Isotopic evidence for geochemical decoupling between ancient epeiric seas and bordering oceans: Implications for secular curves

Abstract

Isotopic analysis of conodonts and their host limestones sampled between two regionally extensive, altered volcanic ash layers in eastern Laurentia shows that a 454 Ma epeiric sea maintained large lateral differences in Nd and C isotope compositions. This is consistent with inferred temperature-salinity-defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments. Because the majority of old marine fossils and sedimentary rocks are known from epeiric seas, some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses, rather than global-scale changes in the state of the earth-ocean system. Data Repository item 9861 contains additional material related to this article.

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

“…Such changes would be manifested as a decrease in the ␦ 13 C of carbonates toward the more restricted portions of the shelf (52). The mechanism proposed by those workers (21,22,52) could easily complement the model presented here during certain geological time periods, thus making the interpretation of changes in the ␦ 13 C of carbonates even more problematic.…”

“…As aragonite is Ϸ2-3‰ enriched in ␦ 13 C relative to LMC at standard temperatures and pressures, the pattern of decreasing ␦ 13 C with age over periods Ͼ10 Myr, which is evident at Site 1006 (23), may partially reflect a change from predominantly aragonite to calcite deposition by shallow-water organisms. Previous work has suggested that global changes in sea level during the Paleozoic could cause local variation in the recycling of organic carbon, also leading to changes in ␦ 13 C in epeiric seas unrelated to the global carbon cycle (21,22). During high stands of sea level it was suggested that circulation of water would be increased, hence reducing the influence of oxidized organic material.…”

“…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.…”

“…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

“…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. Although it is difficult to attribute the final d 13 C values to the influence of a particular factor, in some cases it can be demonstrated (e.g., Holmden et al 1998;Harzhauser et al 2007;Colombié et al 2011). Some local factors seem to have been involved in the record of the Lau excursion, which is mostly documented by d 13 C carb of limestones from the epicontinental basin developed during the early Paleozoic on Baltica.…”