Paleohydrology of the Upper Miocene Bicorb Lake (eastern Spain) as inferred from stable isotopic data from inorganic carbonates

摘要

The hydrological evolution of the Upper Miocene Bicorb lake (eastern Spain) has been established based on sedimentology, carbonate mineralogy and stable isotopic data from inorganic carbonates from the upper lacustrine unit. Three main subunits have been recognized. In subunit A, the sedimentological features suggest a hydrologically closed lake although the isotopic data are not conclusive. The isotopic compositions of primary carbonates from subunit B display a distribution typical for a hydrologically closed lake, and allow the differentiation of four developmental stages. The first stage was defined by the deposition of gypsum, LMC, dolomite and aragonite, indicating large variations in the water Mg/Ca ratio. During the second and third stages only LMC was deposited suggesting only minor Mg/Ca fluctuations. The δ13C and δ18O pattern of the second stage shows a well-defined covariance (r = 0.8), while in the third stage the isotopic data display a negative covariant relationship (r = -0.8). Changes in dissolved inorganic carbon composition are considered the main factor to explain the variations from positive to negative covariance. In the fourth stage the lake waters underwent large Mg/Ca fluctuations, as indicated by the deposition of LMC, HMC and aragonite. The isotopic trend is also covariant. Finally, subunit C is characterized by LMC and minor amounts of HMC, suggesting minor changes in Mg/Ca ratio. The isotopic features of this subunit are more congruent with a hydrologically open lake. The lowest δ13C and δ18O values are very similar in the three subunits, reflecting the most dilute conditions of the lake water, and a uniform isotopic composition of the local meteoric water during these periods. The changes in water balance and solute inputs recorded in the mineralogy and isotopic signatures of carbonates were controlled by the diapiric evolution and changes in the E/P balance.