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  • 1. Dahl, Martin
    et al.
    Gullström, Martin
    Bernabeu, Irene
    Serrano, Oscar
    Leiva-Dueñas, Carmen
    Linderholm, Hans W.
    Asplund, Maria E.
    Björk, Mats
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Ou, Tinghai
    Svensson, J. Robin
    Andrén, Elinor
    Andrén, Thomas
    Bergman, Sanne
    Braun, Sara
    Eklöf, Anneli
    Ežerinskis, Zilvinas
    Garbaras, Andrius
    Hällberg, Petter
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Löfgren, Elin
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    Kylander, Malin E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Masqué, Pere
    Šapolaitė, Justina
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Switzerland.
    Mateo, Miguel A.
    A 2,000-Year Record of Eelgrass (Zostera marina L.) Colonization Shows Substantial Gains in Blue Carbon Storage and Nutrient Retention2024In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 38, no 3, article id e2023GB008039Article in journal (Refereed)
    Abstract [en]

    Assessing historical environmental conditions linked to habitat colonization is important for understanding long-term resilience and improving conservation and restoration efforts. Such information is lacking for the seagrass Zostera marina, an important foundation species across cold-temperate coastal areas of the Northern Hemisphere. Here, we reconstructed environmental conditions during the last 14,000 years from sediment cores in two eelgrass (Z. marina) meadows along the Swedish west coast, with the main aims to identify the time frame of seagrass colonization and describe subsequent biogeochemical changes following establishment. Based on vegetation proxies (lipid biomarkers), eelgrass colonization occurred about 2,000 years ago after geomorphological changes that resulted in a shallow, sheltered environment favoring seagrass growth. Seagrass establishment led to up to 20- and 24-fold increases in sedimentary carbon and nitrogen accumulation rates, respectively. This demonstrates the capacity of seagrasses as efficient ecosystem engineers and their role in global change mitigation and adaptation through CO2 removal, and nutrient and sediment retention. By combining regional climate projections and landscape models, we assessed potential climate change effects on seagrass growth, productivity and distribution until 2100. These predictions showed that seagrass meadows are mostly at risk from increased sedimentation and hydrodynamic changes, while the impact from sea level rise alone might be of less importance in the studied area. This study showcases the positive feedback between seagrass colonization and environmental conditions, which holds promise for successful conservation and restoration efforts aimed at supporting climate change mitigation and adaptation, and the provision of several other crucial ecosystem services.

  • 2. De Jonge, Cindy
    et al.
    Guo, Jingjing
    Hällberg, Petter
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Griepentrog, Marco
    Rifai, Hamdi
    Richter, Andreas
    Ramirez, Edson
    Zhang, Xinbao
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Peterse, Francien
    Boeckx, Pascal
    Dercon, Gerd
    The impact of soil chemistry, moisture and temperature on branched and isoprenoid GDGTs in soils: A study using six globally distributed elevation transects2024In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 187, article id 104706Article in journal (Refereed)
    Abstract [en]

    Glycerol dialkyl glycerol tetraethers (GDGTs) are microbial membrane-spanning lipids that are produced in a variety of environments. To better understand the potentially confounding effect of soil chemistry on the temperature relationship of branched GDGTs (brGDGTs), isoprenoid GDGTs (isoGDGTs) and GDGT-based proxies MBT’5ME and TEX86, soils from 6 elevation transects (mean annual air temperature 0 – 26 ℃, n = 74) were analyzed. Corroborating earlier work, the MBT’5ME index correlates well with mean annual air temperature in the low pH (pH < 7), non-arid soils under study (r = 0.87, p < 0.001). However, a clear over-estimation of reconstructed temperature in the lowest pH (<3.5) soils is observed, explained by the correlation between brGDGT Ia and free acidity. TEX86 also shows a significant correlation with mean annual air temperature (r = 0.45, p < 0.001), driven by temperature dependent concentration changes of isoGDGTs 3 and cren’. However, an overarching correlation with P/E values dominates concentration changes of all supposed Thaumarchaeotal isoGDGTs lipids (GDGT1-3, cren and cren’), implying a potential impact of soil moisture on TEX86 values. In addition to identifying the impact of these confounding factors on the temperature proxy, GDGT ratios that can be used to constrain changes in soil chemistry, specifically exchangeable Ca2+, sum of basic cations, exchangeable Fe3+ and sum of soil metals are proposed (0.53 < r2 < 0.68), while existing ratios for soil moisture availability are tested for the first time in a dataset of non-arid soils. While the impact of soil chemistry on GDGTs may complicate the interpretation of their temperature proxies, our proposed GDGT ratios can potentially be used to constrain a subset of soil chemistry changes through time.

