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Human activities have contributed to significant disruptions of the phosphorus (P) cycle on Earth’s surface. Yet, there is little information about when and how humans started to influence the global P cycle in the past. In this study, we reconstruct lake-wide P burial rates during the Holocene based on sediment-P data of 108 lakes across the globe. The results indicate the first distinct increases in lake P burial rates after the mid-late Holocene (at around 4000 years before present BP) at global scales and in Europe. Yet, different land-use histories have caused different timings of the first increases in lake P records in other regions, with ~2000 BP in China and ~550 BP in North America. We further show that global lake P-sequestration rate from ~4000 BP to 1850 Common Era (CE) has doubled compared with that in the period before 4000 BP. Since 1850 CE, the value increased ~six-fold compared with the period before 4000 BP. These findings indicate that anthropogenic activities have been affecting the global P cycle over a pre-industrial background for millennia.

The East African (hydro-)climate response to perturbations during the deglacial transition (e.g. Older and Younger Dryas) is complex and expressed heterogeneously in different paleoclimatic records. Lake Victoria (LV), Africa’s largest lake, desiccated entirely during the dry last glacial (>16.3 kyr BP) and subsequently refilled as climate conditions got more humid, reaching a highstand during the Early Holocene. However, existing sediment records from LV do not have sufficient resolution to fully resolve short-term hydroclimate changes during the deglacial transition (especially between 14 and 11 kyr BP). There is little direct evidence of late-glacial lake level fluctuations in LV so far because intermediate water depth coring sites suitable to record intermittent lowstands are missing.By analysing sediment cores along a near-shore/shallow water (current water depth 22 m) to offshore/deep water (current water depth 63 m) coring transect covering the past 16,000 years, we aim at a more accurate spatial and temporal reconstruction of LV’s deglacial lake level history in response to regional hydroclimate changes.Core stratigraphy and geochemical evidence, combined with a robust radiocarbon chronology, demonstrate a stepwise infilling of the Lake Victoria basin after its last complete desiccation (< 16.3 kyr BP). Following the dry late glacial Heinrich 1 event, an intermediate water level prevailed between 16.3 and 14.4 kyr BP, with uninterrupted deposition of fine-grained, organic matter-rich pelagic muds at our deep-water site and coarser, sandy-clay deposits at the near shore site. A second dry episode during the Older Dryas (~14 kyr BP) is marked by an abrupt decline in lake level with deposition of coarse mollusc shell bearing sediments at the near shore site indicating a littoral depositional environment. This shift in hydroclimate in the Lake Victoria basin is congruent with a brief period of cooling and drying during the Bölling/Alleröd (Dansgard Oeschger Event 1), which is also recorded in other East African Lakes. Subsequently, Lake Victoria reached maximum water levels with the onset of the African Humid Period in the early Holocene at around 11 kyr BP, which is expressed by elevated input of chemically weathered material (e.g. Rb/K) and deposition of fine-grained muds at both the near shore and offshore sites.

Pigments produced by anoxygenic phototrophic bacteria are amongst the most reliable indicators of water column anoxia preserved in sediments [1], yet conventional pigment measurement techniques are labor intensive and prohibit high-resolution analyses of these biomarkers. In this study, we assess the potential of hyperspectral imaging (HSI) core-scanning to reconstruct high-resolution variations in anoxygenic phototrophic communities using sediment cores from meromictic Lake Cadagno, Switzerland. Three different pigment groups were detected, and each pigment group is diagnostic of different phototrophic communities. Mixolimnetic aerobic primary production is recorded by chlorophyll a (and derivatives), whereas anoxygenic phototrophs are split into two groups – purple sulfur bacteria (PSB) represented by bacteriochlorophyll a, and green sulfur bacteria (GSB) represented by bacteriochlorophylls c, d, e. Spectrophotometer and high-performance liquid chromatography (HPLC) pigment analyses were used to validate the HSI-inferred pigment data. Near-continuous presence of bacteriochlorophylls confirms previously published geochemical evidence for persistent anoxic/sulfidic conditions at Lake Cadagno throughout the past 9.8 ka [2]. Furthermore, major shifts in the anoxygenic phototrophic communities appear to be related to environmental factors that affect lake stratification and light penetration at the chemocline. PSB and GSB became established after ca. 9.8 ka BP, and high abundances of both groups are inferred from 9.8-8.8 ka BP. PSB became dominant during the mid-Holocene from 8.8-3.4 ka BP, likely indicating a stable and shallow chemocline with high light penetration to the sulfidic chemocline. From 3.4-1.4 ka BP, PSB were substantially reduced and GSB became more dominant, likely due to a combination of cooler temperatures and land cover changes in the catchment that led to weakened lake stratification and more frequent turbid underflows. The high-resolution data show GSB established prior to PSB during the initial development of sulfidic conditions at 9.8 ka BP, and show that GSB tend to re-establish more quickly following mass movements and floods. This study demonstrates for the first time the capability of hyperspectral imaging to detect GSB related pigments, and shows the strong potential for high-resolution reconstruction of anoxygenic phototrophic bacteria communities using this technique.1. Sinninghe Damsté, J. S. and Schouten, S.: Biological markers for anoxia in the photic zone of the water column, in: Handbook of Environmental Chemistry, Volume 2: Reactions and Processes, Springer-Verlag, Berlin, Heidelberg, 127–163, https://doi.org/10.1007/698_2_005, 2006.2. Wirth, S. B., Gilli, A., Niemann, H., Dahl, T. W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M. F., and Anselmetti, F. S.: Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps), Geochim. Cosmochim. Acta, 120, 220–238, https://doi.org/10.1016/j.gca.2013.06.017, 2013.

