Chemical weathering of silicate rocks on continents is thought to have played an important role in the evolution of past atmospheric carbon dioxide over geologic timescales. However, the detailed ...links between continental weathering and climate change over shorter timescales, and their potential impact on sediment records deposited in the ocean, remain poorly understood. Here, we present clay mineralogy and strontium‐neodymium isotopic data for marine sediment records from the Northern Indian Ocean, with the aim of investigating the weathering response of large Himalayan river basins to orbital and millennial climate forcing. We show that past glaciated episodes of the late Quaternary corresponded to periods of increased physical erosion, associated with the preferential export of illite and chlorite assemblages from the Himalayan highlands having relatively radiogenic Sr isotopic signatures. In contrast, the warm periods of enhanced monsoon rainfall coincided with the transport of intensively weathered smectite‐dominated soils derived from the floodplains, characterized by lower 87Sr/86Sr signatures. This finding suggests that the short‐term climatic variability over late Quaternary timescales was accompanied by concomitant changes between high mountain‐ versus floodplain‐dominated weathering regimes, with possible impact on the nature of weathered rocks and, as a consequence, on the carbon cycle.
Key Points
Orbital‐millennial clay records in Northern Indian Ocean of past 190 and 80 kyr are reported
Climate change results in a shift from mountain‐ to floodplain‐dominated weathering regimes
Possible impact of climate‐driven weathering regimes in mountainous watersheds on carbon cycle
Chemical measurement points obtained with energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) were processed by a modified weighted interval rank sort ...(WIRS) method in order to study the solute redistribution during solidification of an additively manufactured 316L stainless steel. Scheil-Gulliver calculations give a good first approximation, although the extent of segregation is lower experimentally than theoretically. These discrepancies are believed to be due to the fast solidification rate involved. STEM-EDS data processed by the modified WIRS method is a strong and reliable technique for characterizing the segregation profile for additively manufactured metals, and the proposed method is shown to reduce the experimental uncertainty associated with the quantification of minor elements.
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Deep atmospheric convection in the Indo‐Pacific Warm Pool (IPWP) represents a major source of heat and moisture, thereby affecting the global climate, but its past changes remain debated. Here, we ...present sub‐millennial clay mineralogy and elemental records spanning the last 40 Kyr from within the IPWP. From these data, we infer millennial‐scale fluctuations in precipitation, with generally lower precipitation during Heinrich Stadials 1–4, corresponding to El Niño‐like conditions. Higher precipitation coincided with the warm interstadials, accompanied by La Niña‐like conditions. Moreover, our record indicates the lowest precipitation occurred during the late Holocene, supporting the hypothesis of a stronger‐than‐modern Walker circulation during the Last Glacial Maximum. In combination with other proxy records and TraCE‐21 modeling results, we recognize a distinct spatial heterogeneity of precipitation within the IPWP, resulting from the dominant influences of the ENSO‐like system and migration of the Intertropical Convergence Zone in the eastern and western IPWP, respectively.
Plain Language Summary
Tropical rainfall systems in the Indo‐Pacific Warm Pool (IPWP) play a significant role in transferring energy and moisture. However, their millennial‐scale variabilities in precipitation during the past remain poorly understood. Here, we provide sub‐millennial sediment mineralogy and elemental reconstructions spanning the last 40,000 years from a sediment core in the IPWP. We infer that variability in precipitation induced by atmospheric convection in the IPWP is the main factor controlling the variability in our data through time. Hence, we interpret our records to reflect millennial‐scale fluctuations in precipitation, with lower precipitation during cool stadial periods and higher precipitation during warm interstadials. Such variability corresponds to the changing zonal state of the tropical Pacific atmosphere‐ocean system, in a pattern similar to the modern‐day. Additionally, our record indicates that precipitation was weaker during the late Holocene than during the Last Glacial Maximum (LGM), suggesting that the Pacific atmospheric zonal circulation was stronger during the LGM than today. Furthermore, a model‐data comparison reveals distinct spatial heterogeneity in the precipitation changes, with precipitation in the eastern IPWP (West Pacific) likely controlled mostly by the tropical Pacific atmosphere‐ocean system, whereas precipitation in the western IPWP (Indian Ocean) was driven by latitudinal movement of the Intertropical Convergence Zone.
