Oliver Lehnert

Carbon isotope record of Late Cambrian to Early Ordovician carbonates of the Argentine Precordillera

A more than 2000-m-thick Cambrian–Ordovician carbonate platform succession developed on the exotic Argentine Precordillera terrane during rifting from Laurentia and drifting towards Gondwana. On base of these carbonates, a carbon isotope curve could be developed for the Cambrian–Ordovician. We measured δ13Ccarb and δ13Corg values on bulk rocks, selected components and diagenetic cements. Whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis, most subtidal carbonates exhibit primary marine values. We can report reliable curves for the Middle–Late Cambrian transition and for the latest Cambrian to earliest Middle Ordovician. The δ13C curve matches well with the published data from global Cambrian–Ordovician boundary sections. Excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy. This indicates the interdependence on sea level which rules the productivity and/or preservation of organic carbon and therefore the partition between Corg and Ccarb burial.

Origin of planktotrophy—evidence from early molluscs

SUMMARY The size of early ontogenetic shells (protoconchs) of ancient benthic molluscs suggests that feeding larvae occurred at about 490 myr (approximately, transition from Cambrian to Ordovician). Most studied Ordovician protoconchs were smaller than Cambrian ones, indicating smaller Ordovician eggs and hatchlings. This suggests substitution of nutritious reserve matter such as yolk by plankton as an energy source for larvae. The observed size change represents the first direct empiric evidence for a late Cambrian to Ordovician switch to planktotrophy in invertebrate larvae. It corroborates previous hypotheses about a possible polyphyly of planktotrophy. These hypotheses were primarily based on molecular clock data of extant clades with different types of larva, change in the overall body size, as well as increasing predation pressure on Early Paleozoic sea floors. The Early Ordovician is characterized by an explosive radiation of benthic suspension feeders and it was suggested that planktotrophy would prolongate escape from benthic predation on hatchlings. This biological escalation hypothesis does not fully explain why planktotrophy and suspension feeding became important at the same time, during a major biodiversification. An additional factor that probably included availability of nutrients must have played a role. We speculate that an increasing nutrient supply and availability of photoautotrophic plankton in world oceans have facilitated both planktotrophy and suspension feeding, which does not exclude a contemporaneous predation‐driven escalation. It is very likely that the evolution of planktotrophy as well as increasing predation contributed to the Ordovician radiation.

Did intense volcanism trigger the first Late Ordovician icehouse

Oxygen isotopes measured on Late Ordovician conodonts from Minnesota and Kentucky (United States) were studied to reconstruct the paleotemperature history during late Sandbian to Katian (Mohawkian–Cincinnatian) time. This time interval was characterized by intense volcanism, as shown by the prominent Deicke, Millbrig, and other K-bentonite beds. A prominent carbon isotope excursion (Guttenberg δ 13 C excursion, GICE) postdates the Millbrig volcanic eruptions, and has been interpreted to reflect a drawdown of atmospheric carbon dioxide and climatic cooling. The oxygen isotope record in conodont apatite contradicts this earlier interpretation. An increase in δ 18 O of 1.5‰ (Vienna standard mean ocean water) just above the Deicke K-bentonite suggests an abrupt and short-lived cooling that possibly initiated a first short-term glacial episode well before the major Hirnantian glaciation. The decrease in δ 18 O immediately after the mega-eruptions indicates warming before the GICE, and no cooling is shown in the GICE interval. The coincidence of the Deicke mega-eruption with a cooling event suggests that this major volcanic event had a profound effect on Late Ordovician (late Mohawkian) climate.

