The lithium isotopic composition (δ⁷Li) of marine carbonates is known as a proxy for the chemical weathering intensity of silicate rocks. To evaluate the role of weathering as a sink for atmospheric CO₂ during the Late Cretaceous, we generated a 20 Ma δ⁷Li record (86.3–66.0 Ma) using chalk from Northern Germany as an archive. The late Santonian to Maastrichtian record shows an overall increase of ~4.5‰ with superimposed increases in the late Santonian, early Campanian and across the Campanian–Maastrichtian transition.

The overall increase in δ⁷Li fits with the Late Cretaceous increase in seawater ⁸⁷Sr/⁸⁶Sr. Further, the δ⁷Li record strongly resembles both the evolution of deep-sea temperatures based on benthic oxygen isotopes and modeled changes in the rate of seafloor spreading. Thereby, increases of δ⁷Li in the early Campanian and across the Campanian–Maastrichtian transition correspond to major sea level falls, and the latter with a negative δ¹³C excursion.

These coherences allow three interpretative approaches: (1) Climate cooling controlled the weathering congruency and the magnitude of Li isotope fractionation by clay mineral formation on the continents. (2) Lowland exposure promoted erosion together with enhanced soil and clay formation after sea level falls. (3) A temperature-dependent fractionation during low-temperature basalt alteration affected the seawater δ⁷Li signal in a time, when more fresh oceanic basalts where available due to higher spreading rates. The best possible interpretation for the Late Cretaceous δ⁷Li record and the often-neglected aspects of basalt alteration and reverse weathering need to be further assessed and discussed.

The evolution of the seawater Lithium isotope composition (δ⁷Li_SW) depends on the prevalent weathering regime. To reconstruct δ⁷Li_SW, and thus, weathering congruency, two main archives are widely used: calcareous fossils and bulk carbonate sediments. Either of these archives has its strong and weak points. While carbonate sediments are prone to diagenesis and clay contamination, biogenic skeletons may exhibit strong vital effects on δ⁷Li_SW. Here, we present δ⁷Li_SW data for the mid-Cretaceous derived from various biogenic carbonates such as bivalves (mainly rudists), cephalopods, and brachiopods from well-known sample sites across Europe.

Additionally, we assessed the extent of taxon-specific vital effects on δ⁷Li for well-preserved Albian molluscs from Madagascar (Mahajanga Basin, age 110.5 +/-0.5Ma) and compared them with data for modern taxonomic groups.

Our most complete data set derives from rudist shells (mainly requieniid rudists) from the mid-Barremian to early Aptian sample locations of Sausset (Urgonian Limestone Formation, France), Ericeira (Crismina Formation, Portugal), Kanfanar (Kanfanar unit, Croatia), and Miravete (Villarova de los Pinares Formation, Spain). We compare their δ⁷Li compositions and trends therein with that of cotemporaneous carbonate-rich sediments. We provide preliminary δ⁷Li_SW for several time intervals which range between 22-25‰ on average with values exceeding 30‰ in the late Barremian.

Relative to published bulk carbonate data, we observe a systematic offset for rudist-shell δ⁷Li values of 3-8‰ towards heavier values. We assign this offset to vital effects similar to those known for modern calcitic molluscs.

