Showing papers by "Per Ahlberg published in 2016"

Ediacaran–Cambrian phosphorites from the western margins of Gondwana and Baltica

Abstract: The late Ediacaran–mid Cambrian occurrence s of phosphorites in the wes-tern Mediterranean region (West Gondwana) and southern Sweden (north -west Baltica) are related to the poleward drift of West Gondwana and thenorthern drift of Baltica. As a result, these regions crossed subtropical andtemperate palaeolatitudes of the Southern Hemisphere, in which oceanicupwelling and high organic productivity were the probable sources of phos-phorus along their western margins. A parallel south-west/north-east migra-tion trend of evaporites and delayed, time transgressive phosphorites pointto distal upwelling environments associated with shallower arid conditions.Concentration of hardground-derived phosphate accumulations took placeby repeated alternations of low sedimentation rates and condensation,in situ early diagenetic and microbially mediated precipitation of amorphousand cryptocrystalline, carbonate-bearing francolite, winnowing and poly-phase reworking of previously phosphatized skeletons and hardground-derived clasts. Thicker phosphorites of economic importance display aspiculate sponge-thromboid consortium that points to a distinct benthiccommunity developed on suboxic, slope to basinal substrates. For mostphosphorite samples an up to ten-fold enrichment in rare earth elementconcentration relative to post-Archaean average Australian shale reflects sig-nificant incorporation of rare earth elements and yttrium after deposition.Considerable scavenging took place from anoxic pore waters, followingreductive dissolution of iron oxides as evidenced from the medium rareearth element enrichment in most samples. This contrasts with the rare pre-servation of primary sea water features, such as cerium depletion andyttrium enrichment, in phosphatized thrombolites from the shallower realms(for example, Montagne Noire). The data are consistent with phosphogenesisafter upwelling and transgression of nutrient-rich, suboxic–anoxic, ferrugi-nous waters over more oxygenated, rifting settings and inland portions of anevolving passive margin.

The dipnoan buccal pump reconstructed in 3D and implications for air breathing in Devonian lungfishes

Abstract: Lungfishes are known for, and indeed take their name from, their bimodal respiratory abilities. All three extant genera can use their lungs to extract oxygen from the atmosphere, although their reliance upon this capability differs among taxa. Lungs are considered primitive for the Osteichthyes, however the distinctive buccal pump mode of air gulping exhibited by extant lungfishes appears to be a specialization. It is associated with a number of derived skeletal characters (cranial ribs, long parasphenoid stalk, midline gap between palatal tooth plates) that first appeared during the Devonian. These have been described individually, but in no Devonian lungfish has their three-dimensional (3D) spatial relationship been reconstructed and analyzed. Here we present the 3D morphology of Rhinodipterus, a Mid-Late Devonian lungfish from Australia and Europe, based on synchrotron tomography and conventional microtomography scans.Unlike less crownward contemporaneous lungfishes such as Griphognathus and Chirodipterus, Rhinodipterus has a full set of skeletal buccal pump components that can be directly compared to those of extant lungfishes, suggesting that it made more extensive use of air breathing than other Gogo or Bergisch Gladbach genera. This is interesting in relation to the environmental context as Gogo and Bergisch Gladbach are both marine, contrasting with the frequently hypoxic tropical to subtropical fresh water environments inhabited by modern lungfishes. The evolution of buccal pump-supported lung ventilation was evidently not necessarily associated with a transition to non-marine habitats.

The cranial endocast of Dipnorhynchus sussmilchi (Sarcopterygii: Dipnoi) and the interrelationships of stem-group lungfishes

Abstract: The first virtual cranial endocast of a lungfish from the Early Devonian, Dipnorhynchus sussmilchi, is described. Dipnorhynchus, only the fourth Devonian lungfish for which a near complete cranial ...

Cambrian stratigraphy of the Tomten-1 drill core, Västergötland, Sweden

Abstract: The Tomten-1 drilling at Torbjorntorp in Vastergotland, southern Sweden, penetrated 29.85 m of Cambrian Series 2, Cambrian Series 3, Furongian, and Lower–Middle Ordovician strata. Lithostratigraphically, the succession includes the File Haidar, Borgholm and Alum Shale formations, and the Latorp and Lanna limestones. The drill core succession is described herein for the first time, with special focus on the biostratigraphy of the Cambrian Alum Shale Formation. In the Cambrian Series 3, through Furongian Alum Shale Formation, agnostoids and trilobites have been identified to species level and the succession is subdivided into nine biozones (in ascending order): the Ptychagnostus gibbus, Ptychagnostus atavus, Lejopyge laevigata, Agnostus pisiformis, Olenus gibbosus, Parabolina spinulosa, Ctenopyge tumida, Ctenopyge bisulcata and Ctenopyge linnarssoni zones. The succession is interrupted by numerous stratigraphic gaps of variable magnitudes, as is evident from the biostratigraphy and conspicuous uncon...

EVENT DEPOSITION IN SHALES – WOODFORD SHALE, OKLAHOMA, USA, AND BJøRKåSHOLMEN FORMATION, SWEDEN

Abstract: Shales from two Paleozoic marine settings, the Devonian Woodford Shale of Oklahoma, USA, and siliciclastic mudstone intercalations into the Ordovician Bjørkåsholmen Formation carbonates of Sweden, exhibit distinct millimeterto sub-millimeter laminae (present thickness) in thin section whereas in outcrop they both appear massive. The Devonian example shows a two-fold subdivision of laminae with a dark-colored lower and a light-colored upper part. The lower portion of the lamina contains abundant pyrite and, locally, silt-size detrital quartz and carbonate grains at its base. In the central to upper part of these laminae, pyrite is a minor constituent, significantly less than at the base. Planolites burrows cluster at the lamina top. Phycosiphon isp. fecal strings and Tasmanites algae are present throughout, but the fecal strings rarely penetrate the pyrite-rich lower part of these lamina. The siliciclastic mudstones in the Bjørkåsholmen Formation carbonates either contain abundant shell debris and phosphate clasts, or consist of intercalated lightand dark-colored bands of irregular geometries that are one to several millimeters thick. Phycosiphon isp. is present throughout. In the Woodford Shale, the laminated texture and the presence of Planolites burrows just at the lamina tops suggests lamina deposition in pulses, followed by non-deposition and limited bioturbation. The siliciclastic mud was carried as bed load indicated by local, basal silt-size lags, or as fluid mud. In either case, deposition was a result of an event, probably storms, from which was deposited one discrete millimeter-thick mudstone lamina. This mudstone lamina was subsequently colonized from the top downward in a dysoxic water column resulting in Planolites traces only at the top, and fecal strings diminishing in abundance downward. Only the basal portion of some of the thicker laminae remained largely undisturbed which is where the pyrite precipitated. The Bjørkåsholmen Formation mudstones also show lag deposits, both shell debris and phosphate clasts, which probably originated from currents that moved by bed-load transport. Similar to observations from the Recent Eel delta (USA), the Bjørkåsholmen Formation mudstones were most likely deposited by a series of storm events. This study therefore demonstrates the significance of event deposition (not suspension settling) in the accumulation of important source rocks and unconventional reservoirs, and under dysoxic-oxic conditions.