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Geology and Geochemistry

Although the existence of amber in Spain was cited by Casal in 1762, the paleobiological content within the amber was not detected until the mid 90s. Study of the amber inclusions from the Early Cretaceous of the Iberian Peninsula began with the discovery of the Peñacerrada I outcrop (the so-called Álava amber). In Spain, there are more than 120 outcrops with Cretaceous amber, but only nine of them have provided bioinclusions: two in Basque Country (Peñacerrada II and Salinillas de Buradón), one in Castilla y León (Peñacerrada I), two in Asturias (El Caleyu and Pola de Siero), one in Cantabria (El Soplao), two in Aragón (San Just and Arroyo de la Pascueta), and one in Comunidad Valenciana (Cortes de Arenoso).

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Identification of these more than 120 outcrops was possible after an exhaustive bibliographic search and the in situ verification of most of the references, some of them from the 18th century, and an exhaustive prospection program to locate new outcrops as well. The stratigraphic and sedimentological study of the nine amber outcrops that have provided bioinclusions was completed, all of which were sampled to recover pollen, plant meso- and macro-remains, and material for geochemical analyses.

The basin development during the Early Cretaceous in the Iberian Plate is associated with the opening of the northern part of the Atlantic and the kinematics between the European and Iberian plates. As a result, amber deposits are distributed along a strip curve from the east to the north of the Iberian Peninsula, following the coastline during the Early Cretaceous. Spanish amber-bearing deposits appear in two basins, the Maestrat (=Maestrazgo) Basin (with Tethian influence) and the Basque-Cantabrian Basin (with proto-Atlantic influence). The geological knowledge on these two basins is updated each year, and syntheses of each one have been carried out by several researches in the last years. Amber has always been found associated with layers rich in industrially-exploited coal or organic matter in both the Escucha Fm. (in the Maestrat Basin and the oriental area of the Basque-Cantabrian Basin) and Las Peñosas Fm. (in the occidental area of the Basque-Cantabrian Basin).

All amber localities are related with transitional continental to transitional marine environments. The Basque-Cantabrian Basin is linked with delta-shore marine environments, more specifically, related to paralic environments in the oriental area (Escucha Fm.) to paralic–marine environments in the occidental area (Las Peñosas Fm.), which provides amber with incrusted marine invertebrates. In contrast, the Maestrat Basin shows amber deposits sedimentologically associated with fluvial swamps.

In relation for geochemistry, fossil resins are considered chemical fossils and preserve certain characteristics of the original composition of the paleoecosystem better than any other sedimentary material. Changes in the molecular structure related to the age provide information about the processes that occurred during fossilization. Therefore, chemical studies on amber represent a very important step for elucidating its botanical affinity and age, and for comparing different amber types found in the same or different outcrops.

Today, most of the Spanish outcrops that have yielded abundant amber are being characterized geochemically, and are being compared to other Mesozoic ambers with new techniques, like thermal analysis using termogravimetric (TG) and differential termogravimetric (DTG) profiles, as well as laser desorption ionization, atmospheric pressure chemical ionization, and atmospheric pressure photoionization mass spectrometry. These techniques qualitatively determine the ages of the fossil resins. The geochemistry of the amber has not only contributed to the geographical characterization of the resins, but it has also allowed us to determine their age and the different botanical sources that produced them. Our last results indicate that the resin-producing forests from which Spanish amber originated show geographical differences. In the Maestrat Basin, the forest was mainly dominated by cheirolepids and cypresses, similarly as in the occidental area of the Basque-Cantabrian Basin (cheirolepids and gynkgoals), whereas in the oriental area of the Basque-Cantabrian Basin the forest was dominated by araucariaceans. Among the geochemical studies for the characterization of amber, IRTF techniques, gas chromatography, and thermal analysis are being implemented.

Fossil resin can retain the original carbon-isotopic signal, even if some degradation occurred during sediment burial. New amber C-isotope data is being collected in order to understand if Spanish amber can be used as a good tool for isotope-geochemistry-based stratigraphic and paleoenviromental studies. The δ13C fingerprint of several amber drops from single deposits is being obtained in order to define the C-isotope variability within samples of the same age. Analyses of δ13C on altered and fresh specimens coupled with GC-MS analyses are being performed to understand whereas degradation processes have modified the original δ13C amber signature.

We are also investigating whether the episodes of massive amber production during the Cretaceous are linked to biological and environmental turnovers like the oceanic anoxic events (OAEs). As OAEs have peculiar δ13C fingerprints, stratigraphic and isotope analyses of selected amber-bearing deposits of Spain are being carried out. Isotope analysis are performed on well preserved plant remains, such as wood, leaves, and amber to infer the carbon isotope composition of Cretaceous atmosphere and to understand whether a δ13C perturbation on the active carbon cycle is linked to the resin production.

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