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The organic matter composition of lake sediments influences important in-lake biogeochemical processes and stores information on environmental changes. Extracting this information is notoriously difficult because of the complexity of the organic matter matrix, which routinely imposes trade-offs between high temporal and analytical detail in the selection of methods of analysis. Here, we demonstrate the potential of diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) for achieving both of these objectives using untreated bulk samples from two Holocene lake-sediment cores from central Sweden. We develop quantitative models for sediment total organic carbon (TOC) with the same predictive abilities as models based on samples diluted with KBr and qualitatively characterize the organic matter using a spectra processing-pipeline combined with principal component analysis. In the qualitative analysis we identified four organic matter sub-fractions and the interpretation of these is supported and further advanced with molecular data from pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Within these organic fractions, compound groups such as aromatics, lignin, aliphatics, proteins and polysaccharides were identified by means of DRIFTS and the analyses and processes outlined here enables rapid and detailed quantitative and qualitative analysis of sediment organic matter. The DRIFTS approach can be used as stand-alone method for OM characterization with high temporal resolution in Holocene sediment records. It may also function as a screening process for more specific analyses of sample subsets, such as when coupled with pyrolysis-GC/MS to further tease apart the OM composition, identify sources and determine degradation status.

Monitoring of surface waters in the boreal region over the last decades shows that waters are becoming browner. This timeframe may not, however, be sufficient to capture underlying trajectories and driving mechanisms of lake-water quality, important for prediction of future trajectories. Here we synthesize data from seven lakes in the Swedish boreal landscape, with contemporary lake-water total organic carbon (TOC) concentrations of 1.4–14.4 mg L⁻¹, to conceptualize how natural and particularly human-driven processes at the landscape scale have regulated lake-water TOC levels over the Holocene. Sediment-inferred trends in TOC are supported by several proxies, including diatom-inferred pH. Before ~ 700 ce, all lakes were naturally acidic (pH 4.7–5.4) and the concentrations of inferred lake-water TOC were high (10–23 mg L⁻¹). The introduction of traditional human land use from ~ 700 ce led to a decrease in lake-water TOC in all lakes (to 5–14 mg L⁻¹), and in four poorly buffered lakes, also to an increase in pH by > 1 unit. During the 20^th century, industrial acid deposition was superimposed on centuries of land use, which resulted in unprecedentedly low lake-water TOC in all lakes (3–11 mg L⁻¹) and severely reduced pH in the four poorly buffered lakes. The other lakes resisted pH changes, likely due to close connections to peatlands. Our results indicate that an important part of the recent browning of boreal lakes is a recovery from human impacts. Furthermore, on a conceptual level we stress that contemporary environmental changes occur within the context of past, long-term disturbances.

Since the implementation of large-scale lake monitoring in the similar to 1980s, water color and dissolved organic carbon (DOC) concentrations have increased in many northern lakes (i.e., lake browning), impacting the functioning of aquatic ecosystems. In regions that formerly experienced high levels of acid deposition, this browning trend has been largely attributed to the recovery from the impacts of past acid deposition. However, the extent to which DOC levels have now returned to naturally higher, pre-industrial conditions is still poorly understood. In this study, we assessed whether DOC levels are still influenced by acid deposition in lakes near Sudbury, Ontario, a region that has been heavily affected by sulfur dioxide emissions from local metal smelting during the 20th century. We analyzed water chemistry monitoring data (1981-2018), together with comparisons between modern and pre-industrial DOC levels inferred from sediment spectroscopy, for 51 acid-sensitive and 24 buffered reference lakes across the Sudbury landscape. Since 1981, DOC concentrations doubled in acid-sensitive lakes, with a mean increase of +1.6 mg/L, whereas in more buffered reference lakes, mean DOC levels increased by only 0.8 mg/L. Similarly, sediment-inferred DOC trends indicate that current DOC levels are, on average, similar to 22% below pre-industrial levels in acid sensitive systems compared to only similar to 10% in buffered lakes. Weakening correlations between DOC and acidification-related water chemistry variables (e.g., pH, alkalinity, metals) further indicate a diminishing influence of acid deposition on DOC in Sudbury lakes. These results highlight the strong impact that acid deposition has historically had on lake-water DOC dynamics in this region, but also suggest that DOC levels are approaching natural baseline levels in less acid-sensitive lakes, and that other drivers, such as changes in climate or vegetation cover, are now becoming the dominant controls on changes in DOC concentrations.

