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Provenance and weathering of sediments in the deep basin of the northern South China Sea during the last 38 kyr
No full textDeep-sea sediments are ideal recorders for studying the provenances and relevant chemical and physical weathering of adjacent lands. Major, trace (Fe, K, Al, Ti, V, Rb, Th, and Cr), and rare earth elements (REEs), grain size, and AMS C-14 ages were studied on a 4-m-long core CS11 collected from a deep basin in the northern South China Sea (SCS). Core CS11 sediments were mainly derived from southwestern Taiwan Island and transported by the southward bottom current during most of the study periods, except from 37 to 35 kyr BP, when northern Luzon inputs temporarily had influence. For the past 35 kyr, the stacked chemical weathering index (SCWI) correlated well with the worldwide Younger Dryas (YD), Heinrich 1 (H1), Heinrich 2 (H2), and Heinrich 3 (H3) events, indicating global climate control on weathering intensity in this area. However, the SCWI showed the strongest chemical weathering intensity during the low sea level at the Last Glacial Maximum (LGM) when Taiwan-derived sediments were subaerially exposed on the shelf, chemically weathered and increasingly transported to the deep basin of the SCS. The major factors influencing chemical weathering intensities for sediments in the deep basin of the northern SCS varied on different geological time scales, with climate changes on millennial scales and sea-level changes on orbital scales
Variability in Indonesian Throughflow Upper Hydrology in Response to Precession-Induced Tropical Climate Processes Over the Past 120 kyr
No full textThe Indonesian Throughflow (ITF), as the sole low-latitude conduit connecting the Pacific and Indian Oceans, regulates the thermohaline balance between these oceans. Thus, investigating the variability in the ITF and its relationship with the precessional forcing is crucial for understanding the drivers of tropical climate change. Here, we reconstruct the history of the ITF over the past similar to 120 kyr based on high-resolution (similar to 400 years) delta O-18 and Mg/Ca records of Globigerinoides ruber and Pulleniatina obliquiloculata from core SO217-18540 retrieved from the Flores Sea upwelling region within the main pathway of the ITF. A comparison of these new records with published paleo-oceanographic and climatological data from the western tropical Pacific Ocean suggests that annual mean conditions in the Flores Sea were controlled by ITF variability rather than by monsoonal upwelling. Our results further indicate that precessional insolation was a major forcing for the hydrological evolution of the ITF during the past 120 kyr. We suggest that the precessional insolation forcing paced ITF variability by modulating the mean state of El Nino-Southern Oscillation-like conditions and latitudinal shifts and/or expansion/contraction of the Intertropical Convergence Zone.
Plain Language Summary The Indonesian Throughflow (ITF) is the only low-latitude current transporting warmer, fresher water masses from the Pacific to the Indian Ocean, regulating the interocean heat and fresh water budgets. The ITF is strongly influenced by shifts in winds and temperatures across the tropical Pacific Ocean (El Nino-Southern Oscillation; ENSO) and by variations in the latitudinal position and intensity of the hottest part of the tropical climate belt (Intertropical Convergence Zone; ITCZ). Hence, understanding past ITF variability is highly relevant to constrain projections of future tropical climate change. Here, we present new upper ocean temperature and salinity records spanning the last 120 kyr from core SO217-18540 retrieved within the main ITF pathway. These records are based on delta O-18 and Mg/Ca analyses of surface and deeper dwelling microscopic zooplankton shells (foraminifera). We found that variations in the ITF intensity and vertical structure were controlled by the similar to 20 kyr wobble of the Earth's axis, which affects the distribution of incoming solar radiation. We ascribe this pattern of variability to shifts in the intensity and/or position of the ITCZ and to changes in winds and temperatures over the tropical Pacific Ocean, similar to the modern ENSO but acting on longer timescales in Earth's history
Increased autumn and winter precipitation during the Last Glacial Maximum in the European Alps
No full textThe culmination of the glaciers in the European Alps during the Last Glacial Maximum (LGM) is one of the most intensively studied paleoglaciological events, but its trigger and forcing remain incompletely understood. Here, we provide evidence that the timing of this glacier maximum coincided within age uncertainties with a 3100 yr-long interval of subsurface warming (26.6 to 23.5 ka BP) as recorded by an archive preserved in caves, cryogenic carbonates. This interval of sustained permafrost degradation during one of the coldest intervals of the last glacial period calls for a fundamental change in the dry Arctic-style precipitation regime. Instead, heavy snowfall during autumn and early winter led to the accumulation of a seasonal snowpack insulating the ground from the winter chill. Combined with thermal modelling, the data provide compelling evidence that the LGM glacier advance in the Alps was fueled by intensive snowfall late in the year, likely sourced from the Mediterranean Sea. What controlled changes of glaciers in the European Alps at the time of their largest extent, about 25,000 years ago, is not well known. Here, the authors use cryogenic carbonates in caves to show that heavy snowfall during autumn and early winter was the main source of glacier growth
