Mobile mud layer underneath the desiccated maar lake of Rincón de Parangueo and insights into its microbial fingerprints

We surveyed a subsurface layer of saturated mobile mud and its microbial fingerprints below the desiccated bottom of a maar lake at Rincón de Parangueo (RP), Mexico. A multi-scale approach was followed using geological fieldwork, coring of the sediments, ground penetrating radar survey, physico-chemical characterization including X-ray diffraction and scanning electron microscope, and high-throughput DNA sequencing methods. The mobile mud is an organic-rich silty clay, with high values of alkalinity, volumetric water content and conductivity. Mud mobility has been attributed to overpressure caused by disequilibrium compaction of sediments related to active subsidence and pore overpressure produced by an input of groundwater and gas content in the sediments and resulted in a diverse set of structures related to mud tectonics such as injection domes and fluid seeps through fractures. Extraction and sequencing of sedimentary environmental DNA in the mud layer were performed for Bacteria and Archaea. Despite the small number of samples obtained, the microbial fingerprint from the sedimentary environmental DNA at subsurface shares similarities with the microbial communities identified on the crater surface. Additionally, we identify the DNA of specific methanogenic microorganisms in the mud, such as Bathyarchaeia, Methanomassiliicoccales, and Methanobacteriales, and we speculate on their probable role in gas production and pore overpressure in the mud layer. The underground mud at Rincón de Parangueo represents a geologically dynamic environment with conditions that are favorable for the thriving of microbial communities.

A myricaceous male inflorescence with pollen in situ from the middle Eocene of Europe

The late middle Eocene lacustrine filling of a maar lake at Eckfeld (Eifel Hills, Rhineland-Palatinate, western Germany) has provided four specimens of male inflorescences (catkins) in different stages of anthesis, each with pollen preserved in situ. The appearance of the successive stages together with triporate pollen showing an irregular surface and a myricoid micro-ornamentation clearly suggests an assignment of the fossil catkins to the Myricaceae. The material is described as a new genus and new species and represents the oldest record of male catkins for the family. The in situ preserved pollen grains are comparable to dispersed grains of Triatriopollenites excelsus.

Composite development and stratigraphy of the Onepoto maar lake sediment sequence (Auckland Volcanic Field, New Zealand)

The accurate and precise reconstruction of Quaternary climate as well as the events that punctuate it is an important driver of the study of lake sediment archives. However, until recently lake sediment-based palaeoclimate reconstructions have largely concentrated on Northern Hemisphere lake sequences due to a scarcity of continuous and high-resolution lake sediment sequences from the Southern Hemisphere, especially from the southern mid-latitudes. In this context, the deep maar lakes of the Auckland Volcanic Field of northern New Zealand are significant as several contain continuous and well-laminated sediment sequences. Onepoto Basin potentially contains the longest temporal lake sediment record from the Auckland Volcanic Field (AVF), spanning from Marine Isotope Stage 6e (MIS 6e) to the early Holocene when lacustrine sedimentation was terminated by marine breach of the south-western crater tuff ring associated with post-glacial sea-level rise. The Onepoto record consists of two new, overlapping cores spanning ca. 73 m combined with archive material in a complete composite stratigraphy. Tephrochronology and 14C dating provide the fundamental chronological framework for the core, with magnetic relative palaeo-intensity variability downcore, and meteoric 10Be influx into the palaeolake to refine the chronology. The µ-XRF (micro X-ray fluorescence) downcore variability for the entirety of the lake sediment sequence has been established with measurement of a range of proxies for climate currently underway. This work will produce the first continuous record of the last 200 kyr of palaeoclimate from northern New Zealand to date.

Holocene Environments in Northeast Africa

This chapter provides an overview of the geography, hydrology, and climate of NE Africa, with particular reference to the complex interactions between river regime, climate, the biota, and human settlement. During the Early (11.7–8.2 ka) and Middle Holocene (8.2–4.2 ka) the climate was far less arid than today across the Nile basin, including Nubia, albeit with sporadic dry phases. Climatic desiccation set in during the Late Holocene (4.2 ka to present), with minor wet phases. Intervals when the Nile flow regime was apparently shifting from high to low flow and flood plain incision have provisional ages of ca. 8.15–7.75 ka, 6.4–6.15 ka, 5.7–5.45 ka, 4.7–4.25 ka, 3.35–2.9 ka, 2.8–2.55 ka, and 1600 ce. In the Kerma area of Nubia there were two periods of relatively dense human occupation in the earlier part of the Holocene from 10 ka to 8 ka and from 7 ka to 6 ka, with two significant gaps in the archaeological record at 7.5–7.1 ka and 6.0–5.4 ka, that coincided with very low levels in Lake Challa, a maar lake on the eastern flank of Mt Kilimanjaro, near the Ugandan headwaters of the White Nile.