Sedimentation in the Three Gorges Dam and the future trend of Changjiang (Yangtze River) sediment flux to the sea

The Three Gorges Dam (TGD) on the upper Changjiang (Yangtze River), China, disrupts the continuity of Changjiang sediment delivery to downstream and coastal areas. In this study, which was based on 54 years of annual water and sediment data from the mainstream and major tributaries of Changjiang, sediment deposition induced by the TGD in 2003–2008 was quantified. Furthermore, we determined the theoretical trapping efficiency of the cascade reservoir upstream of the TGD. Its impact on Changjiang sediment flux in the coming decades is discussed. Results show that about 172 million tons (Mt) of sediment was trapped annually by the TGD in 2003–2008, with an averaged trapping efficiency of 75%. Most of the total sediment deposition, as induced by the TGD (88%), accumulated within the region between the TGD site and Cuntan. However, significant siltation (12% of the total sediment deposition) also occurred upstream of Cuntan as a consequence of the upstream extended backwater region of the TGD. Additionally, the Changjiang sediment flux entered a third downward step in 2001, prior to operation of the TGD. This mainly resulted from sediment reduction in the Jinshajiang tributary since the late 1990s. As the cascade reservoir is put into full operation, it could potentially trap 91% of the Jinshajiang sediment discharge and, therefore, the Jinshajiang sediment discharge would most likely further decrease to 14 Mt/yr in the coming decades. Consequently, the Changjiang sediment flux to the sea is expected to continuously decrease to below 90 Mt/yr in the near future, or only 18% of the amount observed in the 1950s. In the presence of low sediment discharge, profound impacts on the morphology of estuary, delta and coastal waters are expected

Sr-Nd isotopic geochemistry of Holocene sediments from the South Yellow Sea: Implications for provenance and monsoon variability

Elemental geochemical and Sr-Nd isotopic signatures are used to decipher terrigenous sediments provenances and transport mechanisms in the South Yellow Sea during the Holocene. Sr-87/Sr-86 ratios in the Chinese and Korean riverine sediments overlap each other, whereas epsilon Nd values of Korean riverine sediments are generally less radiogenic in comparison to the Changjiang and Huanghe. Moreover, eNd values of these two large rivers appear unaffected by mineral sorting and are relative stable during the Holocene. We propose a three end-members (i.e., the Changjiang, the Huanghe, and Korean rivers) mixing model to explain sediment provenances in the Central Yellow Sea Mud (CYSM). Mixing calculations show that the Huanghe is the major sediment contributor to the CYSM before similar to 8 ka (thousand years before 1950 CE), whereas the Changjiang has become the predominant sediment source after similar to 8 ka. Holocene changes in riverine sediment supplies to the CYSM are closely related to the oceanic circulation, monsoon climate, and drainage changes. After examining several hypotheses to explain the variations in Sr-87/Sr-86 ratios of Core YSC-1 during the past similar to 8 kyr, we tentatively attribute that to changes in the erosion patterns of the Changjiang Basin. This in turn is associated with the asynchronous evolution of monsoon precipitation in the upper (Indian Summer Monsoon) and middle-lower Changjiang (East Asian Summer Monsoon). Therefore, our results highlight significant influences of monsoon climate on erosion patterns within the Changjiang catchment at millennial timescales

Paleoproductivity variations in the southern Okinawa Trough since the middle Holocene: Calcareous nannofossil records

Based on 17 AMS(14)C age data, we reconstructed high-resolution records of sea surface primary productivity (PP) in the southern Okinawa Trough (MD05-2908) over the last 6.8 ka BP using the calcareous nannofossil carbon isotope and the relative percentage contents of Florisphaera profunda indexes. The underlying mechanism controlling the sea surface PP was then discussed. The sea surface PP, indicated by the coccolith delta C-13 and %Fp conversional equations, decreased with some fluctuations since 6.8 ka BP. This decrease may be connected to the decreased terrigenous input resulting from the reduced East Asian Summer Monsoon (EASM) precipitation. Both the periods of 4-2 ka BP (PME) and 6.8-4.8 ka BP were characterized by relatively higher PP. The former was mainly controlled by the weakening of the Kuroshio Current, whereas the latter mainly resulted from the greater terrigenous input associated with the stronger EASM