Factors controlling spatial variation in soil aggregate stability in a semi-humid watershed
No full textSoil aggregate stability (SAS) is a key soil property that affects soil erosion and soil ability to support ecosystem functions. The effects of different environmental factors on SAS are extensively documented. However, the relative importance of the factors that drive variation in SAS at watershed scale is not entirely clear. To investigate the effects of the interactions of environmental variables on spatial variation in SAS, 88 sampling sites were selected across an entire watershed (1.1 km2) on the Chinese Loess Plateau (CLP), from where undisturbed soil samples were collected at the 0-10 and 10-20 cm soil depths. Three indices were used to evaluate the SAS - water-stable aggregates greater than 0.25 mm (WSA>0.25, %), mean weight diameter (MWD, mm) and mean geometric diameter (MGD, mm). The results showed that variation of SAS across the watershed was moderate, with coefficient of variation (CV) of 23.5-38.9 %. From combined Spearman's correlation analysis (r), redundancy analysis (RDA) and structural equation modelling (SEM), it was found that soil intrinsic properties, mainly soil texture and organic carbon content (SOC), were the primary control on SAS variation. Topographic attributes, primarily wetness index (TWI) and altitude, were also important controls on SAS. These controls were either the direct or indirect effect through SOC dynamics, spatial distribution of land use (LUT) or vegetation cover (NDVI). The effect of LUT on SAS was mainly driven by SOC and TWI at the 0-10 cm depth but by NDVI and TWI at the 10-20 cm depth. SAS was positively correlated with sand content and SOC, but negatively correlated with silt content, altitude, TWI and NDVI. For LUT, SAS in the apple orchard was significantly lower than in shrubland and grassland, however, it was comparable with that in forest. Considering the effects of improving soil structure and the related economic cost, natural restoration of grassland was a good choice for preventing soil erosion in the study area. The results of this study could deepen our understanding of the controls on SAS variation and therefore become useful in soil management and vegetation restoration decisions on CLP and other regions with similar conditions
Climatic and edaphic controls over the elevational pattern of microbial necromass in subtropical forests
No full textThe sequestration of soil organic carbon (SOC) in terrestrial ecosystems is determined by the balance between plant- and microbial-derived carbon inputs and losses through soil respiration. However, a consensus on the elevational patterns of soil microbial necromass and its contribution to SOC is rare, and the information on how climatic and edaphic factors affect the accumulation of microbial necromass remains limited. In this study, soil samples were collected with a 50-m interval along an elevational gradient (200-950 m above sea level) to investigate the effects of climatic and edaphic variability associated with elevation and season on microbial necromass in subtropical forests. The concentration of soil amino sugar was measured by high-performance liquid chromatography (HPLC) to characterize soil microbial necromass. Partial least squares path modeling (PLS-PM) was used for testing climatic and edaphic controls over the elevational pattern of microbial necromass. The concentration of soil microbial necromass and its contribution to SOC were affected by elevation and season, with lower concentration and contribution in the wet season than in the dry season. Soil microbial necromass linearly increased or followed a quadratic pattern with elevation, and accounted for 18.9% of SOC on average with a greater contribution from fungal necromass (13.2%) than from bacterial necromass (5.7%). Soil temperature, soil nitrogen and moisture content directly influenced the accumulation of soil microbial necromass with varied effects on fungal and bacterial necromass. Warmer and nutrient-impoverished environments were linked with the depletion of fungal necromass, whereas higher soil moisture and nutrient availability were positively associated with the accumulation of bacterial necromass. Our findings demonstrate that less microbial necromass, especially fungal necromass will accumulate in SOC in response to future climate warming in subtropical forests. Such information is valuable for improving our understanding of the potential impacts of future climatic change on soil carbon cycling in subtropical regions
