Daniel Birgel’s research while affiliated with Hamburg University and other places

The Great Blue Hole is a prominent flooded karst sinkhole, located in the lagoon of Lighthouse Reef atoll off the coast of Belize. Short cores recovered from varved bottom sediments have been used in previous studies as climate and cyclone archives covering as much as the past ca 1.7 ka BP. A new 30 m long sediment core, encompassing the entire Holocene and the latest Pleistocene, allows a reconstruction of the development of this geoscientifically significant site in the light of the postglacial and Holocene sea‐level rise. The sedimentary succession in the core is tripartite. The lowermost sedimentary unit A (30.0–28.6 m) comprises grey‐brown to black organic‐rich carbonate sediments, which contain freshwater snails (Pyrgophorus), tropical forest pollen (Myrtaceae) and a low‐diversity dinocyst assemblage. The intermediate unit B (28.6–24.65 m) is a dark greyish‐green to greyish‐brown, weakly laminated carbonate silt, which comprises marine fossils, that is, euryhaline foraminifera (Elphidium) and Halimeda (codiacean algal) platelets. Unit B contains abundant red mangrove (Rhizophora) pollen and a dinocyst assemblage indicating high productivity in surface waters. The uppermost unit C (24.65–0 m) is an annually layered buff and light green carbonate silt with abundant marine fossils. A total of 574 intercalated, coarser‐grained and lighter‐coloured event (storm) beds usually rich in Halimeda and coral fragments were identified in unit C. Fully marine conditions, including trophic seasonality, are also indicated by the dinocyst spectrum. The pollen spectrum derives from a variety of trees (largely pine, oak, podocarp), shrubs and herbs. The sedimentary succession represents the transition from an initial terrestrial cenote phase on a vegetated limestone island (unit A: 12.5–7.2 ka BP), via a subsequent restricted marine phase on an initially flooded carbonate platform with mangrove swamps (unit B: 7.2–5.7 ka BP), to a fully marine phase in an open, well‐circulated atoll lagoon (unit C: 5.7–0 ka BP).

Predictions of tropical cyclone (TC) frequencies are hampered by insufficient knowledge of their natural variability in the past. A 30-m-long sediment core from the Great Blue Hole, a marine sinkhole offshore Belize, provides the longest available, continuous, and annually resolved TC-frequency record. This record expands our understanding, derived from instrumental monitoring (73 years), historical documentations (173 years), and paleotempestological records (2000 years), to the past 5700 years. A total of 694 event layers were identified. They display a distinct regional trend of increasing storminess in the southwestern Caribbean, which follows an orbitally driven shift in the Intertropical Convergence Zone. Superimposed short-term variations match Holocene climate intervals and originate from solar irradiance–controlled sea-surface temperature anomalies and climate phenomena modes. A 21st-century extrapolation suggests an unprecedented increase in TC frequency, attributable to the Industrial Age warming.

The microbially mediated replacement of sulfate‐bearing evaporites by authigenic carbonate and native sulfur under anoxic conditions is poorly understood. Sulfur‐bearing carbonates from the Monte Palco ridge (Sicily) replacing Messinian gypsum were therefore studied to better characterize the involved microorganisms. The lack of (1) sedimentary bedding, (2) lamination, and (3) significant water‐column‐derived lipid biomarkers in the secondary carbonates implies replacement after gypsum deposition (epigenesis). Allochthonous clasts from the older Calcare di Base and the younger Trubi Formation within these carbonates further evidence epigenetic formation. The sulfur‐bearing carbonates are significantly ¹³ C‐depleted (δ ¹³ C as low as −51‰), identifying methane as a major carbon source. The ¹⁸ O‐enrichment of the carbonates (δ ¹⁸ O as high as 5.4‰) probably reflects precipitation from ¹⁸ O‐enriched fluids transported along adjacent faults or precipitation in a closed system with very little water. Native sulfur with variable ³⁴ S‐enrichment (δ ³⁴ S as high as 18.9‰), a relatively small maximum offset (12.3‰) between the sulfate source (gypsum) and native sulfur, and high δ ³⁴ S values of carbonate‐associated sulfate (as high as 61.1‰) suggest a high conversion to native sulfur in a (semi‐)closed system, with insignificant sulfate removal. Anaerobic methanotrophic archaea (ANME) apparently affiliated with the ANME‐1 clade mediated secondary mineral formation as evidenced by the biomarker inventory, which contains abundant ¹³ C‐depleted isoprenoids including sn3 ‐hydroxyarchaeol as the sole hydroxyarchaeol isomer and glycerol dibiphytanyl glycerol tetraethers (GDGTs). A series of various, tentatively identified ¹³ C‐depleted non‐isoprenoidal dialkyl glycerol diethers (DAGEs), 10me‐C 16 fatty acid, hydroxy C 16 fatty acids, and cyclopropyl‐C 17:0ω7,8 fatty acid agree with sulfate‐reducing bacteria participating in the anaerobic oxidation of methane. Specific conditions during gypsum replacement, unlike those at marine methane seeps, are reflected by the occurrence of ¹³ C‐depleted lipids such as lycopane, 9me‐C 17 fatty acid, and novel DAGEs. As a response to a confined environment probably characterized by high sulfate concentrations, sulfidic conditions, and elevated salinity, ANMEs and sulfate‐reducing bacteria apparently adapted their membrane compositions to cope with such stressors.

