T31E-01
Changes in the Circum-Alpine Climate as a Function of the Alpine Upliftment: Constraints from Isotopic Compositions of Fossils, Sediments, and Vein Quartz
The Tertiary circum-Alpine Molasse sediments were deposited during major periods of Alpine tectonism but also at a time of large global climatic change. They are well suited to study the effects of tectonic forcing on climate, because the sediments were deposited in marginal basins, partly to completely isolated from other major oceanic basins. Hence, a comparison of the past climatic and oceanographic evolution indicated by the sediments to those on a global scale, does allow for a qualitative evaluation of the relationship between tectonism and regional climate. Much is known about the geological-geochronological framework of alpine tectonism, including associated erosional rates and sediment volumes. Estimates of changes in paleoelevation and its direct influence on climate have, however, been less well constrained. Three independent lines of evidence indicate significant altitudes of the Alps during the Miocene: 1) H isotope compositions of clay minerals, formed as weathering products and subsequently deposited as part of the Alpine Molasse, have δD reaching values as low as -97‰. 2) O isotope compositions of retrograde metamorphic vein and fissure quartz and H isotope composition of its included fluids have δ18O values as low as -3.5‰ and δD values of -140‰, respectively. 3) ''Exotic" shark teeth from Swiss Upper Marine Molasse sediments that have δ18O values (VSMOW) around 11‰ (n=2), values unlike those from other teeth of the same locality (20.7 to 21.8‰; n=6), but for which the REE patterns support the same diagenetic history, hence supporting a freshwater formation of the low δ18O teeth (also supported by distinct Sr isotope compositions). Using these three approaches as a basis for estimating the isotopic composition of past precipitation and applying the present-day altitude effects on the compositions, it can be concluded that the Miocene Alps had mean altitudes of about 1500 to 2000 m, that is elevations similar to those of today. Paleoclimatic reconstructions from North Alpine Molasse sediments are based on oxygen isotope compositions of fossil mammalian tooth enamel for freshwater molasse deposits, and shark teeth, marine ostracoda, foraminifera, and mammalian phosphatic fossils for the Upper Marine Molasse deposits. The δ18O values (VPDB) of carbonate in phosphate from Oligocene and Miocene large mammal teeth (n=270), for example, vary over a large range from -11.9‰ to -0.5‰, but these variations parallel the composite O isotope curve of Tertiary benthic foraminifera, thus reflecting major global climatic changes such as the Late Oligocene warming, Mid-Miocene climate optimum, and Middle to Late Miocene cooling trends. The δ18O values (VSMOW) of phosphate in shark teeth (19.8 to 23.3‰; n=130) from Miocene marine molasse sediments as well as those of ostracods and foraminifera from these sediments all have variations that parallel those of composite curves for global changes. Collectively, the data support a Neogene paleogeography with a high mountain belt adjacent to marginal marine or freshwater depositional basins but with a regional climate, at least for the northern Molasse realm, that was strongly coupled to the global climate. The Alps thus appear not have influenced the local climate and/or atmospheric circulation patterns significantly.
T31E-02
Testing Thermodynamic-Based Model Predictions of Stable Isotope-Based Paleo-Altimetry Using Modern Surface Waters
Rowley et al. (2001, EPSL, 188, 253-268) present an atmospheric thermodynamic based calculation of the expected relationship between the oxygen and hydrogen isotopic composition of precipitation and elevation in low-latitude orographic settings. The isotopic lapse rate thus derived normalizes the absolute isotopic composition by differencing isotopic compositions of high elevation samples and low elevation starting composition and expressed as Δ(δ18O). This theoretical approach contrasts with empirical approaches of applying observed isotopic lapse rates to paleo-elevation estimates as it quantifies the major controlling factors (primarily starting low elevation temperature (T) and relative humidity (RH) on the lapse rate and allows them to be assessed at times in the past. Theoretically, the isotopic composition of surface waters should represent the precipitation-weighted, hypsometric mean elevation of the drainage basin above the sampling elevations. Because hypsometry and precipitation are not simple linear functions Δ(δ18O) measured in surface water systems should not increase in a simple linear fashion with increasing elevation, in accord with observation. Comparison of isotopic and hydrographic data from numerous collections of surface waters from the Indo-Gangetic plain, frontal and high Himalaya, and southern Tibet with model predictions suggests that predictions are typically within about plus/minus 500m of reality directly testing the reliability of the model. Data-model comparisons reveal an asymmetry such that predicted elevations more often underestimate real elevations rather than overestimate them. Finally orographic precipitation is typically concentrated at relatively low elevations at orographic fronts resulting in relatively little contribution of high altitude run-off in low land rivers potentially negating the possibility of assessing elevations from isotopic records of foreland basin river systems.
