Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: Evidence from clumped isotope paleothermometry

Abstract

a b s t r a c tThe Wonoka-Shuram Anomaly represents the largest negative carbon isotope excursion recognized in the geologic record and is associated with the emergence and diversification of metazoan life ca. 580 million years ago (Ma). The origin of the anomaly is highly debated, with interpretations ranging from primary to diagenetic, each having unique and potentially transformative implications for early life. Here, we apply carbonate clumped isotope thermometry to three sections expressing the anomaly in order to constrain mineral formation temperatures and thus directly calculate water oxygen isotope compositions (␦ 18 O w ) with which carbonate minerals equilibrated. With ␦ 18 O w known, it is possible to address previous hypotheses for the origin of the anomaly. In each section, precipitation temperatures correlate positively with reconstructed ␦ 18 O w . Previous hypotheses, based on the covariance of ␦ 18 O carb vs. ␦ 13 C carb (uncorrected for temperature effects), suggested a meteoric diagenetic origin for the anomaly. However, reconstructed ␦ 18 O w values do not covary with carbon isotope compositions (␦ 13 C carb ) within anomaly facies. Rather, the oxygen isotope and temperature data are consistent with carbonate recrystallization and equilibration under increasingly rock-buffered conditions. Based on simple modeling and comparison to modern formation fluids, recrystallization may have occurred in an environment far removed from the initial depositional or early diagenetic regime. In addition, although clumped isotope temperatures vary significantly and reach elevated values consistent with burial diagenesis, it is unclear to what degree, if at all, carbon isotope values were reset during recrystallization. Ultimately, these new data indicate that Wonoka-Shuram-aged carbonates experienced equilibration with fluids under increasingly closedsystem conditions. The clumped isotope data do not provide a means to distinguish previous hypotheses outright, but provide additional context for the evaluation of geochemical signatures within these ancient carbonate rocks.

Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China

“…O carb that is generally lower than normal seawater values likely reflects a superimposed effect by both calcite-dolomite fractionation and burial diagenesis as evidenced by clumped isotopes (Loyd et al, 2015). If true, our data support the view that δ…”

“…O carb is less rock-buffered and more sensitive to diagenetic alteration than δ 13 C and 87 Sr/ 86 Sr (Lohmann, 1988;Banner and Hanson, 1990;Jacobsen and Kaufman, 1999;Bishop et al, 2014;Loyd et al, 2015 [Ba] are new data from this study. Other data are from Zhou et al (2012).…”

“…Clumped isotope analyses of other time-equivalent strata (Bergmann, 2013;Loyd et al, 2015) suggest different degrees of diagenetic overprint by increasing fluid-rock interaction during deep burial. This may partially support the late diagenesis model proposed by Derry (2010a), but cannot be reconciled with a variety of sedimentological observations that strongly point to an early authigenic origin .…”

Calibration of the dolomite clumped isotope thermometer from 25 to 350 °C, and implications for a universal calibration for all (Ca, Mg, Fe)CO3 carbonates

“…Carbonate clumped isotope thermometry has thus been particularly useful for reconstructing temperatures in times and locations where the isotopic composition of water is unknown, as for reconstructing paleoenvironmental or diagenetical histories (Came et al, 2007, Bristow et al, 2011, Ferry et al, 2011, Finnegan et al, 2011, Dale et al, 2014, Loyd et al, 2015.…”

Kapcsolt izotópok (clumped isotopes) a földtudományi kutatásokban

“…Átdolomitosodott karbonátokat (FERRY et al 2011, SENA et al 2014, LOYD et al 2015, MILLÁN et al 2016, vagy a diagenetikus fluidumokból kivált dolomitot (BRISTOW et al 2011) vizsgálva a diagenetikus fejlődéstörténetre lehet következtetéseket levonni, míg földfelszíni körülmények között kivált, elsődleges dolomitokból paleohőmérsékleti rekonstrukció készíthető (VANDEVELDE et al 2013, DALE et al 2014. Viszonylag kevés tanulmány foglalkozott a mé -lyen elteme tett karbonátos rezervoárokkal, mivel a hőmér -sékleti kalib rációk eddig a 100 °C alatti hőmérsékleti tartományra kor látozódtak.…”