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The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Sea of Japan and southward to western Australia. High recovery by advanced piston corer (APC) has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C 3 to C 4 vegetation often accompanied the drying but may be more linked to global cooling. Western Australia and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion but not weathering intensity, which either shows no trend or a decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial chemical weathering, erosion, and vegetation proxies are often seen to diverge. Future work on the almost unknown Paleogene is needed, as well as the potential of carbonate platforms as archives of paleoceanographic conditions. [ABSTRACT FROM AUTHOR]

The Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO2 consumption process. Here, we performed a first-order estimate of the CO2 consumption induced by silicate chemical weathering and organic carbon burial in subtropical China related to this climatic reorganization. Our results show that an increase in long-term CO2 consumption by silicate weathering varies from 0.06×1012 to 0.87×1012 mol yr−1 depending on erosion flux reconstructions, with an ~50% contribution of Mg-silicate weathering since the late Oligocene. The organic carbon burial flux is approximately 25% of the contemporary CO2 consumption by silicate weathering. The results highlight the significant role of weathering of the Mg-rich upper continental crust in East China, which would contribute to the rapid decline in atmospheric CO2 during the late Oligocene and the Neogene rise in the seawater Mg content. If this climatic reorganization was mainly induced by the Tibetan Plateau uplift, our study suggests that the growth of the Himalayan-Tibetan Plateau can lead to indirect modification of the global carbon and magnesium cycles by changing the regional hydrological cycle in areas of East Asia that are tectonically less active. [ABSTRACT FROM AUTHOR]

Abstract: Continental chemical weathering is central in Earth''s surface biogeochemical cycles as it redistributes elements across reservoirs such as the crust and the oceans. However the evolution of weathering through time and its response to external forcing such as changes in climate remain poorly constrained. In this work, a composite sediment record from the Bay of Bengal is used to document the evolution of chemical weathering in the Himalayan system (Himalayan range and Indo-Gangetic floodplain), the world largest sediment conveyor to the oceans, since the Last Glacial Maximum (LGM). The degree of weathering of the sediments is documented using mobile to immobile ratios such as K/Si and H2O+/Si as well as detrital calcite abundance. Robust weathering proxies are derived by correcting the chemical composition of sediment for sorting effects that occur during transport and deposition. The Bay of Bengal record is also further compared to the chemical composition of modern river sediments from the Ganga and Brahmaputra basin. Weathering proxies all indicate that the sediments exported by the Ganga and Brahmaputra Rivers became increasingly weathered over the past ∼21kyr, whereas, Sr, Nd and major elements suggest a constant sediment provenance in the system over the last 21kyr. These changes in the degree of weathering of the sediments show that the weathering flux exported by the system to the Indian Ocean during the LGM was significantly lower than at present and demonstrate that chemical weathering in continental scale basins such as the Ganga and Brahmaputra responds to Late Quaternary climate changes. [Copyright &y& Elsevier]

Abstract: We present an extensive river sediment dataset covering the Ganga basin from the Himalayan front downstream to the Ganga mainstream in Bangladesh. These sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column. Mineral sorting is the first order control on the chemical composition of river sediments. Taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain. By comparing sediments sampled at the Himalayan front with sediments from the Ganga mainstream in Bangladesh it is possible to budget weathering in the floodplain. Assuming a steady state weathering regime in the floodplain, the weathering of Himalayan sediments in the Gangetic floodplain releases ca. (189±92)×109 and (69±22)×109 mol/yr of carbonate bound Ca and Mg to the dissolved load, respectively. Silicate weathering releases (53±18)×109 and (42±13)×109 mol/yr of Na and K while the release of silicate Mg and Ca is substantially lower, between ca. 0 and 20×109 mol/yr. Additionally, we show that sediment hydration, [H2O+], is a sensitive tracer of silicate weathering that can be used in continental detrital environments, such as the Ganga basin. Both [H2O+] content and the D/H isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions. By comparing the chemical composition of river sediments across the floodplain with the composition of the eroded Himalayan source rocks, we suggest that the floodplain is the dominant location of silicate weathering for Na, K and [H2O+]. Overall this work emphasizes the role of the Gangetic floodplain in weathering Himalayan sediments. It also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account. [Copyright &y& Elsevier]

