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

The hydrological evolution of the Upper Miocene Bicorb lake (eastern Spain) has been established based on sedimentology, carbonate mineralogy and stab le isotopic data from inorganic carbonates from the upper lacustrine unit. Three main subunits have been recognized. In subunit A, the sedimentologica l features suggest a hydrologically closed lake although the isotopic data are not conclusive. The isotopic compositions of primary carbonates from su bunit B display a distribution typical for a hydrologically closed lake, an d allow the differentiation of four developmental stages. The first stage w as defined by the deposition of gypsum, LMC, dolomite and aragonite, indica ting large variations in the water Mg/Ca ratio. During the second and third stages only LMC was deposited suggesting only minor Mg/Ca fluctuations. Th e delta(13)C and delta(18)O pattern of the second stage shows a well-define d covariance (r = 0.8), while in the third stage the isotopic data display a negative covariant relationship (r = -0.8). Changes in dissolved inorgani c carbon composition are considered the main factor to explain the variatio ns from positive to negative covariance. In the fourth stage the lake water s 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 chang es in Mg/Ca ratio. The isotopic features of this subunit are more congruent with a hydrologically open lake. The lowest delta(13)C and delta(18)O valu es are very similar in the three subunits, reflecting the most dilute condi tions of the lake water, and a uniform isotopic composition of the local me teoric water during these periods. The changes in water balance and solute inputs recorded in the mineralogy and isotopic signatures of carbonates wer e controlled by the diapiric evolution and changes in the E/P balance. (C) 1998 Elsevier Science B.V. All rights reserved.