Varved Sediment Sequence Research Articles

Unravelling a 2300 year long sedimentary record of megathrust and intraslab earthquakes in proglacial Skilak Lake, south‐central Alaska

AbstractSeismic hazards in subduction settings typically arise from megathrust, intraslab and crustal earthquake sources. Despite the frequent occurrence of intraslab earthquakes in subduction zones and their potential threat to communities, their long‐term recurrence behaviour is barely studied. Sedimentary sequences in lakes may register ground shaking from different seismic sources. This study investigates two long sediment cores (13 m and 16 m) from Skilak Lake, a proglacial lake in south‐central Alaska, to evaluate whether different seismic sources leave a distinct imprint. The sedimentary record shows a continuously varved sediment sequence, occasionally interrupted by turbidites, slump deposits and tephra beds. Turbidites and slump deposits were objectively identified using a statistical outlier analysis on varve thickness. The earthquake origin of these deposits was ascertained by resemblance with deposits induced by instrumentally recorded earthquakes (for example, 1964 ce Mw 9.2 megathrust and 1954 ce Mw 6.4 intraslab earthquakes) and correlation with multiple coeval landslide deposits on sub‐bottom profiles. The Skilak Lake record chronicles 19 earthquakes with moderate to very high confidence level in the past 1350 years. The sedimentary evidence of instrumentally‐recorded intraslab and megathrust earthquakes within the past 70 years demonstrates that not only megathrust earthquakes, but also past intraslab events are recorded. Although reported seismic intensities at Skilak Lake are comparable for the 1964 ce megathrust and the 1954 ce intraslab earthquakes, the long duration and low frequency content of seismic ground motion during megathrust earthquakes facilitate the triggering of multiple, voluminous landslides and the generation of megaturbidites. In contrast, the shorter duration and higher frequency source spectrum of intraslab earthquakes may only induce surficial slope remobilization and the generation of thinner turbidites. This study demonstrates that the sedimentary record of Skilak Lake has the potential to decipher multiple seismic sources, which opens possibilities for a comprehensive seismic hazard analysis for south‐central Alaska.

Impacts of late‐Holocene climate variability and watershed‐lake interactions on diatom communities in Lac Brûlé, Québec

AbstractA high‐resolution diatom analysis of a varved sediment sequence from Lac Brûlé, southwestern Québec, was used to study temporal patterns of environmental change in the late Holocene. Key periods of interest in the record included the Medieval Warm Period (~800–1300 CE), the Little Ice Age (~1450–1850 CE), and post‐European settlement (~1850–present). Subfossil diatom assemblages were compared to previously published pollen, cladocera, and sediment records from Lac Brûlé, revealing complex dynamics between terrestrial vegetation succession, nutrient fluxes, and trophic interactions. Generalized additive models showed a response to long‐term climate variability in the diatom record, although it was not the most influential driver of community changes at Lac Brûlé. Catchment‐mediated processes instead played the largest role in governing the structure of diatom assemblages in the lake. For example, nutrient loading following a local fire in the watershed at 1345 CE led to an abrupt and significant increase in Fragilaria spp. Human activity associated with deforestation and the Wallingford‐Back Mine (1924–1972 CE) also had strong impacts on the landscape, which triggered further responses in the aquatic communities of Lac Brûlé.

Climate change in the North – past, present and future

The Nordic countries have produced famous polar explorers and researchers who have generated climate research schools at a variety of locations. The dependence of these countries with respect to the lifelihood of their societies, of their use of lands and seas, the exploitation of marine living and non-living resources have made climate research an eminent topic, and many outstanding discoveries of long- and short-term climate change have been made for the first time in Scandinavia. These include early contributions to our understanding of the geological effects of continent-wide glaciations during the ice ages, the complex postglacial history of the Baltic Sea and the varved sediment sequences preserved from lakes with an extraordinary seasonality in their sediment input, as well as the detailed records of temperature, ice texture and impurities and greenhouse gas variations of the last Glacial and of the Holocene preserved in the ice cores from Greenland. Iceland with its volcanic sequences and intercalated sediment layers not only preserved the history of this subaerial segment of the mid-Atlantic Ridge, but also easily datable paleoclimate records. The fate of the Vikings, who settled during the Medieval climate optimum on Iceland and later on Greenland and who lost their habitat on Greenland at the beginning of the Little Ice Age, illustrates vividly the climate-dependent subsistence of the indigenous and non-indigenous Scandinavian populations. Modern Scandinavian climate research institutions also include sophisticated modelling groups.

