Rapid response of silicate weathering rates to climate change in the Himalaya

Chemical weathering of continental rocks plays a central role in regulating the carbon cycle and the Earth’s climate (Walker et al., 1981; Berner et al., 1983), accounting for nearly half the consumption of atmospheric carbon dioxide globally (Beaulieu et al., 2012). However, the role of climate variability on chemical weathering is still strongly debated. Here we focus on the Himalayan range and use the lithium isotopic composition of clays in fluvial terraces to show a tight coupling between climate change and chemical weathering over the past 40 ka. Between 25 and 10 ka ago, weathering rates decrease despite temperature increase and monsoon intensification. This suggests that at this timescale, temperature plays a secondary role compared to runoff and physical erosion, which inhibit chemical weathering by accel-erating sediment transport and act as fundamental controls in determining the feedback between chemical weathering and atmospheric carbon dioxide.

Rock weathering controls the potential for soil carbon storage at a continental scale

As rock-derived primary minerals weather to form soil, they create reactive, poorly crystalline minerals that bind and store organic carbon. By implication, the abundance of primary minerals in soil might influence the abundance of poorly crystalline minerals, and hence soil organic carbon storage. However, the link between primary mineral weathering, poorly crystalline minerals, and soil carbon has not been fully tested, particularly at large spatial scales. To close this knowledge gap, we designed a model that links primary mineral weathering rates to the geographic distribution of poorly crystalline minerals across the USA, and then used this model to evaluate the effect of rock weathering on soil organic carbon. We found that poorly crystalline minerals are most abundant and most strongly correlated with organic carbon in geographically limited zones that sustain enhanced weathering rates, where humid climate and abundant primary minerals co-occur. This finding confirms that rock weathering alters soil mineralogy to enhance soil organic carbon storage at continental scales, but also indicates that the influence of active weathering on soil carbon storage is limited by low weathering rates across vast areas.

Introduction to the Principles of δ18O, δ13C, and 87Sr/86Sr in the Palaeosciences.

The stable isotopes of oxygen (O), carbon (C), strontium (Sr), hydrogen (H), and nitrogen (N) have all been utilised for great effect in palaeoclimate, palaeoecological and palaeobiological studies. Of these, O and C have been by far the most important and, in many types of study, their use has become routine in universities and research institutions around the world. Stable isotopes provide quantitative data about palaeotemperatures, metabolic rates, food webs, palaeosalinity, palaeoprecipitation and evaporation rates as well as glacial ice volumes, production and burial of organic carbon, and other processes related to palaeoclimatic/biological/ecological change. Except for Sr, all the previously mentioned isotopes (O, C, H, and N) directly record paleoclimatic, biological and palaeoecological processes. Conversely, Sr reflects the composition of rocks at the Earth's surface, and its values reflect on the climate indirectly as it is a proxy for global weathering rates and seafloor spreading. This review will only be focusing on three isotopes commonly deployed by palaeo-researchers: carbon, oxygen, and strontium.

Direct measurement of fungal contribution to silicate weathering rates in soil

Chemical weathering produces solutes that control groundwater chemistry and supply ecosystems with essential nutrients. Although microbial activity influences silicate weathering rates and associated nutrient fluxes, its relative contribution to silicate weathering in natural settings remains largely unknown. We provide the first quantitative estimates of in situ silicate weathering rates that account for microbially induced dissolution and identify microbial actors associated with weathering. Nanoscale topography measurements showed that fungi colonizing olivine [(Mg,Fe)2SiO4] samples in a Mg-deficient forest soil accounted for up to 16% of the weathering flux after 9 mo of incubation. A local increase in olivine weathering rate was measured and attributed to fungal hyphae of Verticillium sp. Altogether, this approach provides quantitative parameters of bioweathering (i.e., rates and actors) and opens new avenues to improve elemental budgets in natural settings.

Co-variation of silicate, carbonate and sulfide weathering drives CO2 release with erosion

Global climate is thought to be modulated by the supply of minerals to Earth’s surface. Whereas silicate weathering removes carbon dioxide (CO2) from the atmosphere, weathering of accessory carbonate and sulfide minerals is a geologically relevant source of CO2. Although these weathering pathways commonly operate side by side, we lack quantitative constraints on their co-variation across erosion rate gradients. Here we use stream-water chemistry across an erosion rate gradient of three orders of magnitude in shales and sandstones of southern Taiwan, and find that sulfide and carbonate weathering rates rise with increasing erosion, while silicate weathering rates remain steady. As a result, on timescales shorter than marine sulfide compensation (approximately 106–107 years), weathering in rapidly eroding terrain leads to net CO2 emission rates that are at least twice as fast as CO2 sequestration rates in slow-eroding terrain. We propose that these weathering reactions are linked and that sulfuric acid generated from sulfide oxidation boosts carbonate solubility, whereas silicate weathering kinetics remain unaffected, possibly due to efficient buffering of the pH. We expect that these patterns are broadly applicable to many Cenozoic mountain ranges that expose marine metasediments.