Understanding modern species´ ecological preferences and distribution are prerequisites for their application as proxies in paleoenvironmental reconstructions. This knowledge is, however, missing for the majority of tropical species. In order to contribute to close this gap we characterize the ostracod fauna of Lago Enriquillo in the Dominican Republic. Located in the Main Development Region of North Atlantic tropical storms it has experienced rapid water level fluctuations in the past that are assumed to be related to storm activities. This highlights the relevance of the lake for reconstructions of changes in precipitation and storm activity. Living ostracods inhabiting Lago Enriquillo were analyzed in March and September (dry and rainy season) 2022 with the aim to understand their temporal and spatial distribution and changes in morphological characteristics (valve size). Lago Enriquillo is a warm (27.8-33.8°C), mesohaline (43.7-46.3 psu) and generally slightly alkaline lake (pH: 7.5-8.1) with a max. water depth of 25 m. The spatially variable ostracod fauna is composed of Cyprideis similis, Perissocytheridea cribrosa and Thalassocypria sarbui. The three species reflect the lake’s seasonality differently in terms of their morphology and population structure. C. similis occurred continuously but is smaller in spring than in autumn. P. cribrosa inhabits the lake only in autumn and shows a distinct distribution dominating littoral areas with seasonal freshwater inflow. T. sarbui forms larger populations only in spring and forms only tentative size clusters. Their occurrence is restricted to water depths above (temporary) minima of oxygen (below 10 – 14 m).

Driving factors of shell growth of sub-fossil Glycymeris nummaria shells deposited by high energy events into layers at the Atlantic coast of the Iberian Peninsula were investigated by stable isotope analysis and analysing their growth patterns.

High energy events deposited layers of cockles along the coast of Cádiz Bay during the Roman Period (313 ± 114 AD) and the Dark Ages (648 ± 108 AD) (Gutiérrez-Mas, 2011). The climate fluctuation of the region between these two periods was largely investigated by analysing sediment cores from the pro-deltas on the Iberian margin (e.g. Bartels-Jónsdóttir et al., 2015), Glycymeris shells, however can provide seasonal data of temperature change and the seasonality in chemical characteristics of the marine ecosystems. Comparing the paleotemperature data reconstructed from oxygen isotope ratios of seasonally sampled shell carbonate indicates no significant cooling for the ‘Dark Ages Cold Period’ (DACP) for the Cádiz Bay. The seasonality of carbon isotope ratios however showed distinct changes between the Roman Period and the DACP. Seasonal maxima of δ¹³C values were twice as higher during the DACP which indicates seasonally intensified biological fractionation connected to enhanced primary productivity. This agrees with other studies implying intensified coastal upwelling during the DACP along the Iberian Peninsula, while they observe no significant cooling of the sea surface (Bartels-Jónsdottir et al. 2015). This case study adds to the growing number of evidence that the DACP can not be interpreted as a Europe-wide cooling event as it affected southern regions differently.

The triple oxygen isotope composition (δ¹⁸O and ∆’¹⁷O) of atmospheric CO₂ provides valuable information about CO₂ sources and carbon exchange fluxes between atmospheric reservoirs [1-2]. For example, stratospheric CO₂ has a large positive ¹⁷O-anomaly due to photochemical processes. The ∆’¹⁷O of tropospheric CO₂ is primarily influenced by carbonic anhydrase-catalyzed oxygen isotope exchange between air CO₂ and water in vegetation, resulting in an air CO₂ ∆’¹⁷O mainly governed by the isotope composition of local meteoric water. Additionally, CO₂ from fossil fuel combustion processes shows a negative ¹⁷O-anomaly inherited from the isotope composition of atmospheric O₂ [3].

Starting in April 2023, we conducted automated triple oxygen isotope measurements of air CO₂ in Göttingen using tunable infrared laser direct absorption spectroscopy (TILDAS; Aerodyne, USA) coupled with a custom-built inlet system. The internal error of the ∆’¹⁷O measurements was < 10 ppm and of the δ¹⁸Omeasurements < 0.01‰.

In this report, we present our initial results and discuss the contribution of various reservoirs to the ∆’¹⁷O of atmospheric CO₂ in Göttingen. We also examine daily and seasonal variations observed in our data.

[1] G. Koren et al., J. Geophys. Res. Atmos. 124, 8808–8836 (2019).

[2] M. E. G. Hofmann et al., Geochim. Cosmochim. Acta. 199, 143–163 (2017).

[3] B. Horváth, M. E. G. Hofmann, A. Pack, Geochim. Cosmochim. Acta. 95, 160–168 (2012).