Silicate Weathering Intensity Research Articles

Evolution of weathering intensity in the Qaidam Basin, northeastern Tibetan Plateau, since the middle Miocene: Insights from clay mineral records

Terrestrial silicate weathering records from the tectonically active Tibetan Plateau are crucial to explore the interactions among climate change, tectonic uplift and chemical weathering during the Cenozoic. However, the complex sedimentary facies changes and frequent occurrence of coarse lithologies related to intense tectonic uplift of the Tibetan Plateau have largely limited the reliability of bulk chemical weathering indexes to reveal the silicate weathering history. Here, we present detailed records of clay minerals and geochemical compositions from clay-sized (< 2 μm) sediments collected from the well-dated Huaitoutala section (ca. 15.3–1.8 Ma) in the NE Qaidam Basin to reconstruct the chemical weathering history of the NE Tibetan Plateau during the late Cenozoic. The relatively high (illite–smectite mixed layers + smectite)/(chlorite + illite) ratio indicates a strong silicate weathering intensity from 15.3 to 12.6. Ma, while a continuous decrease in this ratio since 12.6 Ma reflects a long-term decreasing intensity of silicate weathering. By comparing the clay mineral records with the climatic reconstruction and the tectonic events in the NE Tibetan Plateau, the results reveal that middle Miocene global cooling-related drying exerted a significant influence on the decreasing weathering intensity since ca. 12.6 Ma. Furthermore, the late Cenozoic episodic and persistent uplift of the NE Tibetan Plateau may have exerted an additional influence on the long-term decrease in weathering intensity. Specifically, the dramatic fluctuation in the silicate weathering intensity during ∼11–9.6 Ma is mainly due to uplift-induced provenance changes. Moreover, the fluctuating increase in the silicate weathering intensity during ∼8–5 Ma may have been caused by intensified East Asian summer monsoon precipitation. Our study suggests that both climate change and tectonic deformation controlled the chemical weathering intensity in the NE Tibetan Plateau during the late Cenozoic.

Orbital-paced silicate weathering intensity and climate evolution across the Eocene-Oligocene transition in the southeastern margin of the Tibetan Plateau

The Eocene-Oligocene transition (EOT, ∼33.9 Ma), a period of dramatic global cooling marking the onset of the Antarctic ice sheet. However, paleoclimatic reconstructions indicate a notable spatial heterogeneity in both the marine and terrestrial realms. While limited temporal resolution terrestrial records have hindered the precise understanding of short-term climate events and orbital-scale changes during this transition. Here, we present a chemical weathering data sequence from a fluvial-lacustrine deposition from Lühe Basin, which is located at southeastern margin of the Tibetan Plateau. Based on the radioisotope, paleomagnetic and 405 kyr orbital tuning of the elemental datasets, the deposition age of Lühe Basin is dated to ∼35.5–25.5 Ma. Two weakening stages of the weathering intensity are detected around Eocene-Oligocene boundary: 34.1 to 33.9 Ma and 33.8 to 33.6 Ma, which are temporally equivalent to the two global main climatic steps of EOT (EOT-1 and Oi-1). We find the chemical weathering intensity sequence paced the 1.2 Myr obliquity amplitude modulation and 405 kyr long-eccentricity cycles. Notably, the minima of 1.2 Myr obliquity amplitude and 405 kyr eccentricity are synchronous with the Oil-1 event. These patterns highlight the role of orbital forcing in shaping the cooling trend across the EOT and may trigger Antarctic glaciation.

