Abstract
Some of the most relevant worldwide hypogenic gypsum speleothems are the huge selenite crystals recently discovered at different levels in the caves of the Naica mine (Chihuahua, México). These megacrystals, which grown in underwater thermal conditions, reach up to 11 metres in length in the famous Crystals cave and show euhedral and prismatic shape. This work focuses on trace element patterns and hydrogen isotopic signal in the speleothems of Crystals cave and Swords cave and correlates them with global palaeoclimatic records.
At –290 level in the Crystals cave, Sr/Ca ratio along speleothem growth axis changes considerably in the period dated back between 158 and 34.5 kyrs BP (U/Th dating). This fact suggests variations in the physical and chemical properties of the groundwater, such as temperature, related to changes in the meteoric water input. In fact the partition coefficient of Sr2+ in gypsum is mainly affected by changing in composition and temperature of the solution. On the upper part of the mine, at 120 metres deep, the complex speleothems of Swords cave have been studied. They consist of a large high-purity selenite core covered by different thinner alternating layers of aragonite-gypsum and finally by calcite. The trace element concentration in the outer gypsum layers indicates a decrease in groundwater temperature respected to the inner core during the speleothem formation (57 - 7.8 kyrs BP). Furthermore, the hydrogen isotopic analysis of the different gypsum layers suggests contributions of isotopically lighter water during a cold period (? Last Glaciation) corresponding to a time span of the selenite core growth. However, the outer gypsum layers were formed from isotopically heavier water during a warmer period (Earlier Holocene). All these evidences suggest climate control on the hydrodynamics of the old Naica thermal groundwater since the achieved palaeogroundwater record is consistent with other large-scale palaeoclimatic proxies.
