New isotope constraints on the Mg oceanic budget point to cryptic modern dolomite formation

Abstract

The oceanic magnesium budget is important to our understanding of Earth’s carbon cycle, because similar processes control both (e.g., weathering, volcanism, and carbonate precipitation). However, dolomite sedimentation and low-temperature hydrothermal circulation remain enigmatic oceanic Mg sinks. In recent years, magnesium isotopes (δ26Mg) have provided new constraints on the Mg cycle, but the lack of data for the low-temperature hydrothermal isotope fractionation has hindered this approach. Here we present new δ26Mg data for low-temperature hydrothermal fluids, demonstrating preferential 26Mg incorporation into the oceanic crust, on average by εsolid-fluid ≈ 1.6‰. These new data, along with the constant seawater δ26Mg over the past ~20 Myr, require a significant dolomitic sink (estimated to be 1.5–2.9 Tmol yr−1; 40–60% of the oceanic Mg outputs). This estimate argues strongly against the conventional view that dolomite formation has been negligible in the Neogene and points to the existence of significant hidden dolomite formation.

Understanding controls on hydrothermal dolomitisation: insights from 3D Reactive Transport Modelling of geothermal convection

“…It has been more than 35 years since Land (1985) recognised that seawater is the only widely available fluid with the capacity to form extensive bodies of replacement dolomite, although reaction kinetics can limit the rate of dolomitisation at near-surface temperatures (Arvidson and Mackenzie, 1999). Recent geochemical studies (Ryb and Eiler, 2018;Shalev et al, 2019) have presented isotopic data consistent with large-scale dolomitisation by convection of seawater through carbonate sediments. In contrast, over the last decade a major focus of sedimentological research has been to understand the genesis of dolostones developed along extensional or transtensional fault systems.…”

“…Finally, dolomitisation associated with hydrothermal and geothermal circulation of seawater has been suggested as an important but poorly constrained sink for Mg in seawater (Shalev et al, 2019). These processes thus have fundamental implications for understanding the global budgets of Mg, Ca and C, which are linked by the common controls of weathering, volcanism and carbonate precipitation (Arvidson et al, 2011;Elderfield, 2010;Holland, 2005).…”

Understanding controls on hydrothermal dolomitisation: insights from 3D reactive transport modelling of geothermal convection

“…The economic importance of HTD often lies in the associated hydrothermal dissolution and/or mineralisation, in particular Mississippi Valley-type Pb-Zn ore deposits (Davies and Smith, 2006;Qing and Mountjoy, 1994). More controversially, fluids thought to have formed faultrelated HTDs have also been interpreted as being responsible for the formation or overprinting of large volumes of matrix-replacement dolomite that form important hydrocarbon reservoirs and infer basin-wide hydrothermal activity (Al-Aasm, 2003;Davies and Smith, 2006;Sharp et al, 2010;Weissenberger et al, 2006). Machel and Lonnee (2002) were particularly critical of this suggestion and argued strongly against the use of the term hydrothermal (as distinct from geothermal) without evidence that the dolomites formed at temperatures of at least 5-10 • C higher than the country rock.…”

Fracture dolomite as an archive of continental palaeo-environmental conditions

“…20 and references therein), should be revisited regarding their applicability in palaeo-environmental reconstructions. Furthermore, if one could demonstrate that Quaternary, continental and deep marine, dolomite deposits are more widespread than previously thought, recent uncertainties in the oceanic Mg budget could be much better constrained: Based on an extensive δ 26 Mg isotope dataset it has been proposed that a significant, yet undetermined, dolomitic sink should exist ever since the Neogene 62 . Our study results demonstrate dolomite formation via Ca-carbonate precursors at cool to ambient Earth surface temperatures within a geologically short timeframe, which is consistent with the sub-recent dolomite formation in deepmarine basins proposed recently 62 .…”

“…Furthermore, if one could demonstrate that Quaternary, continental and deep marine, dolomite deposits are more widespread than previously thought, recent uncertainties in the oceanic Mg budget could be much better constrained: Based on an extensive δ 26 Mg isotope dataset it has been proposed that a significant, yet undetermined, dolomitic sink should exist ever since the Neogene 62 . Our study results demonstrate dolomite formation via Ca-carbonate precursors at cool to ambient Earth surface temperatures within a geologically short timeframe, which is consistent with the sub-recent dolomite formation in deepmarine basins proposed recently 62 . It is up to future studies to quantify the fluxes of Mg linked to the formation of dolomite and its precursors in different continental and marine sedimentary basins and to estimate the relative contribution of such environments to the global Mg cycling and budget.…”