  • 3.
    Hällberg, Lars Petter
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Paleoclimate and seasonality on Sumatra during the Late Glacial and Holocene: Insights from biomarkers and climate model simulations2024Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Deep atmospheric convection in the Indo-Pacific Warm Pool (IPWP) is a key driver of the Hadley and Walker Circulations that modulate the Asian-Australian monsoons and the El Niño Southern Oscillation (ENSO). Temperature and rainfall seasonality, i.e., the amount and timing of precipitation, impacts ecosystems, carbon content in soils and peats, and human livelihoods. Yet, past climate variability in the IPWP is poorly constrained. The Maritime Continent, located in the center of the IPWP remains a “quantification desert”, with a scarcity of terrestrial paleoenvironmental reconstructions.

    This thesis investigates the evolution of temperature, precipitation amount and seasonality over the Late Glacial (14.7-11.7 ka BP) and the Holocene (last 11.7 ka). This is achieved by combining climate model simulations and lipid biomarker analyses of terrestrial peat archives from Sumatra. Temperature and seasonality were explored by analysis of climate model simulations for the Late Glacial and Holocene. Microbial membrane-derived glycerol dialkyl glycerol tetraethers (GDGTs) were investigated as temperature and hydro-environmental proxies. Using n-alkane distributions, the abundance of algae, aquatic and terrestrial plants was reconstructed and linked to past hydroclimate variability. The hydrogen isotopic composition (dD) of the n-alkanes was then used to disentangle seasonal and annual precipitation signals.

    The analysis of Sumatran GDGTs revealed that bacterial community shifts of the GDGT producers had a strong impact on reconstructed temperatures, and that H-shaped branched GDGT isomers are good tracers of such community shifts. The branched GDGT temperature reconstruction indicates gradual warming over the Holocene, consistent with models and nearby marine records.

    Rainfall seasonality has shifted drastically over the studied time frame, in particular during the end of the Late Glacial, and between 6-4.2 ka BP. The Late Glacial climate was characterized by a much stronger seasonality, with a cold and dry Asian winter monsoon suppressing atmospheric deep convection in the region. The resulting mean state conditions resembled the atmospheric circulation and sea surface temperature patterns during extreme El Niño events in the modern climate. The Mid-Holocene (6-4.2 ka BP) was characterized by increased seasonality, with alternating droughts and heavy rains due to strong monsoon precipitation and longer dry season.

    The Early Holocene was relatively dry. Wetter conditions started around 7-6 ka BP, and peaked at 4.5-3 ka BP. This is consistent with a dD reconstruction on Sulawesi, but 1.5-2 ka later than indicated by speleothem oxygen isotopic (d18O) records on Sumatra and Sulawesi. However, the speleothem records closely follow algal dD values, interpreted here as a seasonal monsoon signal, suggesting that speleothems in the region reflect monsoonal precipitation rather than an annual signal. Rapid drying was reconstructed for the Late Holocene, starting at 3 ka BP, co-occurring with the onset of strengthened ENSO variability. The Late Holocene drying caused drying out and decomposition of peat in one of the studied cores which resulted in a hiatus of 1700 years, highlighting the importance of hydroclimate for peat and carbon accumulation in tropical wetlands.

