Physical weathering of carbonate host-rock by precipitation of soluble salts in caves: A case study in El Orón-Arco Cave (Region of Murcia, SE Spain)

Abstract

The dissolution of carbonate host-rock by freshwater in phreatic or vadose conditions is the most common mechanism for the formation of caves; however, circulation of saline solutions through carbonate materials and precipitation of soluble salts may also play an important role. We studied the stable isotope composition ( 18 O and  34 S of sulfate,  18 O and D of structurally-bound gypsum hydration water and 87 Sr/ 86 Sr) and salinity of fluid inclusions in gypsum speleothems found in El Orón-Arco Cave (Cartagena, SE Spain). We suggest that physical weathering of carbonate host-rock was driven by precipitation of soluble sea-salts (mostly gypsum and halite), and this process controlled the recent geomorphological evolution of the cave. The Triassic carbonate host-rock shows clear evidence for salt weathering, including gypsum/halite infillings in cracks of the bedrock, mechanical spalling of the carbonate, and detachment of rock fragments that lead to the formation cave voids and in-situ accumulations of piles of unsorted rubble. Sulfur and oxygen isotopes of gypsum sulfate (3.0‰< 18 O<11.6‰ and 16.7‰< S<20.7‰) are generally lower than modern seawater sulfate and suggest contributions from a 34 S-depleted source (i.e. oxidation of pyrite). The  18 O and D of gypsum hydration water are relatively low compared to expected values for the evaporation of pure seawater to gypsum saturation, suggesting that gypsum precipitation involved a secondary calciumsulfate source or recycling of gypsum from previous stages, along with mixing of seawater and meteoric water seepage to the cave. The 87 Sr/ 86 Sr in gypsum shows intermediate values between modern seawater and Triassic carbonate values because of interaction between the solution and the bedrock. The salinities of the speleothem-forming solutions are relatively high (13.2±3.2 wt. % eq. NaCl) compared to gypsum formed from evaporated brackish solutions (i.e. ~4-8 wt. % eq. NaCl) and indicate dissolution of earlier evaporites before secondary gypsum precipitation. This cave-forming mechanism, which is related to saline water circulation and precipitation of evaporitic minerals, may be common in other coastal caves.

The potential of gypsum speleothems for paleoclimatology: application to the Iberian Roman Humid Period

“…Gypsum speleothem formation is mostly controlled by evaporation, unlike carbonate speleothems that largely form because of CO 2 degassing. Although the occurrence of subaerial gypsum speleothems is limited compared to carbonate speleothems 3 , gypsum stalactites, stalagmites, flowstones, coralloids and frostwork can be found in a number of caves in arid and semiarid regions 5 – 7 .…”