Enhanced weathering during glacial periods and its dynamic response to climate and sea-level changes inferred from the northern South China Sea sediments

Secondary weathering of exposed continental shelf sediment at low-latitudes may play a significant role in atmospheric CO2 consumption during glacial periods, which is negative feedback for climate stability. However, more lines of evidences are required to verify the link of glacial weathering with climate. This study presents a comprehensive analysis of geochemical proxies that indicate the silicate weathering intensity in the northern South China Sea (SCS) over orbital timescales since ∼244 ka. Provenance discrimination results, based on the Sr-Nd isotopes values and the patterns of rare earth elements (REEs), suggest that the terrigenous sediment was primarily derived from Taiwan Island. The silicate weathering records revealed two different weathering regimes during glacial periods in the northern SCS, corresponding to changes in monsoon climate and sea-level. During the early period of Marine Isotope Stage 6 (MIS 6), the increase in weathering intensity was in response to climate variability, and the rapid delivery of terrigenous sediment through contour currents allowed for the preservation of climate signals in the continental margin. In contrast, the significant increase in weathering intensity during the glacial periods at late MIS 2 and late MIS 6, was a result of the sedimentary recycling process dominated by sea-level rises. In this case, the marginal sea weathering records did not truly reflect climate changes, due to secondary weathering processes on the exposed continental shelf. Our new geochemical results emphasize the dynamic coupling effects of climate and sea-level changes in transmitting and preserving silicate weathering signals at low-latitude during glacial periods. When using marginal sea sediments for paleoclimatic and paleo-weathering reconstruction, one should be cautious of the complex influence of sediment source-to-sink processes on sediment composition, and a holistic approach may provide more robust constraints.

A relatively warm and humid Oligocene climate in the northeastern Tibetan Plateau based on a high-resolution clay mineralogical and geochemical record

The Oligocene was the earliest cool period in the Cenozoic, and thus its terrestrial climatic evolution is of significant interest but remains poorly constrained for Inner Asia. In this study, we obtained a high-resolution paleoclimate record based on clay minerals and silicate chemical weathering proxies of the clay fraction (<2 μm) in sedimentary rocks from the Xining Basin on the northeastern Tibetan Plateau. We compared these data with those for surrounding basins to reconstruct the Oligocene climatic evolution of the northeastern part of the plateau. Illite, chlorite, smectite (I/S; including irregular mixed-layer illite–smectite), and kaolinite are the four main clay minerals in the Oligocene strata. Multiple proxies indicated three characteristic climatic periods. During the late Eocene to early Oligocene, a weak silicate weathering intensity is indicative of an arid climate in the northeastern Tibetan Plateau. A stronger silicate weathering intensity and enrichment in smectite (I/S) and kaolinite indicate more warm and humid climate conditions during 32–25 Ma. During the Oligocene, before the high topography of the northern and northeastern parts of the Tibetan Plateau had developed, the relatively warm and humid climate conditions since 32 Ma were likely caused by enhanced transport of water vapor in response to relatively warm temperatures. The decrease in the degree of silicate weathering from 25 to 23 Ma was possibly caused by regional tectonism and global climatic changes. These relatively warm and humid conditions would explain the occurrence of large Oligocene mammals in the northeastern Tibetan Plateau.

Strong Coupling Between Carbon Cycle, Climate, and Weathering During the Paleocene‐Eocene Thermal Maximum

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM; ∼56 Ma)–a geologically rapid carbon‐release event similar to anthropogenic carbon emissions–is ideal for investigating weathering responses to rapid carbon release and associated climate change. Here, for the first time, we present high‐resolution lacustrine lithium isotope data that reflect basin‐scale silicate weathering across the PETM in the Nanyang Basin, eastern China. Our results reveal an ∼100% increase in regional silicate weathering intensity through the PETM relative to the pre‐PETM level. Synchronous variations between carbon isotope values and weathering intensity demonstrate a strong coupling between carbon cycle, climate, and chemical weathering on millennial timescales. These findings argue that strong negative weathering feedback and massive removal of CO2 maintained Earth's climate within a habitable range during the PETM.

