1st METECH workshop – From deep-sea to coastal zones: Methods and Techniques for studying Palaeoenvironments (original) (raw)

Microbial deposits in upper Miocene carbonates, Mallorca, Spain

The Santanyí Limestone, a 30–35-m thick upper Miocene limestone succession cropping out in Mallorca, contains abundant microbialite deposits, the shape, microstructure and texture of which was controlled by environmental factors: depth, energy and salinity. Three main types of microbialites are distinguished: (1) domed (DNOS) and stratiform, mostly undulate (UNOS) non-oolitic stromatolites, (2) undulate oolitic laminites (UOL) and (3) domed-oolitic thrombolites (DOTs). Based on lithofacies associations and occurrence of microbialite types, the Santanyí Limestone succession is subdivided into five stratigraphic units (I to V) separated by sharp surfaces. Within units II, III and V, the vertical evolution of microbialites was induced by changes in accommodation space/depth: (1) intertidal/very-shallow subtidal conditions at the base were induced by flooding over a wide area, (2) continued sea-level rise caused submergence to subtidal conditions, and (3) a significant bathymetric decrease created the sharp surface bounding these units. In units II and III, NOS accumulated in variable energy and depth conditions, as buildups with thick, somewhat discontinuous and mostly non-isopachous lamination, surrounded by oolitic grainstones with wave and current structures and oolitic intraclasts. In contrast, thin and generally regular and smooth lamination of NOS in unit V suggests, along with the absence of oolite grainstones and macrobiota, calm and restricted, maybe more saline, conditions. UOL, consisting of oolitic layers separated by thin micritic laminae, developed adjacent to NOS in units II and III and to DOT at the lower part of unit III, in shallow-water and low-energy conditions. Both ooids andmicrite laminae have evidence for biogenesis. Micritized ooids containing microbial remains are common. Micritic laminae in UOL and the dark micritic laminae in NOS are thought to represent bacterially enhanced calcite precipitation and lithification during periods of low sedimentation. Oolitic thrombolites containing macrobiota are only present in unit III. They represent deeper and openmarine conditions affected by high-energy events, in which microbially mediated precipitation favoured microbialite accretion and lithification.

Geomicrobiological investigations in subsaline maar lake sediments over the last 1500 years

Quaternary Science Reviews, 2013

Living microorganisms inhabit every environment of the biosphere but only in the last decades their importance governing biochemical cycles in deep sediments has been widely recognized. Most investigations have been accomplished in the marine realm whereas there is a clear paucity of comparable studies in lacustrine sediments. One of the main challenges is to define geomicrobiological proxies that can be used to identify different microbial signals in the sediments. Laguna Potrok Aike, a maar lake located in Southeastern Patagonia, has an annually not stratifying cold water column with temperatures ranging between 4 and 10 °C, and most probably an anoxic water/sediment interface. These unusual features make it a peculiar and interesting site for geomicrobiological studies. Living microbial activity within the sediments was inspected by the first time in a sedimentary core retrieved during an ICDP-sponsored drilling operation. The main goals to study this cold subsaline environment were to characterize the living microbial consortium; to detect early diagenetic signals triggered by active microbes; and to investigate plausible links between climate and microbial populations. Results from a meter long gravity core suggest that microbial activity in lacustrine sediments can be sustained deeper than previously thought due to their adaptation to both changing temperature and oxygen availability. A multi-proxy study of the same core allowed defining past water column conditions and further microbial reworking of the organic fraction within the sediments. Methane content shows a gradual increase with depth as a result of the fermentation of methylated substrates, first methanogenic pathway to take place in the shallow subsurface of freshwater and subsaline environments. Statistical analyses of DGGE microbial diversity profiles indicate four clusters for Bacteria reflecting layered communities linked to the oxidant type whereas three clusters characterize Archaea communities that can be linked to both denitrifiers and methanogens. Independent sedimentary and biological proxies suggest that organic matter production and/or preservation have been lower during the Medieval Climate Anomaly (MCA) coinciding with a low microbial colonization of the sediments. Conversely, a reversed trend with higher organic matter content and substantial microbial activity characterizes the sediments deposited during the Little Ice Age (LIA). Thus, the initial sediments deposited during distinctive time intervals under contrasting environmental conditions have to be taken into account to understand their impact on the development of microbial communities throughout the sediments and their further imprint on early diagenetic signals.