Key Points
We present sub‐millennial clay mineral and elemental records from the Indo‐Pacific Warm Pool spanning the last 40 Kyr
Our records indicate higher precipitation during La Niña‐like conditions and lower precipitation during El Niño‐like conditions
Different patterns and controls on precipitation variability between the eastern and western Indo‐Pacific regions are revealed
Sediment cores from the Mediterranean Sea have evidenced several periods of Sapropel deposition, which can be explained by events of anoxic bottom conditions. An explanation for such events calls for ...a very stratified sea, possibly related with freshwater input through increased precipitations and runoff discharges. It has been suggested that such a stratified Mediterranean Sea may in turn weaken the Atlantic meridional overturning circulation (AMOC) through changes in the Mediterranean overflow water (MOW). Nevertheless, models used to establish this result were relatively simple and the mechanisms leading to such an impact remained elusive. To improve on those previous studies, we analyse the impact of different freshwater releases with rates of 0.2, 0.1, 0.05 and 0.02 Sv (1 Sv = 10
6
m
3
/s) in the Mediterranean Sea using the IPSL-CM5A-LR model in a few multi-centennial simulations. We focus the analysis on the impact of a decrease in the MOW on the large-scale Atlantic circulation. We find a consistent change in horizontal currents in the upper Atlantic Ocean in all simulations in the first century, with a large enhancement of the northward current west of Rockall in the northeast Atlantic. Concerning the AMOC response, we identify three different processes that impact its fate. The first is related to changes in geostrophic currents at depth induced by the disappearance of the MOW, which tends to weaken the AMOC. On the contrary, the second enhances the AMOC and is associated with the increase in northward currents in the horizontal upper circulation. The last process is due to the spread of surface freshwater anomalies out of the Mediterranean Sea that freshens the North Atlantic convection sites and weakens the AMOC. Depending on the rate of the freshwater release, the strength and balance of these three processes are different. For rates larger than 0.05 Sv, we observe a strong reduction of the AMOC, while for lower rates, we notice an enhancement in the upper cell. The climatic response follows that of the upper AMOC with a warming of the North Atlantic for rates lower than 0.05 Sv and a cooling for higher rates. Given that past estimates of freshwater release in the Mediterranean Sea indicate rates lower than 0.05 Sv, we argue that Sapropel events may have enhanced of the upper AMOC and warmed of the North Atlantic.
Abstract The Indian Coastal Current is the only channel for material exchange between the two largest marginal seas in the northern Indian Ocean: the Bay of Bengal and the Arabian Sea. However, its ...past history is poorly known, limiting accurate predictions of its future changes. Here, we present a new clay mineral record from south of India supported by interpretations of model simulations to trace its variability over the last 18 000 years. Decreased smectite/(illite + chlorite) ratios during the cold intervals suggest that a stronger northeasterly wind led to a mean southward flow of the Indian Coastal Current in the Bay of Bengal. In contrast, increased smectite/(illite + chlorite) ratios during the warm intervals suggest the opposite scenario. Combining the proxy record with model simulations, we infer that atmospheric circulation changes were the main driver of the changes. Moreover, a possible link is observed between a positive Indian Ocean Dipole (IOD) and weakened southward flow of the Indian Coastal Current in the Bay of Bengal during the Holocene. These findings imply that future warming scenarios, if associated with more intense positive IOD events as proposed, may lead to a reduction in fresh water transport from the Bay of Bengal to the Arabian Sea.
The relationship between ocean circulation and monsoon systems over orbital to sub‐millennial timescales is a crucial but poorly constrained component of the climate system. Here, using foraminiferal ...and detrital neodymium (Nd) isotope records from the intermediate‐depth northern Indian Ocean, we provide new evidence revealing that both monsoon‐driven weathering inputs and water mass advection from the Southern Ocean influenced past seawater Nd isotope changes in this region. Our results suggest that Indian Summer Monsoon weakening coincided with enhanced northward Antarctic Intermediate Water (AAIW) advection during the last deglaciation, reflecting a strong interhemispheric coupling. In contrast, the Early Holocene was characterized by enhanced monsoon strength but persistently strong AAIW inflow, indicating a relationship in the opposite sense. These differing interhemispheric relationships indicate asynchronous changes in the global atmosphere—ocean—climate system, and may represent a previously unrecognized component of the ocean‐atmosphere reorganization during the deglacial to Holocene transition.
Plain Language Summary
Deciphering the interactions between processes in the Northern and Southern hemispheres is crucial for understanding the mechanisms of paleoclimate change, and therefore for predicting future climate evolution. Resolving the nature of such interactions requires high‐resolution datasets from key components of the system. To this end, we present a new intermediate‐depth foraminiferal neodymium (Nd) isotope record from the Indian Ocean covering the last 17,000 years at an unprecedented resolution of ∼200 years. Variability in this Nd isotope record reflects both changes in regional Nd inputs related to continental weathering driven by the Indian Summer Monsoon (ISM), and changes in the Nd signal carried northwards from the Southern Ocean by Antarctic Intermediate Water (AAIW). By disentangling these continental weathering and ocean circulation signals, we can compare Northern and Southern Hemisphere climate processes on sub‐millennial timescales. We found that ISM weakening coincided with enhanced northward AAIW advection during the last deglaciation, whereas enhanced monsoonal activity was accompanied by persistently strong intermediate water inflow during the Early Holocene. This variable interhemispheric connection between the summer monsoon and intermediate water advection may have played a crucial role in the last deglacial climate transition.