Modern Silicate Weathering Regimes Across China Revealed by Geochemical Records From Surface Soils

AbstractKnowledge of modern kinetically limited (i.e., silicate weathering depends on temperature and precipitation) and supply‐limited (i.e., weathering is limited mainly by the supply of fresh material) weathering regimes is important for analyzing the weathering–CO2–climate system on geological timescales. However, on the global scale, there are large knowledge gaps in modern silicate weathering regimes and their possible shifts, due to the lack of spatial information on in situ weathering processes. Here, we present the first detailed compilation of the major element concentrations in bulk and fine fraction (&lt;2 μm) samples of surface soils from across China, which we use to investigate modern silicate weathering regimes across China and possible shifts between regimes induced by tectonic uplift and climate changes. The results show that changes in climatic factors, especially precipitation, exert a first‐order control on spatial variations in silicate weathering intensity (expressed by the chemical index of alteration, CIA), with additional contributions from nonclimatic factors. Comparison of silicate weathering regimes with the CIA suggests that for bulk samples the supply‐limited weathering regime prevails in areas with CIA &gt; 80, and the kinetically limited regime prevails in areas with CIA &lt; 80. A significant inference from the results is that tectonic uplift and global warming could cause shifts between supply‐limited and kinetically limited weathering regimes in the active mountain belts and floodplains of southern China and in areas dominated by intermediate chemical weathering. However, no shifts would occur in some of the arid to extremely arid areas of northern China and in highland areas with extreme chemical weathering.

Diverse manifestations of silicate weathering responses to late Neogene cooling within a tectonically active setting

The relationship between tectonic uplift, climate change and continental silicate weathering has long been debated because deconstructing the tectonic and climatic impacts on continental silicate weathering is challenging. To address this issue, we present long-term climatic and silicate weathering records since 7.3 Ma on tectonically active NE Tibet retrieved from the SG-1 and SG-1b lake sediment drill cores in the western Qaidam Basin. We find distinct correlations between silicate weathering and regional climate on long-term (>106 years) and short-term (105–106 years) time scales. On long-term scales, silicate weathering intensity exhibits a consistent evolution with regional climate, both of which are dominantly controlled by global climate change. This evolution resulted in a stable weathering intensity during 7.3–~3.6 Ma and a weakening of weathering intensity since ~3.6 Ma, followed by a continuous decreasing trend after 2.6 Ma. However, on short-term scales, silicate weathering intensity displays out-of-phase changes with the regional climate at 6.9–3.7 Ma and 0.2–0.1 Ma and in-phase changes at 7.3–6.9 Ma and 3.7–0.2 Ma. We attribute this long-term consistency between the weathering intensity and climate to aridification in inland Asia, but the short-term inconsistencies at 6.9–3.7 Ma and 0.2–0.1 Ma to the change in the sediment routing system impacted by the regional climate within a tectonically active setting. Such a change in the sediment routing system along with sediment recycling may bias weathering indication on orbital time scales, and our study therefore suggests a more complex response of silicate weathering to tectonic and climatic forcings.

Shifts in the silicate weathering regime in South China during the Meso-Cenozoic linked to Asian summer monsoon evolution

The scarcity of reliable and relatively continuous Meso-Cenozoic weathering records hinders the accurate temporal tracing of silicate weathering regimes and their possible shifts, which has resulted in a knowledge gap in our understanding of the interactions of the weathering-CO2-climate system on geological timescales. Here, we present a detailed compilation of Meso-Cenozoic silicate weathering records from South China. The results show that a trend of increasing weathering occurred in South China during the Meso-Cenozoic, coupled with increases in the average silicate weathering intensity (expressed by the chemical index of alteration (CIA) and the Kaolinite/(Illite+Chlorite) ratio), characterized by three stages. Comparison of silicate weathering regimes with the CIA reveals that expansions of a supply-limited weathering regime in the study region occurred at ~47 Ma and ~2.6 Ma, potentially caused by increases in Asian summer monsoon (ASM) precipitation. Also, we speculate that an increase in the land-sea thermal contrast induced by the global sea-level fall and the India-Asia suture and the initial retreat of the Neotethys Sea may have caused the initial strengthening of the ASM at ~47 Ma; and that the substantially increased cross-equatorial airflow may have contributed substantially to the increase in interglacial ASM precipitation since ~2.6 Ma. A significant implication of the two expansions is that the ASM may have prevailed over South China since at least 47 Ma.

