Shetlands Islands field trip May 2014 : summary of results

This report provides a record of a field excursion to the Shetland Islands in May 2014 to investigate sediments deposited from tsunamis generated from submarine landslides mainly located off the coast of Norway. The research was funded under a NERC Consortium Grant for a project entitled ‘Will climate change in the Arctic increase the landslide-tsunami risk to the UK?’ It was part of Work Block 2 (WB2): ‘What is the timing of tsunami deposits on the UK coastline, and how is it related to the age of major Arctic slides’? The best known and most studied tsunami from the Norwegian submarine landslides is the Storegga event dated at 8,200BP. Sediments deposited from this tsunami are commonly found along the west coast of Norway, east coast of mainland Scotland, and also on the Shetland and Faeroe islands. However, there are other landslides off of Norway for which no associated tsunami has been identified, which poses the question as to whether these events did not generate a tsunami or whether the evidence for a tsunami has not yet been found. Although evidence for seabed slumping off Norway was first discovered in the 1950’s (Holtedahl, 1955, 1971) and the scale and morphology of a massive submarine landslide, subsequently termed Storegga, mapped in the 1970’s (Bugge, 1983), it was not until 1985 that an associated tsunami was first proposed (Svendsen, 1985). The first supporting sedimentary evidence of the tsunami was first identified on mainland Scotland in 1988 (Dawson et al., 1988) then, subsequently, similar sediments were identified on the Shetland Islands (Smith, 1993a). The Storegga Slide has been dated to 8,150BP (Haflidason et al., 2005), however more recent research on the deposits on the Shetlands suggests that some may not all be from Storegga, because 14C age dating gives younger ages of ~5,000 and 1,500 cal yr BP (Bondevik et al., 2005). A major challenge posed by the ages of these younger dates is that they are confined to the Shetlands; there is no indication of these younger tsunamis on mainland Scotland. If the dating is correct and the sediments are indeed from tsunamis, then the submarine landslides off Norway would be an unlikely source, so a local source seems most likely, but none has yet been identified. Alternatively a non-tsunami source for the sediments may explain their presence. The objectives of WB2 therefore are to investigate the tsunami deposits on Shetland that post-date the Storegga Slide, to validate their ages and, if possible, identify possible source locations of the submarine landslides that generated the tsunamis. On Shetland research on tsunami sediments was mainly based on evidence from coastal exposures around Sullom Voe where tsunami sands are dated as coeval with Storegga. The younger sands are mainly preserved in lake cores at locations on Shetland Mainland (Bondevik et al., 2005) where those of 5,000 BP overlie sands of Storegga age at 8,200 years BP age. At coastal sites along Basta Voe on Yell and at a mainland site at Dury Voe very young age dates of ~1,500 BP suggest an additional and very recent, late Holocene event (Bondevik et al., 2005; Dawson et al, 2006). A preliminary field excursion to the Shetlands carried out in 2013 discovered possible new tsunami deposits preserved in peat on central Yell at Whale Firth, Mid Yell Voe and Kirkabister. Subsequent 14C age dating of these deposits resulted in a variety of ages, many much younger than that of Storegga. The 14C method is known to be subject to major uncertainties because of contamination, for example initial age dating in the 1990’s at sites around Sullom Voe returned ages of around 5,000 years BP, although these were subsequently rejected in favour of the earlier, 8,200 BP Storegga event. Thus, validating the ages of the deposits on Yell, prospectively from a number of deposits laid down successively at one site (thereby reducing the sole reliance on 14C dating) was critical in validating the presence of more than one tsunami event on Shetland. The objective of the 2014 field visit to the Shetlands, therefore, was to return to Yell and validate the preliminary results from 2013; revisiting the sites at Whale Firth, Mid Yell and Kirkabister and searching the coastlines of Unst, Fetlar, Yell and north Mainland for additional sites where tsunami sediments might be preserved. Just before the visit new 14C dates from Mid Yell from samples collected in 2013 confirmed the previous results from other locations that had given a wide range of ages; at Whale Firth a single date gave a ‘young’ age of ~5,000 years BP, a range of ages with the oldest at 8,200 years BP were returned from Mid Yell Voe. We first visited sites on north Mainland around Sullom Voe, as it was here that the first indications of the Storegga tsunami were identified on Shetland in 1992. The deposits are classic as they contain rip-up clasts characteristic of tsunami deposits elsewhere. We then visited the sites at Basta Voe, Whale Firth, Mid Yell and Kirkabister. We carried out reconnaissance surveys on Unst, Fetlar, Yell and north Mainland. Preliminary results: 1. The new evidence supports the presence of tsunami sediments on Yell at Mid Yell Voe and Whale Firth, but the age of these sediments requires further research to confirm previous dating and their possible sources, 2. The youngest dated sediments (~1,500 BP) at Vasta Voe are most likely from a tsunami, but their limited areal extent suggests a local source, as yet undetermined, 3. The presence of three events at Mid Yell Voe based on surveys in 2013 was not confirmed, 4. The similarity of the deposits on Mid Yell with those around Sullom Voe on Mainland are suggestive of a similar source, 5. The wide range of the preliminary age dating at the Mid Yell sites (Whale Firth and Mid Yell Voe) is analogous to the early age dating of coastal deposits around Sullom Voe, suggesting the possibility of contamination of the peat material dated, 6. Whereas the 5,500BP event is identified in lake cores, no strongly supportive evidence for sands of this age were identified in the coastal sections, 7. Of the proposed three tsunami events proposed for Shetland only one, Storegga, has a confirmed source, 8. Further analysis of the peat stratigraphy at the coastal sites, reflects vegetation changes over the past ~8,000 years related to climate change, and these could be used to provide a broader context for the 14C age dating that may resolve the present dating issues, 9. Newly discovered sediments at Kirkabister require further research to determine their origin, 10. The origin(s) of the laminated deposits at Whale Firth, Mid Yell and Vatsetter is/are uncertain, but they are probably not from a tsunami, 11. No additional coastal exposures of peat with tsunami sands were located during the reconnaissance surveys on Mainland, Yell, Unst and Fetlar. Postscript; Immediately after this report was finalised, age dating of peat sections at Whale Firth and Mid Yell Voe confirmed that the sands preserved in the woody peat here are of Storegga age, ~8,200 cal yr BP

