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Title: The influence of seawater carbonate chemistry, mineralogy, and diagenesis on calcium isotope variations in Lower-Middle Triassic carbonate rocks

The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios ( 44 Ca/ 40 Ca, expressed as δ 44/40 Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. But, the δ 44/40 Ca of carbonate rocks is influenced not only by the δ 44/40 Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ 44/40 Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ 44/40 Ca may be expected based on several large carbon isotope (δ 13 C) excursions ranging from -2 to + 8‰. Parallel negative δ 13 C and δ 44/40 Ca excursions were previously identified across the end-Permian extinction horizon. Here, we find a second negative excursion in δ 44/40 Ca of ~ 0.2‰ within Lower Triassic strata in both south China and Turkey; however, this excursion is not synchronousmore » between regions and thus cannot be interpreted to reflect secular change in the δ 44/40 Ca of global seawater. Additionally, δ 44/40 Ca values from Turkey are consistently 0.3‰ lower than contemporaneous samples from south China, providing further support for local or regional influences. By measuring δ 44/40 Ca and Sr concentrations ([Sr]) in two stratigraphic sections located at opposite margins of the Paleo-Tethys Ocean, we can determine whether the data represent global conditions (e.g., secular variations in the δ 44/40 Ca of seawater) versus local controls (e.g., original mineralogy or diagenetic alteration). The [Sr] and δ 44/40 Ca data from this study are best described statistically by a log-linear correlation that also exists in many previously published datasets of various geological ages. By using a model of early marine diagenetic water-rock interaction, we illustrate that this general correlation can be explained by the chemical evolution of bulk carbonate sediment samples with different initial mineralogical compositions that subsequently underwent recrystallization. Although early diagenetic resetting and carbonate mineralogy strongly influence the carbonate δ 44/40 Ca values, the relationship between [Sr] and δ 44/40 Ca holds potential for reconstructing first-order secular changes in seawater δ 44/40 Ca composition.« less
 [1] ;  [1] ;  [2] ;  [3] ;  [4] ;  [2] ;  [5] ;  [6] ;  [7] ;  [8] ;  [9] ;  [10] ;  [1]
  1. Stanford Univ., CA (United States). Dept. of Geological Sciences
  2. Univ. of California, Berkeley, CA (United States). Dept. of Earth and Planetary Science; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. Stanford Univ., CA (United States). Dept. of Geological Sciences; Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States). Dept. of Earth, Atmosphereic and Planetary Sciences
  4. Middle East Technical Univ., Ankara (Turkey). Dept. of Geological Engineering
  5. Helmholtz Centre for Ocean Research, Kiel (Germany)
  6. Stanford Univ., CA (United States). Dept. of Geological Sciences; Exxon Mobil Upstream Research Company, Spring, TX (United States)
  7. Trinity Univ., San Antonio, TX (United States). Geosciences Dept.
  8. Univ. of California, Santa Cruz, CA (United States). Inst. of Marine Sciences
  9. Guizhou Univ. (China). College of Resource adn Environment Engineering
  10. Stanford Univ., CA (United States). Dept. of Geological Sciences; Guizhou Univ. (China). College of Resource and Environment Engineering
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Chemical Geology
Additional Journal Information:
Journal Volume: 471; Journal Issue: C; Journal ID: ISSN 0009-2541
Research Org:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 58 GEOSCIENCES; Calcium isotopes; Carbonates; Early diagenesis; Recrystallization; Calcium cycling
OSTI Identifier: