Martin Grosjean

Worldwide, lacustrine ecosystems are threatened by eutrophication and deoxygenation. Both processes are driven by the combined effects of human activities and climate change. Understanding the pre‐anthropogenic nexus between warming, eutrophication, deoxygenation, and related feedback is essential to determine how lakes respond to rapid climate change. We investigate the responses of primary producers, stratification, and nutrient cycling in a small Central European lake (Übeschisee, Swiss Plateau) to rapid climate change during Late Glacial times (17–11 ka). After 16.2 ka (Heinrich Stadial; HS1) and during the Bølling/Allerød interstadial, Übeschisee experienced natural eutrophication following summer warming and vegetation development in the catchment. Anoxia events with blooms of sulfur bacteria systematically established during colder periods of HS1 (16.5 ka), the Older Dryas (14 ka), the Gerzensee Oscillation (13.2 ka), and the Younger Dryas (12.7–11.7 ka). Anoxia events also caused reductive dissolution of Mn; labile P was always efficiently recycled. Algal communities responded to anoxia events, but the excursions of community composition were reversible. Ti follows NGRIP Dust, suggesting that Übeschisee provides a continuous record of dust deposition with high rates during colder periods. The paired lake approach with adjacent Amsoldingersee shows that the results are robust and regionally reproducible, and that common drivers influenced both lakes simultaneously.

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14 690–11 700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. The primary producer community shifted from an oligotrophic cyanobacterial and low-light Pleniglacial assemblage to a stable, stratified Allerød assemblage, characterized by the planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ∼ 2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorus trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.

Reconstructing past South Asian monsoon dynamics, which governs wind and rainfall patterns across the Indian subcontinent, is crucial for understanding low-latitude climate. Constraining monsoon drivers requires separating summer and winter components in palaeoclimate archives, but this remains challenging because proxy signals usually integrate seasonal signals. The northeastern Arabian Sea provides a unique setting in which sedimentary and geochemical proxies are driven by the summer monsoon, while sea surface temperature is primarily controlled by winter monsoon winds. Here we employed mass spectrometry and hyperspectral imaging on a sediment core off Pakistan to reconstruct subdecadal changes in sea surface temperature and marine primary production during the last deglaciation (~16,000–12,000 years ago). Atmospheric humidity and vegetation changes were additionally assessed by plant-wax isotope analyses on a subset of samples. We show that summer monsoon winds were driven by the Northern Hemisphere high-latitude climate on centennial-to-millennial timescales. Winter monsoon was primarily characterized by a millennial-scale decline in wind strength driven by increasing global temperatures, with superimposed centennial-scale variability. We identified an inverse relationship between winter monsoon wind strength and the amount of winter non-monsoonal precipitation. Mechanistic insights of seasonal monsoon dynamics improved interpretation of regional palaeoprecipitation records and may enhance climate model performances in low latitudes.

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14,690–11,700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. Ecological succession progress from a pioneer community of cyanobacteria to a stable anoxic late-successional community characterized by planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ~2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorous trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14,690–11,700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. Ecological succession progress from a pioneer community of cyanobacteria to a stable anoxic late-successional community characterized by planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ~2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorous trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14,690–11,700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. Ecological succession progress from a pioneer community of cyanobacteria to a stable anoxic late-successional community characterized by planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ~2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorous trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14,690–11,700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. Ecological succession progress from a pioneer community of cyanobacteria to a stable anoxic late-successional community characterized by planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ~2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorous trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.

Monitored seabird populations have declined by up to 70% worldwide since the 1950s. Yet, data on long-term seabird population dynamics prior to the anthropogenic era are largely unknown. This limits our ability to understand future population trajectories, particularly in the Southern Ocean, where seabirds are facing multiple environmental threats. Here, we use mercury (Hg) derived from seabird guano in peatland catchments as a tracer of colony population sizes on sub-Antarctic Bird Island (South Georgia). Peat Hg flux and isotope signature results show that the first sustained seabird colonies after deglaciation were established on the island between 6800 and 6100 years ago, predating evidence for colonization on other sub-Antarctic islands by more than 1,000 y. The four subsequent periods with large local seabird populations occurred during phases of less intense Southern Hemisphere westerly winds. Our study unveils significant and repeated millennial-scale shifts in seabird abundance in response to natural climate changes, implying that the present-day increase in westerly wind intensity may lead to further declines in seabird populations in the Southern Ocean.

Predicting the trajectory of aquatic deoxygenation under global warming requires a mechanistic understanding of lacustrine responses to rapid climate shifts. We investigated how climate-driven changes in catchment vegetation and local iron-rich lithology regulated lake stratification and ecosystem resilience in the maar lake Holzmaar (Central Europe). We focused on the Late Glacial, specifically on transitions during Dansgaard-Oeschger Event 1 (DOE-1; ca. 14,690–11,700 cal yr BP), a period of rapid natural warming and cooling that serves as an analogue for future high amplitude climate variation and for modern Arctic lakes undergoing rapid climate-driven transitions. Combining non-destructive hyperspectral imaging (HSI) of sedimentary pigments with high-resolution XRF geochemistry, we resolved parts of the ecosystem trajectory during DOE-1. Ecological succession progress from a pioneer community of cyanobacteria to a stable anoxic late-successional community characterized by planktonic diatom Stephanodiscus minutulus and anoxygenic purple sulphur bacteria (PSB) in the photic zone. While regional warming (mean summer temperature increased ~2.8 °C) provided the physical potential for lake stratification, our data suggest that intense anoxia was primarily triggered by the expansion of Betula in the watershed. This afforestation stabilized the water column through wind shielding. The termination of the anoxic phase coincided with the onset of the Younger Dryas cooling and increased aridity, which effectively destabilized the existing stratification. While the shift from Betula to Pinus forest may have caused a change in the terrestrial-aquatic linkage, the primary driver of the transition was the physical forcing (lake mixing) of the climatic shift (cooling). Geochemically, the lake exhibited remarkable resilience. Unlike carbonate-dominated systems prone to internal phosphorus loading, Holzmaar efficiently sequesters nutrients via a dual mechanism of reactive iron binding (authigenic vivianite) and stable mineral burial. The phosphorous trap prevents nutrient release by permanently sequestering P in the sediment, allowing rapid ecosystem recovery without delay once the specific climate and vegetation drivers shift. Our findings demonstrate that in volcanic maar lakes, catchment vegetation characteristics and local lithology can modulate, and even override, the direct effects of climate warming on aquatic anoxia.