Assessment the Impacts of Sea-Level Changes on Mangroves of Ceará-Mirim Estuary, Northeastern Brazil, during the Holocene and Anthropocene

Predictions of the effects of modern Relative Sea-Level (RSL) rise on mangroves should be based on decadal-millennial mangrove dynamics and the particularities of each depositional environment under past RSL changes. This work identified inland and seaward mangrove migrations along the Ceará-Mirim estuary (Rio Grande do Norte, northeastern Brazil) during the mid-late Holocene and Anthropocene based on sedimentary features, palynological, and geochemical (δ¹³C, δ¹⁵N, C/N) data integrated with spatial-temporal analysis based on satellite images. The data indicated three phases for the mangrove development: (1°) mangrove expansion on tidal flats with estuarine organic matter between >4420 and ~2870 cal yrs BP, under the influence of the mid-Holocene sea-level highstand; (2°) mangrove contraction with an increased contribution of C₃ terrestrial plants between ~2870 and ~84 cal yrs BP due to an RSL fall, and (3°) mangrove expansion onto the highest tidal flats since ~84 cal yr BP due to a relative sea-level rise. However, significant mangrove areas were converted to fish farming before 1984 CE. Spatial-temporal analysis also indicated a mangrove expansion since 1984 CE due to mangrove recolonization of shrimp farming areas previously deforested for pisciculture. This work mainly evidenced a trend of mangrove expansion due to RSL rise preceding the effects of anthropogenic emissions of CO₂ in the atmosphere and the resilience of these forests in the face of anthropogenic interventions.

Factors affecting average grain size changes in rivers of a catchment area (Ardak catchment area, northeast Iran)

Investigation of sediment control factors in river channels of a catchment can play a very important role in the sustainable management of that catchment. For this purpose, Ardak catchment was selected, on which a reservoir dam was constructed for drinking and agricultural purposes large city of Mashhad in northeastern Iran. Then 57 samples were collected from the surface to the depth of 30 cm along the four rivers: A (14), B (23), C (13), and D (7). The samples were subsequently weighed and sieved for particle-size distribution. Cumulative curves were plotted followed by the determination of statistical parameters. Thirty-four representative fresh samples were selected for study using macroscopic features. Lithofacies were identified in the field using parametersand their mutual association following the classification scheme of Walker and Cant and Miall. Data on the textural parameters of different rivers showed that rivers A, D, B, and C produce coarse sediments. In terms of average grain size, river A has a greater role in the production of gravel sediments and river B has a greater role in the production of sand and mud sediments. According to the studies, the effect of formation of this catchment (Mozduran 3, Tirgan, and Mozduran 1) on the production of gravel sediments and other formation (Chaman Bid, Sarcheshmeh, and Shurijeh) which include more shale layers is more in the production of sand and mud sediments. Examination of sedimentary facies of alluvial terraces shows that river B is more effective in producing gravelly sediments and river A is more effective in producing sandy and mud sediments. Finally, a comparison of average grain size in sediments formed in the riverbeds of this catchment area and a comparison of this parameter in past sedimentary facies can show the effect of tectonic activity as the most important factor controlling textural parameters in this basin over time.

Understanding the geobiology of the terminal Ediacaran Khatyspyt Lagerstätte (Arctic Siberia, Russia)

The Khatyspyt Lagerstätte (~544 Ma, Russia) provides a valuable window into late Ediacaran Avalon-type ecosystems with rangeomorphs, arboreomorphs, and mega-algae. Here, we tackle the geobiology of this Lagerstätte by the combined analysis of paleontological features, sedimentary facies, and lipid biomarkers. The Khatyspyt Formation was deposited in carbonate ramp environments. Organic matter (0.12-2.22 wt.% TOC) displays characteristic Ediacaran biomarker features (e.g., eukaryotic steranes dominated by the C₂₉ stigmastane). Some samples contain a putative 2-methylgammacerane that was likely sourced by ciliates and/or bacteria. 24-isopropylcholestane and 26-methylstigmastane are consistently scarce (≤0.4% and ≤0.2% of ∑C_27-30 regular steranes, respectively). Thus, Avalon-type organisms occupied different niches than organisms capable of directly synthesizing C₃₀ sterane precursors among their major lipids. Relative abundances of eukaryotic steranes and bacterial hopanes (sterane/hopane ratios = 0.07-0.30) demonstrate oligotrophic and bacterially dominated marine environments, similar to findings from other successions with Ediacara-type fossils. Ediacara-type fossils occur in facies characterized by microbial mats and biomarkers indicative for a stratified marine environment with normal-moderate salinities (moderate-high gammacerane index of 2.3-5.7; low C₃₅ homohopane index of 0.1-0.2). Mega-algae, in contrast, are abundant in facies that almost entirely consist of allochthonous event layers. Biomarkers in these samples indicate a non-stratified marine environment and normal salinities (low gammacerane index of 0.6-0.8; low C₃₅ homohopane index of 0.1). Vertical burrowers occur in similar facies but with biomarker evidence for stratification in the water column or around the seafloor (high gammacerane index of 5.6). Thus, the distribution of macro-organisms and burrowers was controlled by various, dynamically changing environmental factors. It appears likely that dynamic settings like the Khatyspyt Lagerstätte provided metabolic challenges for sustenance and growth which primed eukaryotic organisms to cope with changing environmental habitats, allowing for a later diversification and expansion of complex macroscopic life in the marine realm.