substantially more rainfall than today, resulting in favourable environmental conditions for modern human expansion. The encounter and subsequent mixture of Neanderthals and modern humans - which, on genetic evidence, is considered to have occurred between 60 and 50 ka - occurred synchronously with the wet phase between 52.5 and 50.5 ka. Based on genetic evidence the dispersal of modern humans into Eurasia started less than 55 ka ago. This may have been initiated by dry conditions that prevailed in North Africa after 50.5 ka. The timing of a migration reversal of modern humans from Eurasia into North Africa is suggested to be coincident with the wet period between 37.5 and 33 ka.

Reconciliation of the Devils Hole climate record with orbital forcing.

The driving force behind Quaternary glacial-interglacial cycles and much associated climate change is widely considered to be orbital forcing. However, previous versions of the iconic Devils Hole (Nevada) subaqueous calcite record exhibit shifts to interglacial values ~10,000 years before orbitally forced ice age terminations, and interglacial durations ~10,000 years longer than other estimates. Our measurements from Devils Hole 2 replicate virtually all aspects of the past 204,000 years of earlier records, except for the timing during terminations, and they lower the age of the record near Termination II by ~8000 years, removing both ~10,000-year anomalies. The shift to interglacial values now broadly coincides with the rise in boreal summer insolation, the marine termination, and the rise in atmospheric CO2, which is consistent with mechanisms ultimately tied to orbital forcing.