Abstract

The Ediacaran Shuram Excursion (SE) was a major negative δ¹³C_carb excursion (to −12‰) thought to reflect significant atmospheric-oceanic oxidation, although direct evidence is limited. Here, we model changes in seawater sulfate concentrations ([SO₄²⁻]_sw) during the SE by using paired S-isotope data of cogenetic sedimentary pyrite and carbonate-associated sulfate from the Ediacaran Doushantuo Formation (South China), making independent calculations for inner-shelf and upper-slope sections. Our results show a significant increase of [SO₄²⁻]_sw toward the peak of the SE and a large proximal-to-distal gradient across the Yangtze Platform. Our findings provide direct evidence for transient oxidation of marine-shelf water masses during the SE, providing insights into Ediacaran oceanic oxygenation, the origin of the SE, and contemporaneous bioevolutionary events.

Variation in seawater sulfate concentrations ([SO₄²⁻]_sw) during the late Ediacaran Shuram Excursion at Jiulongwan (JLW) and Siduping (SDP) sections in South China and coeval global fossil records. C-isotope intervals (Ediacaran positive excursion 2 [EP2] to Ediacaran negative excursion 3 [EN3c-2]) are as in Figure 1. [SO₄²⁻]_sw (solid lines) was calculated per “rate method” of Algeo et al. (2015), using average Δ³⁴S for time bins based on C-isotope intervals (top). EP2 at SDP yielded [SO₄²⁻]_sw of ∼0 mM owing to Δ³⁴S <0‰, and [SO₄²⁻]_sw is not shown for EN3c at JLW due to low sample total inorganic carbon content (<3%). Results of sensitivity test for age model uncertainties are shown by dashed lines: shifting age anchor points at 575 and 560 Ma by −5 m.y. and +5 m.y. leads to higher and lower [SO₄²⁻]_sw estimates, respectively, relative to baseline values (solid lines); note that these estimates are plotted strictly against carbon-isotope intervals at top instead of time scale at bottom. Fossil data are from Xiao and Laflamme (2009) and Grazhdankin (2004). Max., Avg., Min.—maximum, average, minimum; SSF—small shelly fauna.

Title: Sulfur isotope evidence for transient marine-shelf oxidation during the Ediacaran Shuram Excursion

Authors: Wei Shi, Chao Li*, Genming Luo, Junhua Huang, Thomas J. Algeo, Chengsheng Jin, Zihu Zhang,and Meng Cheng

Source: Geology

DOI: 10.1130/G39663.1.

Link to the paper: https://pubs.geoscienceworld.org/gsa/geology/article/526708/sulfur-isotope-evidence-for-transient-marine-shelf