Arsenio Muñoz

Sunspot-like and ENSO/NAO-like periodicities in lacustrinelaminated sediments of the Pliocene Villarroya Basin (La Rioja,Spain)

This is a study of an extremely good outcrop of 1745 varves inthe Villarroya Pliocene Basin (Spain). The thicknesses of the light and darklayers, which were obtained from enlarged photograph negatives, are dealt withseparately, and three time series, of light, dark, and the sum of both layers,are performed. Periodicities of about 12, 6–7 and 2–3 years havebeen obtained. The origin of the recurrent behaviour of the sediments withthese periods could be related to natural phenomena like sunspot cycles, ElNiño-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO) andQuasi-biennial Oscillation (QBO), as these phenomena are capable of modulatingthe climate in this frequency band. From our point of view, there could be aclose relationship between sedimentation in the Villarroya Basin and thesenatural phenomena, which is moved through climate. The light layers record therun-off of gastropods, ostracods and charophytes from a seasonal productivitycycle, and are generated during late summer and autumn. It seems clear that theclimatic information provided by these cycles corresponds mainly to summertime, which would be translated into a variation of the light layer thicknessdue to a more or less intense development of the flora and fauna, being thickerin years of higher temperatures and thinner in years of lower temperatures.Considering the results, we would suggest that the sedimentation of thesematerials in the Pliocene was driven by solar activity or/and ENSO, NAO, andQBO phenomena. Nowadays these phenomena produce rainfall and temperaturechanges with the cyclicities shown in this study and we think they could haveoperated in the same way in Pliocene times.

Climate and anthropogenic factors affecting alluvial fan development during the late Holocene in the central Ebro Valley, northeast Spain

The late Holocene in the central Ebro Depression is characterized by a number of morphodynamic stages that affected hillslopes and channels. Climate fluctuations and human activities together caused severe erosion and floods, resulting in alternating stages of accumulation and incision in the valley floors. Seven accumulation levels (N6-N0) correlated with Ebro River fluvial terraces have been identified in the secondary fluvial network of the central Ebro Depression. The main accumulation level (N3) corresponds to a long period of increased hillslope erosion and sedimentation in the valleys, from the Neolithic to the late Roman periods. The accumulation levels are evident throughout the valleys and extend to the valley mouths, where many connect with alluvial fans. The study of several sedimentological profiles, 14C AMS dating, and many archaeological remnants, have enabled detailed assessment of sedimentation characteristics in the alluvial fans and the relationship thereof to the presence of human settlements. Four sedimentation stages have been identified: (i) pre-Bronze Age (> 3800 years BP), (ii) Bronze Age/Iron Age (3800-2500 years BP), (iii) Roman/post-Roman period (2500-1500 years BP), and, (iv) the early Middle Ages, mainly during the Visigothic period (sixth century). The sedimentation dynamics of the alluvial fans are related to: (i) climate variability, (ii) human activities in the watershed, and (iii) the meandering character of the Ebro River, which has lateral displacements that change the local base level of tributaries and fans. Level N3 in the valley floors corresponds to stages i-iii of the alluvial fans, which are mostly related to human-induced transformation of characteristics of the secondary basins, coinciding with enhanced erosion under favorable (warm and humid) climatic conditions. In contrast, the Medieval stage, which corresponds to level N2 in the valleys, was caused by floods during a period of high climatic variability.

Upper Oligocene-Lower Miocene shallowing-upwardlacustrine sequences controlled by periodic and non-periodic processes(Ebro Basin, northeastern Spain)

The lacustrine facies from two sections (Candasnos and Fraga) ofthe Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in thecentral part of the Ebro Basin (Spain) have been analysed to determine theinfluence of orbital parameters in lacustrine sedimentation. The unit ispredominantly composed of limestones and marls, and represents a shallowlacustrine freshwater system. The sedimentological features of the faciesstudied demonstrate that the lower part of the Candasnos section representsoffshore lacustrine subenvironments whereas the upper part, and the whole ofthe Fraga section, characterise marginal lacustrine areas. Series of stratalthickness variations of limestone, marl, and limestone/marl couplets fromboth sections have been analysed using spectral analysis. This shows thatinformation corresponding to periodic cycles only appears in the offshorefacies, that is to say, in the lower part of Candasnos section, and disappearsin the marginal facies where non-periodic cycles exist. Furthermore, thespectral analysis of the offshore facies highlights the existence of a peak inthe power spectrum with a period of around 7 (6.8 to 7.8) that can berecognised in the field as shallowing-upward lacustrine sequences.Magnetostratigraphic data from the Candasnos section allow us to establish atime span of 2,808 years for the limestone/marl couplet from the lower partof this section, and between 19,000 and 22,000 years for the periodic cycleidentified, thus representing the climatic precession cycle. Shallowingsequences from marginal areas do not correspond with any periodiccycle.

