Apatite thermochronometry and synthetic maps of ages and rates for thermochronometric data are used to estimate the timing of incision of valley relief in the Andes. Central Patagonia offers a unique location to study the feedbacks between long-term climate, topography, and erosion due to the high relief and well-resolved mid-latitude glacial history. New apatite (U-Th)/He ages from two vertical transects and two 4 He/ 3 He release spectra in the fjord network around 47ºS reveal fast cooling (15-30 ºC/Ma) from ~10 to 5 Ma. Samples currently at the surface cooled below ~35 ºC by ~5 Ma, indicating slow cooling and little erosion in those regions since 5 Ma. We show that these very low-temperature thermochronometric data are useful indicators of changes in topography, and insensitive to deep thermal processes, such as migration of the Chile triple junction. Map-based predictions of the thermochronometric signatures of disparate topographic scenarios show the distribution of sample data necessary to resolve the timing of relief change. Comparisons to predicted cooling ages and rates indicate that our new apatite He data are most consistent with a pulse of early glacial incision, with much of the observed valley relief in Patagonia carved between 10 and 5 Ma. Early onset of glaciation in Patagonia is supported by glacial till with bracketing ages of 7.4 and 5 Ma. We therefore conclude that the observed thermochronometric signal of fast cooling from 10 to 5 Ma is likely due to an increase in valley relief coinciding with these early glaciations in the Andes. In other glaciated areas at lower latitudes, studies have found a dramatic increase in valley relief at ~1 Ma. This timing has generated the idea that incision of glacial valleys may be related to the mid-Pleistocene transition, when the global glacial cycle changed from 40 to 100 ka periods. Our results from a higher latitude indicate an alternative, that glacial valleys incised rapidly after the onset of alpine glaciation.

We report a mountain-scale record of erosion rates in the central Patagonian Andes from >10 million years (Ma) ago to present, which covers the transition from a fluvial to alpine glaciated landscape. Apatite (U-Th)/He ages of 72 granitic cobbles from alpine glacial deposits show slow erosion before ~6 Ma ago, followed by a two- to threefold increase in the spatially averaged erosion rate of the source region after the onset of alpine glaciations and a 15-fold increase in the top 25% of the distribution. This transition is followed by a pronounced decrease in erosion rates over the past ~3 Ma. We ascribe the pulse of fast erosion to local deepening and widening of valleys, which are characteristic features of alpine glaciated landscapes. The subsequent decline in local erosion rates may represent a return toward a balance between rock uplift and erosion.

Keywords:Messinian salinity crisis ACE index TEX 86 Mediterranean sea deep sea drilling project Scientific drilling of the abyssal evaporites beneath the deepest parts of the Mediterranean basin gave rise to the idea that the Mediterranean sea completely evaporated at the end of the Messinian. Herein, we show, using new organic geochemical data, that those evaporites were deposited beneath a deepwater saline basin, not in a subaerial saltpan, as originally proposed. Abundant fossil organic lipids were extracted from evaporites in Mediterranean Deep Sea Drilling Project cores. The archaeal lipid distribution and new analyses, using the ACE salinity proxy and TEX 86 temperature proxy, indicate that surface waters at the time of evaporite deposition had normal marine salinity, ranging from ∼26 to 34 practical salinity units, and temperatures of 25-28 • C. These conditions require a deep-water setting, with a mixed layer with normal marine salinity and an underlying brine layer at gypsum and halite saturation. After correction for isostatic rebound, our results indicate maximum drawdown of ∼2000 m and ∼2900 m relative to modern sea level in the western and eastern Mediterranean basins, respectively. Our results are consistent with previously proposed scenarios for sea level drawdown based on both subaerial and submarine incision and backfilling of the Rhone and Nile rivers, which require Messinian sea level drops of ∼1300 m and ∼200 m, respectively. This study provides new evidence for an old debate and also demonstrates the importance of further scientific drilling and sampling of deeper part of the abyssal Messinian units.

No abstract