Mikael Calner

Silurian global events – at the tipping point of climate change

Mass extinction events affect a wide breadth of ecosystems and are one of the major driving mechanisms behind evolution, origination, and diversification of taxa. Such dramatic turnovers have therefore played a significant role in the history of life. The ‘major five’ mass extinctions (Raup and Sepkoski 1982) have received intense attention over the last 25 years, starting with the K-Pg impact hypothesis launched by Luis Alvarez and others in Science in 1980. By comparison, few studies have concerned the about sixty smaller-scale bioevents that are scattered throughout the last ca 543 million years (Barnes et al. 1995), although their architecture often resembles that of the large scale mass-extinctions and they therefore may hold important information relevant to events in general. The Silurian Period is the shortest time period of the entire Phanerozoic (~443-416 Ma) but yields a significant record of ocean-atmospherebiosphere changes. The period has traditionally been described as an environmentally and faunally stable period in Earth history – a greenhouse period with a moderate latitudinal climate gradient and impoverished marine faunas slowly recovering from the end-Ordovician mass extinction. It has more recently, however, been shown that the Silurian was no quieter than any other time interval during the Phanerozoic Eon, rather the opposite. The results of an immense number of studies, primarily in the fields of taxonomy, biostratigraphy and stable isotope stratigraphy clash fundamentally with the old view. The new studies have unequivocally shown that repeated marine biodiversity crises took place, affecting e.g. graptolites, conodonts, chitinozoans, acritarchs, brachiopods and reefs, and that these turnovers were closely linked to abrupt and significant changes in oceanography and the global carbon cycle. Accordingly, today the Silurian can be regarded as one of the most volatile periods of the entire Phanerozoic when considering the ocean-atmosphere system (Cramer and

The Silurian Mulde Event and a scenario for secundo–secundo events

Graphic correlation using graptolites and conodonts provides a high-resolution timescale for correlating from coastal to deep oceanic sections and, thereby, also a detailed record of the sequence of changes during the Mulde Secundo-Secundo Event. That interval includes sedimentary facies otherwise unknown in older Wenlock to early Ludlow strata on Gotland. The identified sequence of changes includes a detailed record of, in order: two extinctions (Datum points 1 and 1-5); widespread deposition of carbon-rich sediments extensive enough to cause a delta(13)C increase of c. 4.8parts per thousand, the onset, maximum and end of a sea-level fall and rise of at least 16m during 30 kyr; a third extinction (Datum 2); a disaster fauna; and a slow faunal recovery. Thus, a secondary result of the event was a weakened greenhouse effect triggering a glaciation: the Gannarve Glaciation (new term). The order of changes proves that regression did not cause the extinctions. Faunal and sea-level changes, as well as the sedimentary succession, fit well with predictions based on an oceanic model. Extinctions were primarily caused by a severe drop in primary planktonic productivity, causing starvation among planktonic larvae in non-coastal settings. The Grotlingbo Bentonite (new term), the thickest in the Wenlock of Gotland, was deposited across the basin shortly after Datum 2. Temporal resolution is high enough to permit some comparison with Quaternary glaciations. (Less)

A latest Llandovery to latest Ludlow high-resolution biostratigraphy based on the Silurian of Gotland - a summary

Abstract A succession of 26 conodont zones and 63 successive subzones and conodont faunas have been identified in the exposed latest Landovery to latest Ludlow strata of Gotland, Sweden. All zones, and several of the subzones and faunas, have been identified elsewhere on Baltica, as well as on other palaeocontinents, indicating that this zonation serves well as the standard zonation for calcareous successions. The average duration of the 24 Wenlock and Ludlow zones was 396±208 ka, using the latest radiometric timescale. However, most zones and subzones formed during the widely recognised Silurian oceanic events correspond to less time per unit—in the order of 30 to 100 ka—and most of those formed during intervening episodes correspond to a few 100 ka. The average duration of the 61 Wenlock and Ludlow zones, subzones, and faunas was only 156±82 ka, i.e., among the highest biostratigraphical resolution available for Phanerozoic epochs. On Gotland the combined known maximum thickness of the strata has increased to more than 750 m.

Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden

Evidence from sedimentology and conodont biostratigraphy is used to reinterpret the mid-Homerian (Late Wenlock) succession on Gotland, Sweden. A new conodont zonation includes from below: the Ozarkodina bohemica longa Zone (including five subzones), the Kockelella ortus absidata Zone and the Ctenognathodus murchisoni Zone (two taxa are named, Ozarkodina bohemica longa and Pseudooneotodus linguicornis). These new zones are integrated with facies in order to correlate strata and infer the major depositional environments and the controls on deposition during the mid-Homerian Mulde Event. Reef-associated and skeletal carbonate deposition predominated before and after the event, i.e. during the uppermost O. s. sagitta Zone and, again, in the C. murchisoni Zone. These periods are characterized by the expansion of reefs and shoal facies across marls in the topmost Slite Group on eastern Gotland and in the lower parts of the Klinteberg Formation on western Gotland, respectively. The intervening O. b. longa and K. o. absidata zones are initially characterized by rapid facies changes, including siliciclastic deposition, and later stabilisation of a carbonate depositional system. The composition of sediments and depositional rates are closely related to the creation and destruction of accommodation space and reflects a classical case of depositional bias of the carbonate and siliciclastic depositional systems. Based on coastline migration, stratal boundaries, and the stratigraphic position of major reef belts, several facies associations can be fitted into a sequence stratigraphic model for platform evolution. A highstand systems tract (HST) situation prevailed prior to, and during the early part of the event; the upper Slite Group including the lower Frjel Formation. This HST was characterized by prolific skeletal production and regional reef development except for during the latest stage when carbonate production declined at the onset of the Mulde Event. Platform growth was inhibited during a following regressive systems tract (RST) when regional siliciclastic deposition predominated; the Gannarve Member. The subsequent lowstand resulted in regional emersion and karstification, i.e. a complete termination of the platform. The post-extinction transgressive systems tract (TST) is exclusively composed of non-skeletal carbonates; the Bara Member of the Halla Formation. Re-occurrence of reefs and a prolific skeletal production marks platform recovery during a second HST; the remaining Halla and the lower Klinteberg formations. Integration of high-resolution biostratigraphy and sequence stratigraphy reveals that the major physical control on platform evolution was a 5th order eustatic sea-level change during an early part of the Mulde Event, and that the bulk of the strata accumulated when the platform aggraded and prograded during the highstand systems tracts. Thus, Silurian oceanic events and associated sea-level changes had profound impact on the neritic carbonate system. The Gotland-based middle and late Homerian sea-level curve shows two rapid regressions, both leading to truncation of highstand systems tracts. The first lowstand occurred at the very end of the C. lundgreni Chron, and the second at the end of the Co.? ludensis Chron. The intervening interval was characterized by stillstand or possibly slow transgression.

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 shoreline to deep basin correlation chart for the middle Silurian coupled extinction-stable isotopic event

Abstract A shoreline to deep basin correlation chart for the middle Silurian coupled extinction-stable isotopic event is presented. The chart is a first attempt to synthesise new stratigraphic, faunal and stable isotopic data, as well as diversity changes among organic-walled microplankton (acritarchs), from the Grötlingbo-1 and Hunninge-1 cores on Gotland (Sweden), along with previously published data from the Bartoszyce IG 1 core (Poland), the Gräfenwarth section (Germany), and the Ruhnu (500) and Ohesaare cores (Estonia). Accordingly, the relative timing of biotic, sedimentary, and stable isotopic changes associated with the event can be studied in high stratigraphic resolution across a basin area of ∼120 000 km2 (in the epicontinental Baltic Basin) and further off the Silurian continental margin into the Rheic Ocean (Gräfenwarth section). A graptolite biozonation is presented for the Late Wenlock of the Grötlingbo-1 core and the middle Silurian carbon isotope excursion (CIE) is for the first time detected in stratigraphically continuous sections on Gotland. It starts close above the LAD of Monograptus flemingii and peaks at δ13C values of 2.4‰ in the Grötlingbo-1 core and at 3.8‰ in the Hunninge-1 core. The correlation chart forms the basis for several important conclusions for the mid-Silurian event: (a) it is associated with major and synchronous sedimentary changes across the basin independent of palaeodepth, i.e., the entire marine realm, from shoreline to deep basin, was affected; (b) the onset of the CIE is within the latest part of a regressive systems tract and related to carbonate platform exposure and termination; (c) the CIE peak interval correlates with the early transgressive systems tract and the development of laminated strata in the deeper parts of epicontinental seas, as well as with increased accumulation of organic matter in even deeper basins; (d) the major diversity anomalies among planktonic and nektonic taxa precede or occur close to the onset of the CIE.