Predicting long-term hydrological change caused by climate shifting in the 21st century in the headwater area of the Yellow River Basin
No full textThe Qinghai-Tibetan Plateau (QTP) is one of the amplifiers of global climate change. The headwater area of the Yellow River Basin (HYRB) on the QTP is the dominant water source region for the whole Yellow River Basin. However, the sensitive responses of hydrological processes to the intensifying climate change are exerting high uncertainties to the water cycle in the HYRB. The aim of this study was to investigate the potential climate change under three Representative Concentration Pathways (RCP 2.6, 4.5, and 8.5) and their hydrological impacts in this region using the ensemble climate data from eight general circulation models (GCMs) and the Soil and Water Assessment Tool (SWAT). Compared to the baseline (1976-2015), the projected climate indicated a rise of 7.3-7.8% in annual precipitation, 1.3-1.9 degrees C in maximum air temperature, and 1.2-1.8 degrees C in minimum air temperature during the near future period (2020-2059), and an increment of 9.0-17.9%, 1.5-4.5 degrees C, and 1.3-4.5 degrees C in precipitation, maximum and minimum temperature, respectively, during the far future period (2060-2099). The well-simulated SWAT modeling results suggested that due to a wetter and warmer climate, annual average actual evapotranspiration (AET) would increase obviously in the future (31.9-35.3% during the near future and 33.5-54.3% during the far future), which might cause a slight decrease in soil water. Water yield would decrease by 16.5-20.1% during the near future period, implying a worsening water crisis in the future. Till the end of this century, driven by the increased precipitation, water yield would no longer continue to decrease, with a decline by 15-19.5%. Overall, this study can not only provide scientific understanding of the hydrological responses to the future climate in both semi-arid and alpine areas, but also contribute to the decision support for sustainable development of water resources and protection of eco-environment in the HYRB
Timing of river capture in major Yangtze River tributaries: Insights from sediment provenance and morphometric indices
No full textThe eastern margin of the Tibetan Plateau represents one of the morphologically most active regions on Earth, where the interplay of recent crustal deformation and subsequent fluvial landscape adjustment has affected the course of continental-scale rivers by river piracy events. Based solely on field observations, such an event has been hypothesised for two of the largest tributaries of the Yangtze River: the Jialing and Hanjiang Rivers. To test this hypothesis, we employ a novel combination of independent methods including a provenance study based on age distributions of detrital zircons from both modern riverbeds and river terraces and a morphometric analysis of river channels and drainage divides. We supported the morphometric analysis with a time-dependent numerical model describing the evolution of river channel long profiles and drainage divides in a succession of river capture events. Analysed zircon ages show clearly distinguishable distributions for the modern Jialing and Hanjiang Rivers, but similar distributions for the recent Hanjiang River up to its topmost terraces. This suggests that the capture of the Hanjiang headwaters by the Jialing River is unlikely to have taken place during the last 1.2 million years. However, several knickpoints in the main stem and the tributaries of the Jialing River cluster at an elevation of about 900 m and separate steeper (downstream) from less steep channel segments (upstream), which is consistent with the morphological expression of a major capture event. chi mapping indicates drainage divide asymmetry at catchment scale with on average steeper rivers on the Jialing side, whereas Gilbert metrics show a symmetric divide at hillslope scale. This numerical model explains this apparent contradiction by the travel time of capture-related knickpoints from the capture point towards the watershed, where chi mapping indicates divide asymmetry immediately after the river capture, while Gilbert metrics are only affected as soon as the knickpoints reach the channel heads and the divide effectively starts moving. Based on knickpoint travel distances and constraints on regional incision / uplift rates, we estimate the possible date of river capture to be the Pliocene. This is earlier than the formation of the terraces investigated in the provenance study but recent enough that most of the drainage divides are still unaffected and currently almost stable. Only the wind gap located in the almost dry valley connecting the two competing drainage systems is likely to have shifted towards the Hanjiang side. We suggest that this resulted in the capture of another important tributary of the Hanjiang River (the Heishui River) by the Jialing drainage system. Our results illustrate the complex evolution of drainage networks along the eastern margin of the Tibetan Plateau, and highlight the importance for combining provenance and morphometric analyses in regions of active landscape rejuvenation where river captures are likely to occur. (c) 2021 Elsevier B.V. All rights reserved
