1. Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: testing the effects of paleolatitudinal gradient
- Author
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Azmy, Karem and Jin, Jisuo
- Subjects
Canadian Shield -- Natural history ,Brachiopods -- Chemical properties -- Observations ,Geochemistry -- Methods ,Ordovician period ,Earth sciences - Abstract
Dalmanelloid brachiopod shells were collected from the Upper Ordovician Lexington Formation (lower Katian) of Kentucky, Sheguindah Shale (middle Katian) on Manitoulin Island, Ontario, and the Stony Mountain Formation (upper Katian) in the Winnipeg area, Manitoba. They were investigated to test the hypothesis of paleo-latitudinal zonation of the shelly benthos. A multi-technique approach was applied to evaluate the petrographic and geochemical (isotopic and elemental) preservation of the secondary layer of shells. Preliminary conventional microscopy, cathodoluminescence (CL), and scanning electron microscopy (SEM) confirmed the retention of primary shell ultrastructure (prismatic low-Mg calcite). The geochemical diagenesis proxies (e.g., Sr, Mn, Fe, and [summation]REE) show insignificant correlations with the [delta][.sup.18]O and [delta][.sup.13]C values, thus supporting the preservation of at least near-primary geochemical compositions. Among the three lots of shells, the mean [delta][.sup.18]O value is the highest in those from the Lexington Formation (-4.5[per thousand] [+ or -] 0.3[per thousand] VPDB), lowest from the Stony Mountain Formation (-6.8[per thousand] [+ or -] 0.4[per thousand] VPDB), and intermediate from the Sheguindah Shale (-6.0[per thousand] [+ or -] 0.8[per thousand] VPDB). The relative gradient in [delta][.sup.18]O increase is in agreement with the paleo-latitudinal gradient, with Kentucky in subtropical, southern Ontario in mid-tropical, and southern Manitoba in subequatorial latitudes. The Lexington Formation shells also have the highest mean [delta][.sup.13]C value (0.8[per thousand] [+ or -] 0.2[per thousand] VPDB) and relatively high P contents (170 [+ or -] 27 ppm), suggesting higher organic productivity, which is consistent with previous interpretation of frequent upwelling of nutrient-rich cool waters along the southeastern margin of Laurentia during the Katian. The Lexington shells also have a lower mean Th/U (0.6 [+ or -] 0.6), which is consistent with blooming organic productivity that likely led to more consumption of oxygen in the water column. Des coquilles de brachiopodes dalmanelloides ont ete recueillies de la Formation de Lexington de l'Ordovicien superieur (Katien inferieur) du Kentucky, du shale de Sheguindah (Katien moyen) de l'ile Manitoulin (Ontario) et de la Formation de Stony Mountain (Katien superieur) de la region de Winnipeg (Manitoba). Elles ont ete examinees pour verifier l'hypothese d'une zonation paleolatitudinale du benthos a coquilles. Une approche faisant appel a plusieurs techniques a ete utilisee pour evaluer la preservation petrographique et geochimique (isotopique et elementaire) de la couche secondaire des coquilles. La microscopie classique preliminaire, la cathodoluminescence (CL) et la microscopie electronique a balayage (MEB) ont confirme la retention d'une ultrastructure primaire dans les coquilles (calcite pauvre en Mg prismatique). Des caracteres geochimiques temoignant de la diagenese (p. ex. Sr, Mn, Fe et [summation]ETR) montrent des correlations significatives avec les valeurs de [delta][.sup.18]O et de [delta][.sup.13]C, appuyant l'interpretation de la preservation de compositions geochimiques au moins quasi primaires. Parmi les trois lots de coquilles, la valeur de [delta][.sup.18]O moyenne est la plus elevee pour celui de la Formation de Lexington (-4,5 [per thousand] [+ or -] 0,3 [per thousand] VPDB), la plus faible pour la Formation de Stony Mountain (-6,8 [per thousand] [+ or -] 0,4 [per thousand] VPDB) et intermediaire pour le shale de Sheguindah (-6,0 [per thousand] [+ or -] 0,8 [per thousand] VPDB). Le gradient relatif d'augmentation du [delta][.sup.18]O concorde avec le gradient paleolatitudinal dans lequel le Kentucky se trouve a des latitudes subtropicales, le sud de l'Ontario, a des latitudes mid-tropicales et le sud du Manitoba, a des latitudes subequatoriales. Les coquilles de la Formation de Lexington presentent egalement le [delta][.sup.13]C moyen le plus eleve (0,8 [per thousand] [+ or -] 0,2 [per thousand] VPDB) et des concentrations de P relativement fortes (170 [+ or -] 27 ppm), ce qui indiquerait une plus grande productivite organique, en accord avec l'interpretation anterieure de remontees frequentes d'eaux froides riches en nutriments le long de la bordure sud-est de la Laurentie au Katien. Les coquilles de la Formation de Lexington ont aussi un rapport Th/U moyen plus faible (0,6 [+ or -] 0,6), ce qui concorde avec une productivite organique fleurissante qui s'est vraisemblablement traduite par une consommation d'oxygene accrue dans la colonne d'eau. [Traduit par la Redaction], Introduction The abundance of planktonic and benthic foraminifers in the Cenozoic marine carbonates provided suitable material for the geochemical studies that reconstructed the environmental variations during that time interval (e.g., [...]
- Published
- 2019
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