T31E-03 INVITED
A New Approach to Paleoaltimetry Based on Abundances of 13C-18O Bonds in Soil Carbonates
The elevation of the earth's surface is among the most difficult environmental variables to reconstruct from the geological record. We describe a new approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters, constrained by measurements of abundances of 13C-18O bonds in paleosol carbonates. The concentration of 13C18O16O in CO2 produced by phosphoric acid digestion of carbonates is proportional to abundances of 13C-18O bonds in reactant carbonates, and abundances of those bonds increase with decreasing carbonate growth temperature due to the isotope exchange equilibrium: 13C16O3 + 12C18O16O2 = 13C18O16O2 + 12C16O3. The paleothermometer based on this reaction constrains carbonate growth temperature independent of the d18O of waters from which they grew because the reaction is a homogeneous equilibrium involving only isotopologues of the carbonate ion. We refer to this method as the 'clumped isotope thermometer' because it measures the temperature-dependent 'clumping' of 13C and 18O into bonds with each other in the carbonate mineral lattice. This thermometer can aid paleoaltimetry by: (1) constraining the growth temperatures of soil carbonate, which can be compared to a known altitudinal gradient in surface temperature; (2) by rigorously constraining the d18O of water from which carbonate grew, which can be compared to the altitude dependence of the d18O of meteoric water; and, in a new approach to this problem, (3) by constraining the correlation between soil temperature and the d18O of soil water. Such correlations can discriminate between the effects of altitude, climate, latitude and seasonality in driving changes in T and d18O of water, and thus provide an opportunity for quantifying the contributions from altitude changes alone. We use our approach to show that 3700 ± 500 m of surface uplift occurred in the Bolivian Altiplano over 3.6 Ma between 10.3 Ma and 6.7 Ma, at an average rate of 1.03 ± 0.14 mm/yr. This rate is most consistent with removal of dense lower lithosphere as the cause of elevation gain. Surface uplift of the Altiplano coincides with a decrease in the rate of contractional deformation in the Andean plateau, the eastward propagation of deformation into the Subandean zone, and a decrease in the convergence rate between the Nazca and South American plates.
T31E-04
Oligocene - Miocene Rise of the Bolivian Altiplano and Eastern Cordillera: Implications for Andean Lithospheric Evolution
The siliciclastic sedimentary record, magnetostratigraphy, and isotopic composition of carbonate deposited in the northern Altiplano and Eastern Cordillera are used to reconstruct the Oligocene to Miocene paleoenvironment and paleoelevation of the Andean plateau. Based on relatively positive δ18O values (-7.4‰ to -6.2‰) for paleosol carbonates in the Salla Formation, the Eastern Cordillera resided at low elevation (<500 m) between ~26 and 29 Ma. By ~25 Ma, there is a negative shift in δ18O of carbonate to values ~2 to 5‰ more negative than Salla Formation paleosols. Paleoleaf physiognomy, δ18O paleoaltimetry, and Δ_{47}$ paleothermometry suggest that the Altiplano had attained no more than ~1600 m of elevation between 11.3 Ma and 10.3 Ma. Both δ18O paleoaltimetry and Δ_{47}$ paleothermometry suggest that the Altiplano was raised to its current elevation by ~6.8 Ma. These results suggest that the Andean plateau rose during two stages: early surface uplift on the order of ~1 to 1.5 km took place between ~25 and 11.3 Ma and later surface uplift of ~2 to 3 km, took place between ~10 and 7 Ma. An intriguing aspect of this elevation history is that the late Oligocene Salla Formation in the Eastern Cordillera suggests very low elevations despite evidence that most upper crustal shortening had already taken place in the Altiplano and Eastern Cordillera. Several possible mechanisms for maintaining low elevations at this time are 1) dynamic subsidence related to a shallow dip of the subducting Nazca slab and/or 2) distributed shortening of dense mantle lithosphere that counters the effect of upper crustal shortening. The former mechanism is supported by a shut down of the central Andean magmatic arc at ~30 Ma. This was followed by widespread eruption of mafic volcanics and ignimbrites in the northern and central Altiplano beginning at ~25 Ma indicating a change in the thermal structure of the lithosphere. The ~2 to 3 km of surface uplift that occurred between ~10 Ma and 7 Ma could have only been generated by removal of mantle lithosphere. The long-term elevation history of the central Andes suggests that most of the surface rise of Andean plateau occurred in discrete pulses associated with the removal of mantle lithosphere.
