Hyperpycnal Discharge of Fluvial Sediment to the Ocean: Impact of Super‐Typhoon Herb (1996) on Taiwanese Rivers

Hyperpycnal events (when suspended sediment concentrations exceed 40 g/L) occur in small‐ and medium‐sized rivers throughout the world but are particularly common in Taiwan; they are often related to landslides or debris flows initiated and transported by typhoon floods. Super‐Typhoon Herb, which swept across Taiwan on July 31–August 2, 1996, triggered floods and landslides throughout the southern part of the island. Sediment concentrations in at least seven rivers (Taan, Choshui, Pachang, Erhjen, Tsengwen, Kaoping, and Peinan) approached or exceeded 40 g/L. Calculated sediment discharged from nine rivers (these seven as well as the Wu and Houlung, neither of which apparently reached hyperpycnal concentrations) during these 3 d was 217 million tons (MT)—most of it on August 1—of which ∼80% was discharged at hyperpycnal concentrations. Presumably, most of the sediments discharged by the Peinan River (to the southeast) and the Kaoping, Erhjen, Tsengwen, and Pachang rivers (to the southwest) were transported directly to the Huatong Basin and the South China Sea (via the Penghu Canyon system), respectively. The bulk of the typhoon‐derived sediment (142 MT), however, was discharged to the northwest (primarily by the Choshui River), and its fate remains unknown. It may have ultimately reached the Penghu Canyon system and thereby the South China Sea, but more probably it was transported northward (via the Taiwan Warm Current) toward China, the East China Sea, or (perhaps) the Okinawa Trough

Calculating highly fluctuated suspended sediment fluxes from mountainous rivers in Taiwan

Small drainage basins, highly fractured rock, high relief, and steep gradients make Taiwan watersheds particularly sensitive to episodic events such as typhoons and earthquakes, and to various types of anthropogenic disturbance. Here we analyze the characteristics of a long-term hydrological dataset from Taiwan and re-evaluate methods used to calculate sediment loads for Taiwan\u27s event-driven rivers. We suggest using the rating curve method stratified down to seasonal levels to reflect the rapid changes in the relationship between water discharge and suspended sediment load. A program is developed to determine the optimal time-interval for constructing rating curves, and is used to calculate hourly, daily, yearly, and long-term mean suspended sediment loads. Seasonal rating curves applied to hourly discharges are particularly critical to calculate sediment fluxes and concentrations in response to episodic events, particularly typhoons. The calculated cumulative long-term mean sediment fluxes for the Jhou-Shuei and Bei-Nan Rivers are considerably smaller than those calculated using monthly weighted average (MWA) method (Dadson et al. 2003). The MWA method likely over-estimates the mean load due to more frequent sediment observations during high-flow events

Widespread dispersal and aging of organic carbon in shallow marginal seas

The occurrence of pre-aged organic carbon (OC) in continental margin surface sediments is a commonly observed phenomenon, yet the nature, sources, and causes of this aged OC remain largely undetermined for many continental shelf settings. Here we present the results of an extensive survey of the abundance and radiocarbon content of OC in surface sediments from the northern Chinese marginal seas. Pre-aged OC is associated with both coarser (>63 µm) and finer (<63 µm) sedimentary components; measurements on specific grain-size fractions reveal that it is especially prevalent within the 20–63 µm fraction of inner shelf sediments. We suggest that organic matter associated with this sortable silt fraction is subject to protracted entrainment in resuspension-deposition loops during which it ages, is modified, and is laterally dispersed, most likely via entrainment within benthic nepheloid layers. This finding highlights the complex dynamics and predepositional history of organic matter accumulating in continental shelf sediments, with implications for our understanding of carbon cycling on continental shelves, development of regional carbon budgets, and interpretation of sedimentary records

Stochastic procedure to extract and to integrate landslide susceptibility maps: an example of mountainous watershed in Taiwan

International audienceThe Generalized Likelihood Uncertainty Estimation (GLUE) is here incorporated into a deterministic landslide model (SHALSTAB) to generate 4000 landslide susceptibility maps which enclose various combinations of full range parameters. Furthermore, an improved index is adopted into GLUE as a criterion to measure model performance, and through that, 200 maps holding top 5% performance are retrieved. Proper ranges for parameters are obtained through GLUE yet they only perform well if combined appropriately. The 200 better maps are overlapped to construct an integrated landslide susceptibility map. Instead of giving a single parameter set or a single susceptibility map, the merit of extracting and integrating procedure is to envelope uncertainties inherited in model structure and input parameters. Bias due to subjective parameter input is potentially reduced. The entire procedure is applied to the Chi-Jia-Wan, a mountainous watershed in Taiwan. The integrated map shows high-risk area (>50% predicted landslide probability) only occupies 16.4% of the entire watershed while able to correctly identify 60% of the actual landslides. For areas above 2100 m height the map is even more successful (projects 77 of the 98 actual landslides). Interactions among parameters are discussed to highlight the unsolvable equifinality problem and improperness of presenting a single model result

