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Long‐term perspectives on terrestrial and aquatic carbon cycling from palaeolimnology

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Lakes are active processors and collectors of carbon (C) and thus recognized as quantitatively important within the terrestrial C cycle. Better integration of palaeolimnology (lake sediment core analyses) with limnological C budgeting approaches has the potential to enhance understanding of lacustrine C processing and sequestration. Palaeolimnology simultaneously assimilates materials from across lake habitats, terrestrial watersheds, and airsheds to provide a uniquely broad overview of the terrestrial‐atmospheric‐aquatic linkages across different spatial scales. The examination of past changes over decadal–millennial timescales via palaeolimnology can inform understanding and prediction of future changes in C cycling. With a particular, but not exclusive, focus on northern latitudes we examine the methodological approaches of palaeolimnology, focusing on how relatively standard and well‐tested techniques might be applied to address questions of relevance to the C cycle. We consider how palaeolimnology, limnology, and sedimentation studies might be linked to provide more quantitative and holistic estimates of lake C cycling and budgets. Finally, we use palaeolimnological examples to consider how changes such as terrestrial vegetation shifts, permafrost thaw, the formation of new lakes and reservoirs, hydrological modification of inorganic C processing, land use change, soil erosion and disruption to global nitrogen and phosphorus cycles might influence lake C cycling. WIREs Water 2016, 3:211–234. doi: 10.1002/wat2.1130 This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Science of Water > Water and Environmental Change Science of Water > Water Quality
Schematic showing the origin and processing pathways of commonly used palaeolimnological proxies in lake sediments (grey), including the influence of modifications by anthropogenic and natural processes (agriculture, fossil fuel combustion, and changes in terrestrial vegetation cover). Carbon (C) pools in lake waters (blue) include particulate inorganic C (PIC), dissolved inorganic C (DIC), particulate organic C (POC), and dissolved organic C (DOC).
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Sediment core measurements from Blelham Tarn, a small lake in the English Lake District taken from showing changes in (a) organic C (OC) burial rates (focusing‐corrected) and loss‐on‐ignition (LOI) at 550°C as an estimate of the % organic content and (b) concentrations of the cyanobacterial pigment aphanizophyll expressed relative to the organic content of the sediment. The proportional increase in aphanizophyll relative to OM in the sediment indicates that cyanobacteria associated with lake eutrophication, became proportionally more important within the lake C budget after ca. 1950 when piped water and sewerage systems were installed in a village within the lake catchment. The general increase in C accumulation rate (CAR) started before 1950 and is associated with agricultural intensification, with peak CARs during the 1980–1990s associated with high livestock densities, and soil erosion. Soil erosion led to a decline in LOI, due to dilution of the %OM by minerogenic material, but the CARs show that it had the net effect of stimulating C burial, probably from enhanced eutrophication. The upper sediments have very high LOI, possibly due to incomplete C degradation, but also because of the decline in soil erosion when livestock densities were reduced.
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Inorganic and organic carbon (C) accumulation rates (not focusing‐corrected) over the past ca. 8000 years in (a) freshwater Lake SS2 and (b) closed‐basin oligosaline lake Braya Sø, which are located with a few kilometers of each other in the Kangerlussuaq area of West Greenland. The high inorganic calcium carbonate deposition around 8000 years ago Lake SS2 sediments is most likely cause by intense weathering of carbonate minerals derived from glacial till, which declined when soils established. The absence of this feature in Braya Sø indicates heterogeneity of carbonate deposition in this area. Afterwards, mean uncorrected C accumulation rates (CARs) were similar in both lakes, but the much greater variability in Braya Sø is caused by fluctuations in water levels, and hence altered conditions for organic C production and deposition, including periods of anoxia in the bottom of the lake when autotrophic purple sulfur bacteria (that leave purple bands in the sediment—inset) were important in C processing. Distinct carbonate laminations in the Braya Sø sediments (inset) indicate periods of intense IC deposition, and yet IC is proportionally much less important than organic C in sediments of this lake. In both lakes, although there was a transition from warmer to cooler conditions ca. 4500 years ago, this had little impact on CARs; instead changes in the hydrological balance influenced CARs in Braya Sø, which is closed‐basin and therefore hydrologically sensitive.
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Representation of how temperature and precipitation influence dissolved organic C (DOC) fluxes and processing within lakes and watersheds from mid‐high latitudes during the summer ice‐free growth periods of different climatic zones (after Ref ). The size of the arrows indicates the relative flux or influence of each component. In ‘humid and warm’ boreal regions with forest cover (a) lakes are generally net heterotrophic because allochthonous DOC from terrestrial vegetation and soils is the dominant lakewater C pool, staining waters brown, limiting light penetration for autotrophic photosynthesis and providing an energy source for heterotrophs (‘the microbial loop’). (b) In cooler regions above treeline with high‐moderate precipitation (e.g., shrub tundra, and tundra heath) where permafrost restricts DOC transport and low temperatures impede terrestrial vegetation decomposition, lake waters with intermediately colored with DOC and lakes are usually weakly heterotrophic. (c) In areas of low precipitation with warm growth season temperatures (e.g., continental interiors with prairie/steppe or savannah vegetation), terrestrial vegetation growth and external DOC transport are limited by moisture availability, recalcitrant, and colorless DOC characterizes the lakewaters and dissolved inorganic C (DIC) is usually the most abundant C pool in waters leading to optimal conditions for autotrophic production. (d) In cold regions with low precipitation and minimal vegetation development (High Arctic, polar deserts, and Montane), allochthonous DOC sources are scarce and nutrients may strongly limit autotrophic processes with the C (and nutrient) budgets often supplemented by wind‐blown dusts. Severe lack of DOC may limit algal growth through ultraviolet radiation damage. Note that the diagram represents only growth season conditions, and complete estimates lake autotrophy/heterotrophy must include the winter period which is usually ice‐covered.
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Conceptual model illustrating how palaeolimnology integrates with other methodologies in quantification of terrestrial‐aquatic carbon (C) cycling, and the elements that can be best quantified by each method (arrows). The breadth of the shapes indicates how widely the methodology has been applied at each timescale and the intensity of shading indicates the relative frequency that each type of technique has generally been applied. The majority of C cycling studies in lakes thus far have focused on physiological/chemical and limnological processes, although there are a growing number of palaeolimnological studies dedicated to this topic. In comparison there are fewer quantitative studies of C sedimentation using techniques such as sediment traps to understand whole lake C budgets; such studies are vital to bridge the gap between palaeo/limnological methods. Timescales of operation for a range of global environmental change processes discussed in the latter half of the review are presented for comparison.
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