T31E-05
The Cenozoic Rise and Fall of the Western United States
Stable isotopic records collected from intermontane basins throughout Western North United States record regional climate change in response to surface uplift of major mountain ranges, as well as global climate change. We observe an approximately 5 to 10 per mil decrease in d18O of lacustrine, palustrine and paleosol carbonate and chert that occurs at ~47 Ma in southwestern Montana, at ~40 Ma in northeastern Nevada and central Utah, and at ~20 Ma in southern Nevada. These shifts are not observed in the Wind River Basin sections east of the Rocky Mountains, which suggest that the isotopic shifts are the result of regional rather than global climate changes. We interpret these isotopic shifts to be the result of spatially varying topographic development of Western North America with topography migrating southwestward with time. This pattern of surface uplift is roughly consistent with the timing of onset of magmatism in these areas. Moreover, the spatially and temporally varying shift is also observed in neocrystallized white mica found within detachment faults of the core complexes adjacent to these basins. These data, taken together, support tectonic models that link magmatism, crustal extension and topographic development of a large orogenic plateau. From the mid-Miocene to the Pleistocene there is an ~2 per mil increase in d18O values of authigenic minerals in sections throughout the Great Basin, from eastern Oregon and northern Nevada south to Arizona. This increase is not observed east of the Rocky Mountains or in southwestern Montana. We suggest that this decrease is the result of collapse of the uplifted plateau and reorganization of regional climate patterns as southerly-sourced precipitation is drawn into the Great Basin.
T31E-06
Eocene Topography of the Northern Sierra Nevada: Direct Paleoelevation Evidence from Hydrogen Isotopes in Kaolinite of Paleostream Channels
The links and feedbacks among topography, tectonics, and climate remain a poorly understood yet important problem in Earth Sciences. Large mountains and high-elevation plateaux exert a strong control on global climate and it is, therefore, critical to understand their topographic history. Despite its importance to global climate change relatively little is known of the Cenozoic topographic development of the western North America. For example, there is considerable debate as to when the Sierra Nevada developed as a mountain range, with one view that the bulk of elevation gain took place in the last 3-5 Ma and the other that it already existed as a major topographic feature throughout much of the Cenozoic. To address this debate we examined the hydrogen isotope composition of kaolinite from weathered Eocene fluvial sediments. These sediments, well known because of past gold mining, occur within Eocene river channels cut into the western flank of the northern Sierra Nevada and are found from paleo-sea level upstream into the modern range. Our results show that the deltaD of kaolinite along paleoslopes decreases systematically by up to 25 per mil within different paleodrainage systems from a high of -80 per mil in sediments deposited at the current base of the Sierra to -106 per mil about 60 km eastward on the flank of the Sierra Nevada. The observed isotopic difference between downstream and upstream samples suggests that the highest altitude samples, collected at ca. 1600 m current elevation, were deposited at Eocene elevations of 1100 m to 1300 m. Thus, Eocene topographic gradients may have been lower than todays, but still reflect mountainous topography, consistent with pebble- to cobble-sized clasts that dominate the Eocene fluvial deposits. Viewed in context of other isotopic and geomorphic studies, we therefore suggest that mountainous topography characterized the Eocene northern Sierra Nevada whose western flank was occupied by high discharge river systems draining a range with elevations up to 2000 m above sea level. Nevertheless, our data also indicate uplift of the mid- and high elevations of the western Sierran slope of by a minimum 300-500 m since the Eocene.