Impact of reservoir operations on glycerol dialkyl glycerol tetraether transportation in suspended particulate matter from the Yellow River

The implementation of Water-Sediment Regulation Scheme in the summer season has introduced a new hydrological regime to the Yellow River downstream since AD 2002. The contribution and fate of the soil organic carbon from the Yellow River that reaches the deltaic and coastal environments are important to understand the regional and global carbon cycle. Glycerol dialkyl glycerol tetraethers (GDGTs) are biomarkers with potential implications in the reconstruction of soil OC transportation and paleo-environmental information. In this study, we collected suspended particulate matter samples from downstream waters of the Yellow River from September 2012 to October 2013 to assess 1) the sources of GDGTs, 2) the impact of the operations of reservoirs on the distribution and transportation of GDGTs to the Yellow River estuary region, and 3) the potential implications of GDGTs as proxies for paleoenvironment reconstruction. Our results suggested that over this annual cycle, more than 72.7% of the particulate isoprenoid GDGTs and 82.6% of the particulate branched GDGTs to the Yellow River estuary region were attributed to the intensive discharge during the Water-Sediment Regulation period. The overall GDGT-based BIT (index of branched and isoprenoid tetraethers) values for the particulate matters exported to the Yellow River-dominated continental margin through this annual cycle were higher than 0.5, and thus were capable of tracking soil organic carbon input. As the contribution of soil-derived GDGTs from the Yellow River would potentially influence the GDGT-based TEX86 and CBT-MBT temperature proxies in the estuary-coastal region, a careful examination is required before applying these proxies to temperature reconstruction in deltaic and coastal areas

Evolution of current circulation and bottom redox conditions in the central Okinawa Trough over the last 19 ka

The foraminifera and geochemical element records from the Core CS2 in the center of the Okinawa Trough (OT) revealed a history of current circulation and bottom redox conditions over the last 19 Ka. During the last glaciation, the few Kuroshio Current (KC) and tropical/subtropical species components, the shoaled depth of thermocline (DOT) and high infaunal foraminifera, together suggest the hypoxia in the isolated and semiclosed OT might be related to an absence/shift of the KC. Furthermore, the hypoxia variations in three episodes coincided with changes in the North Pacific Intermediate Water (NPIW), as stronger hypoxia and enhanced NPIW occurred in the Heinrich Stadia 1 and Younger Dryas periods, and rapidly decreasing hypoxia corresponded with weaker NPIW ventilation in the Bølling/Allerød period. Previously recorded hypoxia in the OT occurred broadly at a water depth of between ~1,300 and 1,800 m. Meanwhile, the lowest productivity precluded the possibility of hypoxia caused by organic matter consumption. We thereby infer the enhanced glacial NPIW might be another reason for the hypoxia. After the postglaciation, the OT transitioned into a fully oceanic environment and was reoccupied by main pathway of the KC. This reoccupation greatly alleviated the hypoxia via weakening stratification and increasing vertical mixing. Evolution research of current circulation and bottom redox conditions can help in understanding the changes in the storage/release of carbon as the paleoclimate changed. Moreover, the variations in bottom redox conditions in the OT offer a possible informative window for understanding the influence of glacial NPIW in the mid-latitude zone

Late-Holocene variation in vertical thermal gradient from the coastal East China Sea: Links to monsoon climate and coastal upwelling

