Abstract
AbstractThe Late Cambrian Steptoean Positive Carbon Isotope Excursion marks a time of significant change in ocean chemistry and trilobite faunas. On the lead up to the carbon isotope excursion and at the excursion itself, there is global evidence from Laurentia and Gondwana of cementation by primary aragonite in shallow subtidal environments accompanied by deposition of aragonitic ooids. However, this occurred at a time widely considered to have been characterised by ‘calcite seas’ when the primary inorganic phases (marine cements and ooids) are normally presumed calcitic. This study has investigated the chemostratigraphy of the Middle–Late Cambrian Port au Port Group, Newfoundland, including the early marine cements. Here, the marine cements contain increasing concentrations of strontium towards the peak carbon isotope excursion (up to 5500 ppm at the peak excursion) before dropping off post‐peak excursion, consistent with the original cements having been aragonitic. This trend is accompanied by relict oomouldic porosity, again suggesting an aragonitic precursor. Primary inorganic mineralogy is largely controlled by the Mg/Ca ratio of sea water but estimates of the Mg/Ca ratio of Late Cambrian oceans are variable (0.8–2). At this level, other factors such as water temperature and pCO2 have been shown to affect mineralogy with warm waters and high levels of CO2 favouring aragonite. It is possible that the warm waters and anoxia that caused the carbon isotope excursion created conditions favourable for the precipitation of aragonite at the same time as major trilobite faunal turnover.
Highlights
The Cambrian was a time of significant evolution of life in the oceans with two major evolutionary explosions (Phase 1 and 2) and extinctions during the Sinsk event and, much more significant, at the Steptoean Positive Carbon Isotope Excursion (SPICE) in the Late Cambrian (Furongian) (Zhuravlev & Wood, 2018)
In a study of Furongian marine cements from the Al Bashair Formation, Oman (Gondwana), Neilson et al (2016) concluded that conditions suitable for the precipitation of aragonite and high-Mg calcite (HMC) may have existed around the SPICE, a time generally believed to be of stable ‘calcite’ seas (Stanley et al, 2010)
This study shows that the depositional environment during the SPICE varied slightly between the March Point Section (MPS) and Felix Cove section (FCS) and that a direct lithostratigraphic correlation is not possible
Summary
The Cambrian was a time of significant evolution of life in the oceans with two major evolutionary explosions (Phase 1 and 2) and extinctions during the Sinsk event and, much more significant, at the Steptoean Positive Carbon Isotope Excursion (SPICE) in the Late Cambrian (Furongian) (Zhuravlev & Wood, 2018). The present study adds X-ray fluorescence elemental concentration data and extensive microprobe analyses of the early marine cements found in the PJF This allowed the construction of a detailed chemostratigraphy for the Middle Cambrian Series 3 and Late Cambrian Furongian on the Port au Port Peninsula and, through investigation of the early marine cements, provides important information regarding oceanic chemistry at the time. The PAP is interpreted as being of shallow-m arine origin, deposited on the outer area of a stable carbonate platform adjacent to the Iapetus Ocean (Cowan & James, 1993; Palmer & James, 1979; Williams, 1979) These predominantly carbonate deposits, with episodic siliciclastic incursions, record the gradual transition from siliciclastic deposition in Cambrian Series 2 (Hawke Bay Fm., Labrador Group) to predominantly carbonate sedimentation in the early. Stratigraphy suggests that a potential third locality may exist on the coastline between the FCS and MPS at 48°30′25.6′′N/59°06′29.9′′W, no geochemical data are available (Barili et al, 2018)
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