title: On the stabilizing influence of silt on sand beds

authors: Bartzke, Gerhard

subjects: sediment mixtures

abstract: In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50  =  55 µm) and sand (D50 =  300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed.

text: On the stabilizing influence of silt on sand beds In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50  =  55 µm) and sand (D50 =  300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) “the layering experiment”: in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2–3 mm; 0.5–3.7 wt %, dry weight in a layer 10 cm deep); and (2) “the mixing experiment” where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07–0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed.

year: 2013

title: Short-term climate variability in the mid-Miocene: A Micropalaeontological and sedimentological investigation of a high-resolution maar lake sediment near Hindon, East Otago, New Zealand

authors: Bates, Noel William

subjects: Palaeoclimate

abstract: Maars are deep, steeply-sloped, small-diameter craters formed by phreato-magmatic eruptions. Phreatomagmatic eruptions result when rising magma intersects a body of water such as an aquifer. Maars often form deep closed-basin lake systems that become repositories for detrital material from the local environment. Tens of metres of detritus representing thousands of years of accumulation may be anoxically-preserved in maar lake sediments for millions of years.
The Hindon Maar Complex (HMC) is located in East Otago, on the South Island of New Zealand. The HMC hosts the highest-resolution mid-Miocene climate archive in the Southern Hemisphere. The date of its formation (14.6±0.1 Ma) coincides with the mid-Miocene climate transition (MMCT), a unique period when global temperatures were decreasing following an anomalous short-lived peak at the mid-Miocene climate optimum. Sensitivity to atmospheric pCO₂ levels and the confluence of the three main Milankovitch orbital cycle minima caused the Antarctic ice sheet (AIS) to advance and retreat. Concurrently, the Antarctic circumpolar currents (ACC) and atmospheric circumpolar westerlies also underwent latitudinal shifts while interacting with warm subtropical currents in the vicinity of vestigial Zealandia. With little rain-shadow effect over the low-lying land mass and exclusively ocean between the latitudes of Zealandia and the polar ice cap, the Hindon Maar Complex was ideally placed in the mid-latitudes to experience and record the subtle climate changes that resulted.
An 18.5 m core drilled from Hindon Maar #1 contained ~9.5 m of highly organic lake sediments with varying concentrations of biosiliceous microfossils. Physical properties of the core (magnetic susceptibility, gamma attenuation density, and spectrophotometry) measured using a Geotek multi-sensor core logger identified two lithofacies distinct from each other at ~5.7 mcd. High resolution mm-scale surveys of core split surfaces revealed core attributes that corroborated the Geotek findings. The lower facies is black laminated sapropelic material throughout while the upper facies is dark brown diatomite that exhibits light laminae and bedding structures related to turbidite mass flow events.
An age-depth model was developed by measuring lamina-couplet thickness in 10 lake sediment samples throughout the core. The lake sediments in the HMC M1-2 core represent ~10,400 years of deposition. The lamina-couplet thickness also identifies a boundary between depositional environments in the vicinity of 6.5 mcd, where deposition below represents a high annual flux of organic material to the lake, while deposition above represents biosiliceous deposition and much reduced flux of organic material.
51 approximately-equidistant lake sediment samples were subjected to FTIRS and BioSi wet chemistry compositional analyses and siliceous microfossil smear slide surveys. All analyses identify a change in deposition environment at ~6.5 mcd, from sediments with high organic input with some terrigenous material and few biosiliceous microfossils to sediments with reduced organic and terrigenous input and copious biosiliceous material.
Changes in the biosiliceous microfossil assemblages upcore were used to infer surface water pH changes. Changes in diatom abundance were indicative of access to nutrients sequestered in the profundal zone of the meromictic lake. Accessibility to nutrients was equated to climates that would promote or inhibit the transfer of nutrients from monimolimnion to mixolimnion across the chemocline. Therefore, changes in the lake sediment deposit were equated to climate-driven changes in the meromictic lake dynamics.
This Hindon Maar #1 lake sediment deposit represents two main climate-driven depositional environments. Warm wet climate conditions that promoted deciduous forest growth and inhibited nutrient accessibility in the lake transitioned over ~1300 years (~6.5 to 5.7 mcd) to windy, cool and dry climate conditions that shifted the range of the forests away from the HMC and allowed nutrients sequestered in the lake to become accessible to diatom algae, increasing the productivity of the lake.
The climates represented in the HMC M1 lake sediments are then projected back to the latitudinal shifts of the ACC and circumpolar westerlies, driven by the expansion and retreat of the AIS during the MMCT.