Determining the rate of CO 2 consumption associated with the Himalayan uplift is an essential prerequisite to understanding climate evolution throughout the Cenozoic. The riverine fluxes of dissolved calcium can be used to quantify uptakes of atmospheric CO 2 by chemical weathering, but this approach requires deciphering the silicate-derived Ca flux from the generally dominant carbonate-derived Ca flux. Here we present high-precision radiogenic calcium (40Ca) analyses of bank sediments and dissolved loads from a network of rivers in central Nepal Himalaya and Bangladesh, to constrain the sources and relative contributions of carbonate and silicate-derived calcium to the dissolved loads of Himalayan rivers. Calcium isotope analyses were performed in multidynamic mode by thermal ionization mass spectrometry, yielding an external precision of ±0.4 ε-units (2 S.D.). Our results show that silicate catchments exposed in the Himalayan range have variably radiogenic ε40Ca compositions relative to seawater, ranging from +0.7 in the TSS to +14 in the LH. In contrast, sedimentary carbonates, including metamorphosed dolomites with variably radiogenic 87Sr/86Sr, exhibit uniform ε40Ca identical to modern seawater. The homogeneous ε40Ca composition of sedimentary carbonates confirms the relative resistance of radiogenic Ca signatures to post-depositional alteration and provides a robust baseline against which the contribution of silicate weathering to the riverine Ca flux can be evaluated. Dissolved load compositions of rivers draining LH catchments exhibit ε40Ca values ranging from 0 to +11, reflecting the relative contribution of unradiogenic carbonates and highly radiogenic metapelitic units of the LH sequence. HHC rivers show a more moderate range, from 0.5 to +2, reflecting the less radiogenic composition of HHC paragneisses. Himalayan front rivers draining all three lithotectonic units of the Himalayan range exhibit moderately radiogenic ε40Ca, ranging from +0.5 to +1.4 and overall similar to the average composition of the Ganga mainstream in Bangladesh. Using an approach combining conventional alkalinity budgets with isotopic mass balance calculations, we show that the dissolved silicate Ca flux of major Himalayan front rivers is primarily derived from the weathering of metapelitic rocks of the lesser Himalaya. In contrast, regions of the high range that generate the bulk of the detrital flux to the Gangetic floodplain represent a subordinate source of silicate-derived Ca. This result demonstrates that the chemical weathering of Ca-silicates in the Himalayan system is decoupled from physical erosion. The extensive erosional activity observed in the high range may thus play a limited role in promoting CO 2 consumption and global cooling by silicate weathering. [ABSTRACT FROM AUTHOR]

The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata. [ABSTRACT FROM AUTHOR]