Key Points
We present a 17 kyr record of foraminiferal Nd isotopes from the intermediate‐depth northern Indian Ocean with a resolution of ∼200 years
Both regional Himalayan weathering inputs and intermediate water advection from the Southern Ocean influenced the record
Different interhemispheric relationships are revealed between Antarctic Intermediate Water and the Indian Summer Monsoon during the last deglaciation compared to the Early Holocene
The Laser Metal Deposition (LMD) process is an additive manufacturing method, which generates 3D structures through the interaction of a laser beam and a gas/powder stream. The stream diameter, ...surface density and focal plan position affect the size, efficiency and regularity of the deposit tracks. Therefore, a precise knowledge of the gas/powder streams characteristics is essential to control the process and improve its reliability and reproducibly for industrial applications. This paper proposes multiple experimental techniques, such as gas pressure measurement, optical and weighting methods, to analyze the gas and particle velocity, the powder stream diameter, its focal plan position and density. This was carried out for three nozzle designs and multiple gas and powder flow rates conditions. The results reveal that (1) the particle stream follows a Gaussian distribution while the gas velocity field is closer to a top hat one; (2) axial, carrier and shaping gas flow significantly impact the powder stream’s focal plan position; (3) only shaping gas, powder flow rates and nozzle design impact the powder stream diameter. 2D axisymmetric models of the gas and powder streams with RANS turbulent model are then performed on each of the three nozzles and highlight good agreements with experimental results but an over-estimation of the gas velocity by pressure measurements.
We have investigated provenance and weathering proxies of the clay-size sediment exported from the Nile River basin over the last 110,000 years. Using neodymium isotope composition of sediments from ...both the Nile Deep Sea-Fan and Lake Tana, we show that the Nile River branches draining the Ethiopian Highlands have remained the main contributors of clays to the Nile delta during the Late Quaternary. We demonstrate that fluctuations of clay-size particle contribution to the Nile Delta are mainly driven by orbital precession cycle, which controls summer insolation and consequently the African monsoon intensity changes. Our results indicate that - over the last 110,000 years – the proportion of clays coming from Ethiopian Traps fluctuates accordingly to the intensity of the last 5 precession cycles (MIS 5 to MIS 1). However, there is a threshold effect in the transport efficiency during the lowest insolation minima (arid periods), in particular during the MIS3. Several arid events corresponding to the Heinrich Stadial periods are associated with small or negligible clay source changes while chemical weathering proxies, such as δ7Li, Mg/Ti and K/Ti, vary significantly. This suggests a straightforward control of weathering by hydro-climate changes over centennial to millennial timescales. Our data also suggests a significant but more progressive influence of the temperature decrease between 110kyr and 20kyr. Taken altogether, the observed tight coupling between past climate variations and silicate weathering proxies leads us to conclude that precipitation changes in northeast Africa can impact soil development over a few hundred years only, while the influence of temperature appears more gradual.
•First Li and Nd coupled isotopic source-to-sink approach over the Nile basin.•Clay εNd fluctuates in phase with the last 5 precession cycles intensity.•Transport thresholds are observed at lowest insolation minima.•Synchronicity between Heinrich Stadials and chemical weathering proxies.•Decoupled effect of temperature and precipitation on clay δ7Li values.
Clay mineralogy, siliciclastic grain-size, major elements, 87Sr/86Sr, and εNd analyses of deep-sea sediments cored in the north-eastern Bay of Bengal are used to reconstruct evolution of detrital ...sources and sediment transport to the proximal part of the Bengal deep-sea fan during the last climatic cycle. εNd values (−13.3 to −9.7) and 87Sr/86Sr ratios (0.721–0.733) indicate a mixture of sediments originating from the Ganges-Brahmaputra rivers and the Indo-Burman ranges. Interglacial Marine Isotopic Stages (MIS) 5 and 1 are associated with a higher contribution of sediments from the Ganges-Brahmaputra river system than is the case for glacial MIS 6, 4, 3, and 2. Siliciclasitic grain-size combined with Si/Al and Si/Fe ratios indicate coarser glacial sediments with numerous turbidite layers. Glacial turbidite layers display similar clay mineralogical compositions to hemipelagic sediments. Only few of turbidite layers (MIS 6, 4, and 2) are slightly unradiogenic (εNd −13.3), suggesting a higher contribution of Ganges-Brahmaputra river sediments. Independently of changes in the sedimentary sources, the smectite/(illite + chlorite) ratio of cores located on the NE Bengal Fan indicates higher inputs of primary minerals (illite and chlorite) from the highlands of the river basins (relief) during glacial MIS 6, 4, 3, and 2 and an increased contribution of pedogenic minerals (smectite and kaolinite) during interglacial MIS 5 and 1. Maximum smectite/(illite + chlorite) ratios during the warm sub-stages of MIS 5 suggest an intensification of summer monsoon rainfall associated with higher rates of physical erosion of the Indo-Gangetic flood-plain and/or dominant summer hydrological conditions transporting a higher proportion of sediments deriving from the Ganges-Brahmaputra rivers to the NE Bengal Fan. In addition, a higher production of smectite in soils of the Indo-Gangetic flood-plain during periods of intensification of monsoon rainfall cannot be excluded.
•Source of the sediments from the north-eastern part of the Bengal Fan is proposed.•Multi-proxy approach gives information about the link between climate and erosion.•Geochemistry results indicate a mixing source of the sediments in the studied area.•Clay assemblage presents a climatic control during the last 182 kyr BP.•Turbidite activity influence sedimentation rates during glacial periods.
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