Sulphate concentration in cave dripwater and speleothems: Long-term trends and overview of its significance as proxy for environmental processes and climate changes

Sulphate concentrations in speleothems identify major volcanic eruptions, provide useful information on soil and aquifer dynamics and, in similar fashion to the ¹⁴C bomb peak, its Anthropocene peak can be used to date recent cave formations. However, the transmission of S from the atmosphere to cave dripwater and its incorporation in speleothems is subjected to biogeochemical cycling and accurate studies of each cave site are needed in order to assess how the S atmospheric signal is modified and eventually encoded in speleothems. This study investigates the role of biogeochemical cycling and aquifer hydrology by utilising published and new dripwater and speleothem data from Grotta di Ernesto (ER) in northern Italy. Here we provide the first long-term record of sulphate concentration in cave dripwater based on over 20 years of measurements. Fast drip site st-ER1 is characterised by a continuous decrease in SO₄ concentration from a high of 7.5 ± 0.8 mg/l in 1993-1994 to a low of 2.2 ± 0.2 mg/l in 2013-2014, and replicates with a delay of ~15 years the decline in the atmospheric SO₂ emissions. The S-series of slow flow ER78 site is further delayed by ~4.5 years in relation to the S retention in the aquifer matrix. The dripwater data are used to extend the previously published S record (1810-1998 AD) of stalagmite ER78 and reconstruct the anthropogenic S-peak: this displays a delay of ~20 years with respect to the atmospheric S emission peak due to biogeochemical cycling and aquifer storage. However, sulphur recycling above the cave did not operate with the same degree of efficiency through time, which resulted in a variable time delay between S deposition and incorporation into the stalagmite. In the pre-Anthropocene era, and in particular during the cold Little Ice Age, biogeochemical cycling was far less efficient than today, and the fast transmission of the atmospheric signal allowed capture of S released during major volcanic eruptions by stalagmites.