Conference Agenda

Long-term Holocene temperature records at high temporal resolution extending beyond instrumental recordings are rare in the Caribbean realm. Long time series are required though, for reconstructions of past climate development, and future projections, especially in the light of ongoing increases in sea-surface temperature (SST), sea level, and major hurricane landfall frequency. We present a 5700-year-long record of SST at annual resolution based on bulk carbonate analyses of stable oxygen isotope ratios (δ¹⁸O) along a 30 m-long sediment core extracted from a marine sinkhole offshore Belize, Central America. Absolute SST-reconstruction is based on a combinded d¹⁸O and TEX₈₆ linear regression model. The d¹⁸O values show a long-term decrease in SST following the Holocene Thermal Maximum and a reversal of this trend around 3 ka BP to a long-term late Holocene increase with SST peaks during the Medieval Warm Period and the period of Industrial Warming. The most pronounced d¹⁸O-decrease is centered around the so-called 4.2 ka-event, which coincides with suppressed ENSO-activity in the eastern Pacific. The late Holocene warming trend is accompanied by a change in orbitally-controlled insolation, a southward shift of the Intertropical Convergence Zone, and increases in the intensity of ENSO. Coincidences of negative SST-excursions of the new temperature record with major tropical volcanic events during the Common Era are apparent, and underline the significance of atmospheric aerosol loading on short-term climatic cooling. The distinct temperature increase along the youngest part of the record is obviously an expression of the anthropogenic impact on SST via greenhouse gas emission.

Marine climate proxies, such as the TEX₈₆ and U^K₃₇, are well established and provide vital tools for marine paleoclimate reconstruction. In contrast, organic temperature proxies that are applicable to study past continental climate changes on a global scale are currently only sparse. The heterocyte glycolipid (HG)-based proxy HDI₂₆ (as well as other HG-based indices) provides a novel tool to access climate information that are stored in global lacustrine sediment sequences. HGs are constituents of the cell envelope of diazotrophic heterocytous cyanobacteria. The heterocyte cell envelope regulates oxygen diffusion into the heterocyte cell in response to environmental conditions. Culture studies and environmental samples have demonstrated a temperature dependency of HG distributions (Bauersachs et al., 2009; Wörmer et al., 2012; Evans et al., 2022). Yet, this proxy has only been investigated in some African and European lakes (Bauersachs et al., 2021), while its utility in studying climate change in other regions of our planet has not been tested yet. Here, we investigated HG distributions in surface sediments of 34 lakes collected along an altitudinal transect across Central America. Our study shows that HGs are ubiquitously present in Central American lakes, albeit with large variation in their distribution. This provides the exciting opportunity to investigate changes in cyanobacterial community composition. Strikingly, the HDI₂₆ showed a strong linear correlation with mean annual air temperatures in the set of lakes investigated in this study indicating that the proxy, besides being applicable in African and European lakes, allows reconstructing climate change in the northern Neotropics.

Microbial life in the Atacama Desert faces several challenges: intense UV radiation, significant diurnal temperature variations and a severe water limitation. Microbial communities developed diverse strategies of adapting to these extreme conditions. One widespread approach is the colonization of habitats within the surface crust and below. We analyzed the features of surface crusts combining sedimentological and biogeochemical approaches, characterizing the organic composition within and below the crusts. Even in close vicinity, microbial colonization and biogeochemical processes change significantly depending on the mineralogical composition of the substrate and the properties of the surface sediment. The crusts show minor differences in their lipid composition, but vary clearly in the abundance of different types of carbohydrates presumably produced as response to increased salinity and desiccation stress. Furthermore, our investigations shed new light on recent and ancient microbial activity and how it contributes to surface protection due to its relationship to and interaction with the surrounding mineral matrices. Even if those processes are slow and occur at a small scale, they sustainably contribute to the surface shaping and biotic communities (i.e. creating preconditions for higher vegetation) over geological time scales and thus may serve as analogue for the early colonization on land.

