Reconstructing Neoproterozoic seawater chemistry from early diagenetic dolomite

Abstract

The pairing of calcium and magnesium isotopes (δ44/40Ca, δ26Mg) has recently emerged as a useful tracer to understand the environmental information preserved in shallow-marine carbonates. Here, we applied a Ca and Mg isotopic framework, along with analyses of carbon and lithium isotopes, to late Tonian dolostones, to infer seawater chemistry across this critical interval of Earth history. We investigated the ca. 735 Ma Coppercap Formation in northwestern Canada, a unit that preserves large shifts in carbonate δ13C values that have been utilized in global correlations and have canonically been explained through large shifts in organic carbon burial. Under the backdrop of these δ13C shifts, we observed positive excursions in δ44/40Ca and δ7Li values that are mirrored by a negative excursion in δ26Mg values. We argue that this covariation is due to early diagenetic dolomitization of aragonite through interaction with contemporaneous seawater under a continuum of fluid- to sediment-buffered conditions. We then used this framework to show that Tonian seawater was likely characterized by a δ7Li value of ~13‰ (~18‰ lower than modern seawater), as a consequence of a different Li cycle than today. In contrast, δ13C values across our identified fluid-buffered interval are similar to modern seawater. These observations suggest that factors other than shifts in global seawater chemistry are likely responsible for such isotopic variation.

A lithium-isotope perspective on the evolution of carbon and silicon cycles

“…23). Yet, critically, Sr/Ca ratios and δ 44/40 Ca analysis can be used to track the burial offset from seawater in shallow-water carbonates 23,32 (see SI), and there is no evidence for a systematic change in the mode of early marine diagenesis through time that could explain the observed ~ 15‰ increase in mean carbonate δ 7 Li values (or a priori reason to expect such a change). Additionally, we used a suite of commonly employed geochemical filters to constrain primary mineralogy (Sr/Ca, Mg/Ca), track detrital contamination (Al/Ca, Rb/Ca) and diagenetic alteration (Mn/Sr, Pb/Ca) (see SI for further detail).…”

Local and global controls on carbon isotope chemostratigraphy

“…As a consequence, several studies have suggested alternative models for generating such negative δ 13 C values, including (1) short-term transient perturbations to the carbon cycle (e.g., Schrag et al 2002;Rothman et al 2003;Bjerrum and Canfield 2011), (2) carbonate diagenesis (Derry 2010;Knauth and Kennedy 2009) or (3) the formation of authigenic carbonate minerals (Tziperman et al 2011;Schrag et al 2013;Laakso and Schrag 2020). However, these models do not consider the possibility of broadly synchronous shifts in δ 13 C DIC in platform waters (Swart 2008;Ahm et al 2019;Crockford et al 2020), that may differ across individual basins on time-scales >10 5 years, and therefore are unrelated to changes in global DIC -analogous to observations from recent carbonate platforms.…”

“…from the average ocean value (Bold et al 2020;Crockford et al 2020). Various mechanisms have been proposed to explain the origin of very depleted δ 13 C values in Neoproterozoic platform interior carbonates but, as of yet, no single hypothesis is accepted widely.…”

“…Modeling studies of carbonate diagenesis that combine measurements of δ 13 C, δ 44/40 Ca, and Sr/Ca ratios, have demonstrated that by embracing the diagenetic history of ancient carbonate it is possible to derived more accurate records of seawater chemistry (Ahm et al 2018). Specifically, carbonate successions that have experienced early marine diagenesis and/or dolomitization may be important, and yet-to-be explored, archives of ancient seawater chemistry (Ahm et al 2019;Hoffman and Lamothe 2019;Crockford et al 2020). Finally, by accepting that shallow-water sedimentary records largely reflect local carbon cycle dynamics, we may find that δ 13 C measurements instead reflect the local fingerprints of important climatic and evolutionary processes.…”

Isotopic responses of magnesium to two types of dissolution-reprecipitation processes for the growth of the double-carbonate mineral norsethite

“…Magnesium is a common component in carbonates and its isotopes have been applied to paleoenvironmental and paleographic studies (Crockford et al, 2020;Fantle and Higgins, 2014;Gothmann et al, 2017;Higgins and Schrag, 2012;Li et al, 2015;Liu et al, 2018;Pogge von Strandmann et al, 2014). Moreover, isotopes can provide unique information for mineral dissolution, precipitation, as well as ion exchange between minerals and fluids (e.g., Chanda et al, 2019;Frierdich et al, 2019;Gorski and Fantle, 2017;Liu and Li, 2020;Zhu et al, 2020).…”