Chromium isotopes in carbonates — A tracer for climate change and for reconstructing the redox state of ancient seawater

Comment on "Bilaterian Burrows and Grazing Behavior at >585 Million Years Ago"

“…These ichnofossils are identical to the Tacuarí material (1), which is shown here in Fig. 2, A, D, and E. At site CCU 22, the succession is undeformed, flat-lying, and unconformably overlies folded late Ediacaran carbonates and shales of the Arroyo del Soldado Group (13,14). It is in turn overlain by Early Permian sandstones of the Tres Islas Formation (2).…”

Acritarchs of the Ediacaran Frecheirinha Formation, Ubajara Group, Northeastern Brazil

“…Sedimentological evidence of such an event is so far lacking. Chemostratigraphic studies of the carbonates , Chiglino 2013 reported negative δ 13 C values (−3.5 ‰) at the base of the unit that may be related to periods of low plankton bioproductivity or sea level fluctuations (Gaucher et al 2004a, Frei et al 2011. The dark gray to black, laminated limestones of the Araticum section, start with a negative δ…”

“…Instead, a plateau around +3‰ VPDB characterizes most of the unit and post-dates a negative excursion, similar to that observed for the late Ediacaran Nama and Corumbá groups (Grotzinger et al 1995, Boggiani et al 2010; Table I). Likewise, a negative excursion followed by moderately positive values is also observed in the late Ediacaran upper Polanco Formation (Arroyo del Soldado Group, Uruguay), , Frei et al 2011. Thus, the chemostratigraphic data suggest an Ediacaran age as the most probable depositional age of the Frecheirinha Formation, but so far do not allow a more refined geochronology.…”

Correlations of some Neoproterozoic carbonate-dominated successions in South America based on high-resolution chemostratigraphy

“…This is probably due to their pure calcitic composition, which determined only very limited metamorphic reactions between different mineral phases. In the particular case of the Jucurutu Formation, Nascimento et al (2007) show that near-primary, unaltered 87 (Halverson et al 2005(Halverson et al , 2010, although similar 87 Sr/ 86 Sr values also occur in the Ediacaran (to which ferruginous deep-waters were also associated) as registered in Cloudina-bearing limestones in Uruguay Frei et al 2011) and Argentina (Gómez Peral et al 2007). One cannot discard, however, that seawater has likely undergone important changes of its 87 Sr/ 86 Sr ratios after major Neoproterozoic glacial events due to input of continental waters from ice melting and strong weathering in continents, blurring the true seawater 87 Sr/ 86 Sr values during deposition of carbonates (Zhao et al 2009).…”

“…Chromium stable isotopes in Fe-rich chemical sediments deposited during glacial events are suitable for tracing oxygenation of surface environments through Earth's history. Besides, Cr isotopes in carbonates may be useful as a tracer for climate change and for reconstructing the redox state of ancient seawater (Frei et al 2011;Bonnand et al 2013).…”