Stratigraphy, facies development, and depositional dynamics of the Late Wenlock Fröjel Formation, Gotland, Sweden.

A lithostratigraphic subdivision of the topmost Slite Beds on western Gotland is proposed, viz. the Frojel Formation (9-11 m) including the Svarvare Mudstone Member (2-3 m) and the Gannarve Siltstone Member (7-8 m). The Frojel Formation resulted from increased siliciclastic deposition within an intracratonic carbonate platform setting during the mid-Homerian (Wenlock, Silurian). The temporal and spatial development of sedimentary facies and the characters of formation boundaries can be explained with a sequence stratigraphic approach to depositional dynamics. The accelerating influx of siliciclastic material was connected to a sea-level fall which therefore initiated the formation of a gradationally based parasequence (the Gannarve Member). The depositional trend is supported by both the facies (upward decrease of accommodation space) and faunal development. The Svarvare Mudstone Member reflects a late highstand period, when hemipelagic deposition and weak, small-scale density currents contributed to sedimentation. The subsequent short period of shallowing resulted in deposition of the overlying Gannarve Siltstone Member, consisting of siltstone tempestites and associated fair-weather mudstones deposited in successively decreasing palaeodepth. The shallowing culminated with the formation of an unconformity, which marks the top of the Cyrtograptus lundgreni Biozone, and which is overlain by oolites. This unconformity correlates to the transgressive surface in offshore settings. The siliciclastic deposition is regarded as regional for this part of the Baltic Basin, influencing an area from offshore east Oland to Saaremaa west of Estonia. The depositional history of the Frojel Formation is discussed in terms of relative and eustatic sea-level changes and with respect to regional correlation of the mid-Homerian eustatic regression. (Less)

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.

Transgressive oolites onlapping a Silurian rocky shoreline unconformity, Gotland, Sweden.

Stratigraphical relationships, facies and petrographical evidences define a rocky shoreline in the Late Wenlock of Gotland. The major facies-elements forming the rocky shoreline are 1) several subaerially exposed reefs, which were eroded during falling base-level and which form the topography, and 2), onlap of oolite grainstones which were deposited during the subsequent rise of the base-level. Five localities along the rocky shoreline are discussed in detail here, viz., Svalings 1 where the unconformable boundary is accessible, and Bara 1-4 where the petrography of the onlapping oolite has been studied. A regression at the end of the latest Cyrtograptus lundgreni Biozone resulted in emersion of parts of the Gotland carbonate platform. Erosion during the short time interval of maximum lowstand (top C. lundgreni Biozone) caused the topography of this rocky shoreline and resulted in truncations, corrosion and vadose silt in the exposed reefs. The following parvus-nassa Biozone starts with the transgressive Bara Oolite Member (lowermost Halla Formation) which onlaps the southern flanks of the youngest reef generation of the Slite Group. The rocky shoreline can be traced for 11 km striking SW-NE. Calculated base-level for the erosion is situated more than 16 metres below the top of the highest preserved part of one Slite reef, indicating the probable minimum magnitude of the sea-level drop. Detailed petrographic work on the onlapping oolite is consistent with the interpretation that it was formed in the tidal zone of the shores bordering the palaeocoast. The Slite Rocky Shoreline Unconformity (SRSU) is herein considered as the platform-proximal equivalent to the sequence boundary separating the regressive Frojel Formation and the transgressive Bara Oolite Member (lowermost Halla Formation) in the Klintehamn area on western Gotland. The rocky shoreline development is taken as the most reliable evidence of a rapid regressive-transgressive cycle in the mid-Homerian of Gotland. (Less)

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.