A global temperature control of silicate weathering intensity

Silicate weathering as an important negative feedback can regulate the Earth’s climate over time, but much debate concerns its response strength to each climatic factor and its evolution with land surface reorganisation. Such discrepancy arises from lacking weathering proxy validation and scarce quantitative paleo-constraints on individual forcing factors. Here we examine the catchment-scale link of silicate weathering intensity with various environmental parameters using a global compilation of modern sediment dataset (n = 3828). We show the primary control of temperature on silicate weathering given the monotonic increase of feldspar dissolution with it (0–30 °C), while controls of precipitation or topographic-lithological factors are regional and subordinate. We interpret the non-linear forcing of temperature on feldspar dissolution as depletion of more reactive plagioclase (relative to orthoclase) at higher temperature. Our results hint at stronger temperature-weathering feedback at lower surface temperature and support the hypothesis of increased land surface reactivity during the late Cenozoic cooling.

Swift Weathering Response on Floodplains During the Paleocene‐Eocene Thermal Maximum

AbstractSilicate weathering is thought to increase and offset the rapid, massive input of CO2 into the atmosphere and ocean during the Paleocene‐Eocene Thermal Maximum (PETM), but few nonmarine records have been used to quantify this. We probe changes in silicate weathering intensity by measuring Li isotope ratios of bedrock and ancient floodplain deposits spanning the PETM in the Bighorn Basin, Wyoming (USA). Our results reveal a rapid increase in silicate weathering intensity during the PETM that remained high during at least the initial stage of climate recovery. Additionally, we determine that soils that formed farthest from ancient river channels underwent larger weathering changes than near‐channel soils. Alongside increased temperatures and pCO2, the simplest explanation for this response relates to increased seasonal fluctuations in water table height in the floodplain that promote dissolution and precipitation reactions. These findings newly demonstrate that weathering on floodplains, like mountain hillslopes, responds to climate change.

Trends and Transitions in Silicate Weathering in the Asian Interior (NE Tibet) Since 53 Ma

The relationship between silicate weathering, Tibetan Plateau uplift, and global cooling during the Cenozoic provides a valuable case study for understanding the interaction of tectonics and climate. The Tibetan Plateau uplift is considered to have caused Cenozoic cooling via the atmospheric CO2 drawdown by increased silicate weathering. However, this hypothesis has been intensively debated over the past few decades due to the lack of complete silicate weathering records from the continental interior, which can directly track the effects of uplift on weathering. We provide the first complete long (past 53 Myr) continental silicate weathering record from the NE Tibetan Plateau, combined with a comprehensive analysis on its evolution pattern, critical transitions, and associated driving forces. The silicate weathering intensity in NE Tibet is characterized by a long-term Paleogene decrease, modulated by global cooling, and a Neogene increase that may be related to the East Asian summer monsoon (EASM) intensification. Three major system transitions in regional silicate weathering are identified at ∼26–23 Ma, ∼16 Ma and ∼8 Ma, which are linked to enhanced EASM forced primarily by tectonic uplift at these intervals, with some surbordinate influences from global climate at ∼16 Ma. We also capture an intensification of the 100-kyr cycle at ∼16 Ma and ∼8 Ma in the obtained silicate weathering record, which is in coincidence in time with the enhancement of the EASM. This might suggest some contribution of the Antarctic ice sheets on modulating the regional silicate weathering in the NE Tibetan Plateau on a timescale of 105–106 years, through its influences on the EASM as proposed by previous studies.