    In conclusion, this dissertation enhances our understanding of past climatic conditions in the Maritime Continent and contributes toward constraining the evolution of temperature, precipitation, and monsoon-driven seasonality over the Late Glacial and Holocene in a region that has a scarce coverage of paleoclimate proxy information. Additionally, the methodological aspects of this thesis advance terrestrial paleoclimatological reconstructions by constraining source shifts of GDGTs and proposing a novel approach to disentangle seasonal and annual precipitation signals from dD.

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  • 4.
    Hällberg, Lars Petter
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stevens, Thomas
    Almqvist, Bjarne
    Snowball, Ian
    Wiers, Steffen
    Költringer, Chiara
    Lu, Huayu
    Zhang, Hanzhi
    Lin, Zeng
    Magnetic susceptibility parameters as proxies for desert sediment provenance2020In: Aeolian Research, ISSN 1875-9637, E-ISSN 2212-1684, Vol. 46, article id 100615Article in journal (Refereed)
    Abstract [en]

    Magnetic susceptibility in sediments has been thoroughly studied as a paleoenvironmental proxy over the last decades. However, it is unknown to what extent magnetic susceptibility variation is also a diagnostic of different sediment sources. Here we investigate if multiple magnetic susceptibility-based parameters can effectively be used as sediment source indicators. New magnetic property data from the Mu Us and Tengger Deserts in China are compared to previously known sediment provenance based on other well-established proxies. To assess the magnetic properties of these deserts, magnetic susceptibility and its out-of-phase component, its dependence on frequency, temperature and low-field amplitude are analyzed. Our results indicate similar sources for the western Mu Us Desert and the Tengger Desert and a distinct source for the eastern Mu Us, in-line with previously hypothesized provenance patterns. However, magnetic properties within the Tengger Desert sediments are homogenous, which may suggest a uniform provenance for the entire Tengger Desert, that the sediments are greatly reworked, or similar magnetic properties in potential multiple source regions. Magnetite is the major magnetic mineral in the study area and the dominant causes for divergence in magnetic properties are the magnetic mineral concentration and domain state. The results here, in particular from the Mu Us, suggest considerable promise for using magnetic susceptibility parameters in desert sediment provenance research.

  • 5. Hällberg, Petter
    et al.
    Kylander, Malin
    Villanueva, Joan
    Davtian, Nina
    Hapsari, Anggi
    Sjöström, Jenny
    Jarne-Bueno, Guillermo
    Rifai, Hamdi
    Phua, Marcus
    Smittenberg, Rienk
    Disentangling seasonal and annual precipitation signals in the tropics over the Holocene: insights from δD, alkanes and GDGTsManuscript (preprint) (Other academic)
    Abstract [en]

    Rainfall seasonality in the tropics has a substantial impact on both ecosystems and human livelihoods. Yet, reconstructions of past rainfall variability have so far generally been unable to differentiate between annual and seasonal precipitation changes. Past variations in seasonality are therefore largely unknown. Here, we disentangle hydrogen isotopic (dD) signals from terrestrial leaf waxes and algae in an 8000-year peat core from Sumatra, which reflect annually integrated versus wet season signals, respectively. We validate these results using lipid biomarkers by reconstructing vegetation dynamics via n-alkane distributions and peatland hydrological conditions using glycerol dialkyl glycerol tetraethers (GDGTs). Finally, we reconstruct biomass burning using levoglucosan concentrations in the core. Algal dD indicate stronger winter (~DJF) monsoon precipitation in the Mid-Holocene, between 8-4.2 ka BP. A period of alternating flooding, droughts and wildfires is reconstructed between 6-4.2 ka BP, implicating very strong monsoonal precipitation and drying out and burning during a longer and intensified dry season. We attribute this strong rainfall seasonality in the Mid-Holocene mainly to orbitally forced insolation seasonality, and a strenghtened Indonesian-Australian summer monsoon. In terms of annually integrated conditions, terrestrial plant dD, vegetation composition and GDGTs all indicate wetter conditions peaking between 3-4.5 ka BP, preceded by drier conditions, and followed by drastic and rapid drying in the late Holocene from around 2.8 ka BP. Our multiproxy annual precipitation reconstruction thereby indicates the wettest overall conditions approximately 1500-2000 years later than a nearby speleothem d18O record, which instead follows the dD recorded by algae in our record. We therefore hypothesize that speleothem reconstructions predominantly record the wet season isotopic signal, due to cave groundwater recharge occurring mainly after heavy precipitation. These results resolve the seasonal versus annual components of past rainfall variability in the Indo Pacific Warm Pool region, and highlight the importance of considering seasonality in rainfall reconstructions.

