Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the river's upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO2 and erosion records.

In order to better calibrate pedogenic carbonate as a proxy for past environments, we compared the stable isotopic composition of soil CO 2 , soil water, and pedogenic carbonate in young soils from central New Mexico, USA. Seasonal changes in the δ 13 C value of soil CO 2 , the δ 18 O value of soil water, and the soil temperature were monitored to establish the timing of isotopic equilibrium with the carbonate. Calcite solubility was calculated from measured temperatures and CO 2 concentrations in the soil. This approach allowed us to determine the conditions associated with pedogenic carbonate formation. Carbon isotope equilibrium, oxygen isotope equilibrium, and minimum calcite solubility all occurred simultaneously during warm, dry conditions in May 2008 when soil CO 2 concentrations were low. It is therefore concluded that pedogenic carbonate forms during warm, dry periods and does not record mean growing season conditions as typically assumed. The seasonal bias in pedogenic carbonate formation may explain the occurrence of pedogenic carbonate in monsoon climates and its absence in regions where annual precipitation is more uniformly distributed. The implications of the seasonal bias for stable isotope-based paleoenvironmental reconstructions are that paleoelevations may have been previously over-or underestimated, paleoatmospheric CO 2 concentrations likely have been significantly overestimated, and pedogenic carbonate provides a C 4 -biased record of paleovegetation, especially in dry soils. Downloaded from REFERENCES CITEDAllison, G.B., 1982, The relationship between 18 O and deuterium in water in sand columns undergoing evapo-

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Quantifying atmospheric CO 2 concentrations (½CO 2 atm ) during Earth's ancient greenhouse episodes is essential for accurately predicting the response of future climate to elevated CO 2 levels. Empirical estimates of ½CO 2 atm during Paleozoic and Mesozoic greenhouse climates are based primarily on the carbon isotope composition of calcium carbonate in fossil soils. We report that greenhouse ½CO 2 atm have been significantly overestimated because previously assumed soil CO 2 concentrations during carbonate formation are too high. More accurate ½CO 2 atm , resulting from better constraints on soil CO 2 , indicate that large (1,000s of ppmV) fluctuations in ½CO 2 atm did not characterize ancient climates and that past greenhouse climates were accompanied by concentrations similar to those projected for A.D. 2100.The anthropogenically driven rise in ½CO 2 atm is well established (1) but its effect on future climate is less certain (2). Many recent studies indicate that ½CO 2 atm has controlled or strongly amplified Phanerozoic (542 Ma-present) climate variations (3-8) and therefore understanding the relationship between ½CO 2 atm and climate over geologic time provides crucial empirical constraints on the magnitude of future global warming (1, 9). Estimates of Paleozoic and Mesozoic ½CO 2 atm are largely based on the soil carbonate CO 2 paleobarometer (10), which is the most temporally continuous proxy (indicator) for ½CO 2 atm over the past 400 million years. The CO 2 paleobarometer is also considered the most reliable provider of ½CO 2 atm estimates for times when ½CO 2 atm was significantly above modern values (11). The CO 2 paleobarometer suggests that ½CO 2 atm values exceeded 3,000 parts per million by volume (ppmV) during Permian (289-251 Ma) and Mesozoic (251-65 Ma) greenhouse climates (5,8,12). However, other ½CO 2 atm proxies, which are either considered to be less reliable at high ½CO 2 atm (stomatal index) or are newly developed and therefore less widely utilized (e.g., fossil bryophytes), typically result in ½CO 2 atm estimates for greenhouse climates that are much lower than estimates from soil carbonate (5, 6). The large discrepancy among proxies can be interpreted two ways: 1) large (1,000s of ppmV) variations in ½CO 2 atm occurred over relatively short time periods (in certain cases shorter than the temporal resolution of the proxy records) throughout the Phanerozoic or 2) some of the proxy estimates are inaccurate. In this study we use data from modern soils and incorporate an improved understanding of pedogenic carbonate formation to recalibrate the CO 2 paleobarometer. We report that the most often quoted ½CO 2 atm values, those previously determined from pedogenic carbonate, are too high, and that paleo ½CO 2 atm values did not persist above 1,500 ppmV during the past 400 million years.Pedogenic (soil) carbonate (calcite, CaCO 3 ) forms in soils where potential evapotranspiration exceeds precipitation, typically in arid to subhumid regions that receive less than 100 cm of rain per year. Ca 2þ ...

Surface uplift of the Garzón Massif in the northern Andes formed a criti cal orographic barrier (2500-3000 m elevation) that generated a deep rain shadow and strongly influenced the evolution of the largest river systems draining northern South America. This basement massif and its correspond ing foreland basement high define the headwaters and drainage divides of the Amazon, Orinoco, and Magdalena Rivers. Despite its pivotal role, the exhumation history of the Garzón Massif and its relationships to the struc tural evolution of the broader Eastern Cordillera foldthrust belt remain unclear. The northern Andes underwent major Cenozoic shortening, with considerable thinskinned and thickskinned deformation and topographic development in the Eastern Cordillera focused during late Miocene time. On the basis of widespread coarsegrained nonmarine sedimentation, pre vious studies have inferred that uplift of the Garzón Massif began during the late Miocene, coincident with rapid elevation gain elsewhere in the Eastern Cordillera. We take an integrated, multiproxy approach to better reconstruct Andean topographic growth and distinguish between exhumation and surface uplift of the Garzón Massif. We present new UPb detrital zircon provenance data, sandstone petrographic data, and paleoprecipitation data from upper Mio cene clastic fill of the Neiva Basin within the adjacent Upper Magdalena Valley of the modern hinterland. In addition, six new apatite fission track (AFT) ages from the central segment of the northeasttrending Garzón Massif (Jurassic granite and Proterozoic gneiss and schist) directly constrain its Neogene exhu mation history. The results indicate that early exhumation may have initiated by ca. 12.5 Ma, but a substantial orographic barrier was not fully established until ca. 6-3 Ma, when >1 km/m.y. of material was exhumed. Thermal his tory modeling of the AFT data suggests diminished exhumation thereafter (3-0 Ma), during latest Cenozoic oblique Nazca-South America convergence. This exhumation history is consistent with paleontological data suggesting late Miocene divergence of the three river systems, with associated trans conti nental drainage of the Amazon River.

Records of past topography connect Earth's deep interior to the surface, reflecting the distribution of heat and mass, past crustal structure, and plate interactions. Many tectonic reconstructions of the North American Cordillera suggest the presence of an Altiplano-like plateau in the location of the modern Basin and Range, with conflicting timing and mechanisms for the onset of surface-lowering extension and orogen collapse. Here we show, through a paleotopographic profile, that from the Eocene to the Oligocene a high, broad orogen stretched across Nevada, with a distinct crest that divided a continuous westward-draining slope extending to central California from an internally drained eastern Nevada plateau. This paleo-orogen maintained demonstrably higher-than-modern elevations, reaching 3500 m in the late Oligocene. Despite the long-term high gravitational potential energy of the crust supporting this topography, surfacelowering extension did not occur until the transition to a transform margin changed the external kinematic framework of the system. Maximum surface lowering was spatially decoupled from brittle upper crustal extension, requiring a large component of mid-crustal flow.

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