text: Short-term climate variability in the mid-Miocene: A Micropalaeontological and sedimentological investigation of a high-resolution maar lake sediment near Hindon, East Otago, New Zealand Maars are deep, steeply-sloped, small-diameter craters formed by phreato-magmatic eruptions. Phreatomagmatic eruptions result when rising magma intersects a body of water such as an aquifer. Maars often form deep closed-basin lake systems that become repositories for detrital material from the local environment. Tens of metres of detritus representing thousands of years of accumulation may be anoxically-preserved in maar lake sediments for millions of years.
The Hindon Maar Complex (HMC) is located in East Otago, on the South Island of New Zealand. The HMC hosts the highest-resolution mid-Miocene climate archive in the Southern Hemisphere. The date of its formation (14.6±0.1 Ma) coincides with the mid-Miocene climate transition (MMCT), a unique period when global temperatures were decreasing following an anomalous short-lived peak at the mid-Miocene climate optimum. Sensitivity to atmospheric pCO₂ levels and the confluence of the three main Milankovitch orbital cycle minima caused the Antarctic ice sheet (AIS) to advance and retreat. Concurrently, the Antarctic circumpolar currents (ACC) and atmospheric circumpolar westerlies also underwent latitudinal shifts while interacting with warm subtropical currents in the vicinity of vestigial Zealandia. With little rain-shadow effect over the low-lying land mass and exclusively ocean between the latitudes of Zealandia and the polar ice cap, the Hindon Maar Complex was ideally placed in the mid-latitudes to experience and record the subtle climate changes that resulted.
An 18.5 m core drilled from Hindon Maar #1 contained ~9.5 m of highly organic lake sediments with varying concentrations of biosiliceous microfossils. Physical properties of the core (magnetic susceptibility, gamma attenuation density, and spectrophotometry) measured using a Geotek multi-sensor core logger identified two lithofacies distinct from each other at ~5.7 mcd. High resolution mm-scale surveys of core split surfaces revealed core attributes that corroborated the Geotek findings. The lower facies is black laminated sapropelic material throughout while the upper facies is dark brown diatomite that exhibits light laminae and bedding structures related to turbidite mass flow events.
An age-depth model was developed by measuring lamina-couplet thickness in 10 lake sediment samples throughout the core. The lake sediments in the HMC M1-2 core represent ~10,400 years of deposition. The lamina-couplet thickness also identifies a boundary between depositional environments in the vicinity of 6.5 mcd, where deposition below represents a high annual flux of organic material to the lake, while deposition above represents biosiliceous deposition and much reduced flux of organic material.
51 approximately-equidistant lake sediment samples were subjected to FTIRS and BioSi wet chemistry compositional analyses and siliceous microfossil smear slide surveys. All analyses identify a change in deposition environment at ~6.5 mcd, from sediments with high organic input with some terrigenous material and few biosiliceous microfossils to sediments with reduced organic and terrigenous input and copious biosiliceous material.
Changes in the biosiliceous microfossil assemblages upcore were used to infer surface water pH changes. Changes in diatom abundance were indicative of access to nutrients sequestered in the profundal zone of the meromictic lake. Accessibility to nutrients was equated to climates that would promote or inhibit the transfer of nutrients from monimolimnion to mixolimnion across the chemocline. Therefore, changes in the lake sediment deposit were equated to climate-driven changes in the meromictic lake dynamics.
This Hindon Maar #1 lake sediment deposit represents two main climate-driven depositional environments. Warm wet climate conditions that promoted deciduous forest growth and inhibited nutrient accessibility in the lake transitioned over ~1300 years (~6.5 to 5.7 mcd) to windy, cool and dry climate conditions that shifted the range of the forests away from the HMC and allowed nutrients sequestered in the lake to become accessible to diatom algae, increasing the productivity of the lake.
The climates represented in the HMC M1 lake sediments are then projected back to the latitudinal shifts of the ACC and circumpolar westerlies, driven by the expansion and retreat of the AIS during the MMCT.

year: 2018

title: A collaborative approach to improving youth mental health.

authors: Goodwin, Debbie Waireka Tuhi

subjects: Maori

abstract: Nga Taiohi Whai Marama / Youth Connex was developed collaboratively by three organisations: Health Waikato Mental Health, Te Runanga o Kirikiriroa, and Linkage. These organisations had a vision of creating greater integration and improved pathways between youth and mental health services, as well as providing a service explicitly based on youth-friendly principles. The key aims of Whai Marama Youth Connex are to improve youth’s access to services and contribute to the development of a youth service-provider network. To fulfil this we aim to provide a range of services, integrated pathways and capacity building on several levels, the client level, the provider level and the funder level. Debbie Goodwin (Tuhoe) was employed as the Manager of the service and has a key role in developing the organisation, planning and shaping its services, and employing and managing its’ staff. She will give an overview of how the service is developing in partnership and biculturally, how youth-friendly principles are enacted, and what some of the key issues have been in its early stages.

text: A collaborative approach to improving youth mental health. Nga Taiohi Whai Marama / Youth Connex was developed collaboratively by three organisations: Health Waikato Mental Health, Te Runanga o Kirikiriroa, and Linkage. These organisations had a vision of creating greater integration and improved pathways between youth and mental health services, as well as providing a service explicitly based on youth-friendly principles. The key aims of Whai Marama Youth Connex are to improve youth’s access to services and contribute to the development of a youth service-provider network. To fulfil this we aim to provide a range of services, integrated pathways and capacity building on several levels, the client level, the provider level and the funder level. Debbie Goodwin (Tuhoe) was employed as the Manager of the service and has a key role in developing the organisation, planning and shaping its services, and employing and managing its’ staff. She will give an overview of how the service is developing in partnership and biculturally, how youth-friendly principles are enacted, and what some of the key issues have been in its early stages.

year: 2003