The evolution history of coastal upwelling systems in the East China Sea over the late-Holocene are important yet difficult to be characterized. Here, we present similar to 3000-year temperature records based on both alkenones (U-37(K')) and glycerol dialkyl glycerol tetraethers (TEX86H) from a sedimentary core in the coastal upwelling site of the southern East China Sea. By examining the similarity and difference between the down-core TEX86H-derived temperature and records of sea surface temperature, summer monsoon, and winter monsoon, we suggest that the TEX86H mainly reflects temperature in the subsurface water in the studied site. In this sense, paired temperature records from both surface (U-37(K')) and subsurface (TEX86H) water would yield an estimation on the vertical thermal gradient, which provides insight into the upwelling strength in the summer season. Weak upwelling periods occurred at 2600-2800 yr BP, 2000-2400 yr BP, 1200-1550 yr BP, 350-750 yr BP, and the recent similar to 100 years. Positive correlations among the coastal upwelling, the Asian summer monsoon, and the solar irradiance in centennial scales at similar to 100-3000 yr BP indicate that solar-induced summer monsoonal wind might be responsible for the coastal upwelling in the studied area. However, weak upwelling intensity and weak monsoonal strength under the background of relatively warm global temperature and strong solar irradiance since the end of the Little Ice Age' (LIA) might suggest a different climatic response to the natural irradiance, asking for further work on the underlying mechanisms of this phenomenon in a widespread area

Late Holocene Orbital Forcing and Solar Activity on the Kuroshio Current of Subtropical North Pacific at Different Timescales

The North Pacific subtropical gyre (NPSTG) redistributes heat and moisture between low and high latitudes and plays a key role in modulating the global climate change and ecosystem. Recent evidence suggests intensification and poleward shift of the subtropical gyres over the last decades due to global warming, but insufficient observations have hampered insight into the integrated effects of ocean-atmosphere interactions at longer timescales. Here we present the first high-resolution (similar to 12 years) grain-size record from Core CF1 in the Okinawa Trough, western subtropical North Pacific, to reconstruct the evolution of the western boundary Kuroshio Current (KC) of NPSTG during the Late Holocene. Our results indicate the KC slow-down during 4.6-2.0 ka, followed by quick enhancement after 2.0 ka, with centennial-scale variabilities (500-700 years) superimposed on the long-term trend. Over millennial timescales, gradually increased pole-to-equator thermal gradient, due to orbital forcing mechanisms, resulted in long-term enhanced KC, whereas solar activity triggered phase changes in the tropical Pacific mean state and controlled KC anomalies on centennial timescales. We suggest that both forcing mechanisms resulted in ocean-atmosphere feedback provoking concurrent changes in mid-latitude westerly and subtropical easterly winds over the North Pacific, alternating their dominance as source regions causing the dynamic changes of KC at different timescales. Our findings offer insight into the role of external forcing mechanisms in the NPSTG changes before the Anthropocene, which have profound implications for the deeper understanding of changes in ocean gyres under global warming scenarios

Evolution of Ocean Productivity in the Sub‐Tropical West Pacific Ocean Across the Last Deglaciation

Changes in marine export productivity may have played a prominent role in the atmospheric CO2 rise and associated warming of the last deglaciation, via their impact on the air-sea partitioning of CO2. To study the evolution of marine export production in the western subtropical Pacific Ocean during the last deglaciation and the mechanisms governing these changes, 230Th-normalization has been applied on a sediment core retrieved from the Okinawa Trough. The reconstructed export productivity record shows two prominent peaks, centered around 14 and 17 ka. Given that phytoplankton growth at our study site is limited by the availability of nutrients today, elevated macro-nutrient supply must have been responsible for sustaining the two deglacial productivity peaks. Specifically, higher nutrient supply originating from the subpolar gyre via subsurface water advection, with enhanced vertical mixing in the subpolar gyre during early HS1 and enhanced wind-driven upwelling during the B/A, would have fueled phytoplankton productivity and export. In contrast, reduced nutrient supply to the euphotic zone caused by gyre circulation changes would have lead to decreased export production during the Holocene. These observations lend support to a predominant control of nutrient availability on the evolution of the marine carbon cycle in the subtropical west Pacific Ocean across the last deglaciation