The Indian monsoon and tectonic both control the Himalayan erosion. Tectonic generates uplift and maintain high elevation of the range and earthquakes weaken the mountain front. Monsoon seasonal intensity controls the glacier cover, landslide activity, river incision, vegetation cover, and export of sediments towards the floodplain. The monsoon also exerts a primary control on the chemical erosion as weathering rates are clearly dependent upon river discharge. In the floodplain, sediment export is also tightly controlled by climate. The seasonality and intensity of the monsoon generate discharge high enough to ensure the efficiency of river transport of sand-rich sediments to the delta and shelf (Lupker et al. JGR, 2011). IODP Expedition 354 allowed to drill a series of seven sites along an E-W transect at 8°N in the Bengal fan. This generated a comprehensive record of Himalayan erosion since 20 Ma. Bengal fan sediments are dominated by turbidites characterised by high accumulation rate and variable grain size raging from clayey silt to sand. Plio-Pleistocene turbidites reveal frequent channel levee deposition with massive sand deposition in inter-levee intervals. Overall, sand represents at least a third of the fan deposition. Turbidites are intercalated with hemipelagic sediments characterised by low accumulation rates, 20 to 50% of biogenic carbonates, fine grains and bioturbation. Turbidites have petrographic and geochemical signatures characteristic of sediments derived from the Himalaya such as the Ganga and Brahmaputra rivers that supply the fan (see A. Galy et al. this session). Major and trace element geochemistry show relatively stable compositions throughout the Neogene and Quaternary. They reveal a very weak regime of chemical weathering with no significant variation through time. Concentrations in mobile elements such as Na and K relative to Al are significantly higher than in modern river and northern fan sediments. This suggests that weathering and/or soil erosion are amplified in the modern time. Detrital carbonate concentration vary significantly with higher concentrations during the Miocene then falling during the Plio-Pleistocene. This suggests either a change in the exposition of Tethyan series in Himalaya or significantly lower chemical weathering during Miocene. Finally, clay composition is dominated by illite which also differs with the present Ganga which is rich in smectite. The record at 8°N contrasts with the ODP Leg 116 record at 1°S in the distal fan, which from 7 to 1 Ma shows a high degree of weathering associated to lower accumulation rates. The record at 8°N indicates that these variations do not reflect changes in the continental source basin but rather reflect a change in the turbiditic export. The Bengal Fan record at 8°N suggests that an erosion regime dominated by physical erosion able to drive large amount of sand and silt with low weathering was established at least 20 Ma ago and remained relatively stable in spite of the global climate variations. These observations suggest that erosion sustained by a highly seasonal climate able to ensure rapid transport of sediments such as the monsoon was already active 20 Ma ago. [ABSTRACT FROM AUTHOR]

Enhanced silicate weathering induced by the uplift of the Himalayan-Tibetan Plateau (HTP) has been considered as the major cause of p CO 2 decline and Cenozoic cooling. However, this hypothesis remains to be validated, largely due to the lack of a reliable reconstruction of the HTP weathering flux. Here, we present a 37-million-year record of the difference in the seawater radiogenic neodymium isotopic composition (Δ ε Nd) of Ocean Drilling Program (ODP) sites and Fe-Mn crusts between the northern and central Indian Ocean, which indicates the contribution of regional weathering input from the South Asian continent to the Indian Ocean. The results show a long-term increase in Δ ε Nd and thus provide the first critical evidence of enhanced South Asian weathering input since the late Eocene. The evolution coincided well with major pulses of surface uplift in the HTP and global climatic transitions. Our foraminiferal ε Nd record suggests that tectonic uplift and silicate weathering in South Asia, especially in the Himalayas, might have played a significant role in the late Cenozoic cooling. [ABSTRACT FROM AUTHOR]

Abstract: Major ion composition of waters, δ13C of its DIC (dissolved inorganic carbon), and the clay mineral composition of bank sediments in the Brahmaputra River System (draining India and Bangladesh) have been measured to understand chemical weathering and erosion and the factors controlling these processes in the eastern Himalaya. The time-series samples, collected biweekly at Guwahati, from the Brahmaputra mainstream, were also analyzed for the major ion composition. Clay mineralogy and chemical index of alteration (CIA) of sediments suggest that weathering intensity is relatively poor in comparison to that in the Ganga basin. This is attributed to higher runoff and associated physical erosion occurring in the Brahmaputra basin. The results of this study show, for the first time, spatial and temporal variations in chemical and silicate erosion rates in the Brahmaputra basin. The subbasins of the Brahmaputra watershed exhibit chemical erosion rates varying by about an order of magnitude. The Eastern Syntaxis basin dominates the erosion with a rate of ∼300 t km−2 y−1, one of the highest among the world river basins and comparable to those reported for some of the basaltic terrains. In contrast, the flat, cold, and relatively more arid Tibetan basin undergoes much slower chemical erosion (∼40 t km−2 y−1). The abundance of total dissolved solids (TDS, 102–203 mg/L) in the time-series samples collected over a period of one year shows variations in accordance with the annual discharge, except one of them, cause for which is attributable to flash floods. Na* (Na corrected for cyclic component) shows a strong positive correlation with Si, indicating their common source: silicate weathering. Estimates of silicate cations (Nasil+Ksil+Casil+Mgsil) suggest that about half of the dissolved cations in the Brahmaputra are derived from silicates, a proportion higher than that for the Ganga system. The CO2 consumption rate due to silicate weathering in the Brahmaputra watershed is ∼6 × 105 moles km−2 y−1; whereas that in the Eastern Syntaxis subbasin is ∼19 × 105 moles km−2 y−1, similar to the estimates for some of the basaltic terrains. This study suggests that the Eastern Syntaxis basin of the Brahmaputra is one of most intensely chemically eroding regions of the globe; and that runoff and physical erosion are the controlling factors of chemical erosion in the eastern Himalaya. [Copyright &y& Elsevier]