Lake Victoria is the largest tropical lake on the planet. Located in East Africa at an altitude of 1135 m asl, it lies across the limits between two major climatic zones with a temperature and moisture gradient and associated tropical biomes, the rain forest, and the savanna. At higher altitudes > 1200–2500 m a.s.l. temperatures are significantly lower and vegetation forms an Afromontane belt. Primarily triggered by climate shifts, these three biomes and fire regimes have been dynamically interspersing over the last 17,000 years.Here, we present a robust 14C chronology mainly based on macroscopic charcoal using the MICADAS system of LARA at the University of Bern, new palynological data used as biostratigraphic control, and the first continuous charcoal record in Lake Victoria to establish the fire history.Our pollen and macro–charcoal records, support the assumption that throughout time regional fire dynamics are controlled by biome’s changes, and that climate was the main driver of these vegetation shifts at least until the Iron Age. Our results indicate that during the Last Glacial Maxima and Heinrich Stadial 1, under dry and colder climates the savanna was dominating, with low fire regimes before 15,000 cal yr BP and increased fire occurrence between 15,000 and 14,000 cal yr BP. After this period, the Afromontane forest started to expand, and warmer and humid climates promoted the growth of rain forests and reduced fire events, which is particularly observed in the African Humid Period (between ca. 11,500 and 5000 cal yr BP). Subsequently, our records indicate a global maximum of fire occurrence at 5000 cal yr BP, followed by unexpectedly low fire regimes during the Iron Age and the subsequent periods.This work is part of a SINERGIA project funded by the Swiss National Foundation which seeks to unravel the long-term causes and consequences of Lake Victoria’s ecosystem dynamics with a special focus on the evolution of fish species and other biotas from the late Pleistocene to the present.

Variation in molybdenum (Mo) concentration and isotope composition is an established tracer for redox changes in marine environments. Here we apply Mo as a proxy of past water column oxygenation in ancient ferruginous and hyposulfidic Lake Towuti. Lake Towuti is >1.2 Myrs old, up to 200m deep, weakly stratified and anoxic below ~100 m water depth, and surrounded by an ultramafic bedrock-dominated catchment in South Sulawesi, Indonesia. Despite the current permanent stratification, deeper water mixing and oxygenation occurred periodically in the past due to Towuti’s sensitivity to climate change. To better reconstruct the redox changes through time, we present Mo concentration and isotope data from laterite profiles overlying ultramafic bedrock, lake surface sediments, and a ~30 kyr sediment piston core from Lake Towuti, Indonesia.Although Mo concentrations in laterite profiles are overall higher compared to the underlying bedrock, the absence of a significant Mo isotopic variability with values close to the unweathered ultramafic protolith, varying from –0.16‰ to –0.04‰ δ98MoNIST3134, suggests low aqueous mobility of Mo during weathering due to the formation of laterite Fe-oxides. In contrast to the laterite samples, Mo isotopic variability in lake surface sediments show a larger variability varying from –1.15‰ to –0.13‰ δ98MoNIST3134 with a significantly lighter Mo isotopic composition in sediments deposited under oxic bottom water conditions. This light Mo isotopic composition is likely caused by early diagenetic redox cycling of Mo and Fe at the sediment-water interface. In the deeper, anoxic parts of the basin, Mo isotopic compositions show values close to the laterite input with elevated sedimentary Mo concentrations which are likely the result of an authigenic Mo enrichment from the water column. Mo isotope variability in sediments taken from a piston core in the deep part of the lake is in the range of Mo isotope compositions from modern surface sediments, varying from –0.14‰ to –0.66‰ δ98MoNIST3134. Interestingly, this variability is well correlated with local and global indicators of climate change from previous studies. Sediments deposited between ~30 kyr and ~10 kyr exhibit Mo isotope signatures similar to present-day oxygenated shallow water sites, thereby suggesting enhanced lake mixing and bottom water oxygenation under drier and colder climate conditions of the last glacial period. Sediments deposited since ~10 kyr under wetter and warmer climate conditions exhibit Mo isotope signatures similar to present-day anoxic deeper water settings. These suggest that Mo isotope compositions of lake sediments are potential quantitative indicators of past climate-driven water column oxygenation.

