Seagrass meadows are vital blue carbon habitats, with sedimentary organic carbon (OC) originating from both the seagrass itself and external sources. In this study, lipid biomarkers (n-alkanes), a well-known proxy for tracing OC sources, were used to indicate seagrass presence in sediment records and to correlate with sedimentary OC in cold-temperate seagrass (Zostera marina) sediments. We calculated a Zostera-ratio (seagrass/algae and terrestrial plants-ratio) using identified seagrass biomass n-alkanes (C15, C17, C19, C21, C23) as a fingerprint for seagrass-derived OC. Based on the presence or absence of seagrass plant remains in sediments, we confirmed an overall significant positive correlation (R2 = 0.49, with significant sites ranging from 0.66 to 0.81; p < 0.001) between the Zostera-ratio and OC in sediment profiles down to 2 m depth. The Zostera-ratio ranged from 0.0006 to 0.35 with higher values indicating seagrass plant material. The findings show that n-alkanes can serve as proxies for both seagrass presence and total OC levels in the sediment.

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.

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 (δD) signals from terrestrial leaf waxes and algae in an 8000-year peat core from Sumatra, which reflect annual versus wet season rainfall 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), as well as biomass burning using levoglucosan concentrations in the core. Finally, we compare our proxy results to a transient climate model simulation (MPI-ESM1.2) to identify the mechanism for seasonality changes. We find that algal δD indicates stronger Indonesian-Australian Summer Monsoon (IASM) precipitation in the Mid-Holocene, between 8 and 4.2 cal ka BP. A period of alternating flooding, droughts and wildfires is reconstructed between 6 and 4.2 cal 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 strengthened IASM, consistent with the modeling results. In terms of annual rainfall, terrestrial plant δD, vegetation composition and GDGTs all indicate wetter conditions peaking between 3 and 4.5 cal ka BP, preceded by drier conditions, followed by drastic and rapid drying in the late Holocene from around 2.8 cal ka BP. Our multiproxy annual precipitation reconstruction thereby indicates the wettest overall conditions approximately 1500–2000 years later than a nearby speleothem δ¹⁸O record, which instead follows the seasonally biased algal δD in our record. We, therefore, hypothesize that speleothem reconstructions over the Holocene in parts of the tropics with low but significant seasonality may carry a stronger seasonal component than previously suggested. The data presented here contribute with new insights on how isotopic rainfall proxies in the tropics can be interpreted. Our findings resolve the seasonal versus annual components of Holocene rainfall variability in the Indo-Pacific Warm Pool region, highlighting the importance of considering seasonality in rainfall reconstructions.

Ratios of glycerol dialkyl glycerol tetraethers (GDGT), which are membrane lipids of bacteria and archaea, are at the base of several paleoenvironmental proxies. They are frequently applied to soils as well as lake- and marine sediments to generate records of past temperature and soil pH. To derive meaningful environmental information from these reconstructions, high analytical reproducibility is required. Based on submitted results by 39 laboratories from across the world, which employ a diverse range of analytical and quantification methods, we explored the reproducibility of brGDGT-based proxies (MBT′5ME, IR, and #ringstetra) measured on four soil samples and four soil lipid extracts. Correct identification and integration of 5- and 6-methyl brGDGTs is a prerequisite for the robust calculation of proxy values, but this can be challenging as indicated by the large inter-interlaboratory variation. The exclusion of statistical outliers improves the reproducibility, where the remaining uncertainty translates into a temperature offset from median proxy values of 0.3–0.9°C and a pH offset of 0.05–0.3. There is no apparent systematic impact of the extraction method and sample preparation steps on the brGDGT ratios. Although reported GDGT concentrations are generally consistent within laboratories, they vary greatly between laboratories. This large variability in brGDGT quantification may relate to variations in ionization efficiency or specific mass spectrometer settings possibly impacting the response of brGDGTs masses relative to that of the internal standard used. While ratio values of GDGT are generally comparable, quantities can currently not be compared between laboratories.

Large uncertainties still exist about the long-term mechanisms influencing the hydroclimate variability of southeast Africa where proxy data and model simulations indicate rainfall dipoles between subtropical and tropical areas. The topography of Madagascar, located off the southeastern coast of Africa, modulates these dipoles while its climate is influenced by the position of the Intertropical Convergence Zone (ITCZ) and the Subtropical High as well as the sea surface temperature (SST) of SW Indian Ocean. The island can thus be considered a key location for the understanding of the tropical SE African climatic variability and the interplay between atmospheric patterns. However, the scarcity of continuous records from Madagascar has made the evolution of regional late Quaternary climate and its driving mechanisms difficult to assess. Here, we present a 26-kyr record of the deuterium/hydrogen isotope ratio (δD) of biomarkers (n-alkanes) from the central eastern part of the island at Antananarivo at around 1250 m a.s.l. Preliminary summary pollen data are also presented as a comparison. The δD profiles of aquatic plant and terrestrial plant-derived n-alkanes generally exhibit similar trends implying that they all record changes in the isotope composition of source water, namely meteoric water that recharges soil and lake waters. In this tropical region, the δD variability of precipitation is mainly influenced by the amount effect reflecting the intensity of precipitation associated with the monsoon. We observe: (i) stable and wet conditions during the Last Glacial Maximum, (ii) drier conditions from 18.5 to 15 ka (ka before present) during the Heinrich Stadial 1 (iii) high humidity after 15 ka culminating at the Younger Dryas (YD), (iv) drier conditions from 11.7 ka to 8.2 ka, (v) a return to humid climate until 2.8 ka, and (vi) an arid phase followed by increased wetness after 0.9 ka, although the record is likely influenced by human-induced vegetation changes the last 1.2 ka. This climate signal is similar to other records from the Mozambique Channel but opposite to records from the East African mainland and the subtropical southern Africa, especially between 20 and 25°S. Although there is a good correspondence of our record with insolation- driven migrations of ITCZ during the LGM and the early Holocene, the dipoles are largely consistent with the modern rainfall anomaly and are best explained by the interlinked effects of the SST changes and the variability of the Mozambique Channel Trough.