Relationship of mineralogy to depositional environments in the non-marine Tertiary mudstones of the southwestern Ebro Basin (Spain)

Abstract During the Middle-Late Eocene, Oligocene and Miocene a non-marine succession, about 5000 m thick, was deposited in the western Ebro Basin. In the northern and southern margins of the basin, conglomerate deposits of alluvial fan origin were deposited. Towards the centre of the basin there is a lateral change to flood plain sandstones and mudstones and finally carbonate and evaporitic lacustrine deposits. Two types of saline lakes, where evaporitec precipitation occurred, can be distinguished: central saline lakes of high salinity, and marginal saline lakes, located near the southern margin, with lower salinity than the central ones. X-ray diffraction, TEM, SEM and EDX analysis were carried out on mudstones from central and southern deposits. The clay minerals identified were illite, chlorite, kaolinite, Mg-rich smectite, palygorskite and mixed-layers illite-smectite and chlorite-smectite. Apart from clays, carbonates (calcite, dolomite and magnesite) and other silicates (quartz, plagioclase, K-feldspar and analcime) were recognised. The palygorskite, Mg-rich smectite, analcime, dolomite and magnesite are of diagenetic origin. The other minerals are detrital, and derived from the surrounding mountain chains. The clay mineral assemblage of each environment is related to the diverse various source areas (Iberian Range and Pyrenees) and their compositional variation through time, as well as to diagenetic processes. On the basis of clay mineral distribution, the detrital clay mineral assemblages are preserved in the alluvial fans, the flood plain without saline influence, and the central saline lakes. In the saline flood plains and marginal saline lakes diagenetic palygorskite and Mg-rich smectites were formed. In the marginal saline lakes the formation of magnesium clays prevailed but in the central saline lakes, where no clay mineral diagenesis is observed, the magnesium precipitated as dolomite and magnesite.

Succession and stable isotope composition of gastropods in Holocene semi-arid alluvial sequences (Bardenas Reales, Ebro Basin, NE Spain): Palaeoenvironmental implications

Three alluvial units, ranging from the middle to late Holocene in age, were distinguished in Bardenas Reales Natural Park (Ebro Basin, NE Spain) based on morphosedimentary analysis and radiocarbon data. In this sensitive Mediterranean alluvial system, the succession and isotopic composition of gastropods greatly contribute to establishing the prevailing palaeoenvironmental conditions. The oldest unit (4763±87 to 2848±55 cal. yr BP) represents fluvial channels and swamped areas, includes the greatest proportion of aquatic gastropods (Ancylus fluviatilis and Lymnaea truncatula) and marsh species (Vallonia pulchella) and presents the most negative values of δ18O (mean −2.21‰ PDB). The intermediate unit (1403±60 to 836±65 cal. yr BP) shows a high rate of alluvial activity related to irregular torrential rainfall, with sedimentary features of aridity, and this unit includes only dry open ground gastropods (Xeroplexa sp., Chondrinidae indet. and Granopupa granum) and presents the least negative values of δ18O (mean −1.17‰ PDB). The youngest unit (191±97 to 127±82 cal. yr BP) exhibits a recurrence of flooding events, mostly includes marsh (Truncatellina callicratis and Lauria cylindracea) and aquatic (Lymnae truncatula) species and shows a very heterogeneous isotopic signal, with −1.83‰ PDB as the mean value of δ18O. As a consequence, a palaeoenvironmental pattern comprised of an early wetter/colder phase followed by a warmer/drier period and, finally, an increase in both cold conditions and hydrological variability, is deduced for the middle to late Holocene in the Ebro Basin. Gastropod shell δ13C values, related to vegetation, can also be linked to palaeoenvironmental changes. The succession and stable isotope composition of gastropods in alluvial sequences are revealed as very sensitive indicators of rapid Holocene climatic changes in fragile semi-arid landscape systems.