Poppe L. de Boer

The mid‐Cretaceous North Atlantic nutrient trap: Black shales and OAEs

Organic-rich sediments are the salient marine sedimentation product in the mid-Cretaceous of the ocean basins formed in the Mesozoic. Oceanic anoxic events (OAEs) are discrete and particularly organic-rich intervals within these mid-Cretaceous organic-rich sequences and are defined by pronounced carbon isotope excursions. Marine productivity during OAEs appears to have been enhanced by the increased availability of biolimiting nutrients in seawater due to hydrothermal alteration of submarine basalts in the Pacific and proto-Indian oceans. The exact mechanisms behind the deposition of organic-rich sediments in the mid-Cretaceous are still a matter of discussion, but a hypothesis which is often put forward is that their deposition was a consequence of the coupling of a particular paleogeography with changes in ocean circulation and nutrient supply. In this study, we used a global coupled climate model to investigate oceanic processes that affect the interbasinal exchange of nutrients as well as their spatial distribution and bioavailability. We conclude that the mid-Cretaceous North Atlantic was a nutrient trap as a consequence of an estuarine circulation with respect to the Pacific. Organic-rich sediments in the North Atlantic were deposited below regions of intense upwelling. We suggest that enhanced productivity during OAEs was a consequence of upwelling of Pacific-derived nutrient-rich seawater associated with submarine igneous events.

Numerical modelling of drainage basin evolution and three-dimensional alluvial fan stratigraphy

Abstract A forward numerical model is presented to study the effects of bedrock erodability, thrust displacement rate, pulsating tectonic activity and sea-level fluctuations on drainage basin morphology and stratigraphy of large alluvial fan systems. A low value of the bedrock erodability coefficient K b (0.5×10 −4 year −2/3 ) vs. thrust rate (2.0 m/ky) is associated with a time lag between cessation of tectonic activity and maximum sedimentation rates observed on the alluvial fans. Applying higher K b values, ranging between 1.0 and 8.0×10 −4 year −2/3 , results in higher sediment yields due to more rapid headward catchment erosion and elaborate sideways branching of drainage networks. Pulsating tectonic activity is reflected in a stratigraphic alternation of prograding and retrograding alluvial fan gravels. During tectonic activity, fan gravel fronts and coastlines retreat because catchment yields are insufficient to fill the accommodation space created by the flexural response due to thrust loading. Phases of tectonic quiescence and cessation of flexural subsidence are indicated by progradation of the gravel front. Depending on the position in the basin, the lag time for arrival of the gravel associated with tectonic cessation is several tens to hundred thousands of years. A combination of pulsed tectonic activity with sinuous sea-level fluctuation leads to a more complex stratigraphic pattern. In that case, stratigraphic response to tectonic pulses is masked by a similar but higher-order frequency-stacking pattern, especially in the more distal parts.

Astronomically induced paleoclimatic oscillations reflected in Pliocene turbidite deposits on Corfu (Greece): Implications for the interpretation of higher order cyclicity in ancient turbidite systems

The growth pattern of fan lobes in the early Pliocene Corfu turbidite system (Greece) demonstrates that regional climatic fluctuations in the land-locked Mediterranean completely concealed global glacioeustatic effects. The temporal evolution of the fan lobes studied closely matches the astronomical precession cycle in this time interval (∼23 ka), strongly suggesting that regular waxing and waning of the sediment supply to the delta-fed turbidite system were directly driven by changes of precipitation and continental runoff. The common lack of high-resolution time control in ancient turbidite systems may lead to mistaken interpretations of higher order cyclicity in ancient turbidite sequences produced by precession-induced variations of sediment supply at low paleolatitudes.

Analogue and numerical modelling of sedimentary systems : from understanding to prediction

Preface. Numerical simulation of the syn- to post-depositional history of a prograding carbonate platform: the Rosengarten, Middle Triassic, Dolomites, Italy ( Axel Emmerich, Robert Tscherny, Thilo Bechstadt, Carsten Buker, Ullrich A. Glasmacher, Ralf Littke and Rainer Zuhlke ). Fine-scale forward modelling of a Devonian patch reef, Canning Basin, Western Australia ( Claude-Alain Hasler, Erwin W. Adams, Rachel A. Wood and Tony Dickson ). Structural, reverse-basin and forward stratigraphic modelling of the Southern Cantabrian Basin, northwest Spain ( Zbynek Veselovsky, Thilo Bechstadt and Rainer Zuhlke ). Numerical modelling of alluvial deposits: recent developments ( John Bridge ). Process-based stochastic modelling: meandering channelized reservoirs ( Simon Lopez, Isabelle Cojan, Jacques Rivoirard and Alain Galli ). Simulation of tidal flow and circulation patterns in the Early Miocene (Upper Marine Molasse) of the Alpine foreland basin ( Ulrich Bieg, Michael Peter Suss and Joachim Kuhlemann ). Predicting discharge and sediment flux of the Po River, Italy since the Last Glacial Maximum ( Albert J. Kettner and James P.M. Syvitski ). Impact of discharge, sediment flux and sea-level change on stratigraphic architecture of river-delta-shelf systems ( George Postma and Aart Peter van den Berg van Saparoea ). Grain-size sorting of river-shelf-slope sediments during glacial-interglacial cycles: modelling grain-size distribution and interconnectedness of coarse-grained bodies ( Xander D. Meijer 0. Modelling the preservation of sedimentary deposits on passive continental margins during glacial-interglacial cycles ( Xander D. Meijer, George Postma, Peter A. Burrough and Poppe L. de Boer ). Modelling source-rock distribution and quality variations: the organic facies modelling approach ( Ute Mann and Janine Zweigel ). Spatial data templates: combining simple models of physical processes with stochastic noise to yield stable, archetypal landforms ( Peter A. Burrough ). Models that talk back ( John C. Tipper ). Index.