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.

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.

The Holocene climate history of Southern Africa remains inconclusive despite the increasing number of proxy records from the region. This might be related to the diversity of proxy records, how the proxies are interpreted, or that proxies may respond to more than one forcing (e.g. hydroclimate, fire, temperature.). Here, a 175-cm peat sequence from Free State, South Africa (28°17′53″S, 29°25′10.9″E), was analyzed using a comprehensive set of novel and conventional proxies, including isotopic (δ¹³C), elemental (CS-XRF), mineral (pXRD), molecular (FTIR-ATR and pyrolysis-GC-MS), grain size (Malvern 3000) and GSSC phytolith composition. The chronology was constructed through AMS radiocarbon dating (n = 7). The early Holocene (10,380–7000 cal yr BP) was characterized by an initial wet phase, followed by relative dryness, at least seasonally, evidenced by slow accumulation rates, low organic content and dominance of terrestrial vegetation in the organic matter matrix. From 7000 cal yr BP, decreasing temperatures, as evidenced in regional climate reconstructions, were associated at Marias Geluk with higher biogenic silica and organic matter content and an increase of moisture-adapted grasses, indicating increasingly mesic conditions. This trend was amplified after 6000 cal yr BP, co-occurring with a southward displacement of the ITCZ. Complex proxy dynamics were observed between 4300 and 2180 cal yr BP, with bulk organic proxies indicating a drier environment (lower carbon content, slow accumulation rates, enriched δ13C values) but the phytolith record pointing towards relatively mesic conditions. The period was also associated with increased fire frequency, that also reached the local wetland. We suggest that the period was associated with seasonally mesic conditions together with increased fire incidence, which affected some of the organic proxies. Increased fire activity was also recorded in the region, while the hydroclimatic indications differed. The last 2000 years, during which human activity is known to have increased in the region, was characterized by lower fire incidence and variable, but relatively moist, conditions. The hydroclimatic inferences for the last 2000 years are in line with previous studies from the region, but additional studies are needed to decipher if the decline in fire incidence was associated to climate forcings, human activities, or a combination of both. The multiproxy approach applied here - in particular the inclusion of FTIR-ATR and pyrolysis GC-MS - revealed a complex interplay between vegetation dynamics, hydrology and paleofire variability. This study confirms that relatively small Holocene temperature variations (compared to northern higher latitudes) were associated with major hydrological variability at Marais Geluk, and reinforces concerns from earlier studies that the hydroclimate of the region is vulnerable to climate change. The result s also show that the southward displacement of the ITCZ, and associated tropical air masses, likely had significant effects on regional hydrology and fire incidence.

Energy from serpentinization processes potentially fuelled the origin of life on Earth. Thus, if serpentinite-impacted sites facilitate microbial habitability, it is important to understand the source and retention of biological signatures in them. Investigating biological signatures in terrestrial serpentinite-impacted environments is also essential for interpreting molecular signature preservation on extraterrestrial bodies. To expand knowledge on the types of biological signatures derived from microorganisms in the serpentinization microbiome, mass spectrometry analysis was performed on Chimaera serpentinite rocks from Antalya Province, Turkey. The presence of diverse types of core and intact glycerol alkyl diether and tetraether lipids in all samples indicates that methanogenic archaea were present, but not in equal abundance in all rock types (carbonate or brucite) or locations (inside and outside the rock). Bacterial derived brGDGTs (branched glycerol dialkyl glycerol tetraethers) and phospholipid fatty acids were also identified in all samples. Comparing targeted lipid data from this study with other studies and locations reveals consistency across the serpentinization lipidome. Additionally, an untargeted lipidomics method was tested on the intact polar lipid (IPL) extracts, This resulted in detection of a wide range of intact lipids, many requiring further elucidation (e.g., diacyl glycerol IPLs, triacylglycerols, glycosphingolipids, sterol esters and cardiolipins), suggesting that there may be diverse bacterial, archaeal and fungal communities in the different rock samples, highlighting the additional complexity of the serpentinization lipidome dark matter.

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.