An alternative model for positive shifts in shallow‐marine carbonate δ¹³C and δ¹⁸O

Abstract

Positive shifts in global seawater δ13CDIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ18O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow‐marine settings. The model involves geochemically altered platform‐top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ13C and δ18O relative to typical oceanic values. During high‐amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and 13C‐depleted platform‐top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate 13C and 18O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.

An Albian demise of the carbonate platform in the Manín Unit (Western Carpathians, Slovakia)

“…The δ 13 C isotope data suggest shallow water realm and more or less continuous diagenesis under marine conditions. Values are comparable with other platform carbonates, especially with Urgonian ones (Immenhauser et al 2003;Godet et al 2006;Föllmi & Godet 2013;Huck et al 2013). The trends in stable isotopes mirror the separation of the succession into three intervals to some degree (the lower and upper parts of the Podhorie Fm and the Manín Fm) (Fig.…”

Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle

“…In this case the ␦ 13 C of the sediments would then become more negative, as the site at a particular location would be more inf luenced by oceanic carbonate. Although this hypothesis cannot be proven at all of the sites, it has been established by using seismic data that the western margin of GBB has prograded toward the west Ϸ20 km, over the past [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. At the other locations, where studies have been less detailed, it is clear based on the sediment description that there is a decrease in the proportion of shallow water-derived sediments with increasing depth (28,30), a change consistent with a change in sea level and proximity to the shallow-water source.…”

“…However, for older time periods, geologists are forced to use records from macrofossils such as brachiopods, belemnites, and mollusks (11, 12) as well as the ␦ 13 C of bulk sediments associated with carbonate ramps or platforms (13-15) and from epicontinental seas (16). Shallow water carbonate sediments deposited in situ are not reliable recorders of global ␦ 13 C as they are known to be affected by short-term changes in sea level, which subject the deposits to the influences of meteoric diagenesis during which time the ␦ 13 C can be altered substantially (17,18). A more diagenetically stable environment is found on the slopes of carbonate platforms, at depths below glacio-eustatic sea-level changes, where mixtures of pelagic and platform-derived sediments are deposited [so-called periplatform sediments (19)].…”

“…For example, it has been suggested that variations in the ␦ 13 C of carbonate sediments deposited in epeiric seas are related to local recycling of organic material, rather than to changes in the global carbon cycle (18,(21)(22)(23). Another study has shown that variations in the ␦ 13 C of carbonate sediments surrounding the Bahamas, an oceanic dominated platform, are not related to global changes in the organic carbon cycle, at least over the past 25 Myr.…”

Facies development and sequence stratigraphy of the Ludfordian (Upper Silurian) deposits in the Zbruch River Valley, Podolia, western Ukraine: local facies overprint on the δ13Ccarb record of a global stable carbon isotope excursion

“…Very good geochemical, biostratigraphical, and paleoecological documentation exists for the Lau excursion, and it is often used as a model to study positive CIEs, which are widely used as chemostratigraphic markers and as indicators of perturbations of the global carbon cycle (Jeppsson 1990;Aldridge et al 1993;Bickert et al 1997;Jeppsson and Aldridge 2000;Munnecke et al 2003;Calner 2005a;Melchin and Holmden 2006;Cramer and Saltzman 2007a, b;Fanton and Holmden 2007;Calner et al 2008;Małkowski et al 2009;Kozłowski and Sobień 2012). Studies of recent and sub-fossil carbonate sediments indicate that sea-level forcing, restricted circulation and laterally variable sediment composition can significantly influence the d 13 C carb values, leading to amplification of positive excursions in carbonate platform interiors during flooding intervals (Immenhauser et al 2003;Swart and Eberli 2005;Swart 2008;Gischler et al 2009;Oehlert et al 2012). The small-scale spatial variability of d 13 C values results from a combination of generic marine-water signal and local factors such as facies, restricted circulation, and diagenetic overprint.…”