Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: testing the effects of paleolatitudinal gradient

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 ΣREE) show insignificant correlations with the δ18O and δ13C values, thus supporting the preservation of at least near-primary geochemical compositions. Among the three lots of shells, the mean δ18O value is the highest in those from the Lexington Formation (–4.5‰ ± 0.3‰ VPDB), lowest from the Stony Mountain Formation (–6.8‰ ± 0.4‰ VPDB), and intermediate from the Sheguindah Shale (–6.0‰ ± 0.8‰ VPDB). The relative gradient in δ18O 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 δ13C value (0.8‰ ± 0.2‰ VPDB) and relatively high P contents (170 ± 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 ± 0.6), which is consistent with blooming organic productivity that likely led to more consumption of oxygen in the water column.

Population structure of Lingula (Bruguière, 1791) in Alue Naga waters, Banda Aceh City, Indonesia

“…This relationship can be estimated through the distribution trends of length and weight data obtained from the measurement of the morphometric components. The estimation of parameter b, the coefficient of the length and weight relationship, is analyzed using a power regression approach simplified through linear transformation (Petetta et al, 2019;Azmy and Jin, 2019).…”

Introduction to “Advances in low-temperature geochemistry, diagenesis, and seawater and climate evolution through the Earth’s history: a tribute to Ján Veizer”

“…In a paper by Azmy and Jin (2019), the authors apply a petrographic and geochemical approach to test the hypothesis of paleolatitudinal zonation of shelly benthos. They did not only utilize the LA-ICPMS for analysing the shell's secondary layer but also introduced it as a technique for cleaning the shell's primary layer, which is a reliable, fast, and effective approach particularly for investigating small fossil shells.…”