The Alps Paleoelevation and Paleoclimate Experiment (APE): Neogene Paleoelevation and Paleoclimate of the Central Alps

Stable isotope paleoaltimetry takes advantage of the relationship between orogen elevation and the stable isotope ratios in meteoric water, which are ultimately recorded in geological archives like foreland basins or orogen-internal shear zones. The δ-δ approach relies on contrasting time-equivalent δ18O and δD records from high- and low-elevation sites to constrain the height of the orogen at the time these geologic archives were formed. However, at the same time, different boundary conditions such as changing paleogeography, atmospheric CO2 concentrations or sea surface temperatures result in complex paleoclimate model outputs, which predict significant changes in the isotopic composition of meteoric water. These changes may be recorded in geological archives and thus complicate the reconstruction of past elevations. The 4DMB Phase 1 project APE aimed at generating a first quantitative estimate for the paleoclimatic signal in Alpine stable isotope records, so that these records may be corrected for and ultimately yield more accurate paleoelevation estimates. We addressed this challenge by integrating isotope-tracking climate model (ECHAM5-wiso) simulations with stable isotope and clumped isotope data from the foreland basin and high-elevation regions of the central Alps. ECHAM5-wiso simulations have been conducted with 1) boundary conditions based on paleogeographic reconstructions of the Last Glacial Maximum (LGM) and the mid-Pliocene (PLIO), and 2) different topographic scenarios for the Alps. The simulations show that modifying environmental conditions can produce similar magnitudes of δ18O change as changes in alpine topography. For example, the climatically induced δ18O changes in the PLIO and LGM experiments correspond to the magnitude of changes created by setting the entire orogen to 50% and 150% of its modern height, respectively (Botsyun et al., 2020). Our modelling results stress the need for the paleoaltimetry community to correct isotopic signals in geologic archives for climate-induced changes in isotope ratios. Pedogenic carbonate proxy data from alluvial megafans of the Swiss Molasse Basin revealed that 1) low-elevation, distal δ18O values are higher than previously assumed and thus, more adequately reflect low-elevation δ18O values required for paleoelevation estimates; 2) Mid-Miocene megafans had considerable topography and an internal elevation gradient; 3) clumped isotope-derived carbonate formation temperatures yield low-elevation paleoclimate estimates and help to embed δ18O data into global climate models. Under consideration of previous work and our modelling results, we conclude that the Central Alps, more specifically the region surrounding the Simplon Fault Zone, attained surface elevations of >4000 m no later than the mid-Miocene (Krsnik et al., 2021). In summary, our approach represents an important methodological advance that allows the disentangling of climatic and surface uplift signals in the geologic stable isotope record. Furthermore, new insights into the Alps elevation history can help to constrain the timing of slab inversion and/or break-off in the Western/Central Alps

Miocene high elevation and high relief in the Central Alps

Reconstructing Oligocene–Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps remain scarce. Here we present stable and clumped isotope measurements to provide a new paleoelevation estimate for the mid-Miocene (∼14.5 Ma) European Central Alps. We apply stable isotope δ–δ paleoaltimetry to near-sea-level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30±5 ∘C) yield a near-sea-level precipitation δ18Ow value of -5.8±1.2‰ and, in conjunction with the high-Alpine phyllosilicate δD value of  -14.6±0.3‰, suggest that the region surrounding the Simplon Fault Zone attained surface elevations of >4000 m no later than the mid-Miocene. Our near-sea-level δ18Ow estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for the Northern Alpine Foreland Basin

Devils Hole Calcite Was Precipitated at +/- 1 degrees C Stable Aquifer Temperatures During the Last Half Million Years

Subaqueous carbonates from the Devils Hole caves (southwestern USA) provide a continuous Holocene to Pleistocene North American paleoclimate record. The accuracy of this record relies on two assumptions: That carbonates precipitated close to isotope equilibrium and that groundwater temperature did not change significantly in the last 570 thousand years. Here, we investigate these assumptions using dual clumped isotope thermometry. This method relies on simultaneous analyses of carbonate increment (47) and increment (48) values and provides information on the existence and extent of kinetic isotope fractionation. Our results confirm the hypothesis that calcite precipitation occurred close to oxygen and clumped isotope equilibrium during the last half million years in Devils Hole. In addition, we provide evidence that aquifer temperatures varied by less than +/- 1 degrees C during this interval. Thus, the Devils Hole calcite delta O-18 time series exclusively represents changes in groundwater delta O-18 values