Hafnium‑neodymium isotope evidence for enhanced weathering and uplift-climate interactions during the Late Cretaceous

The processes initiating the first cooling step of the last greenhouse-to-icehouse transition, from 90 million years ago (Ma) onward still remain enigmatic. While the combination of mountain uplift and continental weathering has been proposed as a major sink for atmospheric CO2 and a climate driver over geological timescales, this hypothesis is much debated and its potential importance in triggering the late Cretaceous global cooling is yet to be explored. In this work, we combined clay mineralogy, trace and major element concentrations, and a new proxy of silicate weathering intensity based on Nd and Hf isotopes (∆ɛHf(t)clay) to explore the potential links between the uplift of the brazilian margin, silicate weathering and climate evolution during the late Cretaceous. Our new ∆ɛHf(t)clay proxy data suggest - for the first time - that marked increase of silicate weathering intensity occurred in southeast Brazil during the late Cretaceous, from ~85 to 70 Ma, related to the tectonic uplift affecting the eastern South American margin at that time. Combined with clay mineralogical analyses, our Hf-Nd isotope data further suggests the existence of a relatively arid local climate during the Turonian-Santonian interval, during which physical disaggregation of rocks most likely prevailed, accompanying the uplift of the Brazilian margin. From the Santonian, we propose that the exposure of new high-elevation regions favored instead locally enhanced precipitations and more hydrolysing conditions, thereby promoting chemical weathering and atmospheric CO2 drawdown. Altogether, our multi-proxy investigation suggests that the uplift of the Brazilian margin could have contributed to the late Cretaceous cooling, potentially playing a key role in the onset of the last greenhouse-to-icehouse transition.

Responses of silicate weathering intensity to the Pliocene-Quaternary cooling in East and Southeast Asia

Silicate weathering plays a key role in maintaining a habitable climate on Earth. Although the climate-weathering link has been verified by observations on river geochemistry and weathering profiles, it remains a big challenge to quantify this link in the geological past mainly because sedimentary records of weathering intensity are complicated by other factors such as tectonics, sediment provenance and sorting effects. To clarify the climate dependence of weathering on different spatial and temporal scales, 4 Myr weathering intensity records determined by Chemical Index of Alteration (CIA), are compiled from 12 sites along the East and Southeast Asian margins ranging from ca. 10°N to 40°N. All the CIA records show overall decreasing trends, corresponding to global cooling. The CIA stacks that indicate regional and continental-scale weathering intensity are produced by binning and averaging the CIA data from each site. To explore the weathering intensity-temperature relationship, literature data on sea surface temperature (SST) anomaly are presented for the paleo-temperature changes at comparable latitudes. The results show that the decrease in CIA stack since 4 Ma is larger in East Asia (71-82 in Pliocene down to ca. 62 in late Quaternary) than in Southeast Asia (73-75 in Pliocene to ca. 72 in late Quaternary), in part due to the amplified midlatitude cooling. The correlation analyses indicate the temperature sensitivity of stacked CIA record in Southeast Asia (ca. 0.8 CIA units/°C) is consistent with modern observations of surface soils and river sediments. In comparison, the response of weathering intensity to temperature change is more sensitive in East Asia, with a magnitude of ca. 2.4 CIA units/°C, which may relate to dynamic source-to-sink processes and weathering regime changes. This study provides the first observation on silicate weathering responses to temperature changes at various latitudes since the Pliocene, showing the similar responses, but with different magnitudes. Our findings provide new insights on continental weathering mechanism and late Cenozoic cooling.