A new upper Middle Ordovician-Lower Silurian drillcore standard succession from Borenshult in Ostergotland, southern Sweden: 2. Significance of delta C-13 chemostratigraphy

A total of 239 isotope samples are used for establishing the δ13C chemostratigraphy in the upper Middle Ordovician to Lower Silurian succession in the approximately 70 m long Borenshult drillcore. The study interval starts in the upper Darriwilian Furudal Limestone and ends in the Rhuddanian Motala Formation. Four named δ13C excursions are recognized in 3–4 formations, namely the Guttenberg isotope carbon excursion (GICE) in the lower-middle Freberga Formation, the Kope (Rakvere) excursion in the uppermost Freberga Formation and possibly the Slandrom Formation, the Whitewater (Moe) excursion in the Lower Member of the Jonstorp Formation and the Hirnantian isotope carbon excursion (HICE) in the Loka Formation. The Middle Darriwilian isotope carbon excursion (MDICE) is missing in the drillcore and it is suggested that the study succession starts just above the interval of this excursion, which is in agreement with the range of MDICE in other Baltoscandic successions. The widespread Waynesville (Saunja) excursion may be cut out by the prominent unconformity below the Fjäcka Shale. The two most conspicuous excursions are the GICE (peak value ∼+1.9‰) and the HICE (peak value ∼+3.7‰), whereas the other excursions are represented by relatively minor perturbations in the δ13C curve. The Borenshult drillcore chemostratigraphy, which has been closely tied to conodont biostratigraphy, is quite similar to that of Estonian drillcores and is useful for not only trans-Baltic but also trans-Atlantic correlations. It is the first drillcore-based δ13C chemostratigraphy from the Swedish Ordovician.

A new upper Middle Ordovician–Lower Silurian drillcore standard succession from Borenshult in Östergötland, southern Sweden: 1. Stratigraphical review with regional comparisons

A recent drilling at Borenshult near Motala resulted in discovery of the stratigraphically most complete succession through the upper Darriwilian–Rhuddanian interval known in Östergötland. The approximately 70 m long drillcore succession is subdivided into eight formations, the oldest being the late Darriwilian Furudal Limestone and the youngest being the Rhuddanian Motala Formation. Conodonts are used for a detailed biostratigraphic classification of the Borenshult drillcore into three subzones of the Pygodus serra Zone, two subzones of the Pygodus anserinus Zone, and three subzones of the Amorphognathus tvaerensis Zone. The base of the Amorphognathus superbus Zone is taken to be ∼10 m above the Kinnekulle K-bentonite, that of the Amorphognathus ordovicicus just below the Fjäcka Shale, and that of the Ozarkodina hassi Zone at the base of the Middle Member of the Loka Formation. Because of its unique lithology and paleontology and its wide geographic occurrence, this member is formally named herein the Skultorp Member. The previously uncertain stratigraphical position of the internationally known “Borenshult fauna” is shown to correlate with the Skultorp Member. A regional comparison of the Borenshult drillcore succession shows it to be most similar to coeval successions in Västergötland and Dalarna but there are some significant regional differences. The average rate of net rock accumulation during late Darriwilian and Sandbian time is calculated to be ∼3–4 mm/ka.

The stratigraphical record of the Argentine Precordillera and its plate-tectonic background

Abstract The stratigraphical record of the Argentine Precordillera from Early Cambrian to Late Devonian times reveals its plate-tectonic history from incipient rifting and the evolution of a marginal platform to the separation from Laurentia, its drift in higher latitudes and amalgamation with Gondwana. This pre-Carboniferous succession can be subdivided into four supersequences bounded by major unconformities and their main features are discussed with respect to plate-tectonic implications. The basal two supersequences which include the carbonate platform deposits are subdivided into 13 third-order sequences, each with a duration of 2–10 Ma. Supersequence A reflects intracratonic rifting, creating a graben system and forming a marginal plateau. In its in higher part a progradational carbonate complex covered the entire platform. Supersequence B shows the development of an aggradiational carbonate succession and the evolution of reef ecosystems comparable to those that developed around the margins of the Ouachita embayment. It also shows the demise of the carbonate platform by drowning. Deposits of Supersequence C reflect crustal extension and rifting, which led to the final separation of the Precordillera from mainland Laurentia. Supersequence D reveals the approach, and probably the accretion, of the Argentine Precordillera to Gondwana.