  • 6.
    Hällberg, Petter Lars
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Meteorological and Hydrological Institute, Sweden.
    Yamoah, Kweku A.
    Kuang, Xueyuen
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Seasonal aridity in the Indo-Pacific Warm Pool during the Late Glacial driven by El Niño-like conditions2022In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 18, no 7, p. 1655-1674Article in journal (Refereed)
    Abstract [en]

    Island South-East Asia (ISEA) is a highly humid region that hosts the world's largest tropical peat deposits. Most of this peat accumulated only relatively recently during the Holocene, suggesting that the climate was drier and/or more seasonal during earlier times. Although there is evidence for savanna expansion and drier conditions during the Last Glacial Maximum (LGM, 21 ka BP), the mechanisms behind hydroclimatic changes during the ensuing deglacial period have received much less attention and are poorly understood. Here we use CESM1 climate model simulations to investigate the key drivers behind ISEA climate at the end of the Late Glacial (14.7–11.7 ka BP), with a focus on the last stadial of the Younger Dryas (12 ka BP). We further simulate the preceding Allerød (13 ka BP) interstadial climate and perform a sensitivity experiment to disentangle the climate impacts due to orbital forcing and Late Glacial boundary conditions against a slowdown of the Atlantic Meridional Overturning Circulation (AMOC). A transient simulation (TRACE) is used to track the climate seasonality and orbitally driven change over time during the deglaciation into the Holocene. In agreement with proxy evidence, CESM1 simulates overall drier conditions during the Younger Dryas and Allerød. More importantly, ISEA experienced extreme seasonal aridity, in stark contrast to the ever-wet modern climate. We identify that the simulated drying and enhanced seasonality in the Late Glacial is mainly the result of a combination of three factors: (1) large orbital insolation difference on the Northern Hemisphere (NH) between summer and winter, in contrast to the LGM and the present day, (2) a stronger (dry) East Asian winter monsoon caused by a larger meridional thermal gradient and (3) a major reorganization of the Indo-Pacific Walker Circulation with an inverted land-sea circulation and a complete breakdown of deep convection over ISEA in NH winters. The altered atmospheric circulation, sea surface temperature and sea level pressure patterns led to conditions resembling extreme El Niño events in the modern climate and a dissolution of the Intertropical Convergence Zone (ITCZ) over the region. From these results we infer that terrestrial cooling of ISEA and at least a seasonal reversal of land-sea circulation likely played a major role in delaying tropical peat formation until at least the onset of the Holocene period. Our results also suggest that centennial to millennial shifts in AMOC strength modifies the Pacific Ocean hydroclimate via alteration of the position of the ITCZ, and a modulation of the Pacific Walker Circulation. However, Late Glacial AMOC shifts are overall less important than hydroclimate changes due to orbital forcing and boundary condition changes relative to the modern climate.

  • 7.
    Hällberg, Petter
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). University of Helsinki, Finland.
    Jarne-Bueno, Guillermo
    Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Schankat, Yolanda
    Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). University of Bayreuth, Germany.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI).
    Rifai, Hamdi
    Phua, Marcus
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, The Bolin Centre for Climate Research (together with KTH & SMHI). Swiss Federal Institute for Forest, Snow and Landscape Research, Switzerland.
    Branched GDGT source shift identification allows improved reconstruction of an 8,000-year warming trend on Sumatra2023In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 186, article id 104702Article in journal (Refereed)
    Abstract [en]

    Branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in sedimentary archives are increasingly used for paleotemperature reconstructions due to their strong correlation with mean annual air temperature. However, environmental factors can influence the brGDGT producing bacterial community, potentially affecting the brGDGT-temperature relationship and introducing fundamental errors in reconstructions. Here we assess the reliability of the methylation index of brGDGTs (MBT ' 5ME) in sediments as a paleotemperature proxy by tracking provenance differences based on brGDGT fractional abundances in a short lake core, two peat cores and surface soils on Sumatra (n = 333 in total). Then, we attempt to reconstruct the Holocene paleotemperatures on Sumatra using the two peat cores. Our results indicate distinct brGDGT and H-shaped brGDGT (H-GDGT) compositions in soil, lake and peat environments, suggesting production by different bacterial communities. We introduce a new index, the isomerization of H-GDGTs (IRH) that can distinguish between these environments. In an 11,000-year long peat core from Diatas, we find that brGDGT composition changes are dominated by bacterial community shifts rather than temperature changes. In contrast, a core from the nearby Padang peatland can be robustly used for a brGDGT-based paleotemperature reconstruction since there are no signs of past environmental or brGDGT source shifts. The results from Padang indicate a gradual warming trend over the past 8,000 years, consistent with climate model simulations and nearby sea surface temperature reconstructions. However, current MBT ' 5ME calibrations yield larger warming trends compared to simulations and other proxy studies, suggesting the need for tropical and/or peat-specific brGDGT temperature calibrations. Our findings demonstrate the importance of assessing environmental shifts and bacterial source community changes when employing brGDGT paleothermometry. The methodological framework outlined in this study can be used in future research for reliable down-core brGDGT temperature reconstructions. Our proxy reconstruction over the past 8,000 years offers novel insights into the Holocene temperature evolution from a region with low climate seasonality.

  • 8.
    Kuang, Xueyuan
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Nanjing University, China.
    Schenk, Frederik
    Stockholm University, Faculty of Science, Department of Geological Sciences. Swedish Meteorological and Hydrological Institute, Sweden.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hällberg, Petter
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Seasonal evolution differences of east Asian summer monsoon precipitation between Bølling-Allerød and younger Dryas periods2021In: Climatic Change, ISSN 0165-0009, E-ISSN 1573-1480, Vol. 165, no 1-2, article id 19Article in journal (Refereed)
    Abstract [en]

    The rapid warming trend during the last deglaciation triggered significant global climate instabilities due to a complex non-linear response of the climate system to the gradual increase in insolation over the northern hemisphere. Although climate impacts can be detected globally, major regional imprints such as seasonal evolution and changes of the East Asian Summer Monsoon (EASM) during the last deglaciation are still poorly constrained due to a lack of comprehensive proxy data. In this study, we compare how the extreme climate shifts are linked to changes in EASM precipitation over China between the unusually warm Bølling-Allerød (BA) interstadial and the following strong cooling of the Younger Dryas (YD) stadial. Our analysis is based on the transient atmosphere-ocean simulations of TraCE-21ka, in addition to new results from high-resolution simulations of the CESM1 model for the BA and YD time slices. We find that the earlier onset and stronger intensity of the EASM in the BA interstadial lead to more precipitation in early summer (May–June) but drier conditions during mid-summer (July–August) over Southern China compared to a stadial climate during the YD episode. For Northern China, we find the opposite response. The insolation change in spring and the forced response of the atmospheric system are thought to be responsible for these differences. Relative to the YD episode, the hemispheric temperature gradient during the BA period is enhanced due to the asymmetric warming between the two hemispheres, leading to an intensified northward equatorial cross flow. Combined with a stronger sensible heating of the Tibetan Plateau in spring and the related earlier northward shift of the westerly jet, the early onset of the EASM is triggered. The latent heat release, which is accompanied by the onset of the EASM and the sudden increasing precipitation over Southern China in early summer, contributes to the westward shift of the Western Pacific Subtropical High (WPSH) and eastward movement of the South Asia High (SAH) in mid-summer. Under the above conditions, Southern China experiences a hot and dry climate, while Northern China receives more precipitation. Additionally, the La Niña-like pattern of the equatorial Pacific also partly contributes to the strong EASM in the warm period by influencing the WPSH location and Pacific-North American (PNA) teleconnection pattern.

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