Soil conservation

Erosion decreases the productivity potential of soil except in rare locations where soils are unusually deep and fertile. Crop yields are decreased as topsoil is removed by erosion. Lower crop yields provide less income to sustain farm families and less food for the world. Recent research has identified and defined factors causing soil erosion and has developed management alternatives to combat these factors and conserve valuable soil resources. The most significant recent progress involves controlling erosion caused by irrigation on highly productive lands

The Impact of Irrigation on Ground Water Quality

Most soils in arid regions contain significant quantities of water soluble salts. When these soils are brought under irrigation, soluble salts are leached into the ground water. The salt leaching rate and the annual salt outflow depends upon the quantity of water applied in excess of evapotranspiration each irrigation season. The total quantity of salt leached from a Portneuf silt loam, 5 m deep, was 70 metric tons/ha. The first 14 cm of water passing out the bottom of the soil carried 38 metric tons of soluble salt/ha of soil into the ground water. Essentially all of the residual soluble salt was leached into the ground water after 30 cm of water/m of soil depth had passed from the soil as leachate, regardless of the number of irrigation seasons required for that amount of leaching. After the residual salts were removed, the salt concentration in the newly irrigated soil was the same as in Portneuf silt loam that had been irrigated for 70 years with a high leaching fraction. Subsequent salt outflow from the soil into the groundwater was directly related to the quantity of water leaching through the soil, indicating that more minerals dissolved with more leaching, but the salt concentration in the soil did not change significantly with the leaching fraction. Salt outflows from both newly irrigated and old irrigated lands can be predicted based upon prevailing laNd conditions and given irrigation practices. The impact of these outflows upon ground water quality can be estimated

Furrow irrigation erosion lowers soil productivity

Recent research efforts have shown that soil erosion decreases soil productivity. Erosion-caused crop production decreases of 15-40% are commonly reported with some values over 50%. Furrow erosion on irrigated land in Idaho decreases topsoil depth on the upslope approximately 33% of the field area and may increase topsoil depth on the downslope 50-55%. Crop yields arc generally decreased where topsoil depths are decreased, but yields are not generally increased where topsoil depths are increased beyond a critical depth. Crops vary in their sensitivity to decreases in topsoil depth, but all crops studied exhibited lower yields on the eroded areas. Soil productivity potential of one area representing several million ha of furrow irrigated land was reduced at least 25% by furrow erosion over 80 irrigation seasons. Technology is not available to restore soil productivity potential to the level that would exist had there been no erosion except for returning topsoil to eroded areas. Research and technology applications arc needed to reduce or eliminate topsoil loss and redistribution by irrigation erosion