Disentangle the hydrodynamic sorting and lithology effects on sediment weathering signals

River sediment can be used to quantitatively investigate the silicate weathering intensity only if the weathering signal preservation and propagation are well understood, and especially the effects of hydrodynamic sorting and provenance lithology can be disentangled properly. Rivers entering the East China Sea have different climatic and geological settings and sediment source-to-sink transport patterns, which allow us to quantify the link between the chemical composition of river sediments and controlling factors of silicate weathering, hydrodynamic sorting, source rock natures and sedimentary recycling. We examine the geochemical evolution of sediments from these fluvial networks in weathering diagrams of A-CN-K [Al2O3-(CaO⁎ + Na2O)-K2O, Nesbitt and Young, 1984] and MFW [Mafic-Felsic-Weathering, Ohta and Arai, 2007]. The seeming “weathering trends” in these weathering diagrams are greatly influenced by mineral sorting given that the river sediment transport, deposition and resuspension are mainly subject to hydrodynamic conditions. The MFW diagram can better distinguish the influence of felsic and mafic source rocks, on condition that the weathering contribution from recycled sedimentary and metamorphic rocks in the catchment is minor. Nevertheless, both the A-CN-K and MFW diagrams are insensitive to sedimentary recycling induced quartz addition. Here, we propose a diagram of A-CNK-Si to disentangle the major factors controlling the river sediment geochemistry like chemical weathering, hydrodynamic sorting and provenance inheritance. This novel approach can decode the silicate weathering signals preserved in sedimentary records after removing the mineral sorting and quartz dilution effects, which will allow us to better reconstruct the geochemical evolution of Earth's surface.

Deciphering the signatures of weathering and erosion processes and the effects of river management on Li isotopes in the subtropical Pearl River basin

Lithium (Li) isotopes signatures in sedimentary archives are promising proxies to reconstruct paleo-weathering rates and paleo-seawater composition. Nonetheless, the interpretation of the seawater Li isotope composition evolution over the Cenozoic is still debated. In this aim, the study of large rivers can provide constraints for continental weathering flux to the ocean. However, in the recent past, human activities like flow regulation via reservoir construction, have significantly modified the hydrodynamic of riverine systems, impacting the transfer of matter from the continent to the ocean. Hence, these effects need to be considered and investigated when unravelling the signature of natural processes from river borne materials.We report Li concentrations and Li isotope compositions (δ7Li) for the dissolved and sediment loads of subtropical river waters from the Xijiang basin, the largest tributary of the Pearl River (also known as the Zhujiang), Southern China. These rivers display low dissolved Li concentrations. Although they are potentially impacted by carbonate dissolution in the upstream karst terrains, the Li budget of these rivers is dominated by silicate weathering. The dissolved Li isotope compositions of rivers vary over a range from +16‰ to +27‰. These values are high compared to the estimated composition of the bedrock (~0‰), and are explained by the preferential incorporation of the light 6Li in solid weathering products formed during the incongruent dissolution of silicate rocks. We show that the variations of the dissolved δ7Li values across the basin reflect changes in silicate weathering intensity which controls the formation of secondary phases. The high δ7Li values in the headwaters reflect greater degree of secondary phase formations associated with the higher erosion rates in the mountainous part of the basin, whereas the lower δ7Li values in the lowlands characterized by low soil erosion rates and more abundant rainfall reflect a more intensive weathering. This interpretation is supported by the positive relationship between soil erosion rates and the Li dissolved fluxes. The isotope compositions of the suspended particulate material (SPM) transported by rivers in the Xijiang Basin (from −3.8‰ to −0.7‰) are systematically lower than the composition of the bedrock. We observe that the δ7Li of most SPM samples collected during the wet season are lower than those of samples collected during the dry season. We interpret this seasonality as a consequence of seasonal changes in the sediment properties due to water regulation. In spite of the influence induced by river managements on the river sediments, our data also highlight that the imprint of weathering and erosion processes is still preserved in the sedimentary load of the Xijiang as suggested by the relationship between the SPM δ7Li values and the weathering intensity defined as the ratio between the chemical weathering rates and the total denudation rates. Altogether, our study confirms the robustness of Li isotopes as a proxy of weathering and erosion processes, even in this heavily-managed river system.

