This contribution examines the climate variations reflected by a mainly lacustrine succession spanning from 17.73 to 14.0 Ma in north‐east Iberia, thus encompassing the Miocene Climatic Optimum (MCO). The study is based on the δ¹³C and δ¹⁸O composition of an array of carbonate facies and marl samples, complemented with sedimentological analysis, illite crystallinity index and magnetic susceptibility data. The onset and ending of the MCO have been detected at ca 17.10‐17.06 Ma and 14.56 Ma, roughly equivalent to the boundaries in the marine record, although with relatively short lags. The variability of the data series evidenced changes in humidity and air temperature through the MCO, some of which coincided with similar variations in other records. Specifically, an evolving positive shift in δ¹³C values, from 16.5 to 14.5 Ma, seems to fit the Monterey excursion observed in marine records. Likewise, increases in δ¹⁸O values between 16.8 and 16.5 Ma and between 14.85 and 14.56 Ma in the study succession concurred with warming intervals recorded in palaeosols of Central Europe, emphasising the coincidence with the temperature maximum at ca 16.6 Ma. A general decline in temperature and an increase in humidity are detected from 14.56 Ma, both with steeper trends until 14.41 Ma then more gradual onwards, indicating the beginning of the Middle Miocene Climatic Transition. These results shed light upon the tightly coincidental features between terrestrial and marine records over those time intervals and, more importantly, highlight the earlier warming and the faster cooling experienced by the lake system as compared with the marine record. These findings provide further evidence to help infer palaeoclimate conditions on a much broader reach than the regional context.

The utility of the Mg/Ca elemental ratio of calcite ((Mg/Ca)_calcite) as a temperature indicator in continental carbonate deposits is a matter of debate due to the different results obtained by diverse authors. In this study, we aimed to test the reliability of the (Mg/Ca)_calcite in fluvial carbonates. We selected the recent tufa stromatolite records of four rivers on the Iberian Peninsula for the trace element analysis based on six-monthly sampling. Previous sedimentary and hydrological studies on these fluvial basins provided the information for this work. The water temperature estimates for the stromatolite (Mg/Ca)_calcite substantially differed from the measured water temperatures in most of the studied cases. We thus assessed other factors that participate in the control of the Mg partitioning between water and calcite. The correction of the detrital Mg content yielded water temperatures that matched the measured ones in one of the rivers. The (Mg/Ca)_water, water discharge and calcite precipitation rates may also occasionally influence the (Mg/Ca)_calcite. The six-month behaviour of some of these parameters could interfere with the relationship between the (Mg/Ca)_calcite and water temperature. According to these results, and their comparison with other non-marine carbonates, the wide variety of parameters that are involved in the (Mg/Ca)_calcite limit it as a geochemical thermometer in continental sedimentary environments.