Tectonic and climaticsetting of lithographic limestone basins

Abstract Lithographic limestones consist largely of micrite, they generally lack bioturbation, and may contain well preservedwhole body fossils. The presence of contemporaneous strike-slip faults, the related en echelon arrangement of basins, characteristic fault patterns, and high local subsidence rates indicate that part of the known lithographic limestone basins were formed by syn-sedimentary strike-slip tectonics. Lithographic limestones have been formed in both hemispheres, at palaeolatitudes of 15° to 40°. The arid climate in this zone enhanced the development of hypersaline waters in the (semi)-enclosed and/or silled basins, inhibiting many forms of biologic life, and easily leading to stratification of the water column, thus enhancing anoxic conditions in deeper parts of the basins. The arid climate, moreover, prevented a significant fluvial runoff, so that terrigenous input was largely absent, and fine-grained carbonate, produced by algae and bacteria in the upper part of the water column, and possibly on surrounding platforms, could dominate the sedimentary system. The conditions which are apparently needed for the formation of lithographic limestone also would favour the development of evaporite deposits. Their common absence in lithographic limestone basins could be due to chemical conditions in the basins and/or to the presence of a permeable subsurface, through which dense brines could flow back to adjacent open marine deep waters.

Six commercially viable ways to remove CO2 from the atmosphere and/or reduce CO2 emissions

BackgroundThe burning of fossil fuels is the main cause of rising CO2 levels of the atmosphere. This will probably result in climate change. Another consequence is ocean acidification. Although these consequences are not yet proven beyond doubt, the risk of doing nothing is too large. The simplest response is the removal and sustainable storage of CO2. By reaction with basic minerals, nature has sequestered almost all of the CO2 that has ever been released by the planet. This weathering continues to play a role but nature cannot cope with the ongoing much (30 to 60 times) higher rates of anthropogenic greenhouse gas emissions.ResultsIn this paper six approaches are described which take advantage of the natural process of weathering and solve other problems as well, thereby making them cost-effective. All six make the maximum use of natural conditions (climate, tides, currents), natural materials (olivine, serpentine), and organisms (diatoms, hyperaccumulator plants).ConclusionsImpacts on the environment are minimized or even turned into benefits.

Geological record of marine tsunami-backwash: the role of the hydraulic jump

Tsunamis are marked by distinct phases of uprush during coastal inundation and backwash when tsunami water recedes. Especially in the case of a steep coastal profile, the return flow may operate in a Froude‐supercritical regime, eroding the flooded area and transporting large volumes of sediment seawards. Important sediment accumulation occurs when the supercritical flow goes through a hydraulic jump where it becomes subcritical upon deceleration. An inferred example in coarse‐grained, mixed carbonates from the Lower Pleistocene on Rhodes (Greece) is described, with offshore bars up to 10 m long with scour‐and‐fill structures and steep antidune stratification. In finer‐grained sandy depositional systems such structures may be much longer, up to hundreds of metres. It is suggested here that, analogous to some turbidite beds, the apparent lack of structures or the presence of faint stratification that is common for graded sand layers within marine tsunamiites may in fact consist of extremely low‐angle, landward‐dipping backset‐strata that formed under a landward‐migrating hydraulic jump during the basinward retreat of tsunami water. Numerical simulations that focus on the internal stratification of backwash‐generated offshore bars support this hypothesis. The recognition of such deposits in the sedimentary record enlarges the toolbox for assessing the past frequency of tsunamis in coastal areas.

Fault-block rotation controlling the distribution of fluvial sediments; a quantitative test on a Lower Pennsylvanian (Carboniferous) cyclothem succession

Depositional models of axial fluvial systems in half‐grabens predict that the fluvial‐sandstone percentage increases towards the downthrown side of a fault, because channel systems tend to migrate to the area of maximum subsidence. This migration is at the expense of mudstone, but floodplain deposition occurs near faults occasionally. The models assume gradual, transverse tilting and no external base‐level change, and their applicability to cases involving tectonics and/or sea‐level change may therefore be restricted. Here, a quantitative analysis is presented on a subsurface data set from a Lower Pennsylvanian cyclothem succession, which formed under conditions of differential subsidence and fluctuating sea‐level. The studied interval is wedge‐shaped and shows a systematic thickness increase from 165 to 245 m, controlled by syndepositional fault‐block tilting. It comprises three depositional units, bounded by coal groups. These units display an upward change from wedge‐shaped (75 to 120 m) to tabular (42 to 55 m). Despite their variable thickness, the units contain almost equal amounts of ca 45 m of floodplain deposits, plus ca 5 m of encased channel sandstones, in all boreholes. Where units are thicker, the remaining thickness comprises fluvial‐braidplain sandstone. This arrangement indicates that the units represent equal time periods, during which background subsidence allowed the deposition of thin channel sands and overbank mud on a level floodplain. Occasional tilting produced additional accommodation space, which was completely filled by sand‐dominated braided systems. The temporary cessation of floodplain‐mud deposition suggests that aggradation of the river system could not keep up with floodplain tilting. In addition, bypass of floodplain fines may have been promoted by a basin parallel tilting component. It is shown that (i) cases in which the standard models fully apply, and (ii) cases in which differential subsidence is too strong or too abrupt, can be distinguished by analysing cross‐plots of cumulative‐sandstone and cumulative‐mudstone thickness.