Persistent orbital influence on millennial climate variability through the Pleistocene
No full textAbundant evidence from marine, ice-core and terrestrial records demonstrates that Earth's climate has experienced co-evolution of orbital- and millennial-scale variability through the Pleistocene. The varying magnitude of millennial climate variability (MCV) was linked to orbitally paced glacial cycles over the past 800 kyr. Before this interval, global glaciations were less pronounced but more frequent, yet scarcity of a long-term integration of high-resolution continental and marine records hampers our understanding of the evolution and dynamics of MCV before the mid-Pleistocene transition. Here we present a synthesis of four centennial-resolved elemental time series, which we interpret as proxies for MCV, from North Atlantic, Iberian margin, Balkan Peninsula (Lake Ohrid) and Chinese Loess Plateau. The proxy records reveal that MCV was pervasive and persistent over the mid-latitude Northern Hemisphere during the past 1.5 Myr. Our results suggest that the magnitude of MCV is not only strongly modulated by glacial boundary conditions on Earth after the mid-Pleistocene transition, but also persistently influenced by variations in precession and obliquity through the Pleistocene. The combination of these four proxies into a new MCV stack offers a credible reference for further assessing the dynamical interactions between orbital and millennial climate variability.
Orbital forcing consistently influenced the magnitude of millennial-scale climate variability through the Pleistocene, according to an analysis of four high-resolution Northern Hemisphere proxy records covering the past 1.5 Myr
Species specific Sr/Ca-delta O-18 relationships for three Tridacnidae species from the northern South China Sea
No full textSkeletal remains of marine bivalves, Tridacna spp., can provide multi-proxy records of environmental variables. Sr/Ca ratio, which has been widely used in corals as a paleo-temperature proxy, has also been explored in Tridacna spp. shells in recent decades, but some controversies remain, especially regarding the different Sr/CaSST relationships across Tridacnidae species. In this study, ten specimens of three different species (Tridacna gigas, Tridacna squamosa and Tridacna derasa) were collected from the northern South China Sea and the monthly resolution Sr/Ca and delta O-18 ratios were investigated. Almost all high-resolution Sr/Ca profiles, determined by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer), show pronounced annual cycles and are significantly correlated with paired delta O-18 value. However, Sr/Ca ratios of T. gigas are positively correlated with paired delta O-18 values, suggesting a negative correlation between T. gigas Sr/Ca and in-situ SST, while Sr/Ca ratios of T. derasa and T. squamosa are negatively correlated with paired delta O-18, suggesting a positive correlation between SST and Sr/Ca in T. derasa and T. squamosa. These interspecies differences highlight the necessity of species identification before using Tridacnidae Sr/Ca ratios in paleoclimate reconstructions. Although the Sr/Ca-delta O-18 relationship has obvious interspecies differences, the clear annual cycles in all these specimens indicate that the Sr/Ca ratios of Tridacna spp. may have the potential to be used in reconstructing the past climate seasonality, ENSO variability and so on
Cirques of the central Tibetan Plateau: Morphology and controlling factors
No full textCirque morphology represents the characteristics of palaeoglaciations and palaeoclimate. This study mapped and analysed 70 cirques in the central Tibetan Plateau (TP) with dominant continental climate. The results show that from northwest to southeast, cirque dimensions (i.e., length, width, and area) increase, while cirque floor altitudes decrease. A likely reason is the high precipitation rate caused by the Indian summer monsoon in the southeastern part. The diversity of cirque aspect values indicates weak or inconsistent prevailing winds during cirque development. Cirques enlarge with altitude, which might imply that cirques at high altitudes developed early and thus increased in size. Cirque aspect diversity increases with altitude from 5100 to 5600 m above sea level (asl), which indicates the capacity of high altitudes to support cirques at less favourable slopes. If altitude increases above 5600 m asl, the diversity of cirque aspect first increases and then decreases, which may be because of local topography. Mountain orientation and lithology had an effect on cirque aspect/size. Cirque numbers and sizes were compared between western, central, and eastern sectors of the Gangdise Mountains in the southern TP with those in the central TP. This comparison showed that a strengthening Indian summer monsoon can raise the value of `cirque density' (i.e., cirque number per unit area), promote glacier development into valley-type, and limit cirque enlargement