Search

Paleoclimate data provide important information about the character of natural climate variability. However, records with sufficient length and resolution to resolve high‐frequency (decadal‐scale) variability across the Holocene are scarce. We present a 10,800‐year reconstruction of spring and summer temperature at three‐year resolution based on biochemical varves from Lake Żabińskie, Poland. The reconstruction is based on Ca/Ti ratio, which are significantly correlated with instrumental spring and summer temperature spanning 240 years. Major climate events of the Holocene period are represented in the reconstruction, including the Holocene Thermal Maximum, 8.2 ka Event, Medieval Climate Anomaly, and Little Ice Age. A low‐frequency 8,000‐year decreasing trend in warm‐season temperatures is driven by declining summer insolation. Temperature variability is highest during the early Holocene, likely related to warmer and drier conditions. The rate of warming during the past 90 years is extremely unusual, if not unprecedented for the Holocene, based on our reconstruction. Plain Language Summary: Studying past climate change helps us understand how the climate system works. One aspect of past climate that is not well understood is how much temperatures changed from year to year and over decades during warm periods of Earth's history. In this study, we measured chemical properties of lake sediments from Poland and found that chemical changes were related to spring and summer temperature changes. We used this data to estimate spring and summer temperatures during the past 10,800 years. Relatively warm summers occurred around 9,000–7,500 years ago, a period known from other studies to be relatively warm in the northern hemisphere. We also found greater variability in temperatures during this warm period. Our long record of spring and summer temperatures shows that the warming rate of the past 90 years is most likely faster than any time during the past 10,800 years, showing that human‐caused global warming is more extreme than natural climate variations. Key Points: 10,800‐year‐long, 3‐year‐resolution, warm‐season temperature reconstruction from biochemical varves from Lake Żabińskie, PolandPeak summer temperatures occurred during the early Holocene with declining summer insolation driving cooling during the past 8,000 yearsThe rate of modern warming is highly unusual, at least 4 standard deviations above the mean temperature trend of the past 10,800 years [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

Varved lake sediments are exceptional archives of paleoclimatic information due to their precise chronological control and annual resolution. However, quantitative paleoclimate reconstructions based on the biogeochemical composition of biochemical varves are extremely rare, mainly because the climate–proxy relationships are complex and obtaining biogeochemical proxy data at very high (annual) resolution is difficult. Recent developments in high-resolution hyperspectral imaging (HSI) of sedimentary pigment biomarkers combined with micro X-ray fluorescence (µ XRF) elemental mapping make it possible to measure the structure and composition of varves at unprecedented resolution. This provides opportunities to explore seasonal climate signals preserved in biochemical varves and, thus, assess the potential for annual-resolution climate reconstruction from biochemical varves. Here, we present a geochemical dataset including HSI-inferred sedimentary pigments and µ XRF-inferred elements at very high spatial resolution (60 µ m, i.e. >100 data points per varve year) in varved sediments of Lake Żabińskie, Poland, over the period 1966–2019 CE. We compare these data with local meteorological observations to explore and quantify how changing seasonal meteorological conditions influenced sediment composition and varve formation processes. Based on the dissimilarity of within-varve multivariate geochemical time series, we classified varves into four types. Multivariate analysis of variance shows that these four varve types were formed in years with significantly different seasonal meteorological conditions. Generalized additive models (GAMs) were used to infer seasonal climate conditions based on sedimentary variables. Spring and summer (MAMJJA) temperatures were predicted using Ti and total C (Radj2=0.55 ; cross-validated root mean square error (CV-RMSE) = 0.7 ∘ C, 14.4 %). Windy days from March to December (mean daily wind speed >7 m s -1) were predicted using mass accumulation rate (MAR) and Si (Radj2=0.48 ; CV-RMSE = 19.0 %). This study demonstrates that high-resolution scanning techniques are promising tools to improve our understanding of varve formation processes and climate–proxy relationships in biochemical varves. This knowledge is the basis for quantitative high-resolution paleoclimate reconstructions, and here we provide examples of calibration and validation of annual-resolution seasonal weather inference from varve biogeochemical data. [ABSTRACT FROM AUTHOR]

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 Żabińskie, 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. Unlabelled Image • Aquatic productivity and anoxia were inferred from a 10,800-year sediment core. • Sedimentary pigments were quantified using hyperspectral imaging (HSI). • Anoxia was persistent during the Holocene prior to human-mediated deforestation. • HSI on varves enables sub-annual reconstruction of past productivity and anoxia. [ABSTRACT FROM AUTHOR]