Varved lake sediments are recognized as valuable archives of paleoclimatic information due to their precise chronological control. However, paleoclimate reconstructions based on the composition of biochemical varves are relatively rare (Zolitschka et al., 2015). We applied novel high-resolution scanning techniques to the varved sediments of Lake Żabińskie, Poland to obtain spatially resolved geochemical data at a resolution of 60 μm covering the period 1966-2019. Relative abundances of elements were measured in resin-embedded sediment slabs using a Bruker M4 Tornado micro-XRF scanner. Chloropigments-a and bacteriopheopigments-a were measured on a wet sediment core using a Specim Hyperspectral core scanner (Butz et al., 2015). The high resolution of the scanning data, and the relatively thick well-preserved varves (average thickness = 6.4 mm), enables a close examination of seasonal scale sediment composition and varve formation processes. Time series of geochemical variables within each varve year were classified into 4 varve type groups based on the dissimilarity measure ψ for multivariate time series (Benito and Birks, 2020; Gordon and Birks, 1974). Based on a Multivariate Analysis Of Variance test, these groups of years experienced significant (p ReferencesBenito, B. M. and Birks, H. J. B.: distantia: an open‐source toolset to quantify dissimilarity between multivariate ecological time‐series, Ecography (Cop.)., 43(5), 660–667, doi:10.1111/ecog.04895, 2020.Butz, C., Grosjean, M., Fischer, D., Wunderle, S., Tylmann, W. and Rein, B.: Hyperspectral imaging spectroscopy: a promising method for the biogeochemical analysis of lake sediments, J. Appl. Remote Sens., 9(1), 096031, doi:10.1117/1.jrs.9.096031, 2015.Gordon, A. D. and Birks, H. J. B.: Numerical methods in Quaternary palaeoecology: II. Comparison of pollen diagrams, New Phytol., 73(1), 221–249, doi:10.1111/j.1469-8137.1974.tb04621.x, 1974.Zolitschka, B., Francus, P., Ojala, A. E. K. and Schimmelmann, A.: Varves in lake sediments - a review, Quat. Sci. Rev., 117, 1–41, doi:10.1016/j.quascirev.2015.03.019, 2015.

Lake Victoria (LV), Africa’s largest lake is situated in the African Great Rift Valley. Due to its shallowness (max.68 m; mean 40 m) and limited river inflow, LV is very sensitive to variations in climate and lake level fluctuations. As a result, LV has undergone repeated low stand periods, or even complete desiccation during the Late Pleistocene with profound effects on the aquatic ecosystem. One example is the emergence of a unique biodiversity of endemic cichlid species following the lake’s last desiccation event during the last glacial and subsequent refilling commencing ~15,000 years ago.In an interdisciplinary project we aim at reconstructing linkages between paleoenvironmental variability, disturbances and adaptive species radiation by combining approaches from paleogenomics, paleoecology and paleolimnology. For this purpose, four sediment cores along a depth-transect (near-shore to offshore), covering ca. the past 14,000 years, are analyzed.We present first paleolimnological results of long-term changes of using (isotope-)geochemical indicators including: Sedimentary pigments and biogenic silica to infer aquatic productivity supported by micro X-ray Fluorescence (XRF) derived element geochemistry, 13C and 15N, and sedimentary phosphorus fraction analyses providing information on changes in sediment composition.The results suggest that the infilling of the LV basin was a long-term step-wise process. This is shown by elevated and variable indicators for lithogenic input (e.g Ti, Zr and K) and interpreted as mobilization of substrate from the shorelines by a dynamic lake level prior to its stabilization in the Early and Mid-Holocene. This process is mainly reflected in the core taken at the greatest water depth (65 m). Simultaneously, the aquatic productivity (BSi and chloropigments) increased rapidly after the refilling of the lake basin in the Late-Glacial. A gradual drying of the climate and a following shift to a more oxygenated water column is observed in the Mid-to Late Holocene indicated by a decline in chemically weathered material (e.g Rb/K & K/Al ratios) and abundance of Mn.