New Ordovician-Silurian drill cores from the Siljan impact structure in central Sweden: an integral part of the Swedish Deep Drilling Program

New drill cores from the largest known impact structure in Europe, the relict of the Siljan meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia. The impact crater is an important target of the project “Concentric Impact Structures in the Palaeozoic” within the framework of the “Swedish Deep Drilling Program”. Two core sections, Mora 001 and Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus, this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the Caledonian collision between Baltica and Laurentia.

Origin of planktotrophy—evidence from early molluscs: a response to Freeman and Lundelius

Based on the study of Early Paleozoic protoconchs, we (Nutzel et al. 2006) concluded that planktotrophic mollusc larvae are unknown from the Cambrian but became common in the Ordovician. Therefore, we assumed that planktotrophy itself evolved at the Cambrian/Ordovician transition and that it is unlikely that planktotrophy represents the original state in Metazoa. This is in agreement with other authors who followed different lines of evidence (see Nutzel et al. 2006). Freeman and Lundelius (FL Riedel 2000). However, such gastropods are known from the Cretaceous onwards only. Scissurellids and some other Vetigastropoda represent another important exception. They are nonplanktotrophic (as all vetigastropods) but have a small initial shell (scissurellids are known from the Triassic onwards). However, all these exceptions do not change the general picture that at a shell length of about 100mm, most planktotrophic molluscs are smaller than most nonplanktotrophic molluscs. Even in recent gastropods, it is commonly hard or impossible to determine the boundary between embryonic shell (protoconch I) and larval shell (protoconch II) and it becomes even more difficult in fossil material that is almost always recrystallized so that fine structures are obscured. Generally, protoconch preservation is rare in the Paleozoic. Most Late Paleozoic to extant gastropods have a tightly coiled initial whorl so that it is possible to measure the diameter of the first whorl if it is preserved. Most planktotrophic gastropods hatch with about one whorl (Figs. 1A and 2J) (e.g., Bandel 1975) and therefore the diameter of the initial whorl can be used to infer the approximate size of the hatchling even if the transition from protoconch I to II is not visible (Nutzel 1998). The oldest gastropods with well-preserved larval shells have been EVOLUTION & DEVELOPMENT 9:4, 313 –318 (2007)

Cambrian phosphatic brachiopods from the Precordillera of western Argentina

Abstract The first Cambrian phosphatic brachiopods (Subphylum Linguliformea, Class Lingulata) are described from the San Juan Precor-dillera of western Argentina, including Neotreta tumida Sobolev, Quadrisonia minor Rowell & Henderson, Prototreta cf. mimica Bell, Curticia sp., Opisthotreta sp. and the new species Dactylotreta sanjuan-ensis and Picnotreta robusta. The faunas are of Middle to Late Cambrian age and originate from Cambrian allochthonous material within the Ordovician Los Sombreros Formation, as well as from material from within the parautochthonous Upper Cambrian/Lower Ordovician Cerro Pelado and El Relincho formations. The described Argentinean faunas are most similar to contemporaneous assemblages from southeastern Siberia, but are also comparable with lingulate faunas from North America and Greenland.

d(13)C chemostratigraphy in the Lower-Middle Ordovician succession of Oland (Sweden) and the global significance of the MDICE

Based on the Tingskullen drillcore, we present the first continuous carbon isotope stratigraphy from the Lower–Middle Ordovician “orthoceratite limestone” of Öland, Sweden. The extremely condensed Tremadocian and Floian stages include large gaps as well as the Ceratopyge Regressive Event and the widespread Evae transgression accompanied by prominent shifts in the δ 13C record. The Dapingian and Darriwilian stages are characterised by low sedimentation rates and a relatively complete sedimentary record. A total of 99 whole-rock samples were analysed for carbon isotope geochemistry from the Ordovician part of the succession (46 m thick). The most striking anomaly detected is the middle Darriwilian isotope carbon excursion (MDICE) that appears unusually well developed and complete for the region, and thus forms an important proxy for intercontinental correlation of the succession.