Daniel N. Schillereff

Hydrological thresholds and basin control over paleoflood records in lakes

The scarcity of long-term hydrological data is a barrier to reliably determining the likelihood of floods becoming more frequent and/or intense in a warmer world. Lake sediments preserve characteristic event layers, offering the potential to develop widely distributed and unique chronologies of historical floods. Inferring flood magnitude remains a greater challenge, previously overcome in part by analyzing sharply laminated polar or alpine sequences. Here we demonstrate an approach to obtain flood frequency and magnitude data from an unexploited resource, the largely visually homogeneous, organic sediments that typify most temperate lakes. The geochemical composition and end-member modeling of sediment trap and adjacent short core particle size data for Brotherswater (northwest England) discriminates the signature of infrequent, coarse-grained flood deposits from seasonal and longer term allogenic (enhanced discharge and sediment supply during winter) and autogenic (summer productivity, thermal mixing) depositional processes. Comparing the paleoflood reconstruction to local river discharges shows that hydrological thresholds censor event signature preservation, with 4 yr recurrence intervals detectable in delta-proximal sediments declining to 9 yr in the lake center. Event threshold (discharge) and process characterization are essential precursors to discerning flood magnitude from sediment archives. Implementation of our approach in globally prevalent temperate lakes offers a vast, unique repository of long-term hydrological data for hydrologists, climate modelers, engineers, and policy makers addressing future flood risks.

First human impacts and responses of aquatic systems: A review of palaeolimnological records from around the world:

Lake sediments constitute natural archives of past environmental changes. Historically, research has focused mainly on generating regional climate records, but records of human impacts caused by land use and exploitation of freshwater resources are now attracting scientific and management interests. Long-term environmental records are useful to establish ecosystem reference conditions, enabling comparisons with current environments and potentially allowing future trajectories to be more tightly constrained. Here we review the timing and onset of human disturbance in and around inland water ecosystems as revealed through sedimentary archives from around the world. Palaeolimnology provides access to a wealth of information reflecting early human activities and their corresponding aquatic ecological shifts. First human impacts on aquatic systems and their watersheds are highly variable in time and space. Landscape disturbance often constitutes the first anthropogenic signal in palaeolimnological records. While the effects of humans at the landscape level are relatively easily demonstrated, the earliest signals of human-induced changes in the structure and functioning of aquatic ecosystems need very careful investigation using multiple proxies. Additional studies will improve our understanding of linkages between human settlements, their exploitation of land and water resources, and the downstream effects on continental waters.

Long-term macronutrient stoichiometry of UK ombrotrophic peatlands

In this paper we report new data on peat carbon (C), nitrogen (N) and phosphorus (P) concentrations and accumulation rates for 15 sites in the UK. Concentrations of C, N and P measured in peat from five ombrotrophic blanket mires, spanning 4000-10,000years to present were combined with existing nutrient data from ten Scottish ombrotrophic peat bogs to provide the first UK perspective on millennial scale macronutrient concentrations in ombrotrophic peats. Long-term average C, N and P concentrations (0-1.25m) for the UK are 54.8, 1.56 and 0.039wt%, of similar magnitude to the few published comparable sites worldwide. The uppermost peat (0-0.2m) is enriched in P and N (51.0, 1.86, and 0.070wt%) relative to the deeper peat (0.5-1.25m, 56.3, 1.39, and 0.027wt%). Long-term average (whole core) accumulation rates of C, N and P are 25.3±2.2gCm-2year-1 (mean±SE), 0.70±0.09gNm-2year-1 and 0.018±0.004gPm-2year-1, again similar to values reported elsewhere in the world. The two most significant findings are: 1) that a regression model of N concentration on P concentration and mean annual precipitation, based on global meta data for surface peat samples, can explain 54% of variance in N concentration in these UK peat profiles; and 2) budget calculations for the UK peat cores yield an estimate for long-term average N-fixation of 0.8gm-2year-1. Our UK results, and comparison with others sites, corroborate published estimates of N storage in northern boreal peatlands through the Holocene as ranging between 8 and 15Pg N. However, the observed correlation of N% with both mean annual precipitation and P concentration allows a potential bias in global estimates that do not take this into account. The peat sampling data set has been deposited at the NERC Data Centre (Toberman et al., 2016).