Analytical solution of the nitracline with the evolution of subsurface chlorophyll maximum in stratified water columns

In a stratified water column, the nitracline is a layer where the nitrate concentration increases below the nutrient-depleted upper layer, exhibiting a strong vertical gradient in the euphotic zone. The subsurface chlorophyll maximum layer (SCML) forms near the bottom of the euphotic zone, acting as a trap to diminish the upward nutrient supply. Depth and steepness of the nitracline are important measurable parameters related to the vertical transport of nitrate into the euphotic zone. The correlation between the SCML and the nitracline has been widely reported in the literature, but the analytic solution for the relationship between them is not well established. By incorporating a piecewise function for the approximate Gaussian vertical profile of chlorophyll, we derive analytical solutions of a specified nutrient-phytoplankton model. The model is well suited to explain basic dependencies between a nitracline and an SCML. The analytical solution shows that the nitracline depth is deeper than the depth of the SCML, shoaling with an increase in the light attenuation coefficient and with a decrease in surface light intensity. The inverse proportional relationship between the light level at the nitracline depth and the maximum rate of new primary production is derived. Analytic solutions also show that a thinner SCML corresponds to a steeper nitracline. The nitracline steepness is positively related to the light attenuation coefficient but independent of surface light intensity. The derived equations of the nitracline in relation to the SCML provide further insight into the important role of the nitracline in marine pelagic ecosystems

Trace metal fluxes to the ocean: The importance of high‐standing oceanic islands

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94592/1/grl16149.pd

Sediment Budget in the Taiwan Strait with High Fluvial Sediment Inputs from Mountainous Rivers: New Observations and Synthesis

The shallow Taiwan Strait at the southern opening of the East China Sea (ECS) receives abundant sediments from turbid mountainous rivers in Taiwan. The volume of sediment is among the highest sediment yields on the global surface. This large amount of sediment discharged from modern Taiwan (range: 175 - 380 Mt y-1 based on 50-yr data) is comparable to that discharged from Changjaing (500 Mt y-1-decreasing in recent decades), underscoring the importance of sediment budget in the Taiwan Strait and sediment flux from Taiwan into the ECS.We documented fluvial mud and sand concentrations during flash flooding with our observations indicating that fluvial materials in Taiwan¡¦s rivers are chiefly composed of mud (> 70 and up to 98 ). By contrast, sand fraction dominates (> 85 for most stations) surface sediments in the Taiwan Strait. Super typhoon Herb alone delivered 130 Mt of sediments from Choshui, the largest river in Taiwan, yet only insignificant amounts of mud were found at the river mouth six months later. The actions of waves, tides, and currents apparently prevent the deposition of fine grained sediments. Assuming sand occupied 30 (the maximum) of the 60 Mt y-1 total sediment input from major western Taiwanese rivers, our annual budget estimate shows that the amount of sand input (18 5 Mt y-1) is comparable to the burial output of sand (12 10 Mt y-1). However, mud burial (6 5 Mt y-1) in the strait is far below the estimated mud input (42 11 Mt y-1), resulting in a significant shortfall. Hydrodynamic conditions were synthesized to explain the distribution pattern of limited mud patches in the strait and to reveal potential pathways by which fine-grain sediment transportation takes place in the seas surrounding Taiwan. A significant shortfall in the mud budget in the Taiwan Strait suggests that ~85 of the fluvial mud left the strait. Alternatively, the 50-year modern sediment flux data used in this study reflects exacerbated sediment flux due to human activity and is possibly too high to represent loads during pre-Anthropocene. Additional studies are needed to explore the flux and fate of mud in and surrounding Taiwan over a longer time scale

