DEP - Artículos

High winter precipitation in Southern Europe during the Last Glacial Maximum as inferred from pollen sequences

High winter precipitation in Southern Europe during the Last Glacial Maximum as inferred from pollen sequences Zumaque, Jena; Vernal, Anne de; Eynaud, Frédérique; Camuera, Jon; Jiménez Moreno, Gonzalo; Combourieu Nebout, Nathalie The climate of the Last Glacial Maximum (LGM) in southern Europe has been thoroughly documented. Still, discrepancies exist between the climatic information provided by pollen data, which mainly indicate steppe vegetation and thus arid conditions, and some model simulations that rather suggest large amounts of precipitation, notably in winter. Here, we investigate the regional vegetation and seasonal climate from 28 ka to 10 ka using published pollen sequences from five sites, including core SU81-18 located on the western Iberian Margin, ODP Site 976 in the Alboran Sea, Padul in Southeastern Spain, Lago Grandi di Monticchio (Monticchio) in southern Italy and Xinias in Greece. Pollen-based estimates of seasonal temperatures and precipitation are reconstructed using the Modern Analogue Technique with the latest version of the Eurasian Modern Pollen Database (EMPD2), which comprises numerous samples from cold environments and provides potential analogues for the LGM. The results highlight a coherent regional pattern of vegetation dynamics that appears tightly linked to winter temperature and precipitation influenced by the westerlies. Most importantly, we reconstruct high winter and annual precipitation during the LGM at all study sites, with values comparable to, or even exceeding, those estimated for the Bølling/Allerød. Our results indicate high variability in rainfall, particularly along the Atlantic margin, and lower variability further east, notably in the Alboran Sea, suggesting a climatic front located between the two regions. Two phases of increased precipitation are evidenced in our records, one from 24 to 22.5 ka and the other from 21.5 to 19.5 ka. The timing of these humid intervals is in phase with two main glacier advances in the European Alps, suggesting a consistent regional atmospheric pattern. Increased precipitation is attributed to the southward displacement of the jet stream and the westerlies due to the maximum extension of the Northern Hemisphere ice sheets. Conversely, the dry phases might correspond to a weakening or latitudinal shift of the westerlies. Indications of tenuous arboreal vegetation growth during the LGM are observed at the Iberian sites, but not at Monticchio and Xinias, where a cold climate may have prevented forest development. We hypothesize that the classic Mediterranean gradient with increasing dryness and temperature from west to east may have been amplified by the influence of the Fennoscandian and Alpine ice sheets due to the Rossby wave breaking of the jet stream west of the Alps.

Organic periostracum preserved in Cretaceous ammonoids from the Andean Neuquén Basin

Organic periostracum preserved in Cretaceous ammonoids from the Andean Neuquén Basin Aguirre Urreta, Beatriz; Marin, Luciana S.; Checa González, Antonio G.; Grenier, Christian; Tunik, Maisa; Lescano, Marina; Castro, María A.; Lazo, Darío G.; Vennari, Verónica V.; Rogel, Martín N. Exceptionally preserved 135-million-year-old ammonoids from the Neuquén Basin at the Andean foothills revealed a fossilised structure never recorded before. Ammonoids are cephalopods that inhabited the oceans for about 400 million years until they became extinct 66 million years ago. Their shells are composed of aragonitic layers bounded externally by an organic periostracum. The latter plays an essential role in initiating shell biomineralisation and protecting minerals from dissolution and abrasion. Here we describe a preserved periostracum in Cretaceous ammonoids, an extremely fragile yet flexible layer, with an approximate thickness of 2 µm and an internal horizontal lamination. The external surface appears mostly smooth, while the internal surface displays a reticulated appearance, interpreted as the casts of aragonite prisms of the calcareous shell. Our results reveal that the ammonoid periostracum contains proteins, polysaccharides, and lipids, consistent with the composition of the periostracum in modern-day molluscs. This study sheds light on a previously little-known organic structure in ammonoids. Its morphological and chemical characteristics allow us to establish that it is a highly conservative structure among molluscs. Furthermore, we show that such a delicate organic structure can be preserved for 135 million years in favourable environmental conditions, opening up the possibility of future discoveries.