Soil erosion on irrigated lands

Erosion on the upper portion and sedimentation on the lower portion of fields redistributes topsoil. The results of these processes become visible when the color of the subsoil differs from that of the topsoil (Fig. 37-1). The visual evidence of topsoil redistribution would be lacking where subsoil and topsoil are nearly the same color. Furrow erosion can cause a major topsoil redistribution on any field and have a simultaneous, severe, negative impact on crop production. Typical fields that have been irrigated for about 80 yr are illustrated in Fig. 37-1, showing the color change as whitish subsoil is mixed with darker_ topsoil. The topsoil distribution varies depending upon the field length and irrigation practice used over the 80 yr. The deepest topsoil areas, resulting from deposition, vary from field to field from about the midpoint to the extreme lower end. Also, there has been a net topsoil loss from most fields, thereby negatively impacting crop yield

Salinity and Plant Productivity

Plant productivity is limited on an estimated one third of the irrigated land in the world or approximately 4 x 10? ha by soluble salt accumulations in the soil, often referred to as soil salinity or salinity. As irrigated agriculture expands, more salinity problems will develop because there are millions of hectares of potentially irrigable land that could become saline. Every year new salinity problem areas develop and are identified. Salinity is the most important problem facing irrigated agriculture, and solving salinity problems is one of the greatest challenges to agricultural scientists. Much research has been conducted during the past 30 to 40 years to determine the relative tolerance of crops to salinity. Most of the salinity tolerance data available through the early 1960s was compiled into useful relationships by Bernstein in 1964, and these data have been cited and applied throughout the world. Since then, many new salinity tolerance studies have been conducted, and many new management practices have been proposed, evaluated, and some of them practiced to reclaim salt-affected soils for improved crop production. Recently, Maas and Hoffman evaluated existing salinity tolerance data for agricultural crops and presented the data graphically so that the relative tolerance among crops could be easily compared

Effects of erosion on soil productivity

Research efforts across the United States have shown that soil erosion decreases soil productivity. Erosion-caused crop production decreases up to 50% have been measured with decreases of 15 to 30% commonly reported. Furrow erosion on irrigated land redistributes topsoil, decreasing topsoil depth on the upslope 33% and increases topsoil depth on lower 50 to 55% of fields. Crop yields are decreased where topsoil depths are decreased, but yields are not increased where topsoil depths are increased above the original depth of 38 cm in a large study area representative of several million hectares of furrow irrigated land. Crops vary in their sensitivity to decreases in topsoil depth. Soil productivity of the entire study area was decreased at least 25% by furrow erosion over 80 irrigation seasons. Technology is not available to restore crop production to the potential level that would have existed without erosion. Research and technology application are needed to reduce or eliminate topsoil loss and redistribution by furrow irrigation to preserve our soil resources in irrigated areas. Application of conservation tillage to furrow irrigated land is suggested as the best known practice to reduce furrow erosion

Furrow Irrigation Erosion Effects on Crop Production

Furrow erosion and sediment deposition redistributes topsoil within fields. Both of these processes are directly proportional to the energy of the furrow irrigation stream. This stream must be large enough at the application point to provide sufficient water for infiltration along the entire furrow length to meet the purposes of irrigation. Where slopes exceed about 0.7% on many silt loam soils, the flow velocity combined with the stream size at the upper ends of fields has sufficient energy to erode soil (Berg and Carter, 1980). As the furrow stream size decreases from infiltration along the furrow, the energy to erode and transport soil also decreases. At some point along the furrow the stream energy reaches a level where it no longer erodes soil. Then, further down slope, the energy reaches a level where the stream will no longer carry the accumulated sediment from upstream erosion. At that point sedimentation begins and continues downslope. The quantity of eroded soil actually leaving the field through the furrow depends upon the sediment load in the furrow stream at the entry point into the drain ditch at the lower end of the field and the duration of the flow at that point

Partnership research with older people: moving towards making the rhetoric a reality

As nursing develops closer partnerships with older people in delivering care, it also needs to develop partnerships in order to create the knowledge base for practice in a way that challenges professional hegemony and empowers older people. However, the process of developing partnerships in research takes place against a background of academic research traditions and norms, which can present obstacles to collaboration. This paper is a reflection on the issues that have arisen in three projects where older people were involved in research at different levels, from sources of data to independent researchers. It points to some of the areas that need further exploration and development

A Buried Drain Erosion and Sediment Loss Control System

The lower ends of most furrow irrigated fields have become convex shaped, meaning the slope progressively increases from a point 20 to 60 feet from the field end to the tailwater ditch. This increasing slope is the result of maintaining tailwater ditches too deep and keeping them cleaned so runoff from these fields is not restricted. The process of forming a convex field end continues yearly at an increasing rate. With each passing year, the slope at the end of the field becomes greater so that runoff water runs faster and has more energy to erode. Over many years, large quantities of soil have been lost from the lower ends of furrow irrigated fields. Field ends 1.5 to 2.0 feet lower than the furrow elevation 20 to 60 feet upslope are common. Much of the soil loss is from the lower ends of fields