The history of mining and smelting and the associated pollution have been documented using lake sediments for decades, but the broader ecological implications are not well studied. We analyzed sediment profiles covering the past similar to 10,000 years from three lakes associated with an iron blast furnace in central Sweden, as an example of the many small-scale furnaces with historical roots in the medieval period. With a focus on long-term lake-water quality, we analyzed multiple proxies including geochemistry, pollen and charcoal, diatom composition and inferred pH, biogenic silica (bSi), visible near-infrared spectroscopy (VNIRS)-inferred lake-water total organic carbon (LW-TOC), and VNIRS-inferred sediment chlorophyll (sed-Chl). All three lakes had stable conditions during the middle Holocene (similar to 5000 BCE to 1110 CE) typical of oligo-dystrophic lakes: pH 5.4-5.6, LW-TOC 15-18 mg L-1. The most important diatom taxa include, for example, Aulacoseira scalaris, Brachysira neoexilis, and Frustulia saxonica. From similar to 1150 CE, decreases in LW-TOC, bSi, and sed-Chl in all three lakes coincide with a suite of proxies indicating disturbance associated with local, small-scale agriculture, and the more widespread use of the landscape in the past (e.g. forest grazing, charcoal production). Most important was a decline in LW-TOC by 30-50% in the three lakes prior to the 20th century. In addition, the one lake (Fickeln) downstream of the smelter and main areas of cultivation experienced a shift in diatom composition (mainly increasing Asterionella formosa) and a 0.6 pH increase coinciding with increasing cereal pollen and signs of blast furnace activity. The pH did not change in the other two lakes in response to disturbance; however, these lakes show a slight increase (0.3-0.5 pH units) because of modern liming. LW-TOC has returned to background levels in the downstream lake and remains lower in the other two.

Dissolved organic carbon (DOC) concentrations and water colour are increasing in many inland waters across northern Europe and northeastern North America. This inland-water "browning" has profound physical, chemical and biological repercussions for aquatic ecosystems affecting water quality, biological community structures and aquatic productivity. Potential drivers of this "browning" trend are complex and include reductions in atmospheric acid deposition, changes in land use/cover, increased nitrogen deposition and climate change. However, because of the overlapping impacts of these stressors, their relative contributions to DOC dynamics remain unclear, and without appropriate long-term monitoring data, it has not been possible to determine whether the ongoing "browning" is unprecedented or simply a "re-browning" to pre-industrial DOC levels. Here, we demonstrate the long-term impacts of acid deposition and climate change on lake-water DOC concentrations in low and high acid-deposition areas using infrared spectroscopic techniques on similar to 200-year-long lake-sediment records from central Canada. We show that acid deposition suppressed naturally higher DOC concentrations during the 20th century, but that a "re-browning" of lakes is now occurring with emissions reductions in formerly high deposition areas. In contrast, in low deposition areas, climate change is forcing lakes towards new ecological states, as lake-water DOC concentrations now often exceed pre-industrial levels.

We conducted a multidisciplinary study to provide the stratigraphic and palaeoclimatic context of monsoonal rainfall dynamics and their responses to orbital forcing for the Bay of Bengal. Using sediment lightness we established an age model at orbital resolution for International Ocean Discovery Programme (IODP) Core U1452C-1H that covers the last 200 ka in the lower Bengal Fan. The low-resolution delta O-18 of G. sacculifer is consistent with global delta O-18 records, at least for major glacial-to-interglacial transitions. The variability of total organic carbon, total nitrogen, and the delta C-13 composition of organic matter indicate the marine origin of organic matter. Marine primary productivity likely increased during insolation minima, indicative for an enhanced NE monsoon during glacials and stadials. Pristine insolation forcing is also documented for wet-bulk density, red green color variability, and grain-size variations, indicating that darker and coarser-grained material deposited at higher sedimentation rates during insolation minima. Stronger NE monsoon likely amplified ocean-atmosphere interactions over the Indian Ocean, leading to stronger upwelling through shoaling the thermocline, and higher delivery of sediment to the Bay of Bengal due to higher soil erosion on land. In addition, lower glacial and stadial sea levels as well as stronger westward surface circulation favored delivery of coarser-grained fluvial material to the lower Bengal Fan. At the same time the stronger NE monsoon might have increased the aeolian supply. Total inorganic carbon, the Ca/Ti ratio, and biogenic silica vary dominantly on obliquity frequencies, suggesting mobilization and transport of lithogenic material primarily during lowered sea levels and/or higher influence of the Northern Hemisphere westerlies on the dust transport from the Tibetan Plateau. The close resemblance of sediment lightness and the climate record of Antarctic ice cores over multiple glacial cycles indicate close relationship between high southern latitude and tropical Asian climate through shifts in position of the Intertropical Convergence Zone. The Bengal Fan monsoonal record shows very clear and strict responses to insolation forcing in the lower part from -200 ka to the Younger Toba Tuff during Marine Isotope Stage (MIS) 7 - 5, and less distinct response patterns after deposition of the ash during MIS 4- 2, consistent with low-amplitude changes in insolation.