Blue mesophotic coral bioherms in the pre-evaporitic Messinian of the western Mediterranean (Almería–Níjar Basin, SE Spain)

Blue mesophotic coral bioherms in the pre-evaporitic Messinian of the western Mediterranean (Almería–Níjar Basin, SE Spain) Sola, Fernando; Braga Alarcón, Juan Carlos Bioherms are common in the pre-evaporitic Messinian in Neogene basins in SE Spain. Coral bioherms reported to date in the region have been interpreted as shallow-water reefs based on coral morphology (stick-like Porites and Tarbellastraea domes) and surrounding sediments. However, at the western margin of the Almería–Níjar Basin, coral bioherms grew in relatively deep waters with low turbidity on the eastern shelf of the upland precursor to the Sierra de Gádor. Bioherms occur in the lower part of a reef unit, in some cases growing directly on the erosion surface separating this unit from the underlying older Miocene rocks. They are formed mainly by Porites, with minor Tarbellastraea, encrusted by foraminifera, coralline algae and microbialite. The thickness of bioherms ranges from less than 1 m to about 20 m in the largest bioherm. Mesophotic conditions can be inferred from direct palaeodepth measurements at the outcrop, showing that bioherms started to develop at several tens of metres water depth. The dominance of thin, laminar, contorted Porites colonies with few finger-like projections is also indicative of low light levels. Facies laterally equivalent to the bioherms—namely algal/bivalve wackestone to packstone, rhodolith rudstone rich in coralline algae of the order Hapalidiales, and Halimeda floatstone to rudstone—are all characteristic of relatively deep waters in the Mediterranean Neogene. The low siliciclastic content of bioherms and surrounding deposits (<8%) suggests that low light conditions in which bioherms grew were primarily due to water depth. The Almería bioherms are the first record of Upper Miocene blue mesophotic reefs in the world.

North Pacific ocean–atmosphere responses to Holocene and future warming drive Southwest US drought

North Pacific ocean–atmosphere responses to Holocene and future warming drive Southwest US drought Todd, Victoria L.; Shanahan, T.M.; DiNezio, P.N.; Klavans, J.M.; Fawcett, P.J.; Anderson, R.S.; Jiménez-Moreno, Gonzalo; LeGrande, A.N.; Pausata, F.S.R.; Thompson, A.J.; Zhu, J. The Southwest United States is experiencing severe and persistent drought, although uncertainties regarding the causes limit our ability to predict changes in water availability. The severity of the current drought has been attributed to a combination of warming and natural changes in atmospheric circulation, suggesting that current rainfall deficits may improve as natural oscillations reverse sign. Here we use new leaf-wax stable isotope reconstructions and simulations for the mid-Holocene (6 thousand years ago) and demonstrate that moderate warming of the Northern Hemisphere can produce drought over the Southwest United States through an ocean–atmosphere response originating in the North Pacific. The patterns of ocean warming and rainfall change resemble the negative phase of the Pacific Decadal Oscillation, indicating that this mode can be excited by external forcings. A similar response to warming is evident in future projections, leading to sustained winter precipitation deficits through the mid-twenty-first century. However, the magnitudes of past and current precipitation deficits associated with this North Pacific response are systematically underestimated in models, possibly due to a weak coupling of ocean–atmosphere interactions. Projections may also underestimate the magnitude of this precipitation response to changes in the North Pacific, leading to greater drought risk in this already water-poor region.

Periostracum Formation in Sepia officinalis and Loligo vulgaris and Homology with Other Molluscs

Periostracum Formation in Sepia officinalis and Loligo vulgaris and Homology with Other Molluscs Ruiz Villaespesa, Ernesto; Checa González, Antonio G.; Lucena Serrano, Cristina; Salas, Carmen The periostracum is the outermost shell layer and the first produced during shell formation in molluscs. This organic layer isolates the extrapallial space from the external environment and provides a scaffold for subsequent calcification. In cephalopods with an internal shell, some organic shell structures are putatively homologous to the periostracum of other molluscan groups. However, neither their detailed structure nor their mode of formation has been described, leaving the extent of this homology unresolved. To address this issue, we investigated the morphology and formation of the organic layer of the dorsal shield and the gladius in embryos of the cuttlefish Sepia officinalis Linnaeus, 1758, and the squid Loligo vulgaris Lamarck, 1798, respectively, using light microscopy and transmission electron microscopy. In both species, the periostracum forms within a periostracal groove located along the lateral and anterior margins of the shell sac. As in other molluscs, secretions from columnar cells at the bottom of the groove produce a dense layer, while a translucent layer is subsequently added beneath it through secretions from cuboidal cells. The main difference is the absence of both a pellicle and of the specialized glandular cells that typically secrete it at the bottom of the periostracal groove.