scholarly journals Temporal Temperature Distribution in Shallow Sediments of a Large Shallow Lake and Estimated Hyporheic Flux Using VFLUX 2 Model

Water ◽  
2021 ◽  
Vol 13 (3) ◽  
pp. 300
Author(s):  
Yong Li ◽  
Na Li ◽  
Jiacheng Feng ◽  
Jianing Qian ◽  
Yajie Shan
Keyword(s):  
Amplitude Ratio ◽  
Lake Taihu ◽  
Eastern China ◽  
Central Zone ◽  
Sediment Surface ◽  
Ratio Method ◽  
Depth Sensors ◽  
Shallow Sediments ◽  
Exchange Flux ◽  
Temperature Amplitude

Identifying and quantifying exchange flux across sediment-water interface is crucial when considering water and nutrient contributions to a eutrophic lake. In this study, observed temporal temperature distributions in shallow sediment of Lake Taihu (Eastern China) based on three-depth sensors at 14 sites throughout 2016 were used to assess temporal water exchange patterns. Results show that temporal temperature in shallow sediments differed with sampling sites and depths and the temperature amplitudes also clearly shrunk as the offshore distance increasing. Exchange fluxes estimated using the VFLUX 2 model based on temperature amplitude show that alternating-direction temporal flow exists in the eastern zone of Lake Taihu with averages of −13.0, −0.6, and 3.4 mm day−1 (negative represents discharging into the lake) at three nearshore sites (0.5, 2.0, and 6.0 km away from the shoreline, respectively). Whereas downwelling flow occurred throughout almost the entire year with averages of 37.7, 23.5, and 6.6 mm day−1 at the three southern nearshore sites, respectively. However, upwelling flow occurred throughout almost the entire year and varied widely in the western zone with averages of −74.8, 45.9, and −27.0 mm day–1 and in the northern zone with averages of −76.2, −55.3, and −51.1 mm day−1. The estimated fluxes in the central zone were relatively low and varied slightly during the entire year (−15.1 to 22.5 mm day−1 with an average of −0.7 mm day−1). Compared with the sub sensor pair (at 5 and 10 cm), the estimated hyporheic fluxes based on the top sensor pair (at 0 and 5 cm) varied within wider ranges and exhibited relatively larger values. Effects of upwelling flow at the western and northern zones need to be paid attention to on nearshore water quality particularly during winter and spring seasons. Estimated flow patterns at the four zones summarily reflect the seasonal water interaction near the sediment surface of Lake Taihu and are beneficial to improve its comprehensive management. Thermal dispersivity usually used for estimating the thermal diffusivity is more sensitive for upward hyporheic flux estimating even if with a low flux. Temperature amplitude ratio method can be used to estimate the exchange flux and suitable for low flux conditions (either upwelling or downwelling). A better evaluation of the exchange flux near inclined nearshore zones might need an optimized installation of temperature sensors along with the potential flow path and/or a vertical two-dimensional model in the future.

Algal activity of dissolved organic nitrogen (DON) in the sediments of Lake Taihu, Eastern China

2016 ◽  
Vol 75 (24) ◽  
Author(s):  
Tianen Zhai ◽  
Jingtian Zhang ◽  
Shouliang Huo ◽  
Beidou Xi ◽  
Mingzhou Su ◽  
...  
Keyword(s):  
Dissolved Organic Nitrogen ◽  
Organic Nitrogen ◽  
Lake Taihu ◽  
Eastern China

Different mechanisms of the pre- and co-eruptive tremor during the 2018 eruption at Sierra Negra volcano, Galapagos

2020 ◽  
Author(s):  
Ka Lok Li ◽  
Meysam Rezaeifar ◽  
Christopher J. Bean ◽  
James Grannell ◽  
Andrew Bell ◽  
...  
Keyword(s):  
Frequency Band ◽  
Velocity Structure ◽  
Amplitude Ratio ◽  
Volcanic Tremor ◽  
Time Lapse ◽  
P Wave ◽  
Ratio Method ◽  
Sierra Negra Volcano ◽  
Sierra Negra ◽  
Active Volcanoes