Molybdenum (Mo) isotopes are known as sensitive recorders for changes in redox conditions because the oxidized form of Mo (Mo VI) is more soluble, whereas its reduced form is more particle reactive. Previous studies suggest that Mo isotopic fractionation during the weathering process is controlled by atmospheric input, Mo host, and bedrock composition. However, Mo isotopic variation and processes influencing fractionation in weathering profiles overlying ultramafic bedrock, the early Earth analog, have yet to be explored. This study explores for the first time (1) Mo behavior and (2) isotopic fractionation in two representative and intensely-weathered lateritic profiles overlying ultramafic bedrock of the East Sulawesi Ophiolite, Indonesia. Mo concentrations measured on samples obtained from laterite successions studied here range between 60 - 537 ppb and are overall higher compared to bedrock values ranging between 9 - 45 ppb. The Mo isotope compositions of laterite samples vary between -0.043‰ to -0.161‰ δ98MoNIST3134. The overall close to mantle Mo isotopic composition of the laterite samples, their small Mo isotope variability, and the covariation between Mo and Ti concentrations suggest low mobility of Mo during chemical weathering and laterite formation. This low Mo mobility is likely a consequence of a) the low Mo concentration of the ultramafic protolith and b) adsorption of Mo to secondary Fe-Oxides during laterite formation under oxic weathering conditions.

Global spread of hypoxia and less frequent mixing in lakes is a major growing environmental concern. Climate change and human impact are expected to increasingly deteriorate aquatic ecosystems. The study of processes and drivers of such changes in the past provides a great asset for prevention and remediation in the future. We used a multiproxy approach combining high-resolution bulk pigment data measured by hyperspectral imaging (HSI) with lower-resolution specific chlorophyll types and carotenoids measured by HPLC to examine Holocene trophic state changes and anoxia evolution in the meromictic Lake Jaczno, NE Poland. A redundancy analysis (RDA) including pollen-inferred vegetation cover, temperature and human impacts provides insight into specific conditions and drivers of changing trophic and redox states in the lake. Anoxic and sulfidic conditions were established in Lake Jaczno after initial basin infilling 9500 years ago. Until 6700 cal BP, lake trophy was relatively low, water turbidity was high and green sulfur bacteria (GSB) were abundant within the phototrophic community, suggesting a deep oxic–anoxic boundary and weak stratification. The period between 6700–500 cal BP is characterized by constantly increasing lake production and a gradual shift from GSB to purple sulfur bacteria (PSB), suggesting a shallower oxic–anoxic boundary and pronounced stratification. Yet, the presence of spheroidene and speroidenone in the sediments indicates intermittent anoxia. After 500 cal BP, increasing human impact, deforestation and intensive agriculture promoted lake eutrophication, with a shift to PSB dominance and establishment of permanent anoxia and meromixis. Our study unambiguously documents the legacy of human impact on processes determining eutrophication and anoxia.

Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgäschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe∕Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe∕Mn-P fraction comprised ∼50 % of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca–P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca–P fraction became the primary P fraction in sediments (representing ∼39 % of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgäschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones., Biogeosciences, 17 (10), ISSN:1726-4170

The climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Due to the temporally very limited instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability and its stability over time. Here, we use proxy-based reconstructions and climate model simulations to quantify changes in tropical-extratropical SH teleconnections as represented by the correlation between the ENSO and SAM indices. Reconstructions indicate mostly negative correlations back to around 1400 CE confirming the pattern seen in the instrumental record over the last few decades. An ensemble of last millennium simulations of the model CESM1 confirms this pattern with very stable ensemble mean correlations around −0.3. Individual forced simulations, the pre-industrial control run and the proxy-based reconstructions indicate intermittent periods of positive correlations and particularly strong negative correlations. The fluctuations of the ENSO-SAM correlations are not significantly related to solar nor volcanic forcing in both proxy and model data, indicating that they are driven by internal variability in the climate system. Pseudoproxy experiments indicate that the currently available proxy records are able to reproduce the tropical-extratropical teleconnection patterns back to around 1600 CE. We analyse the spatial temperature and sea level pressure patterns during periods of positive and particularly strong negative teleconnections in the CESM model. Results indicate no consistent pattern during periods where the ENSO-SAM teleconnection changes its sign. However, periods of very strong negative SH teleconnections are associated with negative temperature anomalies across large fractions of the extra-tropical Pacific and a strengthening of the Aleutian Low.