Macronutrient processing by temperate lakes: A dynamic model for long-term, large-scale application

We developed a model of the biogeochemical and sedimentation behaviour of carbon (C), nitrogen (N) and phosphorus (P) in lakes, designed to be used in long-term (decades to centuries) and large-scale (104-105km2) macronutrient modelling, with a focus on human-induced changes. The model represents settling of inflow suspended particulate matter, production and settling of phytoplankton, decomposition of organic matter in surface sediment, denitrification, and DOM flocculation and decomposition. The model uses 19 parameters, 13 of which are fixed a priori. The remaining 6 were obtained by fitting data from 109 temperate lakes, together with other information from the literature, which between them characterised the stoichiometric incorporation of N and P into phytoplankton via photosynthesis, whole-lake retention of N and P, N removal by denitrification, and the sediment burial of C, N and P. To run the model over the long periods of time necessary to simulate sediment accumulation and properties, simple assumptions were made about increases in inflow concentrations and loads of dissolved N and P and of catchment-derived particulate matter (CPM) during the 20th century. Agreement between observations and calculations is only approximate, but the model is able to capture wide trends in the lakewater and sediment variables, while also making reasonable predictions of net primary production. Modelled results suggest that allochthonous sources of carbon (CPM and dissolved organic matter) contribute more to sediment carbon than the production and settling of algal biomass, but the relative contribution due to algal biomass has increased over time. Simulations for 8 UK lakes with sediment records suggest that during the 20th century average carbon fixation increased 6-fold and carbon burial in sediments by 70%, while the delivery of suspended sediment from the catchments increased by 40% and sediment burial rates of N and P by 131% and 185% respectively.

An Inter-comparison of µXRF Scanning Analytical Methods for Lake Sediments

The acquisition of high-resolution geochemical data from wet sediment cores through µXRF scanning is an increasingly important analytical tool. A number of µXRF core scanners are in use today that measure elemental concentrations using slightly different methods and several correction methods have been proposed in order to generate more precise quantitative geochemical data. However, only few inter-device comparisons have been undertaken to date and this contribution addresses this deficiency. The main focus is based on a sediment core 1.44 m in length extracted from Loch of the Lowes, Southern Uplands of Scotland, and analysed using a Geotek XZ MSCL carrying an Olympus Delta XRF (University of Liverpool) and an ITRAX core scanner housed at the National Oceanography Centre—Southampton (NOCS). The core is strongly laminated but layers are not consistent in terms of thickness or frequency, thus it provides a good test of the comparative analytical resolution of the devices. The cores were measured at 5 and 1 mm intervals using the Geotek scanner and steps of 0.3 mm on the ITRAX instrument. Seven elements were selected for this inter-comparison: the Geotek 5 mm measurements of Ti, Zr, K and Rb concentration picked up all phases of enhanced mineral supply to the lake as characterised by light-coloured sediments. The finer detail (mm-scale laminations) was more effectively captured by the Geotek 1 mm and ITRAX scans, however Zr, Sr and Rb measured on the ITRAX were significantly more spikey or noisy in character; in particular, peaks and troughs in ITRAX count rate of similar amplitude to the light/dark layers also appeared across intervals with no visible stratigraphic variability. The Geotek less effectively discriminated thin layers within a sediment matrix of geochemically-different lithology. The decision as to which scanning resolution to choose depends on the research question being asked. If abrupt sedimentological or climatic transitions are not under investigation, then lower resolution scans, which are well within the capability of the 5 mm Geotek runs, may be sufficient to capture palaeoenvironmental change. The Itrax can also be operated to run in such a low-resolution mode but on this occasion this was not selected. Another approach investigated was to externally calibrate the Geotek data using a regression correction method (proposed by Boyle et al. 2015, this volume) was tested and consistent dry-mass equivalent concentrations were found for both techniques despite variable statistical relationships. We thus recommend that future research presents µXRF data in terms of dry-mass concentration to facilitate more effective method and field site intercomparison. A final instrument comparability test was performed by measuring a short laminated sandstone section and a homogenous, parallel faced obsidian piece on the ITRAX (NOCS), the Geotek and an Eagle III μXRF system (NOCS). As an example, the Ti profiles through the laminated sandstone output from the three instruments showed some differences: clear peaks and troughs in the ITRAX measurements effectively tracked the alternating light and dark layers while the Geotek returned a more smoothed signal. Interestingly, the Eagle III data exhibit several Ti peaks that do not fully correspond to layer thickness. For the obsidian piece, the signal-to-noise ratio was good in all cases but the relative major element composition varied between devices, possibly due to different instrument configurations’ keeping this firmly in mind when comparing geochemical data thus seems critical.

Simulating a century of soil erosion for agricultural catchment management

Agricultural land management requires strategies to reduce impacts on soil and water resources while maintaining food production. Models that capture the effects of agricultural and conservation practices on soil erosion and sediment delivery can help to address this challenge. Historic records of climatic variability and agricultural change over the last century also offer valuable information for establishing extended baselines against which to evaluate management scenarios. Here, we present an approach that combines centennialscale reconstructions of climate and agricultural land cover with modelling across four lake catchments in the UK where radiometric dating provides a record of lake sedimentation. We compare simulations using MMF-TWI, a catchment-scale model developed for humid agricultural landscapes that incorporates representation of seasonal variability in vegetation