Speleothems are secondary cave carbonate deposits and an established terrestrial paleoclimate archive. Moreover, they have the potential to record conditions in the ecosystem overlying the cave, particularly the vegetation and soil. Coupled records of paleoclimate and -ecological information from a single speleothem would be an invaluable source of information to understand the past and future sensitivity of terrestrial ecosystems to climate change, particularly under the lens of anthropogenic forcing. In order to develop proxies that record unambiguous and quantifiable ecosystem processes, it is necessary to i) understand and characterize how processes in the soil-karst-cave system will affect geochemical tracers that may be sensitive to ecosystem conditions, and ii) develop transfer functions and frameworks that allow the interpretation of such tracers in speleothems over time periods longer than observations.

In this talk, I will discuss ways to approach these problems using different methods. I will show how combining multiple proxy measurements and forward modeling can be used to quantify the temperature sensitivity of soil respiration in the past from speleothem δ¹³C records. Finally, I will discuss the potential of speleothem organic carbon isotopes (δ¹³C and ¹⁴C) as direct tracers for surface ecosystem conditions, and the challenges and opportunities associated with these measurements.

Changes in the surface ocean pH and temperature caused by the uptake of anthropogenic CO₂ are posing a threat to calcifying marine organisms. Recent studies have observed significant impacts on coral reef ecosystems with impaired carbonate skeletal growth and decreased calcification due to acidifying oceans. The current coverage of observations for the northwestern Cuban coastal waters provides an incomplete picture of natural climate variability over interannual to interdecadal timescales, showing the need for high resolution climate archives. Cuba is situated between densely populated landmasses of North and South America offering a unique environment to study multiple aspects of anthropogenic activity across the region as well as their interconnectivity. Sub-seasonally resolved sea surface temperature (SST), δ¹⁸O of seawater, and carbonate chemistry parameters were reconstructed from a massive Siderastrea siderea coral from Cuba’s northwestern coast through a multi-proxy approach since the preindustrial era. Trace element ratios as proxies of SST indicate no significant increase in temperature over the past 160 years since 1845. Over the same time period, coral skeletal δ¹¹B ratio decreased by ca. 1.6 ‰, translating into a decrease of 0.1 on the pH scale, reflecting the acidification of the northwestern Cuban coastal waters. Furthermore, an accelerating depletion of coral δ¹³C from the 1850s to 2005 of 1.5 ‰ demonstrates the changes in seawater CO₂ with an anthropogenic imprint due to increased fossil fuel combustion. Further investigation and the comparison to trace elements indicate possible baseline shifts in regional seawater carbonate chemistry that has been affected by anthropogenic activity.

High-resolution speleothem paleoclimate records with annual to seasonal resolution are helpful for detailed analysis of climatic changes with limited duration (e.g., volcanic climate impacts), but also for longer-duration events (e.g., at 8.2 ka or 4.2 ka). A high resolution paleoclimate data set is also mandatory for meaningful comparison with archaeological or historical records.

Low stalagmite growth rates could impede the acquisition of high-resolution isotope data with the classical micro-milling approach and isotope ratio mass spectrometry (IRMS) analysis. We therefore compared secondary ionization mass spectrometry (SIMS) stable isotope measurements at 7-15 µm resolution with the lower-resolution micro-milling results at 90 µm steps. For the investigated stalagmite from Kleine Teufelshöhle (Frankonia, Germany) the SIMS resolution corresponds to annual resolution, whereas IRMS only reaches a 4-13 year resolution.

Albeit a constant offset, SIMS and IRMS δ¹⁸O data match very well (r = 0.61, p < 0.001, n = 84). This significant correspondence suggests that SIMS stable isotope analysis could be a promising alternative for high-resolution carbonate studies. An adjacent second δ¹⁸O SIMS profile reproduced high-resolution features (r=0.59, p < 0.001, n = 982) and allows a detailed assessment of paleoclimate variability on an annual scale and a comparison with independent climate records, e.g., weather information from historical documents or tree rings.

Stalagmite δ¹⁸O trends closely follow the low-frequency fluctuations of a regional tree-ring record (Land et al., 2019), additionally constraining the stalagmite chronology. The different climate sensitivities of both records enable a more robust and detailed discussion of paleoclimatic variations.

A cave monitoring sets the basis of speleothem studies. It helps to understand changing cave conditions on seasonal scale and site-specific geochemical differences such as hydrological conditions and heterogeneities in bedrock respectively. Here we present a set of parameters measured in La Vallina Cave (NW Iberian Speleothem Archive, NISA). Over the course of 16 months cave air conditions (pCO₂ and δ¹³C) were examined to understand calcite precipitation, ventilation processes and mixing of soil air and atmospheric air. From stalactitic soda straws the δ¹³C_DIC of dripwater was determined to learn about the carbon cycle in the overlaying soil and karst driven by seasonal vegetation and microbial degradation changes. We capture seasonal changes in δ¹³C_DIC independent of degassing/PCP effects. Further, the δ¹⁸O and δD composition of dripwater and hydrology help to understand water reservoir and recharge processes related to precipitation. Finally, the elemental geochemistry of dripwater was investigated by analyzing 16 elements. We present data of commonly used elements such as Mg, Sr, Ca or Ba but also elements sparsely reported in cave dripwater monitoring studies such as Y, Na or As. Controlling factors are related to bedrock leaching, particulate import and potentially redox conditions.

Here, we map a series of stalagmites from Asturias, Spain, by laser ablation inductively-coupled-plasma mass spectrometry and confocal laser scanning (fluorescence) microscopy and discuss the origins of trace element and fluorescence variations. Seasonal banding is evident with both methods and may be attributed to lignins/humic acids based on fluorescence absorption and emission characteristics.
Some lateral variations in fluorescence present as saw-tooth “spires” and demonstrate disruptions of seasonal banding, corroborated by trace element variations (most prominently Mg and Na). Such features likely reflect the differential partitioning of trace elements by sectoral zoning as a result of low supersaturation and/or high organic matter load, combined with the effects of dissolved organic matter on the calcite growth surface and the association of each element with colloidal organic matter. The lateral variability of trace elements demonstrates the pitfalls of obtaining trace element information from one-dimensional transects without prior reconnaissance mapping. It is, however, possible that traditional drilling with ~1 mm holes homogenizes these features and provides reliable trace element estimates.