High-resolution reconstructions of climate variability that cover the past millennia are necessary to improve the understanding of natural and anthropogenic climate change across the globe. Although numerous records are available for the mid- and high-latitudes of the Northern Hemisphere, global assessments are still compromised by the scarcity of data from the Southern Hemisphere. This is particularly the case for the tropical and subtropical areas. In addition, high elevation sites in the South American Andes may provide insight into the vertical structure of climate change in the mid-troposphere. This study presents a 3000 yr-long austral summer (November to February) temperature reconstruction derived from the 210Pb- and 14C-dated organic sediments of Laguna Chepical (32°16' S, 70°30' W, 3050 m a.s.l.), a high-elevation glacial lake in the subtropical Andes of central Chile. Scanning reflectance spectroscopy in the visible light range provided the spectral index R570/R630, which reflects the clay mineral content in lake sediments. For the calibration period (AD 1901–2006), the R570/R630 data were regressed against monthly meteorological reanalysis data, showing that this proxy was strongly and significantly correlated with mean summer (NDJF) temperatures (R3 yr = −0.63, padj = 0.01). This calibration model was used to make a quantitative temperature reconstruction back to 1000 BC. The reconstruction (with a model error RMSEPboot of 0.33 °C) shows that the warmest decades of the past 3000 yr occurred during the calibration period. The 19th century (end of the Little Ice Age (LIA)) was cool. The prominent warmth reconstructed for the 18th century, which was also observed in other records from this area, seems systematic for subtropical and southern South America but remains difficult to explain. Except for this warm period, the LIA was generally characterized by cool summers. Back to AD 1400, the results from this study compare remarkably well to low altitude records from the Chilean Central Valley and southern South America. However, the reconstruction from Laguna Chepical does not show a warm Medieval Climate Anomaly during the 12–13th century, which is consistent with records from tropical South America. The Chepical record also indicates substantial cooling prior to 800 BC. This coincides with well-known regional as well as global glacier advances which have been attributed to a grand solar minimum. This study thus provides insight into the climatic drivers and temperature patterns in a region for which currently very few data are available. It also shows that since ca. AD 1400, long-term temperature patterns were generally similar at low and high altitudes in central Chile.

High-resolution records of calibrated proxy data for the past 2000 yr are fundamental to place current changes into the context of pre-industrial natural forced and unforced variability. Although the need for regional spatially explicit comprehensive reconstructions is widely recognized, the proxy data sources are still scarce, particularly for the Southern Hemisphere and South America. We provide a 1500 yr long warm season temperature record from varved Lago Plomo, a proglacial lake of the Northern Patagonian Ice field in southern Chile (46°59' S, 72°52' W, 203 m). The thickness of the bright summer sediment layer relative to the dark winter layer (measured as total brightness; % reflectance 400–730 nm) is calibrated against warm season SONDJF temperature (1900–2009; r = 0.58, p(aut) = 0.056, RE = 0.52; CE = 0.15, RMSEP = 0.28 °C; five-year triangular filtered data). In Lago Plomo, warm summer temperatures lead to enhanced glacier melt and suspended sediment transport, which results in a thicker light summer layer and to brighter sediments (% total brightness). Although Patagonia shows pronounced regional differences in decadal temperature trends and variability, the 1500 yr temperature reconstruction from Lago Plomo compares favourably with other regional/continental temperature records but also emphasizes significant regional differences for which no data and information existed so far. The reconstruction shows pronounced sub-decadal–multi-decadal variability with cold phases in the 5th, 7th and 9th centuries, during parts of the Little Ice Age chronozone (16th and 18th centuries) and in the beginning of the 20th century. The most prominent warm phase is the 19th century which is as warm as the second half of the 20th century, emphasizing a delayed recent global warming in the Southern Hemisphere. The comparison between winter precipitation and summer temperature (inter-seasonal coupling) from Lago Plomo reveals alternating phases with parallel and contrasting decadal trends of winter precipitation and summer temperature and positive and negative running correlations(winterPP;summerTT). In the 20th century the trend of this correlation changes at 1920, 1945 and 1975 AD, and the phases with positive (negative) correlations inferred from the lake sediments are also found as a regional robust pattern in reanalysis data, and coincide with the changes of the instrumental PDO index. Enhanced circumpolar flow around 60° S is proposed for positive phases of PDO which leads to the reversed coupling and contrasting decadal trends of winter precipitation and summer temperature during PDO positive phases. Our reconstruction of the inter-seasonal coupling back to 1530 AD reproduces many features of existing PDO reconstructions from the Pacific suggesting that Lago Plomo provides a record for the regional expression of the PDO in Patagonia.