Authigenic carbonate formation rates in marine sediments and implications for the marine δ13C record

Abstract

Carbon isotope (δ13C) variations measured in carbonates have been attributed to large-scale phenomena throughout Earth history, such as changes in atmospheric oxygen or global glaciations. These interpretations follow from a model wherein the δ13C of marine dissolved inorganic carbon (DIC) is controlled by the relative sedimentary burial rates of biogenic carbonate (BC) and organic carbon (OC). A new model proposes authigenic carbonate (AC) as a third major sedimentary C pool, implying that δ13C anomalies are not necessarily indicative of extreme changes in the global carbon cycle and/or atmospheric oxygen. Two conditions are required for AC formation to significantly alter bulk carbonate δ13C: the AC isotopic composition must be at least ∼3‰ different from that of BC and the AC/BC ratio must be >0.1. We use pore fluid Ca and Sr concentrations to estimate rates of AC formation in Late Cenozoic marine sediments, then calculate relative fractions of AC, OC, and BC. Today AC is not expected to constitute a significant fraction of total sedimentary carbon (AC+OC+BC) globally; however, there are modern sites where local conditions promote elevated AC/BC and anaerobic metabolisms can alter the δ13C of pore fluids. We investigate these sites to determine what conditions might enable AC to alter δ13C of marine DIC. We find there is very little net addition of AC relative to BC, but large quantities of AC form today across many settings via recrystallization. In settings where remineralization of organic matter causes recrystallized carbonate to form with modified δ13C, AC/BC is generally too low for this recrystallization to significantly shift the δ13C of the bulk carbonate. However, exceptions are found in sites with very low BC and extensive methane oxidation, suggesting that this environment type would need to be globally extensive in the past in order for AC formation to change the δ13C of marine DIC.

Authigenic carbonate burial in the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA)

“…The first crucial assumption in the authigenic carbon model (Schrag et al ., ) is that the burial of authigenic carbonate is comparable to organic carbon and marine carbonate fluxes. Accurately estimating this term, however, is difficult: significant authigenic burial fluxes can manifest as subtle changes in bulk geochemistry, and studies targeting pre‐Jurassic periods lack the deep sea sedimentary record which is an important locus of authigenic carbonate burial in the modern ocean (Mitnick et al ., ; Bradbury & Turchyn, ). For this reason, calculating the global authigenic carbon sink from Palaeozoic epicontinental records may represent only a conservative estimate.…”

“…The authigenic carbonate revision of the global carbon cycle model (Schrag et al ., ) was based on observations made from modern sites of authigenesis and assumes that similar processes would have been more prevalent during intervals with low pO 2 , a large dissolved inorganic carbon (DIC) reservoir and reduced biological pump (Higgins et al ., ). Ongoing efforts to measure carbonate authigenesis rates in sea floor sediments (Mitnick et al ., ; Bradbury & Turchyn, ) provide important constraints on modern ƒ AC , but represent an imperfect analogue for Palaeozoic sediments deposited in poorly oxygenated basins prior to the advent of pelagic calcification. Based on modern expectations, the Bakken black shales were deposited in conditions conducive to the formation of isotopically offset authigenic carbonates, but instead host large volumes of disseminated carbonates (7 to 9 wt.%) which are isotopically intermediate ( ε AC ≈ 0·3‰ to 3·7‰).…”

“…Peckmann et al ., ; Formolo et al ., ). The global burial of authigenic carbonate is therefore suppressed in the well‐oxygenated modern ocean (Mitnick et al ., ; Bradbury & Turchyn, ). However, Higgins et al .…”

Atmospheric CO<sub>2</sub> estimates for the Miocene to Pleistocene based on foraminiferal <i>δ</i><sup>11</sup>B at Ocean Drilling Program Sites 806 and 807 in the Western Equatorial Pacific

“…Microfossils in sediments at these sites, as with any sedimentary sequences, have the potential to be influenced by diagenesis. Despite evidence of authigenic carbonate formation, recent modeling work concluded that the influence of dissolution and reprecipitation at Sites 806 and 807 was relatively minor (Mitnick et al, 2018). Prior work has also found minimal impacts on the B/Ca ratio of Pliocene foraminifera from Site 806 (White and Ravelo, 2020) and on the Mg/Ca ratio of Miocene Dentoglobigerina altispera shells at Site 806 (Sosdian et al, 2020).…”

Methane Occurrence and Quantification in a Very Shallow Water Environment: A Multidisciplinary Approach

“…, contributing to the atmospheric CH 4 -pool and acting as a powerful greenhouse gas (Mitnick et al, 2018). In very shallow waters (<30 m water depth), this aspect is particularly important since gas bubbles can quickly reach the atmosphere (Borges et al, 2016).…”