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Global spread of anoxia in aquatic ecosystems has become a major issue that may potentially worsen due to global warming. The reconstruction of long-term hypolimnetic anoxia records can be challenging due to lack of valid and easily measurable proxies. The sedimentary Mn/Fe ratio measured by X-ray fluorescence (XRF) is often used as a proxy for past lake redox conditions. Yet the interpretation of this ratio can be problematic when Fe and Mn accumulation is not solely redox driven. We used the varved sediments of Lake Moossee (Switzerland) to examine the partitioning of Fe and Mn in seven fractions by sequential extraction under various oxygen conditions over the last 15,000 years. We combined this data with XRF scans and an independent diagnostic proxy for anoxia given by a hyperspectral imaging (HSI)-inferred record of bacteriopheophytin, to validate the use of the XRF-Mn/Fe ratio as redox proxy. In the 15,000-year long record, Fe was bound to humins and amorphous, crystalline, sulfide and residual forms. Mn was mainly present in carbonate and amorphous forms. Higher erosion, prolonged anoxia, diagenesis and humic matter input affected Fe and Mn accumulation. Under holomixis the XRF-Mn/Fe ratio successfully reflected lake redox conditions. Periods with higher detrital Fe input obscured the applicability of the ratio. During phases of permanent anoxia, intensified early diagenetic processes trapped Mn in the sediments in carbonate, crystalline oxide and humic forms. Our study shows that the single use of the XRF-Mn/Fe ratio is often not conclusive for inferring past lake redox conditions. The application of the XRF-Mn/Fe as a proxy for anoxia requires taking into account the individual lake characteristics and changes in lake environmental conditions, which affect the accumulation of Fe and Mn in the sediments., Science of The Total Environment, 755 (Part 2), ISSN:0048-9697, ISSN:1879-1026

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 Żabinskie, 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.

Varved lake sediments provide a unique opportunity to validate results of isotope dating methods. This allows testing of different numerical models and constraining procedures to produce reliable and precise chronologies. Here we present results of the model testing and validation by multiple dating approaches for the varved sediment record from Lake Żabińskie (northeastern Poland). Our goal was to assess possible deviations of 210Pb-derived ages from true sediment ages provided by varve chronology and to check how different numerical procedures can improve the consistency of the chronologies. Different methods for age estimation were applied including varve counting, 210Pb, 137Cs, 14C and tephra identification. Microscopic analysis of the varve microfacies revealed that laminations found in Lake Żabińskie were biogenic (calcite) varves. Three independent counts indicated a good preservation quality of laminae in the sediment profile which contained 178+4/−8 varve years. The calendar-year time scale was verified with two maxima of 137Cs activity concentrations in the sediments (AD 1963 and 1986) and a terrestrial leaf dated to AD 1957–1958 by the 14C method. Additionally, geochemical analysis of the glass shards found in the sediments indicated a clear correlation with the Askja AD 1875 eruption of Iceland which provided an unambiguous verification of the varve chronology. For testing 210Pb dating we used two routinely applied models: Constant Flux Constant Sedimentation (CFCS) and Constant Rate of Supply (CRS). None of the models in their standard forms produced a chronology consistent with varve counts and independent chronostratigraphic markers. Both models yielded ages much younger than the calendar age with a difference of ca. 50 years at the bottom part of the profile. However, a significant improvement was introduced after using the composite CFCS model with sediment accumulation rates calculated for different zones of the sediment profile. This provided highly consistent values along the time scale with deviations 137Cs peaks as well as testing the correction values for incomplete 210Pb inventory provided results that finally deviated from the calendar age only by ≈ 10 yrs for the 120 years old sediments.

This chapter reviews the history of study and the current status of Mid-Holocene climatic and cultural change in the South Central Andes, which host a wide range of different habitats from Pacific coastal areas up to extremely harsh cold and dry environments of the high mountain plateau, the altiplano or the puna. Paleoenvironmental information reveals high amplitude and rapid changes in effective moisture during the Holocene period and, consequently, dramatically changing environmental conditions. Therefore, this area is suitable to study the response of hunting and gathering societies to environmental changes, because the smallest variations in the climatic conditions have large impacts on resources and the living space of humans. This chapter analyzes environmental and paleoclimatic information from lake sediments, ice cores, pollen profiles, and geomorphic processes and relates these with the cultural and geographic settlement patterns of human occupation in the different habitats in the area of southern Peru, southwest Bolivia, northwest Argentina, and north Chile and puts in perspective of the early and late Holocene to present a representative range of environmental and cultural changes. It has been found that the largest changes took place around 9000 cal yr BP when the humid early Holocene conditions were replaced by extremely arid but highly variable climatic conditions. These resulted in a marked decrease of human occupation, “ecological refuges,” increased mobility, and an orientation toward habitats with relatively stable resources (such as the coast, the puna seca, and “ecological refuges”).

Publisher Summary Lacustrine records that encompass the full, and late glacial periods, extends into the early Holocene document climate changes through the effects on limnology. Wind intensity, temperature, and water balance are the principal climate effects recorded by lake sediments. A selection of lake records along the Pole-Equator-Pole: Americas transect from Alaska to southern Patagonia reveals broad zonal regions of climate-controlled paleolimnology. In the Arctic, lakes record climate mostly as temperature signals that change the effective moisture balance, and their productivity. South of the Laurentide ice sheet, lakes also responded to warming temperatures, and reduced anticyclonic wind systems between the full glacial, and the early Holocene. A related effect, the disruption of Intertropical Convergence Zone (ITCZ), and the moisture-bearing trade winds responsible for maintaining lakes in Central America, and northern South America, caused basins in this region to be shallow, saline, or dry during the full glacial period, and to become re-established only by the latest glacial, and early Holocene. The single site in southern South America apparently did not receive moisture in significant amounts until the early Holocene, as westerly precipitation finally became established in the region. This analysis indicates that lakes recorded generally synchronous climate changes by similar but sometimes opposite responses. This synchroneity does not imply the same causality, even though the dynamics of climate change between the full glacial and the Holocene were clearly linked and mutually interdependent.

Publisher Summary This chapter discusses human dimensions of late Pleistocene/Holocene arid events in southern South America, where, examples from late glacial, and early and mid-Holocene archaeological sites in southern South America provide information about the human dimensions of past climate changes. Particularly in semiarid areas with marginal food, and water resources, changes from humid to arid conditions coincided with major changes in human occupational, and cultural patterns. The end of the first period of human occupation in central Chile coincided with rapidly increasing aridity, when trees were replaced by shrubs and herbs, the Pleistocene paleolakes disappeared, and the Pleistocene megafauna became extinct. The Paleo-Indian hunters occupied ecological refuges around lakes, where the resources were concentrated in an environment under general water stress. A similar process was observed 2000 years later in the Atacama Desert, and Altiplano of northern Chile, when the first period of human occupation came to an abrupt end, synchronously with the rapid desiccation of early Holocene paleolakes, and the disappearance of abundant water, vegetation, and animal resources. The subsequent extremely arid mid-Holocene environments resulted in a general hiatus of human occupation in the Atacama basin, whereas widespread resettlement coincided again with a rise of the paleolake levels to the modern levels.