Lennart Jeppsson

The optimal acetate buffered acetic acid technique for extracting phosphatic fossils

When extracting or cleaning phosphatic fossils using acetic acid, the solutions must be buffered with calcium acetate to a pH greater than 3.6 for conodont and many fish taxa, and to a slightly greater pH for some conulariid, shark, and some fish taxa. Diagrams of the changes in pH and density during dissolution, rate of dissolution, and productivity per liter of acid provide a means for selecting the economically optimal process, monitoring the progress, and adjusting the composition of the solution when necessary.

A buffered formic acid technique for conodont extraction

Formic acid must be buffered with both calcium carbonate and calcium phosphate to be suitable for extraction of conodont elements and other phosphatic fossils from dolomites. All unbuffered solutions and those buffered with only calcium carbonate or calcium phosphate destroy the phosphatic fossils. Experimental effects using different concentrations and different combinations of phosphate and calcium carbonate are diagrammatically illustrated. A 10 percent formic acid solution with 20 g calcium carbonate and 1.2 g tricalcium phosphate per liter leaves conodont elements undamaged even for a period of 129 days. One liter of this solution dissolves 95 g dolomite.

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.

Silicification and bentonites in the Silurian of Gotland

Abstract A list of 25 localities with silicified fossils is given. Silicification was selective. At most of the localities, only the bryozoans and brachiopods are affected. Their skeletons originally consisted of low-magnesium calcite. Silicified gastropods (original shells mainly aragonite) and crinoids (original skeletons probably high-magnesium calcite) occur only at three localities. Seven other taxa are also represented. The origin of the silica was tephra, as is indicated by the close connection between the bentonites and the levels with siHcified fossils. The source area for the tephra was probably south of Gotland.

Conodont-based revisions of the late Ludfordian on Gotland, Sweden

Abstract The Late Ludlow topmost Hemse-Burgsvik succession includes more substantial and more rapid facies changes than most older and younger intervals on Gotland. A revised conodont zonation for this interval includes three zones, the Polygnathoides siluricus, the Icriodontid, and the Ozarkodina snajdri zones and four subzones, the Upper P. siluricus, the Lower, the Middle and the Upper Icriodontid subzones. The three zones are approximately coeval with the Neocucullograptus kozlowskii graptolite fauna, the succeeding impoverished graptolite fauna and the Monograptus balticus/Pseudomonoclimacis latilobus Zone (= the lower M. formosus Zone), respectively. The new zonation permits the first high-resolution correlations across Gotland, despite very large differences in facies from SW to NE. Another result is a more detailed stratigraphic subdivision and revised boundaries of most of the units. The Mill-klint, the main and the Botvide members of the När Fm. (new, the upper part of the Hemse Group), the lower, middle, and upper Eke Fm., and the Burgsvik Fm. are distinguished. Well known names are kept, as far as possible, but the lateral extent of their boundaries is revised, resulting in a considerable increase in precision and very different thickness data, e.g. the Ludlow is calculated to be somewhere between 337 and 425 m instead of the 215 m given hitherto. Faults or disturbances with similar effects are identified for the first time on Gotland. Silurognathus maximus is named.

Notes on Some Upper Silurian Multielement Conodonts

Seventeen types of conodont elements, fifteen previously described as form species, have been grouped into three multielement species. The type species of the form genera Ozarkodina Branson and Mehl, 1933, Plectospathodus Branson and Mehl, 1933, Spathodus Branson and Mehl, 1933 (non Boulenger, 1900), and Spathognathodus Branson and Mchl, 1941 are all elements of the same conodont apparatus. Hindeodella Bassler, 1925 is probably the oldest generic name applicable to this type of multielement species. The apparatuses of the multielement species investigated appear to consist of five or six different types of elements that can readily be homologized from species to species. This fact appears to offer a most useful basis for the continued grouping of form species into multielement species within the genera Hindeodella. and Ligonodina.

Wenlock metabentonites from Gotland, Sweden: geochemistry, sources and potential as chemostratigraphic markers

Two metabentonite suites occur within the Wenlock limestones and marls of Gotland, one within the Slite Formation (M. belophorus to the C. ellesae biozones) and the other in the Mulde Formation (G. nassaM. dubius Biozone). Their geochemical characteristics based on rare-earth element (REE) distributions in apatite crystals separated from the metabentonites suggest origins from three separate volcanic sources. One of these sources has an alkaline affinity, reflected in relatively high levels of Th, Nb and Zr, which suggests that it may have lain over thickening continental crust during the waning stages of the closure of the Tornquist Sea, while the other two represent calc-alkaline magmas. The source of the volcanic ash is placed some 400 km to the southwest of Gotland, on the TornquistTeysseyre Zone, which is the northernmost expression of the Trans-European Suture Zone. Distinctive differences in REE distribution in apatite from these metabentonites should assist in future correlation studies in Wenlock stratigraphy, both within the Baltic region and further afield. (Less)

A new technique to separate conodont elements from heavier minerals

The density of conodont elements is between 2.90 g cm-3 and 3.04 g cm-3, probably very close to 3.00. Removing all traces of dolomite and calcium carbonate before density separation keeps the viscosity of sodium polytungstate solution low enough to make it useful for density separations as high as 3.04 g cm-3. In most cases, a nearly clean phosphatic fraction can be produced by adding this method to existing standard techniques. The oxidizing technique to remove pyrite is also discussed. In all kinds of physical concentration work consideration of the “bulk density” is very important for good results, especially regarding coniforms.