Low and seasonally variable salinity in the Pennsylvanian equatorial Appalachian Basin

Marie Y. Jimenez, Linda C Ivany, Emily J. Judd, Gregory Henkes

Research output: Contribution to journalArticle

Abstract

Use of the δ 18 O thermometer in deep time investigations is complicated by uncertainty in the oxygen isotopic composition of seawater and an increasing potential for diagenetic alteration with age. These concerns are particularly important when considering that δ 18 O values from Paleozoic marine carbonates tend to be low and increasingly depleted with age. Demonstrating the preservation of original chemistry and thus eliminating diagenesis as a factor is a primary hurdle in reconstructing climatic conditions in deep time. Here, we report stable isotope data from serially sampled, Middle Pennsylvanian mollusks from the tropical Appalachian Basin of Kentucky, USA. X-ray diffraction and scanning electron microscopy indicate an aragonite mineralogy with retention of primary microtextures, elemental chemistry is consistent with the shells of living mollusks, and δ 18 O data reveal regular cyclic variation over ontogeny, together suggesting that original shell carbonate is preserved and records environmental conditions over the life history of the animal. However, values are depleted, centering around −4.6‰, and intraannual variation is significant, spanning up to 2.2‰, calling for significant summertime runoff of fresh water to the basin in which the shell-producing organisms grew. Our data extend a documented trend toward more depleted carbonate values eastward across the midcontinent sea with increasing distance from Panthalassa, reflecting a salinity gradient driven largely by isotopically depleted and seasonal runoff from the Central Pangaean Mountains. Mollusk data support the primary nature of low δ 18 O values from brachiopod shells in the midcontinent, but they also highlight the problems with calculating paleotemperatures from δ 18 O values of carbonates precipitated in epicontinental seas by assuming a marine seawater composition. While carbonate clumped isotope paleothermometry is a potential solution to this seawater δ 18 O ‘problem’, we demonstrate that such data from these aragonitic samples are related to burial diagenesis and not the Pennsylvanian paleoenvironment. The bias toward samples from such settings in the Paleozoic, because continental margins are more likely to be deformed, suggests that postulated regional and global temperature histories should be treated with caution until epeiric oxygen isotope seawater values can be better constrained.

Original languageEnglish (US)
Pages (from-to)182-191
Number of pages10
JournalEarth and Planetary Science Letters
Volume519
DOIs
StatePublished - Aug 1 2019

Fingerprint

Carbonates
Pennsylvanian
salinity
mollusks
carbonates
Seawater
carbonate
shell
seawater
basin
drainage
Runoff
Isotopes
Oxygen Isotopes
Paleozoic
isotopes
ontogeny
histories
chemistry
runoff

Keywords

  • Appalachian Basin
  • mollusk
  • oxygen isotope
  • Pennsylvanian
  • salinity
  • seasonality

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science

Cite this

Low and seasonally variable salinity in the Pennsylvanian equatorial Appalachian Basin. / Jimenez, Marie Y.; Ivany, Linda C; Judd, Emily J.; Henkes, Gregory.

In: Earth and Planetary Science Letters, Vol. 519, 01.08.2019, p. 182-191.

Research output: Contribution to journalArticle

Jimenez, Marie Y. ; Ivany, Linda C ; Judd, Emily J. ; Henkes, Gregory. / Low and seasonally variable salinity in the Pennsylvanian equatorial Appalachian Basin. In: Earth and Planetary Science Letters. 2019 ; Vol. 519. pp. 182-191.
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