Geological controls on the geometry of incised-valley fills: Insights from a global dataset of late-Quaternary examples

Incised valleys that develop due to relative sea-level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised-valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database-driven statistical analysis of 151 late-Quaternary incised-valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised-valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised-valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised-valley fills, likely through effects on relative sea-level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley-fill geometry is positively correlated with the associated drainage-basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley-fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley-margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised-valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley-fill thickness and present-day coastal-prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of source-to-sink sediment route-ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs

Central Santa Catarina coastal dunefields chronology and their relation to relative sea level and climatic changes

During the past decades, there have been contrarian explanations for the formation and stabilization of coastal dunefields: while many authors believe the dunes formation would be enhanced by falling sea level, others argue that a rising or stable sea level context would be favorable. For Brazilian coastal dunefields, the second hypothesis seems to be more consistent with the luminescence ages found so far; however, most of these data were obtained without using the SAR protocol. Another point of concern is the role of climate change in the aeolian system, which is still not very clear. The aim of this paper is to try to clarify these two questions. To this end, five coastal dunefields were selected in central Santa Catarina coast. The remote sensing and dating results allowed the discrimination and mapping of at least four aeolian generations. Their age distribution in relation to the global curve of relative sea level variation during the Late Pleistocene allows us to suggest that the formation of Aeolian dunefields in the coastal context is supported by stable relative sea level. However, relative sea level is not the only determinant for the formation and preservation of the aeolian coastal dunes. Evidences of climatic control indicate that the initiation of dunefields would be favored by periods of less humidity while their stabilization would occur preferably during the periods of rain intensification, connected to monsoon activity

Introduction to the early-Anthropocene Special Issue

The first records of Holocene CO2 and CH4 concentrations in the atmosphere were published two decades ago (Barnola et al., 1987; Chappellaz et al., 1990), and the observation that both gas concentrations had increased during the last several thousand years has drawn considerable attention from scientists. Compet- ing hypotheses attribute these increases either to natural forcing or anthropogenic intervention in the climate system. This ongoing debate is the focus of this volume. Papers in Part 1 of this issue explore the history of Holocene population growth and land clearance. Part 2 examines possible sources of the CH4 increase during the last 5000 years, and Part 3 examines possible sources of the CO2 increase over the last 7000 years. Part 4 investigates whether or not the drops in CO2 and CH4 concentrations proposed in the early anthropogenic hypothesis would have been sufficient to cause glacial inception in the Northern Hemisphere. Part 5 explores possible causes of the decreases in CO2 concentrations during the last 1000 years. A final paper provides one assessment of whether the natural or anthropogenic hypotheses for the late-Holocene greenhouse-gas increases can be rejected based on currently available evidence. When this issue was first planned, the editors sent out invita- tions to all scientists who had first-authored papers relevant to the topic or had otherwise played a prominent role in the discussion. This list was closely balanced among those favoring natural explanations, those favoring the anthropogenic explanation, and those who had not taken a strong position on either side of the debate. Roughly half of the scientists invited chose to participate, and papers that support the natural and anthropogenic explana- tions can be found in each of the five major parts of the issue. Some of the most prominent proponents of natural forcing did not choose to participate, but their views are frequently discussed in the 14 papers that form this issue