Monsoon‐Enhanced Silicate Weathering as a New Atmospheric CO2 Consumption Mechanism Contributing to Fast Late Miocene Global Cooling

AbstractFast late Miocene global cooling since ∼7 Ma accompanied by less changeable atmospheric CO2 levels revealed by existing proxy reconstructions has suggested an intriguing tectonic‐climate link that remains controversial. Here, we present late Cenozoic clay mineral records of the silicate weathering intensity from the Chinese Loess Plateau and northeastern Tibetan Plateau to demonstrate a remarkable increase in silicate weathering intensity at ∼9–7 Ma induced by enhanced monsoon. This change caused CO2 consumption ranging from 0.18 to 1.8 × 1011 mol C yr−1 over the East Asian monsoon region, accounting for 0.2%–2% of the modern continental silicate weathering flux, thus providing an additional atmospheric CO2 sink. Moreover, we propose that this additional sink may have contributed to the large atmospheric CO2 consumption and fast global cooling in the late Miocene, which ultimately caused the onset of the Northern Hemisphere glaciation at ∼7 Ma.

Glacial-interglacial variation in catchment weathering and erosion paces the Indian summer monsoon during the Pleistocene

Silicate weathering via CO2 consumption is considered to regulate the long-term climate change. Discriminating silicate versus carbonate weathering from sedimentary records is helpful to understand the coupling relationship between weathering and past climate. However, owing to the paucity of well-dated and continuous records of weathering and erosion, the relationship between silicate and carbonate weathering and their links with mountain uplift and glacial-interglacial climate remain elusive. Geochemical analysis of a 666 m core from the Heqing basin, a paleolake situated at the southeastern margin of the Tibetan Plateau, demonstrates silicate versus carbonate weathering and erosion changes under Indian summer monsoon (ISM) conditions spanning the entire Pleistocene, via respective tracing between Rb/Sr and Al/Na ratios. The Rb/Sr record is interpreted as reflecting a competition between clayey silicate-origin Rb versus carbonate-origin Sr fluxes, whereas Al/Na ratio is a balance between weathering and erosion of silicate minerals, both pacing the glacial-interglacial cycles of the ISM. During interglacials, strong silicate weathering intensity reflected by low Al/Na values is mainly controlled by warmer temperatures, with the reduced variability during 1.82–0.92 Ma, recalling the pacing of the Arabian Sea surface temperature amplitudes. Within glacials, the weak carbonate leaching and clay erosion within the older and younger intervals are interrupted by strong clay erosion within the middle interval. In conclusion, the Heqing Rb/Sr and Al/Na records reveal the sensitivity of catchment weathering and erosion to ISM changes over glacial-interglacial timescales with warmer temperature enhancing silicate weathering and clay formation during interglacials and greater physical erosion during glacials.

Temperature Control on Silicate Weathering Intensity and Evolution of the Neogene East Asian Summer Monsoon

AbstractAccurately reconstructing the evolution of the Asian monsoon is predicated on understanding the impact of temperature, precipitation, and tectonic paleogeography on silicate weathering proxies over million year timescales. We find that decreasing trends in chemical weathering proxies in both the northern and southern Tibetan Plateau match those from benthic oxygen isotope and sea surface temperature stacks since the late Miocene. In contrast, a synthesis of magnetic parameter‐based records and marine clastic flux records reveals stronger monsoon precipitation during both the warm middle Miocene and cool late Miocene, supporting model simulations showing that both paleogeography and atmospheric CO2 content are important in controlling the summer monsoon. This trend of increasing monsoon precipitation contrasts with the record of decreasing chemical weathering, suggesting that chemical weathering proxies are mainly regulated by temperature at million year timescales, rather than precipitation. These findings clarify reconstructions of the Cenozoic evolution of the East Asian summer monsoon.

