Search

The Last Glacial Maximum (LGM) is widely used to refer to the episode when global ice volume last reached its maximum and associated sea levels were at their lowest. However, the boundaries of the interval are ill-defined and the term and acronym have no formal stratigraphical basis. This is despite a previous proposal to define it as a chronozone in the marine records on the basis of oxygen isotopes and sea levels, spanning the interval 23–19 or 24–18 ka and centred on 21 ka. In terrestrial records the LGM is poorly represented since many sequences show a diachronous response to global climate changes during the last glacial cycle. For example, glaciers and ice sheets reached their maximum extents at widely differing times in different places. In fact, most terrestrial records display spatial variation in response to global climate fluctuations, and changes recorded on land are often diachronous, asynchronous or both, leading to difficulties in global correlation. However, variations in the global hydrological system during glacial cycles are recorded by atmospheric dust flux and this provides a signal of terrestrial changes. Whilst regional dust accumulation is recorded in loess deposits, global dust flux is best recorded in high-resolution polar ice-core records, providing an opportunity to define the LGM on land and establish a clear stratigraphical basis for its definition. On this basis, one option is to define the global LGM as an event between the top (end) of Greenland Interstadial 3 and the base (onset) of Greenland Interstadial 2, spanning the interval 27.540–23.340 ka (Greenland Stadial 3). This corresponds closely to the peak dust concentration in both the Greenland and Antarctic ice cores and to records of the global sea-level minima. This suggests that this definition includes not just the coldest and driest part of the last glacial cycle but also the peak in global ice volume. The later part of the LGM event is marked by Heinrich Event 2, which reflects the onset of the collapse of the Laurentide at c. 24 ka, together with other ice sheets in the North Atlantic region. A longer and later span for the LGM may be desirable, although defining this in chronostratigraphical terms is problematic. Whichever formal definition is chosen, this requires the contribution of the wider Quaternary community. [ABSTRACT FROM AUTHOR]

Abstract: The shallow-marine to coastal deposits of Alghero have been dated using the Optically Stimulated Luminescence (OSL) method, and studied to define the palaeoenvironmental evolution of coastal areas in northwest Sardinia. Dating, coupled with sedimentological and stratigraphical analyses, allowed the subdivision of the succession in four unconformity-bounded units (U2, U3a, U3b, U4). Unit U2 of penultimate glacial stage (MIS 6; ca. 150ka) is composed of dune deposits formed intermittently along rocky coastal areas under strong wind conditions. Subunit U3a is composed of cliff-bounded gravelly dominated pocket beach deposits, commonly highly fossiliferous, and is assigned to the interglacial MIS 5e (ca. 125ka). Subunit U3b is characterized by sandy beach to coastal systems of the isotopic sub-stage MIS 5c (ca. 100ka). The unconformity between U3b and U4 marks the initiation of last glacial regression (MIS 5a/4; ca. 77ka) during which shelf areas were exposed and small incised valleys developed. Along the valley flanks, colluvial or sheetflood sediments were deposited and, in the central part thick reddish paleosols formed. Valleys were eventually filled by thick aeolian dune deposits (MIS 4–3) indicating persistent inland sand transport. Around Alghero, coastal carbonate-rich dunes preferentially formed and migrated inland during lowstand and falling stages when the climate is relatively cold, vegetation is quite open, winds are fairly strong and sediment supply from exposed shelf areas is high. [Copyright &y& Elsevier]

Abstract: The Ecemiş River developed during the Pleistocene and Holocene in the Ecemiş basin. The basin developed because of tectonic movement on the Ecemiş Fault, which is west of the Aladağ Mountains. In the Pleistocene, glacial–interglacial climate changes directly affected the regimes and discharges of rivers, causing changes in river levels along the Ecemiş channel. Fluvial formations became extremely complicated when active tectonic movements accompanied the changes resulting from the climatic conditions. The rivers embedded in their beds according to new local base levels in interglacials. Terraces formed due to interruptions in the cycle of morphological development along the Ecemiş River. From the morphologic structure of these terraces and their positions in the Ecemiş Valley two groups and five levels of terraces were defined. In the Ecemiş Valley, modern terraces (1–5m) formed in the Holocene, while low terrace systems (5–10m and 15–20m) and high terrace systems (30–40 and 70–80m) formed in the Pleistocene. [Copyright &y& Elsevier]

Abstract: The Piànico-Sèllere is a lacustrine succession from northern Italy that records a sequence of climatic transitions across two Pleistocene glacial stages. The intervening interglacial stage is represented by well-preserved varves with calcitic (summer) and clastic (winter) laminae. There is a tight coupling between climate-driven lithologic changes and magnetic susceptibility variations, and stable paleomagnetic components were retrieved from all investigated lithologies including the largely diamagnetic calcite varves. These components were used to delineate a sequence of magnetic polarity reversals that was interpreted as a record of excursions of the Earth''s magnetic field. Comparison of the magnetostratigraphic results with previously published data allows discussion of two possible models which have generated controversy regarding the age of the Piànico Formation. The data indicates that the Piànico Formation magnetostratigraphy correlates to geomagnetic field excursions across the Brunhes/Matuyama transition, and consequently the Piànico interglacial correlates to marine isotope stage 19. This correlation option is substantially consistent with K–Ar radiometric age estimates recently obtained from a tephra layer interbedded in the Piànico Formation. The alternative option, considering the Piànico interglacial correlative to marine isotope stage 11 within the Brunhes Chron as supported by tephrochronological dating reported in the literature, is not supported by the magnetostratigraphic results. [Copyright &y& Elsevier]

Abstract: The chronostratigraphic position of the Piànico carbonate sequence, Oroebic Alps, Italy remains controversial, notwithstanding that it is one of the best-preserved records of an alpine interglacial. Two distal tephra were discovered in the sequence. The lowermost (t21d) is a rhyolitic tephra dated by K–Ar at 779±13ka, different from the uppermost phonolitic tephra (t32) with an assigned age of 393±12ka. The volcanic source of t32 was recently interpreted as being the Brown Leucitic Tuff of the Roccamonfina volcano, southern Italy. New trace element and Nd and Sr isotopic composition of t32 and of 420–200ka volcanic products of the Roccamonfina clearly indicate that this volcano is not the source of t32, but that it likely originated from the northern Roman Magmatic Province. The products of the initial explosive activity of Sabatini Mts., dated ca. 780–800ka, isotopically match t32 and are proposed as its source. This indicates that the previously inferred age of ∼400ka for t32 is erroneous. New isotopic and geochemical data of t21d confirm it as being associated with the first pyroclastic eruptions (790–710ka) of the Puy de Sancy volcano, French Massif Central. The Early-Middle Pleistocene K–Ar ages obtained previously for t21d, together with the isotopic attribution of volcanic sources to both tephra layers presented here, confirm Piànico as one of the best interglacial sequences of Cromerian age in the Western Alps. [Copyright &y& Elsevier]

Abstract: In the Alps, interglacial and interstadial deposits are rarely preserved due to the intense erosive effect of glaciers in the valleys. Fortunately, some outcrops and cored sequences located in the field area ranging from Lyon to Evian provided sedimentary profiles datable by palynostratigraphy in a highly documented geomorphological context. An overview of several palynological sequences studied in this large area is proposed, and their position in a general chronostratigraphical pattern is discussed. Particular attention is paid to palynostratigraphical evidence whose relevance is tested with systematic comparisons with long reference European pollen sequences spanning several glacial cycles. Minimum ages are suggested for non-glacial episodes corresponding to the deposits studied. [Copyright &y& Elsevier]