High-Resolution Historical Record of Plant Protection Product Deposition Documented by Target and Nontarget Trend Analysis in a Swiss Lake under Anthropogenic Pressure.

A multiproxy workflow was used to assess >60 plant protection products (PPPs) in sediment samples from a Swiss lake under heavy agricultural pressure. The results show the appearance of PPPs for the first time in the early 1960s with an overall detection of 34 PPPs and with herbicides and fungicides found in equal proportions. Paleolimnological data [e.g., chronology, hyperspectral imaging of sedimentary green pigments, and semiquantitative elemental composition (µXRF scans)] suggest that PPP concentrations and fluxes to the sediment over time are not related to land surface processes such as soil erosion or lake biogeochemistry but are attributed mainly to PPP application (inferred from sales) or regulatory measures (bans). Additional compounds with similar sources of contamination as the target PPPs captured by nontarget trend analysis (≥2000 unknown profiles) reveal significant inputs of contaminants to the lake starting in the 1970s, followed by a decrease of contamination at the beginning of the 1990s and a constant increase by ∼28% of the unknown compounds since the year 2000. An ecological risk assessment conducted on detected PPPs indicates that since the 1980s, the sediment quality is insufficient with risk quotient values displaying maximum levels in the most recent sediments (∼2010) despite bans of specific PPPs and environmental regulations.

Scanning Hyperspectral Imaging for In Situ Biogeochemical Analysis of Lake Sediment Cores: Review of Recent Developments.

Hyperspectral imaging (HSI) in situ core scanning has emerged as a valuable and novel tool for rapid and non-destructive biogeochemical analysis of lake sediment cores. Variations in sediment composition can be assessed directly from fresh sediment surfaces at ultra-high-resolution (40−300 µm measurement resolution) based on spectral profiles of light reflected from sediments in visible, near infrared, and short-wave infrared wavelengths (400−2500 nm). Here, we review recent methodological developments in this new and growing field of research, as well as applications of this technique for paleoclimate and paleoenvironmental studies. Hyperspectral imaging of sediment cores has been demonstrated to effectively track variations in sedimentary pigments, organic matter, grain size, minerogenic components, and other sedimentary features. These biogeochemical variables record information about past climatic conditions, paleoproductivity, past hypolimnetic anoxia, aeolian input, volcanic eruptions, earthquake and flood frequencies, and other variables of environmental relevance. HSI has been applied to study seasonal and inter-annual environmental variability as recorded in individual varves (annually laminated sediments) or to study sedimentary records covering long glacial−interglacial time-scales (>10,000 years).

Linking the formation of varves in a eutrophic temperate lake to meteorological conditions and water column dynamics.

Despite varved sediments being widely used for paleolimnological studies, little information is available about how climate and meteorological signals are recorded in varves at sub-seasonal to annual scale. We investigate links between meteorological and limnological conditions and their influence on biochemical varve formation and preservation of sub-seasonal climate signals in the sediments. Our study site is postglacial Lake Zabinskie located in NE Poland, in which thick and complex varved sediments have been studied for the last decade. These sediments provide an excellent material for studying the influence of short-term weather conditions on geological records. For this, we use an almost decade-long (2012-2019) series of observational data (meteorological conditions, physicochemical water parameters, and modern sedimentation observations) to understand varve formation processes. Then we compare these results with a high-resolution biogeochemical proxy dataset based on µXRF and hyperspectral imaging (HSI) measurements of a varved sediment core spanning the same period. Here we show direct links between the meteorological and limnological conditions and varve formation processes. This is particularly the case for air temperature which governs calcite laminae formation and primary production. We further show that calcite grain size is influenced by lake mixing intensity resulting from the wind activity, and that holomixis events lead to the formation of distinct manganese (Mn) peaks in the typically anoxic sediments. Our findings show that high-resolution non-destructive spectroscopy methods applied to complex biochemical varves, in combination with long observational limnological datasets, provide useful information for tracking meteorological and limnological processes in the past.

A high-resolution record of Holocene primary productivity and water-column mixing from the varved sediments of Lake Zabinskie, Poland.

Eutrophication and anoxia are increasing in lakes worldwide. However, our understanding of variations of primary productivity and anoxia in lakes over thousands of years is limited. Long-term records are needed to understand the natural variability of lake ecosystems and to improve our understanding of drivers of productivity and anoxia. In this study, we used the varved sediment record of Lake Zabinskie, Poland to answer the following research questions: 1) How have primary production and water column oxygen concentrations varied during the past 10,800 years?; 2) what role did natural and anthropogenic forces have in driving changes in primary production or lake mixing regime? Recently developed hyperspectral imaging (HSI) techniques were used to quantify sedimentary chloropigments-a and bacteriopheopigments-a (Bphe-a) at sub-annual resolution. These data, combined with elemental data from micro X-ray fluorescence (µ-XRF) and pigment assemblage data from high-performance liquid chromatography (HPLC) measurements, were used to reconstruct paleolimnological conditions. Bphe-a was used as an indicator of anoxia, and its presence suggests that an extensive anoxic zone was present nearly continuously from 10.8 to 2.8 ka BP. Anoxic conditions, driven by thermal stratification, were promoted by closed forest cover during that time, which limited wind-driven mixing of the water column. After 2.8 ka BP, water column oxygenation occurred more frequently, particularly during periods of increased human agricultural activity and forest opening. Pronounced anoxia was again present continuously from ~610 to 1470 CE, concurrent with a period of reforestation. After ~1610 CE, deforestation caused increases in erosion rates, algal production, and water column oxygenation. Pigment assemblages indicate that the algal community during the past 150 years was different from any other time during the Holocene. This study demonstrates a clear link between lake biogeochemical processes and forest cover and shows the potential of HSI to produce extremely high-resolution records of past productivity and redox conditions from varved lake sediments.