T31E-07
The Isotopic Rain Shadow and Elevation History of the Sierra Nevada Mountains, California
Research on the elevation history of the Sierran Nevada Mountains has yielded conflicting results. Some studies argue for substantial uplift within the last 3 to 5 million years (Ma); others propose that high elevations may have existed since the Cretaceous. Because heavier isotopes of oxygen are concentrated in rain and snow as they form from clouds, the rain shadow generated by the Sierra Nevada creates a strong isotopic gradient, with 18O-depleted precipitation on the eastern, leeward side of the range. Thus, stable oxygen isotope ratios can be used to monitor the development of the Sierran rain shadow as a proxy for high mountains blocking the flow of moisture from the Pacific Ocean. Prior work using this approach has focused exclusively on samples from the eastern side of the Sierras. When looking solely at the leeside of a mountain range, however, it is difficult to preclude alternate explanation for shifts in the isotopic composition of local precipitation (e.g., climate change, changes in moisture sources). We address this problem by analyzing the oxygen isotope composition of tooth enamel bioapatite from contemporaneous mammalian fossils on either side of the present Sierran range. Mammals ingest meteoric water, incorporating its oxygen into their tooth enamel, so isotopic analysis of enamel bioapatite can be used to monitor the isotopic composition of surface water. By sampling on both sides of the range, differences in oxygen isotope composition induced by a rain shadow can be isolated from regional and global factors like climate fluctuations. Our results indicate that the Sierras have generated a strong rain shadow for at least 18 Ma. Unfortunately, resolution for locality ages is not fine enough to differentiate between glacial and interglacial climate intervals during the last 2 Ma. As a consequence, we cannot determine whether the Sierras experienced a Late Cenozoic pulse of uplift, but we can conclude that very substantial elevation was present across the range prior to the Late Cenozoic.
T31E-08 INVITED
The roles of tectonics in erosion: Fracturing and fragmentation are key, rock uplift is not
As Gilbert and Dutton recognized in the 19th century, erosion consists of two processes: "the disintegration of the rocks, reducing them to fragments, pebbles, sand, and clay" [Dutton, 1882] and then their transport. Tectonics contributes to both but more importantly to the first. Although many in the geomorphic community subscribe to "the emerging view that erosion rates adjust to high rates of tectonically driven rock uplift" [Montgomery and Brandon, 2002], numerical models are not needed to see that rather than ''driving" erosion, most "rock uplift" results from erosion via isostatic compensation. Relegation of rock uplift to consequence, not cause, of erosion, however, does not deny tectonics a role in erosion. Tectonics plays its key role by fracturing rock. Fractures not only provide avenues for water flow and thus promote weathering of rock, but also generate erodible fragments that can be extracted and transported on hillslopes or by rivers and glaciers. Tectonics does the first part of erosion (as defined by 19th century geologists): disintegration of massive rocks. Faults are not perfect planes; both local roughness and larger scale bends require straining of the adjacent rock masses upon slip on the fault, as shown well by aftershocks of major earthquakes. Although aftershocks of great earthquakes commonly occur on the faults that rupture in mainshocks, within continents many, if not most, aftershocks occur within the larger volume of rock of adjacent blocks that slipped past one another in mainshocks. Thus, they contribute to the dismemberment of these rock volumes into smaller blocks. Scaling rules for earthquakes suggest that dimensions of ruptures for very small earthquakes, Magnitude < -2, can be meters or less. The Gutenberg-Richter recurrence relationship implies that such earthquakes are common, as recordings by high-magnification seismographs in low-Noise environments show. The large differences among fault plane solutions of aftershocks and of microearthquakes in intracontinental settings show that the small faults that rupture in microearthquakes are not parallel to one another and that some faults must intercept others. Thus, it seems likely that the upper crust in tectonically active regions is fractured and fragmented into blocks on the scale of boulders (if not smaller) and ready for the next part of erosion - transportation by slope, river, or glacial processes. A corollary is that deeply exhumed lower crust, which was not recently deformed by brittle fracturing and faulting, will be difficult to erode.