<p>Volcanic tremor are persistent seismic signals observed near active volcanoes. They are often associated with eruptions, although the exact relationships are not well constrained. To gain a better insight into the generation mechanisms of volcanic tremor, we study tremor that occurred during the 2018 eruption at Sierra Negra volcano, Galapagos. Located 1000 km west of continental Ecuador, Sierra Negra is a shield volcano with a large summit caldera and is one of the most active volcanoes in the Galapagos archipelago. The 2018 eruption started at about 19:55 UTC on 26th June and lasted about two months. Two tremor phases with very different frequency characteristics are identified before and after the eruption onset. The pre-eruptive phase is characterized by a narrow frequency band (2.5 – 4 Hz) and the co-eruptive phase has a broad frequency band (1 – 15 Hz). Location of the two phases by a seismic amplitude ratio method suggests that they are likely to be generated by different physical processes. The pre-eruptive phase is likely generated by dike opening while the co-eruptive phase is associated with lava flow. This interpretation is consistent with a time-lapse P-wave velocity structure of the volcano imaged by local-earthquake travel-time tomography.</p>

scholarly journals Seismo-acoustic characterisation of the 2018 Ambae (Manaro Voui) eruption, Vanuatu

2021 ◽  
Vol 83 (9) ◽  
Author(s):  
Iseul Park ◽  
Arthur Jolly ◽  
Robin S. Matoza ◽  
Ben Kennedy ◽  
Geoff Kilgour ◽  
...  
Keyword(s):  
Amplitude Ratio ◽  
Seismic Station ◽  
Volcanic Tremor ◽  
Acoustic Signals ◽  
Ratio Method ◽  
Seismic Signals ◽  
Reverse Time ◽  
Seismic Stations ◽  
Time Migration ◽  
Eruptive Episode

AbstractA new episode of unrest and phreatic/phreatomagmatic/magmatic eruptions occurred at Ambae volcano, Vanuatu, in 2017–2018. We installed a multi-station seismo-acoustic network consisting of seven 3-component broadband seismic stations and four 3-element (26–62 m maximum inter-element separation) infrasound arrays during the last phase of the 2018 eruption episode, capturing at least six reported major explosions towards the end of the eruption episode. The observed volcanic seismic signals are generally in the passband 0.5–10 Hz during the eruptive activity, but the corresponding acoustic signals have relatively low frequencies (< 1 Hz). Apparent very-long-period (< 0.2 Hz) seismic signals are also observed during the eruptive episode, but we show that they are generated as ground-coupled airwaves and propagate with atmospheric acoustic velocity. We observe strongly coherent infrasound waves at all acoustic arrays during the eruptions. Using waveform similarity of the acoustic signals, we detect previously unreported volcanic explosions at the summit vent region based on constant-celerity reverse-time-migration (RTM) analysis. The detected acoustic bursts are temporally related to shallow seismic volcanic tremor (frequency content of 5–10 Hz), which we characterise using a simplified amplitude ratio method at a seismic station pair with different distances from the vent. The amplitude ratio increased at the onset of large explosions and then decreased, which is interpreted as the seismic source ascent and descent. The ratio change is potentially useful to recognise volcanic unrest using only two seismic stations quickly. This study reiterates the value of joint seismo-acoustic data for improving interpretation of volcanic activity and reducing ambiguity in geophysical monitoring.

scholarly journals Nutrient removal efficiency of lake wetlands:A case study of Sanshan Wetland in Lake Taihu, eastern China

2016 ◽  
Vol 28 (1) ◽  
pp. 124-131 ◽  
Author(s):  
WANG Yan ◽  
◽  
HUANG Jiacong ◽  
YAN Renhua ◽  
GAO Junfeng
Keyword(s):  
Removal Efficiency ◽  
Nutrient Removal ◽  
Lake Taihu ◽  
Eastern China ◽  
Nutrient Removal Efficiency

224Ra/228Th disequilibrium in sediments of Lake Taihu: Implications of nitrogen fluxes across the sediment–water interface

2020 ◽  
Author(s):  
Xiaoyan shi ◽  
Xin luo ◽  
Jimmy.Jiu jiao ◽  
Jing huang ◽  
Meiqing lu ◽  
...  
Keyword(s):  
Lake Sediments ◽  
Molecular Diffusion ◽  
Lake Taihu ◽  
Inorganic Nitrogen ◽  
Eastern China ◽  
Water Remediation ◽  
Water Interface ◽  
Benthic Fluxes ◽  
Eutrophic Lakes ◽  
Nitrogen Fluxes