Climate has been proposed to control both the rate of terrestrial silicate weathering and the export rate of associated sediments and terrestrial organic carbon to river-dominated margins – and thus the rate of sequestration of atmospheric CO 2 in the coastal ocean – over glacial–interglacial timescales. Focused on the Ganges–Brahmaputra rivers, this study presents records of post-glacial changes in basin-scale Indian summer monsoon intensity and vegetation composition based on stable hydrogen ( δ D ) and carbon ( δ 13 C ) isotopic compositions of terrestrial plant wax compounds preserved in the channel–levee system of the Bengal Fan. It then explores the role of these changes in controlling the provenance and degree of chemical weathering of sediments exported by these rivers, and the potential climate feedbacks through organic-carbon burial in the Bengal Fan. An observed 40‰ shift in δ D and a 3–4‰ shift in both bulk organic-carbon and plant-wax δ 13 C values between the late glacial and mid-Holocene, followed by a return to more intermediate values during the late Holocene, correlates well with regional post-glacial paleoclimate records. Sediment provenance proxies (Sr, Nd isotopic compositions) reveal that these changes likely coincided with a subtle focusing of erosion on the southern flank of the Himalayan range during periods of greater monsoon strength and enhanced sediment discharge. However, grain-size-normalized organic-carbon concentrations in the Bengal Fan remained constant through time, despite order-of-magnitude level changes in catchment-scale monsoon precipitation and enhanced chemical weathering (recorded as a gradual increase in K/Si ⁎ and detrital carbonate content, and decrease in H 2 O + /Si ⁎ , proxies) throughout the study period. These findings demonstrate a partial decoupling of climate change and silicate weathering during the Holocene and that marine organic-carbon sequestration rates primary reflect rates of physical erosion and sediment export as modulated by climatic changes. Together, these results reveal the magnitude of climate changes within the Ganges–Brahmaputra basin following deglaciation and a closer coupling of monsoon strength with OC burial than with silicate weathering on millennial timescales. [ABSTRACT FROM AUTHOR]

Atmospheric CO 2 and global climate are closely coupled. Since 800 ka CO 2 concentrations have been up to 50% higher during interglacial compared to glacial periods. Because of its dependence on temperature, humidity, and erosion rates, chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition. Cooler and drier conditions and lower non-glacial erosion rates suppressed in situ chemical weathering rates during glacial periods. However, using systematic variations in major element geochemistry, Sr–Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea spanning the last 1.1 Ma, we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. We attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. Our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for ∼9% of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial–interglacial variation of ∼80 ppmv. As a result, if similar magnitudes can be identified in other tropical shelf-slope systems, the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric CO 2 during cold stages of the Quaternary. This in turn would have caused an intensification of glacial cycles. [ABSTRACT FROM AUTHOR]

The erosion of major mountain ranges is thought to be largely cannibalistic, recycling sediments that were deposited in the ocean or on the continents prior to mountain uplift. Despite this recognition, it has not yet been possible to quantify the amount of recycled material that is presently transported by rivers to the ocean. Here, we have analyzed the Li content and isotope composition (δ7Li) of suspended sediments sampled along river depth profiles and bed sands in three of the largest Earth's river systems (Amazon, Mackenzie and Ganga-Brahmaputra rivers). The δ7Li values of river-sediments transported by these rivers range from +5.3 to -3.6‰ and decrease with sediment grain size. We interpret these variations as reflecting a mixture of unweathered rock fragments (preferentially transported at depth in the coarse fraction) and present-day weathering products (preferentially transported at the surface in the finest fraction). Only the finest surface sediments contain the complementary reservoir of Li solubilized by water-rock interactions within the watersheds. Li isotopes also show that river bed sands can be interpreted as a mixture between unweathered fragments of igneous and sedimentary rocks. A mass budget approach, based on Li isotopes, Li/Al and Na/Al ratios, solved by an inverse method allows us to estimate that, for the large rivers analyzed here, the part of solid weathering products formed by present-day weathering reactions and transported to the ocean do not exceed 35%. Li isotopes also show that the sediments transported by the Amazon, Mackenzie and Ganga-Brahmaputra river systems are mostly sourced from sedimentary rocks (>60%) rather than igneous rocks. This study shows that Li isotopes in the river particulate load are a good proxy for quantifying both the erosional rock sources and the fingerprint of present-day weathering processes. Overall, Li isotopes in river sediments confirm the cannibalistic nature of erosion and weathering. [ABSTRACT FROM AUTHOR]