Hypersalinity accompanies tectonic restriction in the eastern Mediterranean prior to the Messinian Salinity Crisis

This study describes the hydroclimate evolution of the eastern Mediterranean Basin during the early Messinian (7.2 to 6.5 Ma) time-interval based on analysis of a succession at Agios Myron (Crete, Greece), prior to the onset of the Messinian Salinity Crisis (5.96–5.33 Ma). Specifically, we report sea surface temperature and salinity reconstructions based on a combined analysis of biomarkers and oxygen isotopes of planktonic foraminifera. Data reveal that a negative water budget and strong hydrologic and climate variability characterized the eastern Mediterranean Basin at this time, and we identify three distinct phases. In Phase 1 (7.2–6.9 Ma), a shift to more positive oxygen isotope values in planktonic foraminifera at ~7.2 Ma is attributed to progressive gateway restriction of Mediterranean–Atlantic corridors and subsequent cooling until 6.9 Ma. In Phase 2 (6.9–6.7 Ma), distinct warm and hypersaline events (at 6.9–6.82 and 6.72 Ma) resulted in stressed marine microfauna during periods of strong evaporation. An important step-change in the Mediterranean restriction at 6.72 Ma may have resulted from shallowing of the Mediterranean gateways and reduced Mediterranean marine outflow. During Phase 3 (6.7–6.5 Ma) this gateway shallowing reduced the oceanic input into the Mediterranean Basin causing significant hydrological changes, reflected in a wide range of temperature and salinity fluctuations accompanied by enhanced water-column stratification. The data presented here counterbalance the general lack of quantitative temperature and especially salinity estimates available for the Mediterranean Messinian, time interval where we still highly rely on modelling for such evaluations. This study highlights the severity of preconditioning stages leading to the Messinian Salinity Crisis in the Mediterranean and sets values for extreme salinity conditions that could still host marine life

Hypersalinity accompanies tectonic restriction in the eastern Mediterranean prior to the Messinian Salinity Crisis

This study describes the hydroclimate evolution of the eastern Mediterranean Basin during the early Messinian (7.2 to 6.5 Ma) time-interval based on analysis of a succession at Agios Myron (Crete, Greece), prior to the onset of the Messinian Salinity Crisis (5.96–5.33 Ma). Specifically, we report sea surface temperature and salinity reconstructions based on a combined analysis of biomarkers and oxygen isotopes of planktonic foraminifera. Data reveal that a negative water budget and strong hydrologic and climate variability characterized the eastern Mediterranean Basin at this time, and we identify three distinct phases. In Phase 1 (7.2–6.9 Ma), a shift to more positive oxygen isotope values in planktonic foraminifera at ~7.2 Ma is attributed to progressive gateway restriction of Mediterranean–Atlantic corridors and subsequent cooling until 6.9 Ma. In Phase 2 (6.9–6.7 Ma), distinct warm and hypersaline events (at 6.9–6.82 and 6.72 Ma) resulted in stressed marine microfauna during periods of strong evaporation. An important step-change in the Mediterranean restriction at 6.72 Ma may have resulted from shallowing of the Mediterranean gateways and reduced Mediterranean marine outflow. During Phase 3 (6.7–6.5 Ma) this gateway shallowing reduced the oceanic input into the Mediterranean Basin causing significant hydrological changes, reflected in a wide range of temperature and salinity fluctuations accompanied by enhanced water-column stratification. The data presented here counterbalance the general lack of quantitative temperature and especially salinity estimates available for the Mediterranean Messinian, time interval where we still highly rely on modelling for such evaluations. This study highlights the severity of preconditioning stages leading to the Messinian Salinity Crisis in the Mediterranean and sets values for extreme salinity conditions that could still host marine life