Aerosols as a source of dissolved black carbon to the ocean

溶解黑碳（dissolved black carbon, DBC）在开阔大洋的年龄可达上万年，是海洋中到目前为止已知的年龄最老、最大的惰性溶解有机碳。因此海洋溶解黑碳的源汇问题是全球碳循环研究的重要部分。该研究团队通过测定2015年春季东、黄海以及西北太平洋的气溶胶中水溶性有机碳(water soluble organic carbon, WSOC)、水溶性黑碳 (water soluble black carbon, WSBC)的含量，并结合超高分辨质谱---傅里叶变换离子回旋共振质谱 (FT-ICR-MS)，解析了海洋气溶胶中溶解黑碳的浓度以及分子组成。进一步分析发现气溶胶中溶解黑碳的浓度与水溶性有机碳浓度显著高度相关。预测的未来生物质燃烧以及沙尘输送的变化可能增加大气沉降溶解黑碳的通量，影响区域甚至全球碳库。该研究结果将有助于把溶解黑碳纳入到海洋碳循环研究模型中。 近海海洋环境科学国家重点实验室助理教授鲍红艳为论文第一作者，高树基教授为共同通讯作者。【Abstract】Dissolved black carbon (DBC) is the largest known slow-cycling organic carbon pool in the world’s oceans. Atmospheric deposition could significantly contribute to the oceanic DBC pool, but respective information is lacking. Here we estimate that, during the dust outbreak season, the atmospheric dry deposition of water-soluble black carbon (WSBC) is～ 40% of the riverine input to the China coastal seas. The molecular composition of atmospheric WSBC determined by ultrahigh-resolution mass spectrometry, reveals similar soil-derived sources as for riverine discharge. WSBC is significantly positively correlated with water-soluble organic carbon (WSOC) in marine aerosols, and water-soluble black carbon contributes on average 2.8 ± 0.65% to the total WSOC. Based on this relationship, the global atmospheric deposition of DBC to the ocean is estimated to be 1.8 ± 0.83 Tg yr−1. Anticipated future changes in biomass burning and dust mobilization might increase these numbers, with consequences for regional ecosystems and global carbon reservoirs.This study was funded by the Major State Basic Research Development Program of China (973 program) (No. 2014CB953702) and the China Postdoctoral Science Foundation (No. 2013M540529). Hanse-Wissenschaftskolleg (the Institute for Advanced Study in Germany) is acknowledged for its support of Dr. H. Bao. 本研究受到973项目“不同营养水平下海洋氮循环关键过程及其对大气物质沉降的响应”（No.2014CB953702）以及中国博士后科学基金（No.2013M540529）的资助； 鲍红艳博士在德国的工作受到德国Hanse-Wissenschaftskolleg奖学金的资助

Ambient nitrate switches the ammonium consumption pathway in the euphotic ocean

高树基教授研究团队的研究论文，以现场原位速率观测与环境因子调查，证明了环境硝酸盐浓度是调控海洋真光层不同氨氮消耗途径的关键因子。硝酸盐通过影响浮游植物对于氨氮吸收的能力，决定了真光层内氨氮的氧化路径强弱，刻画了海洋真光层氮循环的微结构。高树基团队利用同位素示踪培养技术，围绕南海和西北太平洋真光层内硝化作用和浮游植物氨氮吸收速率的垂直分布特点及其动力学特征进行研究。首次提出硝酸盐浓度通过影响不同浮游植物类群对于氨氮的吸收能力，从而调控真光层的氨氮消耗途径。研究团队的万显会博士为成果的第一作者，近海海洋环境科学国家重点实验室的戴民汉教授，史大林教授、张瑶教授、盛华夏博士后、祝依凡，以及澳大利亚联邦科学与工业研究组的Tom Trull教授和美国Bigelow海洋实验室的Mike Lomas教授为成果的共同完成人。【Abstract】Phytoplankton assimilation and microbial oxidation of ammonium are two critical conversion pathways in the marine nitrogen cycle. The underlying regulatory mechanisms of these two competing processes remain unclear. Here we show that ambient nitrate acts as a key variable to bifurcate ammonium flow through assimilation or oxidation, and the depth of the nitracline represents a robust spatial boundary between ammonium assimilators and oxidizers in the stratified ocean. Profiles of ammonium utilization show that phytoplankton assemblages in nitrate-depleted regimes have higher ammonium affinity than nitrifiers. In nitrate replete conditions, by contrast, phytoplankton reduce their ammonium reliance and thus enhance the success of nitrifiers. This finding helps to explain existing discrepancies in the understanding of light inhibition of surface nitrification in the global ocean, and provides further insights into the spatial linkages between oceanic nitrification and new production.This work is supported by the National Natural Science Foundation of China through grants 91328207, Science Fund for Creative Research Groups of the National Natural Science Foundation of China through grants 41721005 and National Basic Research Program of China (973 Program) through grants 2014CB953702, 2015CB954003. T.W.T. was supported by an MEL Visiting Scholar award. 成果获得了国家重点基础研究发展计划（2014CB953702, 2015CB954003），国家自然科学基金委“创新研究群体项目”（41721005）以及重点基金（91328207）的资助

Monsoon hydrography and productivity changes in the East China Sea during the past 100,000 years : Okinawa Trough evidence (MD012404)

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 24 (2009): PA3208, doi:10.1029/2007PA001577.We analyzed the high-resolution foraminifer isotope records, total organic carbon (TOC), and opal content from an Okinawa Trough core MD012404 in order to estimate the monsoon hydrography and productivity changes in the East China Sea (ECS) of the tropical western Pacific over the past 100,000 years. The variability shown in the records on orbital time scales indicates that high TOC intervals coincide with the increases of boreal May–September insolation driven by precession cycles (∼21 ka), implying a strong connection to the variations in monsoons. We also observed possibly nearly synchronous, millennial-scale changes of the ECS surface hydrography (mainly driven by salinity changes but also by temperature effects) and productivity coincident with monsoon events in the Hulu/Dongge stalagmite isotope records. We found that increased freshening and high productivity correlate with high monsoon intensity in interstadials. This study suggests that the millennial-scale changes in monsoon hydrography and productivity in the ECS are remarkable and persistent features over the past 100,000 years.Y.Y.’s work was partly supported by Global Environmental Research Fund (RF-081) and JSPS Kakenhi (21674003)