Abstract: Large river sediments are mostly derived from tectonically active mountain belts, but then undergo a series of sedimentation, temporary storage and reworking episodes on their journey to the ocean. The long transfer time of these sediments through active floodplains might result in a significant chemical maturation via weathering reactions, which is of critical importance for biogeochemical cycles at the Earth surface. This study reports the chemical composition of river sediments from different locations throughout the courses of the main tributaries of the Amazon Basin. Sampling along river depth-profiles yields access to the whole grain size and chemical composition range of river sediment. Here, weathering intensities (i.e. losses of Na, K, Mg and Ca) associated with chemical weathering in floodplains are (1) examined as a function of grain size, and (2) integrated over the whole grain size range, for three selected long river reaches flowing through the foreland and the lowland of the Amazon Basin: the upper Marañon, the Beni and the lower Madeira rivers. A relatively small Na loss through plagioclase dissolution is observed in the Madeira reach, while an important Ca loss due to carbonate dissolution occurs in the two foreland reaches (Marañon and Beni). No significant loss of K and Mg is observed in any of the reaches, showing the low alterability of primary K and Mg-bearing minerals and suggesting retention of K and Mg in the particulate phase by secondary minerals. The combination of these findings with previously reported data on the downstream change of dissolved Na, K, Mg and Ca fluxes suggests that chemical weathering of “stable” alluvial deposits could also significantly contribute to theweathering flux generated in foreland and lowland areas. The comparison between the three Amazon reaches and the Gangetic plain tends to show that the features observed in the Amazon are valid on a global scale. Finally, we show that, although resulting in a relatively small change in the chemical composition of the river sediment, silicate weathering in the lower Madeira floodplain can lead to a CO2 drawdown equivalent to ca. 10% of the total CO2 consumption flux of the whole Madeira basin. [Copyright &y& Elsevier]

The chemical weathering of rocks with sulfuric acid is usually not considered in reconstructions of the past evolution of the carbon cycle, although this reaction delivers cations and alkalinity to the ocean without involvement of atmospheric CO2. The contribution of sulfuric acid as a weathering agent is still poorly quantified; the identification of riverine sulfate sources is difficult. The use of δ34S and δ18O of dissolved sulfate allows us to demonstrate that most of the sulfate in surface waters of the Mackenzie River system, Canada, derives from pyrite oxidation (85% ± 5%) and not from sedimentary sulfate. The calculated flux of pyrite-derived sulfate is 0.13 × 1012 mol/yr, corresponding to 20%–27% of the estimated global budget. This result suggests that the modern global ocean delivery of sulfide-derived sulfate, and thus chemical weathering with sulfuric acid, may be significantly underestimated. A strong correlation between sulfide oxidation rates and mechanical erosion rates suggests that the exposure of fresh mineral surfaces is the rate-limiting factor of sulfide oxidation in the subbasins investigated. The chemical weathering budget of the Mackenzie River shows that more than half of the dissolved inorganic carbon discharged to the ocean is ancient sedimentary carbon from carbonate (62%) and not atmospheric carbon (38%). The subsequent carbonate precipitation in the ocean will thus release more CO2 in the atmosphere-ocean system than that consumed by continental weathering, typically on glacial-interglacial time scales. [ABSTRACT FROM AUTHOR]