Lithium isotope compositions of the Yangtze River headwaters: Weathering in high-relief catchments

Tectonically active high relief areas are proposed to have a substantial impact on climate regulation throughout the Cenozoic. Here, we present lithium (Li) isotope data of the dissolved load and suspended particulate material from the basins of the Yalong River and the Jinsha River, which drain the high-relief borders of the eastern Tibetan Plateau and constitute the headwaters of the Yangtze River. Dissolved δ7Li values are relatively low and constant in the Jinsha River mainstream (+6.8‰ to +9.7‰) compared to the higher and downstream increasing values in the Yalong River mainstream (+10.8‰ to +17.5‰). Furthermore, dissolved δ7Li values in the major tributaries of the Jinsha and Yalong River are highly variable and increase downstream from +11.4‰ to +29.4‰ and from +9.0‰ to +23.6‰, respectively. Robust correlations between Li, Na, and Cl concentrations identify that evaporite dissolution in the uppermost headwaters of the Jinsha River basin controls the Li budget and riverine δ7Li values of the mainstream and has a significant impact on dissolved Li in the Yangtze River. In contrast to the Jinsha River mainstream, dissolved Li in the Yalong River mainstream and the Jinsha and Yalong River tributaries are dominantly derived from the dissolution of silicates. δ7Li variations in the Yalong River basin and the Jinsha tributaries reflect little Li uptake by clays in the upper reaches and more Li uptake by clays in the lower reaches of the river basins. SWR/D ratios (where SWR is the chemical silicate weathering rate, and D is the total denudation rate) highlight an intermediate silicate weathering intensity under a kinetically limited weathering regime for the study area. However, positive correlations between dissolved δ7Li value and silicate weathering intensity proxies (such as Si/(Nasil + K) and K/(Nasil + K)) indicate an increasing weathering intensity downstream. A positive correlation between dissolved δ7Li values and the annual precipitation and a negative one between δ7Li values and elevation indicates that larger annual precipitation under a flattening relief results in a stronger silicate weathering intensity, thus, more Li uptake in clays and higher riverine δ7Li values in the lower reaches. Compared to the lower reaches of Yangtze River basin (∼+20‰), lower dissolved δ7Li values in its headwater of this study suggest that the increase in δ7Li of seawater over the Cenozoic is influenced by weathering in floodplains that developed due to erosion of high mountains, rather than weathering of mountain belts itself. Combined with silicate weathering rates of the eastern border of the Tibetan Plateau, the highest riverine δ7Li values in its catchments come along with the highest silicate weathering rates and high CO2 consumption, suggesting that a more elevated topography may have an important role in the drawdown of CO2 during the Cenozoic.

Enhanced chemical weathering triggered an expansion of euxinic seawater in the aftermath of the Sturtian glaciation

The Cryogenian Period comprised two episodes of global glaciation (Sturtian and Marinoan glaciations) separated by a non-glacial interval, which was characterized by early radiations of eukaryotic algae and putative metazoans. Geochemical data indicate that the non-glacial interval might be marked by a transient marine oxygenation, nevertheless oceanic redox conditions varied both in time and space. Further, the links between non-glacial climate and marine redox variations are not well constrained. Here we present high-resolution lithium isotope (δ7Li), Fe speciation and trace element (Mo and U) data for clastic sedimentary rocks from the Cryogenian interglacial Datangpo Formation, South China, in order to track the evolution of continental chemical silicate weathering and driving factors behind marine redox variability during the Cryogenian non-glacial interval. A significantly negative δ7Li excursion of ∼−5‰ is observed in the basal Datangpo Formation, suggesting a dramatic increase in chemical silicate weathering intensity in the aftermath of the Sturtian glaciation. Expansion and contraction of anoxic-sulfidic conditions, as demonstrated by Fe speciation and trace element (Mo and U) data, mirror changes in silicate weathering intensity. Our study provides evidence that greater nutrient and sulfate availability, due to high silicate weathering intensity associated with increased exposure of fresh rocks and a warm climate, facilitated the spread of euxinic waters over the continental margins of the otherwise ferruginous Cryogenian ocean.