History of Holocene fluvial activity in Romania: evidences based on absolute dating

The main objective of this paper is to illustrate how was carried the reconstitution of geomorphological activity of rivers in Romania at different times of the Holocene based on the interpretation sedimentation contexts with dated samples, the sediment quality, their deposition styles, relict morphologies and others. The database compiled was processed using bivariate and multivariate statistical techniques or the probability distribution functions. The secondary objectives relate to: (i) Establishing a chronology of fluvial activity in Romania? (ii) How were felt Holocene climatic thresholds (identified in various proxies) in fluvial activity? (iii) How river chronology correlates with other environmental and geoarchaeological in our geographic area? (iv) In this research stage, we can explore some links with Central Europe fluvial chronologies? Keywords— fluvial chronologies; absolute dating; probability density functions; Holocene I. APPROACHING PREMISES Regarding the knowledge evolution on fluvial landforms and processes, the year 2010 stands out as a reference year in grouping the study objectives: (i) before 2010, the fluvial geomorphology research has focused mainly on present-day and modern channel dynamics, on fluvial processes related to human impact and other driving factors; (ii) after 2010, many notable attempts have occurred, with studies focused on the better understanding of the style and of the magnitude of channels changes for different timescales (secular, millennial, Late Holocene, Holocene, Late Glacial and transition to Holocene). Over the last 6 years several scientific studies based on absolute dating were published, with results correlated with multidisciplinary investigations. Due to the increased number of references on past climate and hydrological events, geomorphologists wanted to correlate them with the magnitude of fluvial activity, responsible for the morphologic and sedimentologic adjustment styles of river channels. The required information are stored in sedimentary archives (litho-stratigraphic units whose morphological, granulometric, geochemical, biological characteristics indicate the mechanisms and physical processes which contributed to their deposition) and are dated by one of the known techniques. Because these structures are rich in biogenic material, the most used method to evaluate their age was radiocarbon dating. In many regions of Europe absolute dating databases of the sedimentary archives were achieved, processed using the methodology elaborated by [1] and [2]. Thus, from Great Britain [3, 4], Spain [5], Germany [6] and Poland [7, 8], several hundreds of absolute dating of sedimentary archives were statistically approached and conclusions regarding the periods with high fluvial activity registered in the considered regions were obtained. In light of these results we want to report our own results throughout the objectives of the present study. In Romania, significant progress was achieved as regards the chronology of the Holocene. [9] summed the palaeoclimatic data sources and climate reconstructions for the last 15000 years in Romania. The fluvial sedimentary archives (from floodplain and floodplain terraces) and sedimentary archives from small sedimentary basins (small lakes, peatbogs, swamps, glacial lakes) which are directly related with a drainage basin and network, were the main focus of our study. By approaching the meta-analysis technique, respectively, ”the use of a systematic review procedure and common set of statistical techniques to combine the results of several studies” [10], we compiled a database of over 400 absolute dating collected from sedimentary units who had at source a hydrological event of a certain magnitude. The aim of this work is the review of the existent records (published or in press), regarding the chronology of fluvial activity of Romanian rivers following some exceptional hydrological events. On this basis we intend to reconstruct the geomorphological fluvial activity of rivers in different moments of Holocene, to identify how some certain climatic thresholds were felt in the answer of fluvial processes, the role of human impact, correlation of other environmental and geoarchaeological chronologies from our geographical area. II. STUDY AREA AND THE DATABASE The compiled database totaled 404 stratigraphic units dated, distributed in Romania and across the border (in the case of Somes, Cris and Mures rivers). The data were collected from over 80 publications, and a large number of dates are provided by our research team (n ~ 90). The samples are originating from fluvial environments adjacent to actual rivers or palaeo-paths DOI 10.15551/prgs.2017.92