Changing lake water total organic carbon (TOC) concentrations are of concern for lake management because of corresponding effects on aquatic ecosystem functioning, drinking water resources and carbon cycling between land and sea. Understanding the importance of human activities on TOC changes requires knowledge of past concentrations; however, water-monitoring data are typically only available for the past few decades, if at all. Here, we present a universal model to infer past lake water TOC concentrations in northern lakes across Europe and North America that uses visible-near-infrared (VNIR) spectroscopy on lake sediments. In the orthogonal partial least-squares model, VNIR spectra of surface-sediment samples are calibrated against corresponding surface water TOC concentrations (0.5-41 mg L-1) from 345 Arctic to northern temperate lakes in Canada, Greenland, Sweden and Finland. Internal model-cross-validation resulted in a R-2 of 0.57 and a prediction error of 4.4 mg TOC L-1. First applications to lakes in southern Ontario and Scotland, which are outside of the model's geographic range, show the model accurately captures monitoring trends, and suggests that TOC dynamics during the 20th century at these sites were primarily driven by changes in atmospheric deposition. Our results demonstrate that the lake water TOC model has multiregional applications and is not biased by postdepositional diagenesis, allowing the identification of past TOC variations in northern lakes of Europe and North America over time scales of decades to millennia.

Organic matter (OM) is a key component of lake sediments, affecting carbon, nutrient, and trace metal cycling at local and global scales. Yet little is known about long-term (millennial) changes in OM composition due to the inherent chemical complexity arising from multiple OM sources and from secondary transformations. In this study we explore how the molecular composition of OM changes throughout the Holocene in two adjacent boreal lakes in central Sweden and compare molecular-level information with conventional OM variables, including total carbon, total nitrogen, C:N ratios, delta C-13, and delta N-15. To characterize the molecular OM composition, we employed a new method based on pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), which yields semiquantitative data on > 100 organic compounds of different origin and degradation status. We identify large changes in OM composition after deglaciation (circa 8500 +/- 500 B.C.), associated with early landscape development, and during the most recent 4050 years, driven by degradation processes. With molecular(-)level information we can also distinguish between natural landscape development and human catchment disturbance during the last 1700 years. Our study demonstrates that characterization of the molecular OM composition by the high-throughput PyGC/MS method is an efficient complement to conventional OM variables for identification and understanding of past OM dynamics in lake-sediment records. Holocene changes observed for pyrolytic compounds and compound classes known for having different reactivity indicate the need for further paleo-reconstruction of the molecular OM composition to better understand both past and future OM dynamics and associated environmental changes.

Understanding the response of sensitive Arctic and subarctic landscapes to climate change is essential to determine the risks of ongoing and projected climate warming. However, these responses will not be uniform in terms of timing and magnitude across the landscape because of site-specific differences in ecosystem susceptibility to climate forcing. Here we present a multi-proxy analysis of a sediment record from the 330-km2 lake Torneträsk to assess the sensitivity of the Fennoscandian subarctic landscape to climate change over the past ~ 9500 years. By comparing responses of this large-lake system to past climatic and environmental changes with those in small lakes in its catchment, we assessed when the magnitude of change was sufficient to affect an entire region rather than only specific sub-catchments that may be more sensitive to localized environmental changes such as, e.g., tree-line dynamics. Our results show three periods of regional landscape alteration with distinct change in sediment composition: i) landscape development following deglaciation and through the Holocene Thermal Maximum, ~ 9500–3400 cal yr BP; ii) increased soil erosion during the Little Ice Age (LIA); and iii) rapid change during the past century coincident with ongoing climate change. The gradual landscape development led to successive changes in the lake sediment composition over several millennia, whereas climate cooling during the late Holocene caused a rather abrupt shift occurring within ~ 100 years. However, this shift at the onset of the LIA (~ 750 cal yr BP) occurred > 2000 years later than the first indications for climate cooling recorded in small lakes in the Torneträsk catchment, suggesting that a critical ecosystem threshold was not crossed until the LIA. In contrast, the ongoing response to recent climate change was immediate, emphasizing the unprecedented scale of ongoing climate changes in subarctic Fennoscandia.

The composition of sediment organic matter (OM) exerts a strong control on biogeochemical processes in lakes, such as those involved in the fate of carbon, nutrients and trace metals. While between-lake spatial variability of OM quality is increasingly investigated, we explored in this study how the molecular composition of sediment OM varies spatially within a single lake and related this variability to physical parameters and elemental geochemistry. Surface sediment samples (0-10 cm) from 42 locations in Harsvatten - a small boreal forest lake with a complex basin morphometry - were analyzed for OM molecular composition using pyrolysis gas chromatography mass spectrometry for the contents of 23 major and trace elements and biogenic silica. We identified 162 organic compounds belonging to different biochemical classes of OM (e.g., carbohydrates, lignin and lipids). Close relationships were found between the spatial patterns of sediment OM molecular composition and elemental geochemistry. Differences in the source types of OM (i.e., terrestrial, aquatic plant and algal) were linked to the individual basin morphometries and chemical status of the lake. The variability in OM molecular composition was further driven by the degradation status of these different source pools, which appeared to be related to sedimentary physicochemical parameters (e.g., redox conditions) and to the molecular structure of the organic compounds. Given the high spatial variation in OM molecular composition within Harsvatten and its close relationship with elemental geochemistry, the potential for large spatial variability across lakes should be considered when studying biogeochemical processes in-volved in the cycling of carbon, nutrients and trace elements or when assessing lake budgets.