Present-day global warming challenges our ability to predict shifts in atmospheric circulation and Hadley cell dynamics. The Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma), a hyperthermal event marked by extreme global warming (~6.5°C) and a profound negative carbon isotope excursion (~3.5-7‰), serves as a critical deep-time analogue of rapid climate shifts. Typically associated with increased humidity in the northwestern Tethys, the low-latitude Central High Atlas of Morocco offers an excellent window into changes in Hadley cell extent.
To investigate Toarcian climate dynamics, detailed stratigraphic logging and high-resolution sampling (~25 cm) were performed on the 155 m-thick Toumliline section, covering the stratigraphy of the T-OAE. To reconstruct detrital input and continental weathering intensity, we integrated mass-specific magnetic susceptibility (n=553), thermomagnetic susceptibility (n=20), clay mineralogy (n=20), and major and trace element geochemistry (n=30).
Our data reveal a complex three-stage sequence of depositional conditions: (i) peak early Toarcian (Tenuicostatum Zone) chemical weathering transitioning into the (ii) more arid Toumliline carbonate ramp (latest Tenuicostatum). At the T-OAE onset (iii), physical erosion increases drastically. Although the T-OAE was likely wetter than the latest Tenuicostatum Zone (ii), it exhibits a lower Chemical Index of Alteration and decreased kaolinite content compared to the initial (i) Tenuicostatum Zone, coupled with an increased influx of magnetic and siliciclastic minerals. These rapid compositional fluctuations highlight a complex response to low latitude climate evolution across the T-OAE. Rather than simple unidirectional shifts in atmospheric circulation, our data suggest low-latitude environments experienced volatile, multi-phase regime shifts preceding and during the T-OAE.

Reconstructing the climate of the past requires archives that bear proxy information on temperature, precipitation, or atmospheric composition, for example. In this study we use contour marks as potential water level marks in mud sediments to reconstruct local evaporation rates at the Bromacker lagerstaette site in the Early Permian. The measured values indicate a daily evaporation of 1.5–3.75 mm with a median value of 2.25 mm/day. The calculated evaporation rates are compared with modern ones to estimating moisture, precipitation and seasonal climate. For evaluating reconstructed precipitation and evapotranspiration rates, values simulated by global climate models and calculated from geochemical datasets are furthermore incorporated. For the first time, quantitative data and models elucidate the climate of the Tambach Formation and Bromacker lagerstaette against the background of the pivotal transition from the Carboniferous Ice House to the Permian Hothouse climate. Our results suggest a strongly seasonal climate with wet summers and dry winters in a mountainous region based on unexpectedly low temperatures (10.9-15.0°C) considering the palaeo-location of the site in the tropical belt during the Early Permian. According to the modern vegetation model by Holdridge a moist forest biome is suggested for the Bromacker lagerstaette.

Weathering on early Earth informs on the conditions under which life evolved; only a few Paleoarchean (>3.2 Ga) regions worldwide, however, expose sufficiently well-preserved strata. We here report on proximal-to-distal compositional changes of conglomerates of the Moodies Group, one of the oldest well-preserved terrestrial-facies siliciclastic sequences. We examined conglomerates at 114 locations in six tectonostratigraphic belts of the central BGB by clast counts, geologic mapping, facies analysis and petrography. Immature, debris-flow-conglomerates eroded from the Onverwacht Anticline (OA) evolve within ca. 10-20 km depositionally downdip to texturally and compositionally supermature, matrix-free chert-pebble conglomerates in fluvial and coastal-plain facies; they also decrease in number, thickness and clast size. The present outcrop area of the source, the larger OA region, is dominated by basalts and komatiites (ca. 73%), interspersed with mostly thin cherts (ca. 6%), ferruginous cherty sediments (ca. 15%), and (sub-)volcanics (rhyo-)dacites (ca. 6%). Even the most proximal Moodies conglomerates, unconformably overlying the OA region, however, are almost exclusively composed of (rhyo-)dacites. This population decreases at ca. 10-15%/km over up to 25 km, replaced by chert clasts. We exclude hydraulic sorting, mechanical attrition, and chemical weathering in temporary storage as dominant agents; we infer that aggressive chemical weathering decomposed ultramafic rocks nearly completely in-situ and (rhyo-)dacitic clasts during transport in short drainages of low to moderate steepness and in short time. Although high precipitation, elevated temperature and a CO₂-rich atmosphere may have contributed, the widespread presence of pedogenic sulfate concretionary beds in Moodies sandstones also suggest significant silicate weathering through sulfuric-acid rain.