Vegetation and climate effects on soil production, chemical weathering, and physical erosion rates

Weathering of bedrock to produce regolith is essential for sustaining life on Earth and global biogeochemical cycles. The rate of this process is influenced not only by tectonics, but also by climate and biota. Here we investigate these interactions with new observations of soil production, chemical weathering, and physical erosion rates from the large climate and vegetation gradient of the Chilean Coastal Cordillera (26° to 38° S). These findings are compared to a global compilation of published data from similar settings. The four Chilean study areas span (from North to South): arid (Pan de Azúcar), semi-arid (Santa Gracia), mediterranean (La Campana) and temperate humid (Nahuelbuta) climate zones. We test the hypotheses that: 1) soil production as well as chemical weathering rates increase with increasing mean annual precipitation; 2) physical erosion rates stabilize as vegetation cover increases; and 3) the contribution of chemical weathering to total denudation is constant over the climate gradient.We find observed soil production rates range from ~7 to 290 t/(km2 yr) and are lowest in the sparsely vegetated and arid North, increase southward toward the vegetated mediterranean climate, and then decrease further South in the temperate humid zone. This trend is discussed and compared with global data from similar catchments underlain by granitic lithologies. Calculated chemical weathering rates range from zero in the arid North to a high value of 211 t/(km2 yr) in the mediterranean zone. Chemical weathering rates are comparable in the semi-arid and temperate humid zones (~20 t/(km2 yr). Physical erosion rates are low in the arid zone (~11 t/(km2 yr)) and increase towards the South (~ 40 t/(km2 yr)). Combined total chemical weathering and physical erosion rates indicate that denudation rates are lowest in the arid North and highest in the Mediterranean climate zone. The contribution of chemical weathering to total denudation rates increases and then decreases with increasing mean annual precipitation from North to South. The observation that the calculated chemical weathering rates in the southernmost location, with the highest mean annual precipitation and the highest chemical index of alteration, are not the highest of all four study areas is found to be consistent with the global data analysis.

Do Martian slopes with Recurring Slope Lineae (RSL) have a distinct topographic signature?

<p>Recurring Slope Lineae (RSL) are dynamic, low-albedo, slope-parallel surface features on Mars that occur mainly on steep (>25°) slopes. RSL typically display seasonal dynamics as they appear during late Martian spring, progressively grow during summer, and subsequently fade as summer ends. RSL formation mechanisms remain under debate with proposed mechanisms involving either water/brines (‘wet theories’) vs. dry granular flows within a surficial dust layer (‘dry theories’). In an attempt to distinguish between plausible RSL mechanisms, this study compares the topographic and morphologic characteristics of hillslopes with and without RSL. We suggest that a distinct topographic signature for RSL hillslopes would argue against the ‘dry’ RSL mechanisms, as RSL dynamics within a thin dust layer are not expected to significantly impact the hillslope-scale topography. In contrast, the presence of fluids on RSL hillslopes could conceivably accelerate rock weathering rates, which in turn may impact the hillslope-scale topography. Our analyses are based on HiRISE, CTX and HRSC digital terrain models (DTMs) together with geomorphic mapping using high-resolution orbital images. We focus on inner crater hillslopes and compare the topographic characteristics of RSL vs. non-RSL slopes. In addition, in order to account for the potential influence of aspect-dependent solar irradiation on hillslope processes, we also applied our analysis on adjacent ‘control’ craters that are devoid of RSL activity. Preliminary results from Palikir (-41.6°/ 202.1°E) and Rauna (35.2°/ 328°E) craters reveal that the topographic slope distribution along crater walls with RSL activity is distinct from the slope distribution along crater walls which are devoid of RSL activity. Our results appear to support increased rock-weathering rates on crater walls that presently experience RSL activity.</p><p> </p><p> </p>

Hydrology without Dimensions

<div> </div><div> <div> <div>Dimensional analysis offers an ideal playground to tackle complex hydrological problems. The powerful dimension reduction, in terms of governing dimensionless groups, afforded by the PI-theorem and the related self-similarity arguments is especially fruitful in case of nonlinear models and complex datasets. After briefly reviewing these main concepts, in this lecture I will present several applications ranging from hydrologic partitioning (Budyko's curve) and stochastic ecohydrology, to global weathering rates and soil formation, as well as landscape evolution and channelization. Since Copernicus-dot-org asks me to add at least 25 words to the abstract, I would like to thank the colleagues who supported my nomination for the Dalton medal and my many collaborators.</div> </div> </div>

A Lithology-based Silicate Weathering Model for Earth-like Planets

<p>Silicate weathering is a key step in the carbonate-silicate cycle (carbon cycle) that draws down</p><p>CO2 from the atmosphere for eventual burial and long-term storage in the planetary interior. This process is thought to provide an essential negative feedback to the carbon cycle to maintain temperate climates on Earth and Earth-like. We implement thermodynamics to determine weathering rates as a function of surface lithology (rock type). These rates provide upper limits that allow estimating the maximum rate of weathering in regulating climate. We model chemical kinetics and thermodynamics to determine weathering rates for three types of rocks inspired by the lithologies of Earth's continental and oceanic crust, and its upper mantle. We find that thermodynamic weathering rates of a continental crust-like lithology are about one to two orders of magnitude lower than those of a lithology characteristic of the oceanic crust. Our results show that the weathering of mineral assemblages in a given rock, rather than individual minerals, is crucial to determine weathering rates at planetary surfaces. We show that when the CO2 partial pressure decreases or surface temperature increases, thermodynamics rather than kinetics exerts a strong control on weathering. The kinetically- and thermodynamically-limited regimes of weathering depend on lithology, whereas, the supply-limited weathering is independent of lithology. Our results imply that the temperature-sensitivity of thermodynamically-limited silicate weathering may instigate positive feedback to the carbon cycle, in which the weathering rate decreases as the surface temperature increases. </p>