&lt;p&gt;Radium-224 /Thorium-228 (&lt;sup&gt;224&lt;/sup&gt;Ra/&lt;sup&gt;228&lt;/sup&gt;Th) disequilibrium in sediments is an advanced proxy of benthic processes and has been gradually used to quantify the fluxes and solute transfer across the sediment&amp;#8211;water interface (SWI). This study makes the first attempt to explore the nitrogen fluxes across the SWI of Lake Taihu, the third largest and highly eutrophic freshwater lake in eastern China, based on the plumbing of &lt;sup&gt;224&lt;/sup&gt;Ra/&lt;sup&gt;228&lt;/sup&gt;Th disequilibrium in the lake sediments. The microscopic sediment cores (0-20 cm) were collected in different parts of the lake, and exchangeable &lt;sup&gt;224&lt;/sup&gt;Ra and &lt;sup&gt;228&lt;/sup&gt;Th in bulk sediments were measured. Dissolved inorganic nitrogen (DIN) in pore water and overlying lake water were also analyzed. Deficits of &lt;sup&gt;224&lt;/sup&gt;Ra compared to its parent isotopes &lt;sup&gt;228&lt;/sup&gt;Th were observed in the lake sediments, suggesting the influences of mixing processes. The deficits were relatively significant in the western and northern parts, which are consistent with the relative high-eutrophicated areas of the lake. One-dimensional (1D) radium-thorium diagenetic model in the sediment was used to estimate the benthic fluxes based on the &lt;sup&gt;224&lt;/sup&gt;Ra deficits. Results show that the benthic fluxes of &lt;sup&gt;224&lt;/sup&gt;Ra varied from -0.428 to 1.170 dpm cm&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;, and the bio-irrigation and molecular diffusion are considered to be the major factors. Specifically, in the severely eutrophicated area of the lake, the bio-irrigation predominates in benthic fluxes, reaching up to 97.1% of the deficit of &lt;sup&gt;224&lt;/sup&gt;Ra. The DIN benthic fluxes were also quantified, leading to a flux estimation of 3.41 mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;, which exceeds riverine input (2.63 mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;) and the loading derived from lacustrine groundwater discharge (0.02~0.03 mol m&lt;sup&gt;-2&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;). This study reveals that sediment processes could be the vital factors for the lake nutrient loadings, and highly contribute to the lake eutrophication. This study is constructive for the water remediation and ecosystem restoration in Lake Taihu and other large eutrophic lakes elsewhere.&lt;/p&gt;

scholarly journals Grounding zone subglacial properties from calibrated active source seismic methods

2020 ◽  
Author(s):  
Huw J. Horgan ◽  
Laurine van Haastrecht ◽  
Richard B. Alley ◽  
Sridhar Anandakrishnan ◽  
Knut Christianson ◽  
...  
Keyword(s):  
Amplitude Ratio ◽  
Coefficient Of Determination ◽  
Ratio Method ◽  
Seismic Sources ◽  
Factors Affecting ◽  
Shear Wave Velocities ◽  
Active Source ◽  
Seismic Methods ◽  
Ice Stream ◽  
Water Layers

Abstract. The grounding zone of Whillans Ice Stream, West Antarctica, exhibits an abrupt transition in basal properties from the grounded ice to the ocean cavity over distances of less than 0.5–1 km. Active source seismic methods reveal the grounded portion of the ice stream is underlain by a relatively stiff substrate (relatively high shear wave velocities) compared to the deformable till found elsewhere beneath the ice stream. Several kilometers upstream of the grounding zone, layers of subglacial water are detected, as are regions that appear to be water layers less than the thickness resolvable by our technique. The presence of stiff subglacial sediment and thin water layers upstream of the grounding zone supports previous studies that have proposed the dewatering of sediment within the grounding zone and the possibility that ocean water is pumped into the subglacial system and upstream. The setting enables calibration of our methodology using returns from the floating ice shelf. This allows a comparison of different techniques used to estimate the sizes of the seismic sources. We find a strong correlation (coefficient of determination = 0.45) between our calibrated method and a commonly used amplitude ratio method, but our results also highlight the incomplete knowledge of other factors affecting the amplitude of seismic sources and reflections in the cryosphere.

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