Early Silurian (Wenlockian) δ¹³C profiles from the Cape Phillips Formation, Arctic Canada and their relation to biotic events

Abstract

Geochemical data from the Cape Phillips Formation, Arctic Canada, are examined in association with three Silurian biotic crises in the graptolite community; the early Wenlockian Ireviken, mid Wenlockian Cyrtograptus lundgreni, and end Wenlockian Colonograptus ludensis extinction events. Positive δ13Corg excursions are associated with the Ireviken and C. lundgreni events, but not the Co. ludensis Event. The Ireviken and C. lundgreni excursions are recognized worldwide and are herein interpreted to be the result of carbonate weathering in response to eustatic sea-level drop. The C. lundgreni excursion is of greater magnitude in the more proximal basin margin section at Abbott River, Cornwallis Island, and is explained by the amplification of a more strongly positive δ13C signature in shallower parts of an epeiric basin during increased exposure and weathering of the carbonate shelf. Excursion C5, within the C. praedeubeli – C. deubeli Zone, is also of regional significance, as it occurs in both the Abbott River section and Twilight Creek section on Bathurst Island, and is also recognized in Estonia, Poland, and England. Excursion C6 is recognized in the Gorstian Stage, yet its regional significance remains equivocal. There is a reasonable general agreement between the shape of the δ13Corg and δ13Ccarb curves, yet the δ13Ccarb curve is largely a record of detrital carbonate derived from the shelf. The δ13Corg curve represents extraction of dissolved inorganic carbon by plankton and thus is more indicative of ambient paleoceanographic conditions. These data are valuable in that they provide a detailed secular marine δ13C curve for the Wenlockian of Arctic Canada from relatively unaltered sections of varying facies whose ages are well constrained by graptolite biostratigraphy.

Carbon isotope chemostratigraphy of the Llandovery in Arctic Canada: Implications for global correlation and sea-level change

“…If the δ 13 C carb data represent the composition of materials transported laterally from the adjacent, shallow shelf setting and the δ 13 C org is measured in organic matter that settled mainly vertically within the deeper-water environment, then the differing trends seen in the δ 13 C carb and δ 13 C org data through the uppermost Rhuddanian and lower Aeronian could be the result of differences in the δ 13 C values of the seawater in the shallow shelf versus more offshore slope environments, respectively. This interpretation is supported by shelf gradients in seawater δ 13 C DIC values that developed in the Cape Phillips Basin during the sea-level low stands in the Hirnantian (Melchin & Holmden in press) and Homerian (Noble et al 2005). The interpretation is based on differences in peak magnitude values of positive δ 13 C excursions reconstructed from proximal shelf and more distal deposits.…”

At what stratigraphical level is the mid Ludfordian (Ludlow, Silurian) positive carbon isotope excursion in the type Ludlow area, Shropshire, England?

“…the sedgwickii Zone (late Aeronian, Llandovery) excursion in Arctic Canada (Melchin & Holmden 2006); early Sheinwoodian excursion in Wales (Loydell & Frýda 2007, Cramer et al 2010) and early Sheinwoodian and mid Homerian excursions in Arctic Canada (Noble et al 2005).…”

First evidence for the Homerian (late Wenlock, Silurian) positive carbon isotope excursion from peri-Gondwana: new data from the Barrandian (Perunica)

“…Because of its typical shape (double-peak) as well as its supposed stratigraphic position (i.e., Homerian) we interpret the δ 13 C excursion as the Homerian δ 13 C excursion. The latter isotopic excursion has been hitherto documented from several sections on Baltica (e.g., Wenzel & Joachimski 1996;Samtleben et al 1996;Kaljo et al 1997Kaljo et al , 2003Kaljo et al , 2007Kaljo & Martma 2006;Calner et al 2006Calner et al , 2012Calner 2008), Laurentia (e.g., Lenz 1993Saltzman 2001;Noble et al 2005Noble et al , 2012Lenz et al 2006;Cramer et al 2006Cramer et al , 2010 and Avalonia (e.g., Corfield et al 1992;Loydell 2007;Ray et al 2011a, b;Marshall et al 2012). It is noteworthy that all hitherto recorded occurrences of the Homerian carbon isotope excursion have been described from palaeoplates that subsequently formed the supercontinent Laurussia (i.e., from Laurentia, Baltica and Avalonia).…”

“…Correlation of the Homerian δ 13 C excursion with the more precise graptolite zonations is not without problems (compare data from Kaljo et al 2007;Cramer et al 2006Cramer et al , 2011Noble et al 2012). Some uncertainties are probably influenced by a low density of chemostratigraphic and/or biostratigraphic samples, by different local graptolite zonations (Kaljo & Martma 2006;Lenz et al 2006;Loydell 2007Loydell , 2012Noble et al 2005) and maybe also by not exactly synchronous appearances of zonal graptolite taxa in different areas of Laurentia, Avalonia andBaltica (e.g., Loydell 1998, 2007;Cramer et al 2006Cramer et al , 2011Noble et al 2005Noble et al , 2012Marshall et al 2012). Present summaries of the stratigraphic position of the Homerian δ 13 C excursion (Cramer et al 2011) suggest that the excursion started in the uppermost part of the Cyrtograptus lundgreni Biozone, reaches its maximum within the Pristiograptus parvus-Gothograptus nassa Biozone and ends within the Colonograptus deubeli-C. praedeubeli Biozone.…”

“…These geochemical events were closely linked to major crises in the marine ecosystem as well as to palaeoclimatic changes (see Jeppsson 1998;Munnecke et al 2003;Noble et al 2005Noble et al , 2012Loydell 2007;and Calner 2008 for reviews). Some of those perturbations in the carbon cycle have been documented globally (see Calner 2008 for review) and as such they have become important stratigraphic tools for global correlation of Silurian successions.…”