Quantitative reconstruction of paleoenvironmental and paleoclimatic changes that lack modern analogues is inherently challenging. A notable example of extreme paleoenvironmental transformation occurred in the Mediterranean Basin at the Messinian–Zanclean boundary (5.33 Ma), marked by the transition from the terminal Lago-Mare phase of the Messinian salinity crisis to the re-establishment of normal marine conditions starting with Zanclean. Despite its significance, the paleoenvironmental conditions of the Lago-Mare phase remain unresolved due to the conflicting fossil record, including both freshwater to brackish assemblages and marine microfossils. In this study, we applied a multivariate statistical approach to interpret a large dataset of terrestrial and aquatic biomarker indices from the Maccarone section in northern Italy. This analytical method was specifically tailored to address complexities associated with records originating from highly dynamic and contrasting, transitional environments (i.e., from lacustrine to marine) bringing the advantage of using statistical techniques to identify features that a traditional, visual, observation of complex array of proxies is unable to identify. Cluster analysis reveals a stepwise evolution of environmental conditions throughout the Messinian–Zanclean transition, while redundancy analysis indicates a sudden shift in the water column structure from stratified to well-mixed conditions at the Messinian–Zanclean boundary. Additionally, a gradual change is inferred in terrestrial vegetation, reflecting a progressive reconfiguration of coastal environments following the Zanclean marine transgression, with inland migration of the coastline and a decrease in wetland aquatic plant indicators.

Some of the carbon removed from Earth’s surface is stored within authigenic carbonate in marine sediments. Methane seeps are crucial sites of global marine carbon cycling sustaining microbial activity, enabling carbonate formation and the transfer of methane-derived carbon to the geosphere. Carbon sequestration rates depend on carbonate precipitation rates, which can be accelerated by mat-forming microorganisms that are ubiquitous at methane seeps and other Earth surface environments today. We investigate a 5-m-long drill core from an active methane seep at 1350 m water depth in the South China Sea with an exceptional abundance of pink and clear aragonite cement derived from the sulfate-driven anaerobic oxidation of methane, yet both cements precipitated under different conditions. Phase-specific ²³⁰Th/U-based ages, lipid biomarker compositions, and calcium isotope data suggest that pink aragonite is a product of in situ biofilm mineralization. First estimated precipitation rates of these individual cements in the seep carbonates range from 0.04 cm/ka for clear aragonite to 1.0 cm/ka for pink aragonite, suggesting an up to 25-fold increase in precipitation rates associated with biofilm mineralization. These results provide first kinetic constraints for future quantitative carbon cycle models, emphasizing the role of biofilms in accelerating carbon sequestration in marine authigenic carbonates.

The central Arabian Sea, a unique tropical basin, is profoundly impacted by monsoon wind reversal affecting its surface circulation and biogeochemistry. Phytoplankton blooms associated with high biological productivity and particle flux occur in the northern part of the central Arabian Sea due to summer-monsoon-induced open-ocean upwelling and winter convection. The core oxygen minimum zone (OMZ) at intermediate water depths is another important feature of the northern central Arabian Sea and fades southward. In this study, we attempt to interlink how these factors collectively impact phytodetrital export to the sediment. Short sediment core-top (1 cm) samples representing the recent particle flux signatures were analysed from five locations (21 to 11° N; 64° E) in the central Arabian Sea. Previously, we used core-top (0–0.5 cm) samples and observed a trend between diatom frustule abundance and diversity with bulk sedimentary parameters indicating a spatial variability in phytodetrital export to the sediment. To verify this observation further, lipid biomarkers of key phytoplankton groups and a sea surface temperature (SST) proxy have been analysed in addition to diatom frustules. The C37 alkenone-based SST proxy indicated cooler SST (27.6 ± 0.25 °C) in the north (21–15° N) mostly due to upwelling (summer) and convective mixing (winter). Warmer SSTs (+0.4 °C) are measured in the south, which usually remains nutrient-poor. This trend was consistent with satellite-derived average SST values (2017–2020). Lipid biomarker analysis suggests that dinoflagellates were likely to be the highest contributor, as indicated by dinosterol and its degradative product dinostanol, followed by brassicasterol and C37 alkenone, likely representing diatoms and coccolithophores, respectively. The north, which largely experiences periodic phytoplankton blooms and is influenced by the thick OMZ, revealed the highest contents of organic matter, diatom frustules (diversity and abundance), dominated by large, thickly silicified cells (e.g. Coscinodiscus and Rhizosolenia) and phytoplankton lipid biomarkers, as well as lower contents of zooplankton biomarkers (cholesterol and cholestanol). In contrast, relatively smaller chain-forming centric (e.g. Thalassiosira) and pennate (e.g. Pseudo-nitzschia, Nitzschia, Thalassionema) diatom frustules along with lower phytoplankton lipid biomarker contents were found in the south, where zooplankton biomarkers and silicious radiolarians were more abundant. The possible impacts of the OMZ on particle flux related to the phytoplankton community, including zooplankton grazing and other factors, have been discussed.