Tropical cyclone frequency and infiltration changes recorded by speleothem trace metal signatures

Trace element abundances such as Mg/Ca ratios are frequently used in speleothems to support climate reconstructions or karst processes. However, signals of short-lived extreme events (e.g., droughts or strong rainfall events and tropical cyclones) are often concealed within the record of the classical stable isotope and elemental records due to dissolution and mixing processes within the soil and karst zone. On the contrary, drip water transition metal ratios and their binding affinities to particles and organic ligands, suggested faster transport in response to strong infiltration events and thus possibly reflect seasonal and/or even event-based rainfall variations (Warken et al., 2022). Here we assess replicated LA-ICPMS records of elemental abundances in two modern speleothems from Larga Cave, Puerto Rico, to explore their potential as indicators of extreme precipitation events. Speleothem Mg/Ca ratios show a broad co-variability with δ¹⁸O and δ¹³C values, and are interpreted as sensitive indicators of regional hydrology (δ¹⁸O) and local water balance (Mg/Ca, δ¹³C). Analogous to the drip water signature, we identify a common pattern of element ratios typically associated with particle and/or organic ligand fluxes (e.g., Cu/Ca, Ni/Ca, Zn/Ca, Al/Ca, Cu/Ni, Cu/Co, …), which differs from the δ¹⁸O and Mg/Ca records. In particular, we observe a similarity of the infiltration-sensitive proxies to historic Hurricane and tropical cyclone occurrence since 1850 AD. This suggests that the trace metal pattern observed in the drip water is recorded by the stalagmites, and that these elements are most promising candidates as indicators for fast infiltration changes, and possibly even tropical cyclone frequency. Furthermore, we explore a long speleothem record covering the last glacial period between 40.8 and 12.2 ka BP. In line with the modern interpretation, trace elements such as Cu/Ca or Zn/Ca indicate higher cyclone activity during the Holocene as compared to the glacial cold phase. In addition, the record reveals strong variations on the millennial scale which coincide with the onsets of warm and wet interstadial phases associated with abrupt warming events in the northern hemisphere and northward shifts of the Intertropical Convergence Zone. These observations support that mobilization and transport of organic ligands and associated elements is effectively coupled to infiltration.

Reference:

Warken, S. F., Kuchalski, L., Schröder-Ritzrau, A., Vieten, R., Schmidt, M., Hoepker, S., Hartland, A., Spötl, C., & Frank, N. (2022). The impact of seasonal and event-based infiltration on transition metals (Cu, Ni, Co) in tropical cave drip waters. Rapid Communications in Mass Spectrometry e9278. https://doi.org/10.1002/rcm.9278