Contemporary sedimentation rates and depositional structures in a montane lake basin, southern Coast Mountains, British Columbia, Canada

AbstractSpatial variability of recent lacustrine sedimentary structures and sedimentation rates are examined for Green Lake, a morphologically complex lake basin of the southern Coast Mountains, British Columbia. A dense, 100 m grid sampling scheme was used for sediment coring within the 2 km2 lake basin. Deltaic, massive, weakly laminated, and varved sediment sequences are identified within the sediment record. Spatial patterns among these sedimentary deposits are related to within‐lake sediment transfer processes, morphometric controls, and the extent of post‐depositional mixing by bioturbation. Unconformities, turbidites, and cohesive slump failure deposits, observed within the contemporary varve sequences, could all be correlated with major flooding events in the catchment area and direct anthropogenic disturbances along the shoreline. There is an overall, non‐linear decrease in sedimentation rates with increasing distance from the lake inflows; however, this pattern is disrupted in deep water sites of intervening lake sub‐basins where locally higher accumulation rates are observed. Spatial sedimentation patterns are quantitatively described by an empirically‐derived model. Systematic variations in the model parameters are observed for different lake sub‐regions and are associated with changing sediment transfer dynamics between proximal and distal sub‐basin settings. Copyright © 2006 John Wiley & Sons, Ltd.

Indication of climatically induced natural eutrophication during the early Holocene period, based on annually laminated sediment from Lake Holzmaar, Germany

A multidisciplinary study is presented for the period from 10,128–9102±10 cal BP from Lake Holzmaar in western Germany. Concurrent signals in sediment chemistry and diatom assemblages are recorded in the varved sediment sequence, and several distinct periods in the lake's response to climatic amelioration during the early Holocene period are revealed. Diatom-based transfer functions were used to estimate the lake's trophic status. In conjunction with high-resolution geochemical analysis, these data were applied to quantify the magnitude of natural nutrient enrichment. The inferred changes in the lake nutrient status are discussed with reference to the changing environmental conditions and development of the aquatic ecosystem. The major perturbation of the lake occurred with the rapid eutrophication at 9820 cal BP. Following a period of decreasing productivity due to a meromictic phase of Lake Holzmaar, an accelerated release of phosphorus from the sediment occurred and diatom-inferred total phosphorus concentrations increased rapidly. Longer hydraulic residence times in Lake Holzmaar are assumedly related to elevated evaporative transpiration rates and increasing temperatures. Indication of milder winter and increasing summer temperatures at this time is in agreement with interpretation of the pollen data and changes in the seasonal characteristics of the diatom assemblages.

A 3000‐year palaeoenvironmental record from annually laminated sediment of Lake Korttajarvi, central Finland

High‐resolution physical analyses (varve thickness and relative X‐ray density) were conducted on a 3000‐year varved sediment sequence in Lake Korttajarvi, central Finland. Climate and the local environment strongly influence the properties of the lake sediments, and, through a combination of physical proxies, severe and favourable climate periods and anthropogenic effects on sedimentation with an annual to decadal resolution could be detected. We observed previously identified historical climate periods in the Lake Korttajarvi varve record. The Medieval Climate Anomaly (often termed the Medieval Warm Period) of AD 980–1250, which is characterized by highly organic sediment and a minor minerogenic flux during mild winters, started and terminated abruptly, but also included a short (30‐year) colder period lasting between AD 1115 and AD 1145. The Little Ice Age, however, was not clear in our record, although there were two minor cooling periods in AD 1580–1630 and AD 1650–1710. Natural variability in the sediment record was disrupted by increased human impact in the catchment area at AD 1720. There is a distinct positive anomaly in mineral matter accumulation between 907 and 875 BC, which indicates more severe climate conditions. This period exists contemporary with a cold event, recorded worldwide, c. 2800 years ago.

A fine‐scale record of 130 years of organic carbon deposition in an anoxic fjord, Saanich Inlet, British Columbia

A varved sediment sequence from an anoxic temperate fjord is examined for a 130‐yr record of organic carbon content and stable carbon isotope composition. Saanich Inlet, British Columbia, receives most detrital input from a logged and reforested watershed. Winter and summer laminae differences are significant and are isotopically depleted with respect to water column organic carbon. Within the core, organic carbon content increased from 2.5% in the 1860s to 3.3% in the 1990s, whereas carbon isotopes become depleted by 1.2‰ toward the late 1990s. Homogenization of large sample volumes yields the most repeatable carbon results. Overlaid on the trends are cycles of roughly a 38‐yr period for both records. River outflow has not changed, but logging rates that peaked midcentury in the watershed match the organic carbon shifts. This influence, plus the interdecadal Pacific‐climate oscillation, likely explain the periodicity observed.